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Dioscorin is one of the major soluble proteins in yam tubers. Unlike other well-known plant storage proteins, such as patatin and sporamin, dioscorin is argued for its function as storage proteins, and the molecular mechanisms underlying its expressional complexity are little understood. In this study, we isolated five dioscorin genes from Dioscorea alata L., comprising three class A (Da-dio1, -3 and -4) and two class B (Da-dio2 and -5) isoforms. Expressions of all dioscorin genes gradually decreased in mother tubers during yam sprouting and regrowth. On the other hand, all dioscorin genes accumulated transcripts progressively with tuber development in new tubers, with Da-dio5 being the most prominent isoform. In yam leaves, the expressions of Da-dio5 were up-regulated by the treatments of five phytohormones (gibberellic acid, salicylic acid, indole-3-acetic acid, abscisic acid, and ethylene), and three abiotic stresses (high-temperature, low-temperature and drought). To further elucidate the regulatory mechanisms of Da-dio5 expressions, transgenic Arabidopsis plants harboring the Da-dio5 promoter-β-glucuronidase (GUS) fusion were generated. GUS staining showed that expressions of the Da-dio5 promoter were detected mainly in the shoot apical meristem (SAM) and hypocotyls, and enhanced by the treatments of the five hormones, and the three abiotic stresses mentioned above. These results suggest diverse roles of Da-dio5 in yam sprouting, regrowth, and tuberization, as well as in response to enviromental cues.
Yams (Dioscorea spp.) are members of the monocotyledonous family Dioscoreaceae. More than 600 yam species have been cultivated worldwide , consumed as an important source of food in some African and Asian countries due to the high contents of carbohydrate and nutritionally relevant proteins (1–3%) in fresh tubers [2,3]. In China, yams are traditionally used as medicinal foods to strengthen the spleen, kidney, liver, and stomach; to reduce phlegm and fatigue; and to cure chronic diarrhea and diabetes . The beneficial properties of yams have recently been attributed to the storage protein dioscorin [5,6,7,8]. Consequently, the functional properties and pharmaceutical potential of dioscorin have attracted increasing attention [9,10].
Dioscorin is the major protein in yam tubers, accounting for approximately 80–85% of the total soluble proteins . A previous study revealed that the major tuber proteins in yam tubers are encoded by members of gene family classes of A and B . The protein sequences of the members from these two classes are 67–75% identical in Dioscorea cayenensis . However, dioscorin has been argued for its function as a storage protein despite its high abundance in yam tubers. Unlike most other storage proteins, dioscorin also has types of enzymatic activities, such as trypsin inhibitor (TI), carbonic anhydrase (CA), dehydroascorbate (DHA) reductase, monodehydroascorbate (MDA) reductase activities, and unique lectin activities [13,14,15,16]. Moreover, dioscorin possesses antioxidant properties, with an ability to scavenge both 1,1-diphenyl-2-picrylhydrazl radicals and hydroxyl radicals . Dioscorin was even found to inhibit angiotensin-converting enzyme activity . The well-known tuber storage proteins, such as sporamin from sweet potato and patatin from potato, also contribute to pest and pathogen resistance, as well as resistance to abiotic stresses [1,17,19,20]. These findings suggest that the so-called storage proteins including dioscorin may have dual roles related to storage and defense in plants.
Although the predominant protein in yam tubers, compared to patatin and sporamin, dioscorin has been less intensively studied. In the present study, we isolated five dioscorin genes (i.e., Da-dio1–5) in a cultivar of D. alata L. and explored their expression characteristics in relation to yam sprouting, regrowth, and tuberization, as well as in response to the treatments of hormones and abiotic stresses. Furthermore, the promoter-β-glucuronidase (GUS) reporter technique was exploited in transgenic Arabidopsis plants to investigate the molecular mechanisms underlying the complex expressional control for Da-dio5, a major Da-dio isoform. It is expected that our results reported here will help understand the roles of dioscorin in yam tubers, and provide useful information for understanding its transcriptional regulations.
