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2.  Plant defence responses in oilseed rape MINELESS plants after attack by the cabbage moth Mamestra brassicae  
Journal of Experimental Botany  2015;66(2):579-592.
Highlight
The larvae of Mamestra brassicae feed less and show reduced growth on plants with ablated myrosin cells, which raises questions about the role of defence cells in Brassicaceae plants.
The Brassicaceae family is characterized by a unique defence mechanism known as the ‘glucosinolate–myrosinase’ system. When insect herbivores attack plant tissues, glucosinolates are hydrolysed by the enzyme myrosinase (EC 3.2.1.147) into a variety of degradation products, which can deter further herbivory. This process has been described as ‘the mustard oil bomb’. Additionally, insect damage induces the production of glucosinolates, myrosinase, and other defences. Brassica napus seeds have been genetically modified to remove myrosinase-containing myrosin cells. These plants are termed MINELESS because they lack myrosin cells, the so-called toxic mustard oil mines. Here, we examined the interaction between B. napus wild-type and MINELESS plants and the larvae of the cabbage moth Mamestra brassicae. No-choice feeding experiments showed that M. brassicae larvae gained less weight and showed stunted growth when feeding on MINELESS plants compared to feeding on wild-type plants. M. brassicae feeding didn’t affect myrosinase activity in MINELESS plants, but did reduce it in wild-type seedlings. M. brassicae feeding increased the levels of indol-3-yl-methyl, 1-methoxy-indol-3-yl-methyl, and total glucosinolates in both wild-type and MINELESS seedlings. M. brassicae feeding affected the levels of glucosinolate hydrolysis products in both wild-type and MINELESS plants. Transcriptional analysis showed that 494 and 159 genes were differentially regulated after M. brassicae feeding on wild-type and MINELESS seedlings, respectively. Taken together, the outcomes are very interesting in terms of analysing the role of myrosin cells and the glucosinolate–myrosinase defence system in response to a generalist cabbage moth, suggesting that similar studies with other generalist or specialist insect herbivores, including above- and below-ground herbivores, would be useful.
doi:10.1093/jxb/eru490
PMCID: PMC4286410  PMID: 25563968
Brassica napus (oilseed rape); defence cells; generalist; glucosinolate; jasmonates; myrosinase; plant–insect interaction; transcriptional profiling.
3.  The ethylene response factor Pti5 contributes to potato aphid resistance in tomato independent of ethylene signalling 
Journal of Experimental Botany  2014;66(2):559-570.
Highlight
This study demonstrated that the ethylene response factor Pti5 contributes to antibiotic defences against aphids, whereas ethylene contributes to antixenotic but not antibiotic aphid resistance.
Ethylene response factors (ERFs) comprise a large family of transcription factors that regulate numerous biological processes including growth, development, and response to environmental stresses. Here, we report that Pti5, an ERF in tomato [Solanum lycopersicum (Linnaeus)] was transcriptionally upregulated in response to the potato aphid Macrosiphum euphorbiae (Thomas), and contributed to plant defences that limited the population growth of this phloem-feeding insect. Virus-induced gene silencing of Pti5 enhanced aphid population growth on tomato, both on an aphid-susceptible cultivar and on a near-isogenic genotype that carried the Mi-1.2 resistance (R) gene. These results indicate that Pti5 contributes to basal resistance in susceptible plants and also can synergize with other R gene-mediated defences to limit aphid survival and reproduction. Although Pti5 contains the ERF motif, induction of this gene by aphids was independent of ethylene, since the ACC deaminase (ACD) transgene, which inhibits ethylene synthesis, did not diminish the responsiveness of Pti5 to aphid infestation. Furthermore, experiments with inhibitors of ethylene synthesis revealed that Pti5 and ethylene have distinctly different roles in plant responses to aphids. Whereas Pti5 contributed to antibiotic plant defences that limited aphid survival and reproduction on both resistant (Mi-1.2+) and susceptible (Mi-1.2–) genotypes, ethylene signalling promoted aphid infestation on susceptible plants but contributed to antixenotic defences that deterred the early stages of aphid host selection on resistant plants. These findings suggest that the antixenotic defences that inhibit aphid settling and the antibiotic defences that depress fecundity and promote mortality are regulated through different signalling pathways.
doi:10.1093/jxb/eru472
PMCID: PMC4286409  PMID: 25504643
Basal resistance; ERF; EREBP; insect resistance; Macrosiphum euphorbiae; Mi-1.2.
4.  A force of nature: molecular mechanisms of mechanoperception in plants 
Journal of Experimental Botany  2013;64(15):4663-4680.
The ability to sense and respond to a wide variety of mechanical stimuli—gravity, touch, osmotic pressure, or the resistance of the cell wall—is a critical feature of every plant cell, whether or not it is specialized for mechanotransduction. Mechanoperceptive events are an essential part of plant life, required for normal growth and development at the cell, tissue, and whole-plant level and for the proper response to an array of biotic and abiotic stresses. One current challenge for plant mechanobiologists is to link these physiological responses to specific mechanoreceptors and signal transduction pathways. Here, we describe recent progress in the identification and characterization of two classes of putative mechanoreceptors, ion channels and receptor-like kinases. We also discuss how the secondary messenger Ca2+ operates at the centre of many of these mechanical signal transduction pathways.
doi:10.1093/jxb/ert204
PMCID: PMC3817949  PMID: 23913953
Calcium; cell-wall integrity; mechanoperception; mechanosensitive ion channels; receptor-like kinases; thigmomorphogenesis.
5.  Amino acid-mediated impacts of elevated carbon dioxide and simulated root herbivory on aphids are neutralized by increased air temperatures 
Journal of Experimental Botany  2014;66(2):613-623.
