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Over 400 species of flowering plants depend entirely on fungi for their nutrition demands. These fascinating plants, often incorrectly referred to as ‘saprophytes’, provide unique insights in plant–fungus associations. In the light of recent advances, Merckx et al. (pp. 1255–1261) summarize our current knowledge about myco-heterotrophic plants and point out controversial aspects of myco-heterotrophy that remain to be resolved.
Arbuscular mycorrhizal fungi (AMF) alleviate salt stress and impart beneficial effects on plant growth and productivity. Evelin et al. (pp. 1263–1280) review the role of AMF in improving salt-tolerance of host plants by employing various biochemical, physiological and molecular mechanisms, and provide benchmark information for development and prioritization of future research programmes.
The slippery waxy zone in the upper part of pitchers has long been considered the key trapping structure of the Nepenthes carnivorous plants; however, the presence of wax is reported to be variable within and between species of this species-rich genus. Gaume and Di Giusto (pp. 1281–1291) study the expression over plant ontogeny of the slippery waxy zone and its role in prey capture in two related species, and show that this zone can be lost when supplanted by more efficient features, thus generating new pitcher forms. The results highlight the roles of selection and developmental mechanisms in the morphological diversity of Nepenthes.
Stem growth habit is an important agronomic trait of soybean. Using near-isogenic lines for stem growth habit, Tanaka and Shiraiwa (pp. 1293–1299) show that indeterminate types exhibit greater stomatal conductance compared with determinate types. The indeterminate types also have relatively smaller cells and finer structure on their leaf epidermis; thus there was a clear effect of stem growth habit on stomatal conductance under favourable conditions.
Endangered species in dwindling habitats may have lower reproductive success when their potential mates are closely related. Pinto-Torres and Koptur (pp. 1301–1311) suggest that pollinators are important in the persistence of Jacquemontia reclinata populations on the south-eastern Florida coast, as self-pollination and crossing of close relatives are less successful than crosses with far neighbours or plants in other populations.
The rare, alpine monocarpic perennial Campanula thyrsoides shows high genetic diversity within populations and considerable genetic differentiation. Ægisdóttir et al. (pp. 1313–1322) show that the natural isolation of suitable habitats for C. thyrsoides restricts gene flow among populations, as expected for a monocarpic species with limited seed dispersal capacities. The species' outcrossing behaviour, long-lived individuals and overlapping generations can explain the observed high genetic diversity within populations.
Soluble sugars are involved in responses to stress; for example exogenous sucrose treatment efficiently induces tolerance to the herbicide atrazine in Arabidopsis thaliana plantlets. Ramel et al. (pp. 1323–1337) find natural variation in pre-stress shoot endogenous sugar levels and responses of plantlets to subsequent atrazine stress between accessions of A. thaliana. These significant relationships point to an important integration of carbon nutritional status and induction of stress tolerance in plants. (Featured article in ContentSelect on p. v.)
The polygalacturonase (PG) gene family is enriched in pollen, but currently little is known about its exact function. Huang et al. (pp. 1339–1351) find that the expression pattern and function assigned by antisense RNA reveals that BcMF9, a novel PG gene isolated from Brassica campestris flower buds, is required both for intine and exine formation, and for subsequent pollen germination and male fertility. (Featured article in ContentSelect on p. v.)
The gametophytes of some homosporous ferns are strap- or ribbon-shaped and have been assumed to have evolved from terrestrial cordate shapes as an adaptation to epiphytic habitats. Takahashi et al. (pp. 1353–1361) show that in Colysis, in addition to the usual apical-cell-based and multicellular meristems that are, respectively, responsible for early growth and formation of cushions with archegonia, there is a marginal meristem that allows indefinite growth, including gametophyte branching. Prolonged gametophyte growth can provide an ecological adaptation to epiphytic habitats.