The full-length cDNAs of five Da-dio genes were cloned and submitted to GeneBank (Da-dio1, KX237676; Da-dio2, KX237677; Da-dio3, KX237678; Da-dio4, KX237676; Da-dio5, KX237676). The open reading frame (ORF) of the five Da-dio genes were 819–831 base pair (bp) long, and predicted proteins of 272–276 amino acids (aa), molecular weights (MW) of ~30 kDa, and theoretical isoelectric point (pI) values of 6.17 to 7.02 (Table 1). Multiple sequence alignments were performed upon the five Da-dio proteins identified here, a dioscorin homolog (dioscorin-5 precursor) from Dioscorea japonica and three carbonic anhydrases (CAs) from human (P00915), mouse (P13634) and Arabidopsis (CAB79100), respectively. As shown in Figure 1A, the five Da-dios contained three putative residues that influence CA activity, and two cysteine residues that relate to activities of TI, DHA reductase, and MDA reductase. In addition, all dioscorin proteins contained a putative signal peptide (amino acids 23–25). A phylogenetic tree (Figure 1B) was constructed for the five Da-dios and 18 NCBI-downloaded dioscorin sequences of five Dioscorea species (D. alata, D. japonica, D. pseudojaponica, D. batatas and D. cayenensis). The five Da-dios identified this study were grouped clearly into class A (Da-dio1, Da-dio3 and Da-dio4) and class B (Da-dio2 and Da-dio5). Amino acid sequence alignments showed a high similarity among the Da-dios of different cultivars, being 91–97% identical within the same subfamily, and 68% similar across the subfamilies (Table S1 and Figure 1B).
D. alata L. propagates mainly via tubers, the modified stem. Tuber development is accompanied by a variety of biochemical and morphological changes. To explore the roles of the five Da-dio genes during yam sprouting and regrowth, we examined their expressions in the mother tubers at 15, 30, 45, 60, 75 and 90 days after planting (DAP). All five Da-dio genes were expressed at highest abundance in mother tubers at 15 DAP. As the vines continued to grow, the expression of the five Da-dio genes gradually decreased until reaching almost undetectable levels at 90 DAP (Figure 2A). To further investigate whether the dioscorin gene expressions are associated with tuber development, the Da-dio transcripts were investigated in new tubers at 120, 150, 180, 210 and 240 DAP, corresponding to different stages of tuber development (Figure 2B). As shown in Figure 2C, the expressions of five Da-dio genes increased in a similar pattern in the new tubers except for a difference in the fold-change. The expressions of Da-dio1–4 were up-regulated by 3–5-fold, whereas almost 300-fold was observed for Da-dio5 at the beginning of the maturation stage (210 DAP). These results suggested that expressions of the five dioscorin genes are closely correlated with yam sprouting, regrowth, and tuber development, of which Da-dio5 seems to be the most important member implicated in tuber formation and development.
To further understand the functions of Da-dio5, its expressions were examined in six D. alata tissues, viz. tubers, stems, roots, bulbils, male flowers and leaves. Da-dio5 exhibited a tissue-specific expression, with transcripts detected mainly in tubers and bulbils but hardly in the other tissues (Figure 2D), further strengthening the importance of Da-dio5 in yam tuber development. To identify the cellular compartment in which Da-dio5 functions, the Da-dio5 protein fused with a green fluorescent protein (GFP) was transiently expressed in rice protoplasts. As shown in Figure 3, Da-dio5-GFP was localized to the vacuole and cytosol of transformed protoplasts.
To further explore the expressional characters of Da-dio5, leaves of young yam plants were treated with five hormones (GA, gibberellic acid; SA, salicylic acid; IAA, indole-3-acetic acid; ABA, abscisic acid; ET, ethylene.) and three stresses (drought, 4 °C and 45 °C). As shown in Figure 4A, expressions of the Da-dio5 in leaves were enhanced at early stages of the hormone treatments (3–6 h), but depressed in different degrees for respective hormones at later stages. The expressions of Da-dio5 in leaves were significantly up-regulated by the drought treatment, and the two temperature stresses, although to a much lesser extent (Figure 4B).
To characterize the spatiotemporal expression patterns of Da-dio5, a 2617-bp putative promoter fragment upstream of Da-dio5 was cloned using a PCR-based genome walking technique. Multiple types of putative cis-acting elements were identified in the Da-dio5 promoter (Table 2). The Da-dio5 promoter contained a GCN4_motif and a Skn-1_motif, which are associated with endosperm expression. The promoter also harbored putative regulatory elements responsible for responses to environmental stresses and hormone signals, such as low-temperature and heat-stress responsive elements, drought-inducible elements, defense-responsive elements, and elements responsive to various hormones (ET, SA, GA, ABA, and IAA). For a precise analysis of the expression patterns of the Da-dio5 promoter, we transformed the Da-dio5 promoter::GUS chimeric construct into A. thaliana. In transgenic Arabidopsis seedlings at 10 days after germination (DAG), GUS staining was predominantly observed in the shoot apical meristem (SAM) and hypocotyls, while the petiole was weakly stained (Figure 5A). In transgenic seedlings at 20 DAG, a similar staining pattern was observed in the SAM, petiole and vasculature, whereas no staining was observed in the hypocotyls. As expected, the blue stain was not observed in control seedlings. There was also a lack of staining in the roots, flowers and stems at any developmental stages (not shown), indicating the tissue/organ-specific activity of the Da-dio5 promoter.