Highlight
Multiple abiotic factors can combine to alter crop quality and rates of herbivore attack. Aphids benefit from elevated CO2 and root damage, but these effects are neutralized by increased temperatures.
Changes in host plant quality, including foliar amino acid concentrations, resulting from global climate change and attack from multiple herbivores, have the potential to modify the pest status of insect herbivores. This study investigated how mechanically simulated root herbivory of lucerne (Medicago sativa) before and after aphid infestation affected the pea aphid (Acyrthosiphon pisum) under elevated temperature (eT) and carbon dioxide concentrations (eCO2). eT increased plant height and biomass, and eCO2 decreased root C:N. Foliar amino acid concentrations and aphid numbers increased in response to eCO2, but only at ambient temperatures, demonstrating the ability of eT to negate the effects of eCO2. Root damage reduced aboveground biomass, height, and root %N, and increased root %C and C:N, most probably via decreased biological nitrogen fixation. Total foliar amino acid concentrations and aphid colonization success were higher in plants with roots cut early (before aphid arrival) than those with roots cut late (after aphid arrival); however, this effect was counteracted by eT. These results demonstrate the importance of amino acid concentrations for aphids and identify individual amino acids as being potential factors underpinning aphid responses to eT, eCO2, and root damage in lucerne. Incorporating trophic complexity and multiple climatic factors into plant–herbivore studies enables greater insight into how plants and insects will interact in the future, with implications for sustainable pest control and future crop security.
doi:10.1093/jxb/eru439
PMCID: PMC4286407  PMID: 25403916
Aboveground–belowground interactions; aphid; climate change; legume; root herbivore; simulated herbivory.
6.  Persistence and transgenerational effect of plant-mediated RNAi in aphids 
Journal of Experimental Botany  2014;66(2):541-548.
Highlight
This work shows that the transgenerational effect of RNA interference in the green peach aphid Myzus persicae contributes to a 60% decline in aphid population growth.
Plant-mediated RNA interference (RNAi) has been successfully used as a tool to study gene function in aphids. The persistence and transgenerational effects of plant-mediated RNAi in the green peach aphid (GPA) Myzus persicae were investigated, with a focus on three genes with different functions in the aphid. Rack1 is a key component of various cellular processes inside aphids, while candidate effector genes MpC002 and MpPIntO2 (Mp2) modulate aphid–plant interactions. The gene sequences and functions did not affect RNAi-mediated down-regulation and persistence levels in the aphids. Maximal reduction of gene expression was ~70% and this was achieved at between 4 d and 8 d of exposure of the aphids to double-stranded RNA (dsRNA)-producing transgenic Arabidopsis thaliana. Moreover, gene expression levels returned to wild-type levels within ~6 d after removal of the aphids from the transgenic plants, indicating that a continuous supply of dsRNA is required to maintain the RNAi effect. Target genes were also down-regulated in nymphs born from mothers exposed to dsRNA-producing transgenic plants, and the RNAi effect lasted twice as long (12–14 d) in these nymphs. Investigations of the impact of RNAi over three generations of aphids revealed that aphids reared on dsMpC002 transgenic plants experienced a 60% decline in aphid reproduction levels compared with a 40% decline of aphids reared on dsRack1 and dsMpPIntO2 plants. In a field setting, a reduction of the aphid reproduction by 40–60% would dramatically decrease aphid population growth, contributing to a substantial reduction in agricultural losses.
doi:10.1093/jxb/eru450
PMCID: PMC4286408  PMID: 25403918
Aphid; dsRNA acquisition by feeding; effector; green peach aphid; Hemiptera; Myzus persicae; saliva; silencing; systemic movement; virulence and fecundity.
7.  Functional characterization of cotton genes responsive to Verticillium dahliae through bioinformatics and reverse genetics strategies 
Journal of Experimental Botany  2014;65(22):6679-6692.
Verticillium wilt causes dramatic cotton yield loss in China. Although some genes or biological processes involved in the interaction between cotton and Verticillium dahliae have been identified, the molecular mechanism of cotton resistance to this disease is still poorly understood. The basic innate immune response for defence is somewhat conserved among plant species to defend themselves in complex environments, which makes it possible to characterize genes involved in cotton immunity based on information from model plants. With the availability of Arabidopsis databases, a data-mining strategy accompanied by virus-induced gene silencing (VIGS) and heterologous expression were adopted in cotton and tobacco, respectively, for global screening and gene function characterization. A total of 232 Arabidopsis genes putatively involved in basic innate immunity were screened as candidate genes, and bioinformatic analysis suggested a role of these genes in the immune response. In total, 38 homologous genes from cotton were singled out to characterize their response to V. dahliae and methyl jasmonate treatment through quantitative real-time PCR. The results revealed that 24 genes were differentially regulated by pathogen inoculation, and most of these genes responded to both Verticillium infection and jasmonic acid stimuli. Furthermore, the efficiency of the strategy was illustrated by the functional identification of six candidate genes via heterologous expression in tobacco or a knock-down approach using VIGS in cotton. Functional categorization of these 24 differentially expressed genes as well as functional analysis suggest that reactive oxygen species, salicylic acid- and jasmonic acid-signalling pathways are involved in the cotton disease resistance response to V. dahliae. Our data demonstrate how information from model plants can allow the rapid translation of information into non-model species without complete genome sequencing, via high-throughput screening and functional identification of target genes based on data-mining and VIGS.
doi:10.1093/jxb/eru393
PMCID: PMC4246195  PMID: 25326626
Arabidopsis; data-mining; Gossypium hirsutum; innate immune response,; Verticillium dahliae; virus-induced gene silencing.