In hypocotyls of flax (Linum usitatissimum), cadmium-induced reorientation of growth coincides with marked changes in the methylesterification and cross-linking of homogalacturonans within various cell-wall domains. Paynel et al. (pp. 1363–1372) find that the temporal regulation of pectin methylesterase (PME) and peroxidase genes and enzymes fits the previously reported Cd-induced structural changes of homogalacturonans. After PME catalyses de-esterification of homogalacturonans, their cross-linking will depend on the activity of peroxidases interacting with Cd-dimerized blocks and reinforcing the cell cohesion during the Cd-induced swelling. (Featured article in ContentSelect on p. vi.)
Callose is deposited in the periclinal walls of stomata in the fern Asplenium nidus in the form of fibrils organized in radial arrays focused on the stomatal pore region. Apostolakos et al. (pp. 1373–1387) find that the time and pattern of callose deposition and subsequent degradation play an essential role in internal stomatal pore formation, and callose participates in deposition of the local guard cell wall thickenings.
In some micro-organisms the exposure of a cell to a slow increase in osmotic pressure preserves cell viability after rehydration while sudden dehydration produces a lower rate of viability, which could be due to membrane vesiculation. Assani et al. (pp. 1389–1395) study cytoplasmic vesicle formation in onion and conclude that it is strongly influenced by the kinetics of osmotic dehydration. Hechtian strand connections between protoplasts and exocytotic vesicles are a prerequisite for successful deplasmolysis, and the results suggest that a decrease in the area-to-volume ratio of a cell could cause cell death following an osmotic shock.
In the UK, fruit-bearing plants in hedgerows are an important source of winter food for birds. Jacobs et al. (pp. 1397–1404) investigate whether recent declines in pollinator populations are likely to threaten fruit-set in five common hedgerow species, and find that the proportions of flowers setting fruit in Prunus spinosa, Crataegus monogyna and Hedera helix are significantly reduced when insects are excluded from flowers, whereas fruit-set in Rubus fruticosus and Rosa canina are unaffected. Ensuring strong populations of insect pollinators may thus be essential to guarantee a winter fruit supply for birds in UK hedgerows. (Featured article in ContentSelect on p. vi.)
Reduction in female fitness in large clones can occur as a result of increased geitonogamous self-fertilization and its influence through inbreeding depression. Liao et al. (pp. 1405–1412) find that female reproductive success at the ramet level decreases with clone size in the clonal herb Aconitum kusnezoffii. This pattern is attributable to the negative effects of self-pollination, which arise primarily from a strong early-acting inbreeding depression leading to the abortion of selfed embryos prior to seed maturation.
Many plants reproduce both clonally and sexually, and the balance between the two modes of reproduction will vary among populations. Wilk et al. (pp. 1413–1419) characterize clonal growth in three populations of wild strawberry, Fragaria virginiana, in north-eastern Illinois. Each site shows remarkably different recruitment patterns, ranging from a site with only two widespread clones to one with 19 distinct genets. Such site-to-site variation in clonality may be common in clonal plant species.
Knowledge of pollen dispersal patterns and variation of fecundity is important to understanding evolutionary processes and to formulating strategies to conserve forest genetic resources. Tani et al. (pp. 1421–1434) find that pollen dispersal patterns of two dipterocarp species of Shorea in Malaysia are affected by differences in conspecific tree flowering density, reductions in which lead to an increased selfing rate. The magnitude of general flowering, male fecundity variation and distance between pollen donors and mother trees needs be taken into account when attempting to predict the effects of management practices on the self-fertilization and genetic structure of key tree species in tropical forests.
Alternanthera philoxeroides (C3) and Hemarthria altissima (C4) are riparian species that can survive long-term submergence. Luo et al. (pp. 1435–1444) report remarkable capacities of these plants to rapidly adjust the photosynthetic apparatus to sudden changes in irradiance and/or O2, the two factors varying dramatically upon submergence/de-submergence. This ability for photosynthetic acclimation may ensure quick recovery of growth and carbohydrate accumulation during flood intervals.