To determine whether the cis-acting elements as predicted in hormone and stress responses (Table 2) were functional, histochemical GUS staining and GUS activity assay were conducted in transgenic seedlings. GUS activity was significantly induced in seedlings at 9 h after the treatments of GA, IAA, SA, or ABA. In contrast, in ACC-treated seedlings, GUS gene expression increased at 6 h, and then decreased at 9 h (Figure 5B). GUS activity was enhanced by both high-temperature and drought treatments, with the latter being more striking, but inhibited markedly by low-temperature treatment (Figure 5C). The GUS staining observations correlated well the assays of GUS activity (Figure 5). These data were consistent with some of the Da-dio5 expression patterns (Figure 4) and clearly indicated that the Da-dio5 has crucial functions in responding to various hormone and environmental cues.
Dioscorin, which is the major soluble protein in yam (Dioscorea spp.) tubers, has been isolated from various yam species . In previous reports, phylogenetic analysis of dioscorins from various yam species revealed that the dioscorins were grouped into two classes (A and B) . Here, we showed the presence of five dioscorin genes in D. alata cv. Hainan No. 56 that consists of three class A (Da-dio1, -3 and -4) and two class B (Da-dio2 and -5) isoforms. Phylogenetically, the five dioscorin genes are highly conserved, similar to their homologs in other Dioscorea species, implying the subjection of these genes to a similar selection pressure during evolution. Moreover, complete conservation of two cysteine residues that relate to activities of TI, DHA reductase, and MDA reductase suggest additional roles besides functions as storage proteins.
Tuber development includes three major steps: induction, initiation and tuberization. Each of these steps is accompanied by a drastic change in the expression of genes encoding a set of proteins abundantly present in tubers . However, it is unclear which genes are required for tuber development. In contrast to the established roles of potato (patatin) and sweet potato (sporamin) storage proteins in tuber development [21,22,23,24,25,26], dioscorins have been studied mainly for their biological activities and pharmaceutical potentials . In this study, five dioscorin genes exhibited similar expression patterns during yam sprouting, regrowth and tuberization (Figure 2). The dioscorin gene expression levels gradually decreased until they were undetectable with the process of tuber germination, sprouting and regrowth (Figure 2A). Tuber growth is reported to be accompanied by protein (dioscorin) depletion along with sugar mobilization, which is controlled by the redox status of the tubers . Increasing evidences have shown that dioscorins possess CA, DHA reductase and MDA reductase activities that are important for control of redox status in D. alata L. tubers . Thus, these results together conclude that dioscorins function as the major storage protein to support tuber germination, and provide nutrients for the growth of new plants from reproductive organs. Additionally, all five dioscorin isoforms were detected at 120 DAP in accordance with tuber formation, and were highly abundant in maturing tubers (at 210 days), with Da-dio5 being the most prominent isoform (Figure 2C). Previous studies showed that the process of tuberization is accompanied by increased oxidative stress, evidenced by elevated levels of H2O2 and reactive nitrogen species. Therefore, it is suggested dioscorins have an role in controlling redox status that is also important for the process of tuberizaion . Further study revealed the expressional predominance of Da-dio5 in the types of reproductive organs: tubers and bulbils (Figure 2D). Together, these results suggest the importance of dioscorins in both vegetative growth and reproductive development in yam plants.