8.  Knockout of the two evolutionarily conserved peroxisomal 3-ketoacyl-CoA thiolases in Arabidopsis recapitulates the abnormal inflorescence meristem 1 phenotype 
Journal of Experimental Botany  2014;65(22):6723-6733.
Summary
Simultaneous mutation of two peroxisomal thiolase enzymes shows that fatty acid β-oxidation is required for the normal development of inflorescences in Arabidopsis and for successful fertilization to produce seed.
A specific function for peroxisomal β-oxidation in inflorescence development in Arabidopsis thaliana is suggested by the mutation of the ABNORMAL INFLORESCENCE MERISTEM 1 gene, which encodes one of two peroxisomal multifunctional proteins. Therefore, it should be possible to identify other β-oxidation mutants that recapitulate the aim1 phenotype. Three genes encode peroxisomal 3-ketoacyl-CoA thiolase (KAT) in Arabidopsis. KAT2 and KAT5 are present throughout angiosperms whereas KAT1 is a Brassicaceae-specific duplication of KAT2 expressed at low levels in Arabidopsis. KAT2 plays a dominant role in all known aspects of peroxisomal β-oxidation, including that of fatty acids, pro-auxins, jasmonate precursor oxophytodienoic acid, and trans-cinnamic acid. The functions of KAT1 and KAT5 are unknown. Since KAT5 is conserved throughout vascular plants and expressed strongly in flowers, kat2 kat5 double mutants were generated. These were slow growing, had abnormally branched inflorescences, and ectopic organ growth. They made viable pollen, but produced no seed indicating that infertility was due to defective gynaecium function. These phenotypes are strikingly similar to those of aim1. KAT5 in the Brassicaceae encodes both cytosolic and peroxisomal proteins and kat2 kat5 defects could be complemented by the re-introduction of peroxisomal (but not cytosolic) KAT5. It is concluded that peroxisomal KAT2 and KAT5 have partially redundant functions and operate downstream of AIM1 to provide β-oxidation functions essential for inflorescence development and fertility.
doi:10.1093/jxb/eru397
PMCID: PMC4246196  PMID: 25297549
3-Ketoacyl-CoA thiolase; Arabidopsis thaliana; β-oxidation; flowering; germination; peroxisome; seed development.
9.  Leaf hydraulic conductance declines in coordination with photosynthesis, transpiration and leaf water status as soybean leaves age regardless of soil moisture 
Journal of Experimental Botany  2014;65(22):6617-6627.
Summary
Short statement: Field and chamber studies show a decline in leaf hydraulic conductance as soybean leaves age that is independent of decreases in soil moisture.
Photosynthesis requires sufficient water transport through leaves for stomata to remain open as water transpires from the leaf, allowing CO2 to diffuse into the leaf. The leaf water needs of soybean change over time because of large microenvironment changes over their lifespan, as leaves mature in full sun at the top of the canopy and then become progressively shaded by younger leaves developing above. Leaf hydraulic conductance (K leaf), a measure of the leaf’s water transport capacity, can often be linked to changes in microenvironment and transpiration demand. In this study, we tested the hypothesis that K leaf would decline in coordination with transpiration demand as soybean leaves matured and aged. Photosynthesis (A), stomatal conductance (g s) and leaf water potential (Ψleaf) were also measured at various leaf ages with both field- and chamber-grown soybeans to assess transpiration demand. K leaf was found to decrease as soybean leaves aged from maturity to shading to senescence, and this decrease was strongly correlated with midday A. Decreases in K leaf were further correlated with decreases in g s, although the relationship was not as strong as that with A. Separate experiments investigating the response of K leaf to drought demonstrated no acclimation of K leaf to drought conditions to protect against cavitation or loss of g s during drought and confirmed the effect of leaf age in K leaf observed in the field. These results suggest that the decline of leaf hydraulic conductance as leaves age keeps hydraulic supply in balance with demand without K leaf becoming limiting to transpiration water flux.
doi:10.1093/jxb/eru380
PMCID: PMC4246190  PMID: 25281701
Development; drought; leaf age; leaf hydraulic conductance; leaf water potential; photosynthesis; senescence; stomatal conductance.
10.  The trehalose pathway in maize: conservation and gene regulation in response to the diurnal cycle and extended darkness 
Journal of Experimental Botany  2014;65(20):5959-5973.
Summary
Extended darkness induces a transient increase in sugars and trehalose pathway gene expression.
Energy resources in plants are managed in continuously changing environments, such as changes occurring during the day/night cycle. Shading is an environmental disruption that decreases photosynthesis, compromises energy status, and impacts on crop productivity. The trehalose pathway plays a central but not well-defined role in maintaining energy balance. Here, we characterized the maize trehalose pathway genes and deciphered the impacts of the diurnal cycle and disruption of the day/night cycle on trehalose pathway gene expression and sugar metabolism. The maize genome encodes 14 trehalose-6-phosphate synthase (TPS) genes, 11 trehalose-6-phosphate phosphatase (TPP) genes, and one trehalase gene. Transcript abundance of most of these genes was impacted by the day/night cycle and extended dark stress, as were sucrose, hexose sugars, starch, and trehalose-6-phosphate (T6P) levels. After extended darkness, T6P levels inversely followed class II TPS and sucrose non-fermenting-related protein kinase 1 (SnRK1) target gene expression. Most significantly, T6P no longer tracked sucrose levels after extended darkness. These results showed: (i) conservation of the trehalose pathway in maize; (ii) that sucrose, hexose, starch, T6P, and TPS/TPP transcripts respond to the diurnal cycle; and(iii) that extended darkness disrupts the correlation between T6P and sucrose/hexose pools and affects SnRK1 target gene expression. A model for the role of the trehalose pathway in sensing of sucrose and energy status in maize seedlings is proposed.
doi:10.1093/jxb/eru335
PMCID: PMC4203130  PMID: 25271261
Maize; shade stress,; trehalose-6-phosphate; trehalose gene family; diurnal cycle,; quantitative RT-PCR.
11.  Fertility of CMS wheat is restored by two Rf loci located on a recombined acrocentric chromosome 
Journal of Experimental Botany  2014;65(22):6667-6677.
Summary
The high potential for an acrocentric chromosome originated from a complex reorganization of chromosomes 1HchS and 6HchS from Hordeum chilense in the development of hybrid wheat technology.
Cytoplasmic male sterility (CMS) results from incompatibility between nuclear and cytoplasmic genomes, and is characterized by the inability to produce viable pollen. The restoration of male fertility generally involves the introgression of nuclear genes, termed restorers of fertility (Rf). CMS has been widely used for hybrid seed production in many crops but not in wheat, partly owing to the complex genetics of fertility restoration. In this study, an acrocentric chromosome that restores pollen fertility of CMS wheat in Hordeum chilense cytoplasm (msH1 system) is studied. The results show that this chromosome, of H. chilense origin and named Hchac, originated from a complex reorganization of the short arm of chromosomes 1Hch (1HchS) and 6Hch (6HchS). Diversity arrays technology (DArT) markers and cytological analysis indicate that Hchac is a kind of `zebra-like′ chromosome composed of chromosome 1HchS and alternate fragments of interstitial and distal regions of chromosome 6HchS. PCR-based markers together with FISH, GISH, and meiotic pairing analysis support this result. A restorer of fertility gene, named Rf 6H ch S, has been identified on the short arm of chromosome 6HchS. Moreover, restoration by the addition of chromosome 1HchS has been observed at a very low frequency and under certain environmental conditions. Therefore, the results indicate the presence of two Rf genes on the acrocentric chromosome: Rf 6H ch S and Rf 1H ch S, the restoration potential of Rf 6H ch S being greater. The stable and high restoration of pollen fertility in the msH1 system is therefore the result of the interaction between these two restorer genes.
doi:10.1093/jxb/eru388
PMCID: PMC4246193  PMID: 25271260
Acrocentric chromosome; cytoplasmic male sterility; Hordeum chilense; restorer gene; Triticum aestivum; zebra-like chromosome.
12.  Accumulation of 5-hydroxynorvaline in maize (Zea mays) leaves is induced by insect feeding and abiotic stress 
Journal of Experimental Botany  2014;66(2):593-602.
Summary
Short statement: 5-Hydroxynorvaline is an abundant, stress-induced non-protein amino acid in maize. At concentrations similar to those in maize leaves, 5-hydroxynorvaline inhibits Rhopalosiphum maidis (corn leaf aphid) reproduction on artificial diet.
Plants produce a wide variety of defensive metabolites to protect themselves against herbivores and pathogens. Non-protein amino acids, which are present in many plant species, can have a defensive function through their mis-incorporation during protein synthesis and/or inhibition of biosynthetic pathways in primary metabolism. 5-Hydroxynorvaline was identified in a targeted search for previously unknown non-protein amino acids in the leaves of maize (Zea mays) inbred line B73. Accumulation of this compound increases during herbivory by aphids (Rhopalosiphum maidis, corn leaf aphid) and caterpillars (Spodoptera exigua, beet armyworm), as well as in response to treatment with the plant signalling molecules methyl jasmonate, salicylic acid and abscisic acid. In contrast, ethylene signalling reduced 5-hydroxynorvaline abundance. Drought stress induced 5-hydroxynorvaline accumulation to a higher level than insect feeding or treatment with defence signalling molecules. In field-grown plants, the 5-hydroxynorvaline concentration was highest in above-ground vegetative tissue, but it was also detectable in roots and dry seeds. When 5-hydroxynorvaline was added to aphid artificial diet at concentrations similar to those found in maize leaves and stems, R. maidis reproduction was reduced, indicating that this maize metabolite may have a defensive function. Among 27 tested maize inbred lines there was a greater than 10-fold range in the accumulation of foliar 5-hydroxynorvaline. Genetic mapping populations derived from a subset of these inbred lines were used to map quantitative trait loci for 5-hydroxynorvaline accumulation to maize chromosomes 5 and 7.
doi:10.1093/jxb/eru385
PMCID: PMC4286406  PMID: 25271262
Aphid; drought; 5-hydroxynorvaline; maize; Rhopalosiphum maidis.
13.  Changes in DNA damage, molecular integrity, and copy number for plastid DNA and mitochondrial DNA during maize development 
Journal of Experimental Botany  2014;65(22):6425-6439.
Summary
During development in maize, chloroplast and mitochondrial DNA damage increases, copy number of unimpeded DNA molecules decreases, and in vitro DNA repair increases, with light causing the greatest change.
The amount and structural integrity of organellar DNAs change during plant development, although the mechanisms of change are poorly understood. Using PCR-based methods, we quantified DNA damage, molecular integrity, and genome copy number for plastid and mitochondrial DNAs of maize seedlings. A DNA repair assay was also used to assess DNA impediments. During development, DNA damage increased and molecules with impediments that prevented amplification by Taq DNA polymerase increased, with light causing the greatest change. DNA copy number values depended on the assay method, with standard real-time quantitative PCR (qPCR) values exceeding those determined by long-PCR by 100- to 1000-fold. As the organelles develop, their DNAs may be damaged in oxidative environments created by photo-oxidative reactions and photosynthetic/respiratory electron transfer. Some molecules may be repaired, while molecules with unrepaired damage may be degraded to non-functional fragments measured by standard qPCR but not by long-PCR.
doi:10.1093/jxb/eru359
PMCID: PMC4246179  PMID: 25261192
DNA damage; DNA repair; organellar DNA; PCR; reactive oxygen species; Zea mays.
14.  Computational and genetic evidence that different structural conformations of a non-catalytic region affect the function of plant cellulose synthase 
Journal of Experimental Botany  2014;65(22):6645-6653.
Summary
Computational modelling of peptide structure, genetic complementation in Arabidopsis thaliana, and confocal microscopy provide evidence that a region between two transmembrane helices may adopt two predominant structural conformations that affect the function of plant cellulose synthase.
The β-1,4-glucan chains comprising cellulose are synthesized by cellulose synthases in the plasma membranes of diverse organisms including bacteria and plants. Understanding structure–function relationships in the plant enzymes involved in cellulose synthesis (CESAs) is important because cellulose is the most abundant component in the plant cell wall, a key renewable biomaterial. Here, we explored the structure and function of the region encompassing transmembrane helices (TMHs) 5 and 6 in CESA using computational and genetic tools. Ab initio computational structure prediction revealed novel bi-modal structural conformations of the region between TMH5 and 6 that may affect CESA function. Here we present our computational findings on this region in three CESAs of Arabidopsis thaliana (AtCESA1, 3, and 6), the Atcesa3 ixr1-2 mutant, and a novel missense mutation in AtCESA1. A newly engineered point mutation in AtCESA1 (Atcesa1 F954L) that altered the structural conformation in silico resulted in a protein that was not fully functional in the temperature-sensitive Atcesa1 rsw1-1 mutant at the restrictive temperature. The combination of computational and genetic results provides evidence that the ability of the TMH5–6 region to adopt specific structural conformations is important for CESA function.
doi:10.1093/jxb/eru383
PMCID: PMC4246192  PMID: 25262226
Cellulose biosynthesis; CESA; genetic complementation; protein structural modelling; rsw1 mutant; transmembrane helix.
15.  Tomato Male sterile 1035 is essential for pollen development and meiosis in anthers 
Journal of Experimental Botany  2014;65(22):6693-6709.
Summary
This study demonstrated that tomato Male sterile 10 35 encodes a basic helix–loop–helix transcription factor involved in meiosis and tapetum development at the early stage of anther development.
Male fertility in flowering plants depends on proper cellular differentiation in anthers. Meiosis and tapetum development are particularly important processes in pollen production. In this study, we showed that the tomato male sterile (ms10 35) mutant of cultivated tomato (Solanum lycopersicum) exhibited dysfunctional meiosis and an abnormal tapetum during anther development, resulting in no pollen production. We demonstrated that Ms10 35 encodes a basic helix–loop–helix transcription factor that is specifically expressed in meiocyte and tapetal tissue from pre-meiotic to tetrad stages. Transgenic expression of the Ms10 35 gene from its native promoter complemented the male sterility of the ms10 35 mutant. In addition, RNA-sequencing-based transcriptome analysis revealed that Ms10 35 regulates 246 genes involved in anther development processes such as meiosis, tapetum development, cell-wall degradation, pollen wall formation, transport, and lipid metabolism. Our results indicate that Ms10 35 plays key roles in regulating both meiosis and programmed cell death of the tapetum during microsporogenesis.
doi:10.1093/jxb/eru389
PMCID: PMC4246194  PMID: 25262227
Anther; male sterility; meiosis; tapetum; tomato (Solanum lycopersicum).
16.  Genome-wide identification and characterization of the Populus WRKY transcription factor family and analysis of their expression in response to biotic and abiotic stresses 
Journal of Experimental Botany  2014;65(22):6629-6644.
Highlight text
This study presents the genome-wide characterization of the Populus WRKY family under biotic and abiotic stresses. Overexpression of an SA-inducible gene, PtrWRKY89, enhanced resistance to pathogens in transgenic poplar.
WRKY proteins are a large family of regulators involved in various developmental and physiological processes, especially in coping with diverse biotic and abiotic stresses. In this study, 100 putative PtrWRKY genes encoded the proteins contained in the complete WRKY domain in Populus. Phylogenetic analysis revealed that the members of this superfamily among poplar, Arabidopsis, and other species were divided into three groups with several subgroups based on the structures of the WRKY protein sequences. Various cis-acting elements related to stress and defence responses were found in the promoter regions of PtrWRKY genes by promoter analysis. High-throughput transcriptomic analyses identified that 61 of the PtrWRKY genes were induced by biotic and abiotic treatments, such as Marssonina brunnea, salicylic acid (SA), methyl jasmonate (MeJA), wounding, cold, and salinity. Among these PtrWRKY genes, transcripts of 46 selected genes were observed in different tissues, including roots, stems, and leaves. Quantitative RT-PCR analysis further confirmed the induced expression of 18 PtrWRKY genes by one or more stress treatments. The overexpression of an SA-inducible gene, PtrWRKY89, accelerated expression of PR protein genes and improved resistance to pathogens in transgenic poplar, suggesting that PtrWRKY89 is a regulator of an SA-dependent defence-signalling pathway in poplar. Taken together, our results provided significant information for improving the resistance and stress tolerance of woody plants.
doi:10.1093/jxb/eru381
PMCID: PMC4246191  PMID: 25249073
Pathogen; Populus; SA (salicylic acid); stress tolerance; transcription factor; WRKY.
17.  Additive effects of two quantitative trait loci that confer Rhopalosiphum maidis (corn leaf aphid) resistance in maize inbred line Mo17 
Journal of Experimental Botany  2014;66(2):571-578.
Summary
At least two genetic loci contribute to the high aphid resistance observed in the seedlings of maize inbred line Mo17. One of these loci increases the biosynthesis of defence-related benzoxazinoids.
Plants show considerable within-species variation in their resistance to insect herbivores. In the case of Zea mays (cultivated maize), Rhopalosiphum maidis (corn leaf aphids) produce approximately twenty times more progeny on inbred line B73 than on inbred line Mo17. Genetic mapping of this difference in maize aphid resistance identified quantitative trait loci (QTL) on chromosomes 4 and 6, with the Mo17 allele reducing aphid reproduction in each case. The chromosome 4 QTL mapping interval includes several genes involved in the biosynthesis of DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a maize defensive metabolite that also is required for callose accumulation in response to aphid feeding. Consistent with the known association of callose with plant defence against aphids, R. maidis reproduction on B73×Mo17 recombinant inbred lines was negatively correlated with both DIMBOA content and callose formation. Further genetic mapping, as well as experiments with near-isogenic lines, confirmed that the Mo17 allele causes increased DIMBOA accumulation relative to the B73 allele. The chromosome 6 aphid resistance QTL functions independently of DIMBOA accumulation and has an effect that is additive to that of the chromosome 4 QTL. Thus, at least two separate defence mechanisms account for the higher level of R. maidis resistance in Mo17 compared with B73.
doi:10.1093/jxb/eru379
PMCID: PMC4286405  PMID: 25249072
DIMBOA; Rhopalosiphum maidis; aphid; benzoxazinoid; callose; maize; quantitative trait.
18.  Multivariate genetic analysis of plant responses to water deficit and high temperature revealed contrasting adaptive strategies 
Journal of Experimental Botany  2014;65(22):6457-6469.
Summary
Plants need to control transpiration and photosynthesis tightly in order to grow under drought and high temperature. Contrasting genetic-by-environment interactions are exploited by Arabidopsis thaliana to improve stress tolerance.
How genetic factors control plant performance under stressful environmental conditions is a central question in ecology and for crop breeding. A multivariate framework was developed to examine the genetic architecture of performance-related traits in response to interacting environmental stresses. Ecophysiological and life history traits were quantified in the Arabidopsis thaliana Ler×Cvi mapping population exposed to constant soil water deficit and high air temperature. The plasticity of the genetic variance–covariance matrix (G-matrix) was examined using mixed-effects models after regression into principal components. Quantitative trait locus (QTL) analysis was performed on the predictors of genotype effects and genotype by environment interactions (G×E). Three QTLs previously identified for flowering time had antagonistic G×E effects on carbon acquisition and the other traits (phenology, growth, leaf morphology, and transpiration). This resulted in a size-dependent response of water use efficiency (WUE) to high temperature but not soil water deficit, indicating that most of the plasticity of carbon acquisition and WUE to temperature is controlled by the loci that control variation of development, size, growth, and transpiration. A fourth QTL, MSAT2.22, controlled the response of carbon acquisition to specific combinations of watering and temperature irrespective of plant size and development, growth, and transpiration rate, which resulted in size-independent plasticity of WUE. These findings highlight how the strategies to optimize plant performance may differ in response to water deficit and high temperature (or their combination), and how different G×E effects could be targeted to improve plant tolerance to these stresses.
doi:10.1093/jxb/eru364
PMCID: PMC4246181  PMID: 25246443
Antagonistic pleiotropy; Arabidopsis thaliana; genotype by environment interactions; G-matrix; mixed-effects model; photosynthesis; QTL; water use efficiency.
19.  A major root-associated acid phosphatase in Arabidopsis, AtPAP10, is regulated by both local and systemic signals under phosphate starvation 
Journal of Experimental Botany  2014;65(22):6577-6588.
Summary
Under phosphate starvation, the root-associated activity of Arabidopsis acid phosphatase AtPAP10 is differentially regulated by local and systemic signalling at transcriptional, post-transcriptional, and post-translational levels.
The induction and secretion of acid phosphatases (APases) is a universal response of plants to phosphate (Pi) starvation. AtPAP10 (Arabidopsis purple acid phosphatase 10) is a major Pi starvation-induced APase that is associated with the root surface in Arabidopsis. So far, the roles of local and systemic signalling in regulating root-associated AtPAP10 activity remain largely unknown. In this work, we show that a decrease of local, external Pi availability is sufficient to induce AtPAP10 transcription in roots in the presence of sucrose, a systemic signal from shoots, whereas the magnitude of the induction is affected by the Pi status of the whole plant. Once the AtPAP10 mRNAs are synthesized in roots, subsequent accumulation of AtPAP10 proteins in root cells and increase in AtPAP10 activity on the root surface are mainly controlled by local signalling. Previously, ethylene has been demonstrated to be a positive regulator of AtPAP10 activity. In this study, we provide evidence that under Pi deficiency ethylene mainly modulates enzymatic activity of AtPAP10 on the root surface, but not AtPAP10 transcription and protein accumulation, suggesting that it functions as a local signal. Furthermore, our work indicates that the effect of ethylene on the induction of root-associated AtPAP10 activity depends on sucrose, but that the effect of sucrose does not depend on ethylene. These results reveal new insights into the distinct roles of local and systemic signalling in the regulation of root-associated AtPAP10 activity under Pi starvation.
doi:10.1093/jxb/eru377
PMCID: PMC4246188  PMID: 25246445
Phosphate starvation responses; purple acid phosphatase 10; local and systemic signalling; ethylene; sucrose; Arabidopsis.
20.  Interaction between parental environment and genotype affects plant and seed performance in Arabidopsis  
Journal of Experimental Botany  2014;65(22):6603-6615.
Highlight text
The genotype-by-environment interactions of five parental environments with seed and plant performance are mediated by distinct genetic and molecular pathways, and the selective pressures that have shaped their natural variation.
Seed performance after dispersal is highly dependent on parental environmental cues, especially during seed formation and maturation. Here we examine which environmental factors are the most dominant in this respect and whether their effects are dependent on the genotypes under investigation. We studied the influence of light intensity, photoperiod, temperature, nitrate, and phosphate during seed development on five plant attributes and thirteen seed attributes, using 12 Arabidopsis genotypes that have been reported to be affected in seed traits. As expected, the various environments during seed development resulted in changed plant and/or seed performances. Comparative analysis clearly indicated that, overall, temperature plays the most dominant role in both plant and seed performance, whereas light has a prominent impact on plant traits. In comparison to temperature and light, nitrate mildly affected some of the plant and seed traits while phosphate had even less influence on those traits. Moreover, clear genotype-by-environment interactions were identified. This was shown by the fact that individual genotypes responded differentially to the environmental conditions. Low temperature significantly increased seed dormancy and decreased seed longevity of NILDOG1 and cyp707a1-1, whereas low light intensity increased seed dormancy and decreased seed longevity of NILDOG3 and NILDOG6. This also indicates that different genetic and molecular pathways are involved in the plant and seed responses. By identifying environmental conditions that affect the dormancy vs longevity correlation in the same way as previously identified naturally occurring loci, we have identified selective forces that probably shaped evolution for these important seed traits.
doi:10.1093/jxb/eru378
PMCID: PMC4246189  PMID: 25240065
Genotypes; light; nitrate; parental environment; phosphate; plant performance; seed performance; temperature.
21.  Gα modulates salt-induced cellular senescence and cell division in rice and maize 
Journal of Experimental Botany  2014;65(22):6553-6561.
Highlight text
This work reveals two distinct functions of Gα in NaCl stress in rice and maize: attenuation of leaf senescence caused by sodium toxicity in leaves, and cell cycle regulation by osmotic/ionic stress.
The plant G-protein network, comprising Gα, Gβ, and Gγ core subunits, regulates development, senses sugar, and mediates biotic and abiotic stress responses. Here, we report G-protein signalling in the salt stress response using two crop models, rice and maize. Loss-of-function mutations in the corresponding genes encoding the Gα subunit attenuate growth inhibition and cellular senescence caused by sodium chloride (NaCl). Gα null mutations conferred reduced leaf senescence, chlorophyll degradation, and cytoplasm electrolyte leakage under NaCl stress. Sodium accumulated in both wild-type and Gα-mutant shoots to the same levels, suggesting that Gα signalling controls cell death in leaves rather than sodium exclusion in roots. Growth inhibition is probably initiated by osmotic change around root cells, because KCl and MgSO4 also suppressed seedling growth equally as well as NaCl. NaCl lowered rates of cell division and elongation in the wild-type leaf sheath to the level of the Gα-null mutants; however there was no NaCl-induced decrease in cell division in the Gα mutant, implying that the osmotic phase of salt stress suppresses cell proliferation through the inhibition of Gα-coupled signalling. These results reveal two distinct functions of Gα in NaCl stress in these grasses: attenuation of leaf senescence caused by sodium toxicity in leaves, and cell cycle regulation by osmotic/ionic stress.
doi:10.1093/jxb/eru372
PMCID: PMC4246186  PMID: 25227951
Abiotic stress; maize; NaCl stress; plant heterotrimeric G protein; rice; sodium perception.
22.  Downregulation of a putative plastid PDC E1α subunit impairs photosynthetic activity and triacylglycerol accumulation in nitrogen-starved photoautotrophic Chlamydomonas reinhardtii  
Journal of Experimental Botany  2014;65(22):6563-6576.
Summary
Impaired carbon precursor supply through cpPDC is detrimental for TAG biosynthesis in Chlamydomonas reinhardtii under conditions of photoautotrophy and nitrogen starvation.
The chloroplast pyruvate dehydrogenase complex (cpPDC) catalyses the oxidative decarboxylation of pyruvate forming acetyl-CoA, an immediate primer for the initial reactions of de novo fatty acid (FA) synthesis. Little is known about the source of acetyl-CoA in the chloroplasts of photosynthetic microalgae, which are capable of producing high amounts of the storage lipid triacylglycerol (TAG) under conditions of nutrient stresses. We generated Chlamydomonas reinhardtii CC-1618 mutants with decreased expression of the PDC2_E1α gene, encoding the putative chloroplast pyruvate dehydrogenase subunit E1α, using artificial microRNA. A comparative study on the effects of PDC2_E1α silencing on FAs and TAG production in C. reinhardtii, grown photoautotrophically and mixotrophically, with and without a nitrogen source in the nutrient medium, was carried out. Reduced expression of PDC2 _E1α led to a severely hampered photoautotrophic growth phenotype with drastic impairment in TAG accumulation under nitrogen deprivation. In the presence of acetate, downregulation of PDC2_E1α exerted little to no effect on TAG production and photosynthetic activity. In contrast, under photoautotrophic conditions, especially in the absence of a nitrogen source, a dramatic decline in photosynthetic oxygen evolution and photosystem II quantum yield against a background of the apparent over-reduction of the photosynthetic electron chain was recorded. Our results suggest an essential role of cpPDC in the supply of carbon precursors for de novo FA synthesis in microalgae under conditions of photoautotrophy. A shortage of this supply is detrimental to the nitrogen-starvation-induced synthesis of storage TAG, an important carbon and energy sink in stressed Chlamydomonas cells, thereby impairing the acclimation ability of the microalga.
doi:10.1093/jxb/eru374
PMCID: PMC4246187  PMID: 25210079
Acetyl-CoA; chlorophyll fluorescence transients; fatty acid synthesis; lipid; microalgae; photosystem II; pyruvate dehydrogenase.
23.  Tudor-SN, a component of stress granules, regulates growth under salt stress by modulating GA20ox3 mRNA levels in Arabidopsis  
Journal of Experimental Botany  2014;65(20):5933-5944.
Summary
Tudor-SN protein accumulates in stress granules in response to salt stress in Arabidopsis. It binds GA20ox3 mRNA in vivo and up-regulates GA20ox3 levels to maintain plant growth under salt stress.
The Tudor-SN protein (TSN) is universally expressed and highly conserved in eukaryotes. In Arabidopsis, TSN is reportedly involved in stress adaptation, but the mechanism involved in this adaptation is not understood. Here, we provide evidence that TSN regulates the mRNA levels of GA20ox3, a key enzyme for gibberellin (GA) biosynthesis. The levels of GA20ox3 transcripts decreased in TSN1/TSN2 RNA interference (RNAi) transgenic lines and increased in TSN1 over-expression (OE) transgenic lines. The TSN1 OE lines displayed phenotypes that may be attributed to the overproduction of GA. No obvious defects were observed in the RNAi transgenic lines under normal conditions, but under salt stress conditions these lines displayed slower growth than wild-type (WT) plants. Two mutants of GA20ox3, ga20ox3-1 and -2, also showed slower growth under stress than WT plants. Moreover, a higher accumulation of GA20ox3 transcripts was observed under salt stress. The results of a western blot analysis indicated that higher levels of TSN1 accumulated after salt treatment than under normal conditions. Subcellular localization studies showed that TSN1 was uniformly distributed in the cytoplasm under normal conditions but accumulated in small granules and co-localized with RBP47, a marker protein for stress granules (SGs), in response to salt stress. The results of RNA immunoprecipitation experiments indicated that TSN1 bound GA20ox3 mRNA in vivo. On the basis of these findings, we conclude that TSN is a novel component of plant SGs that regulates growth under salt stress by modulating levels of GA20ox3 mRNA.
doi:10.1093/jxb/eru334
PMCID: PMC4203129  PMID: 25205572
Arabidopsis; GA20ox3; RNA immunoprecipitation; salt stress; stress granules; Tudor-SN.
24.  Exploring ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions 
Journal of Experimental Botany  2014;65(20):6023-6033.
Summary
Ammonium nutrition is toxic to many plants. Arabidopsis displays high intraspecific variability in ammonium tolerance (shoot biomass), and ammonium accumulation seems to be an important player in this variability.
Plants are dependent on exogenous nitrogen (N) supply. Ammonium (NH4 +), together with nitrate (NO3 –), is one of the main nitrogenous compounds available in the soil. Paradoxically, although NH4 + assimilation requires less energy than that of NO3 –, many plants display toxicity symptoms when grown with NH4 + as the sole N source. However, in addition to species-specific ammonium toxicity, intraspecific variability has also been shown. Thus, the aim of this work was to study the intraspecific ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions. Plants were grown with either 1mM NO3 – or NH4 + as the N source, and several parameters related to ammonium tolerance and assimilation were determined. Overall, high variability was observed in A. thaliana shoot growth under both forms of N nutrition. From the parameters determined, tissue ammonium content was the one with the highest impact on shoot biomass, and interestingly this was also the case when N was supplied as NO3 –. Enzymes of nitrogen assimilation did not have an impact on A. thaliana biomass variation, but the N source affected their activity. Glutamate dehydrogenase (GDH) aminating activity was, in general, higher in NH4 +-fed plants. In contrast, GDH deaminating activity was higher in NO3 –-fed plants, suggesting a differential role for this enzyme as a function of the N form supplied. Overall, NH4 + accumulation seems to be an important player in Arabidopsis natural variability in ammonium tolerance rather than the cell NH4 + assimilation capacity.
doi:10.1093/jxb/eru342
PMCID: PMC4203136  PMID: 25205573
Ammonium; Arabidopsis thaliana; glutamate dehydrogenase; glutamine synthetase; natural variation; nitrate; nitrogen.
25.  Breeding high-yielding drought-tolerant rice: genetic variations and conventional and molecular approaches 
Journal of Experimental Botany  2014;65(21):6265-6278.
Summary
This study discusses improvement of popular rice varieties under drought through the identification and marker-assisted introgression of drought yield QTLs without any adverse effect on yield under normal conditions.
The increased occurrence and severity of drought stress have led to a high yield decline in rice in recent years in drought-affected areas. Drought research at the International Rice Research Institute (IRRI) over the past decade has concentrated on direct selection for grain yield under drought. This approach has led to the successful development and release of 17 high-yielding drought-tolerant rice varieties in South Asia, Southeast Asia, and Africa. In addition to this, 14 quantitative trait loci (QTLs) showing a large effect against high-yielding drought-susceptible popular varieties were identified using grain yield as a selection criterion. Six of these (qDTY 1.1, qDTY 2.2, qDTY 3.1, qDTY 3.2, qDTY 6.1, and qDTY 12.1) showed an effect against two or more high-yielding genetic backgrounds in both the lowland and upland ecosystem, indicating their usefulness in increasing the grain yield of rice under drought. The yield of popular rice varieties IR64 and Vandana has been successfully improved through a well-planned marker-assisted backcross breeding approach, and QTL introgression in several other popular varieties is in progress. The identification of large-effect QTLs for grain yield under drought and the higher yield increase under drought obtained through the use of these QTLs (which has not been reported in other cereals) indicate that rice, because of its continuous cultivation in two diverse ecosystems (upland, drought tolerant, and lowland, drought susceptible), has benefited from the existence of larger genetic variability than in other cereals. This can be successfully exploited using marker-assisted breeding.
doi:10.1093/jxb/eru363
PMCID: PMC4223988  PMID: 25205576
Direct selection; drought; grain yield; marker-assisted breeding; QTLs; rice.

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