The findings in sweet potato illustrate the precise roles of the phytohormone and signaling pathways in regulating expressions of tuber storage proteins (sporamin) upon abiotic stresses . In this study, the expressions of Da-dio5 in yam leaves were up-regulated by the treatments of five phytohormones (GA, SA, IAA, ABA, and ET), and three abiotic stresses (high-temperature, low-temperature and drought) (Figure 4A,B). Thus, the induced expression of Da-dio5 revealed its involvement in adaptation to changes of environmental cues and stresses. Furthermore, under field conditions, induction of tuberization is controlled by a number of environmental conditions such as photo-period, temperature and nitrogen supply, via the change of endogenous phytohormone levels. Considering the expressional predominance of Da-dio5 in yam tubers, and the coincidence of increased dioscorin expression with yam tuberization, characterization of the Da-dio5 promoter will be beneficial to understanding the regulatory mechanisms of dioscorin genes in relation to their physiological functions in yam plants. Transgenic Arabidopsis seedlings harboring the Da-dio5 promoter-GUS construct showed GUS staining mainly in the SAM and hypocotyls at 10 DAG, but absent in the hypocotyls at 20 DAG (Figure 5A), indicating a strict spatial/temporal expression pattern. This is consistent with the findings that the underground stem tubers of yam are derived from swollen hypocotyls . In addition, both GUS activity assay and GUS staining showed that the Da-dio5 promoter was also induced by multiple phytohormones (GA, SA, IAA, ABA, and ACC), high-temperature and drought treatments, but inhibited by low-temperature treatment (Figure 5B,C), suggesting a multifaceted response of Da-dio5 to changeable environmental cues. These responses could be backed up by the relevant cis-elements predicted in the Da-dio5 promoter (Table 2), suggesting our observations were similar to those of tuber storage protein homologs in potato and sweet potato [30,31].
D. alata cv. Hainan No. 56 plants were cultivated in the experimental plantation of the Agricultural College at Hainan University in Danzhou, Hainan, China. To clone the dioscorin genes and analyze their tissue expressions, we harvested the leaves, stems, roots, and tubers at 180 days after planting (DAP), and the male flowers at 210 DAP and the bulbils 240 DAP. To analyze gene expressions in mother tubers during yam sprouting and regrowth, we collected the mother tubers at 15, 30, 45, 60, 75, and 90 DAP after planting. To examine gene expressions during the tuber development, we collected the new tubers at the stage of tuber formation (120 DAP), the early stage of rapidly bulking (150 DAP), the rapidly bulking stage (180 DAP), the maturing stage of tubers (210 DAP), and the harvesting stage (240 DAP) (Figure 2B) . We collected samples from three plants (biological replicates) for analyses. All plant materials collected were immediately frozen in liquid nitrogen and stored at −80 °C prior to RNA extraction.
D. alata L. plants (n = 3) with 6–8 fully expanded leaves were grown in a growth chamber at 28 °C under a 16-h light: 8-h dark photoperiod. Arabidopsis thaliana plants (ecotype Columbia) were grown in a growth chamber at 22 °C under a 16-h light: 8-h dark photoperiod.
For hormone treatments, D. alata L. plants (n = 3) with 6–8 fully expanded leaves were sprayed with 100 μM gibberellic acid (GA), 100 μM indole-3-acetic acid (IAA), 100 μM abscisic acid (ABA), 0.05% ethephon (an ethylene-releasing compound) (ET) and 100 μM salicylic acid (SA) for 12 h ; 10-day-old transgenic Arabidopsis seedlings were transferred to filter papers soaked with 100 μM GA, 100 μM IAA, 100 μM ABA, 100 μM 1-aminocyclopropanecarboxylic acid (ACC) and 100 μM SA for 9 h, and with double-distilled water as the control . For low-temperature and high-temperature stress treatments, D. alata L. plants (n = 3) with 6–8 fully expanded leaves and 10-day-old transgenic Arabidopsis seedlings were transferred to filter papers saturated with MS medium and incubated at 4 °C and 45 °C for 6 h . For drought stress treatment, D. alata L. plants (n = 3) with 6–8 fully expanded leaves were dehydrated for 15 d and 20-day-old transgenic Arabidopsis seedlings were dehydrated on filter paper at 60% humidity for 3 h . Whole transgenic Arabidopsis seedlings were collected after treatments and assayed for GUS activity or histochemically analyzed by GUS staining as described [35,36]. The experiments were repeated three times, and with three technical replicates (10–15 seedlings per replicate).
A D. alata cv. Hainan No. 56 transcriptome database that includes a total of 52,866 unigenes was obtained by RNA-sequencing and de novo assembly using the Illumina HiSeq 2000 platform (data, unpublished). The sequences of dioscorin genes from other yam species were downloaded from the National Center for Biotechnology Information (NCBI) database and used as query sequences for basic local alignment search tool (BLAST) analysis of our transcriptome database. We obtained five D. alata dioscorin (Da-dio) target sequences with complete open reading frames. Subsequently, specific primer pairs were designed to amplify respective Da-dio cDNA sequences (Table S2).
We deposited the obtained sequences to the NCBI database for basic local alignment search tool (BLAST) searches, following the online comparative and bioinformatic analyses of the Da-dio genes (Available online: http://www.ncbi.nlm.nih.gov). The amino acid sequences of the five Da-dios were aligned with homologous sequences downloaded from NCBI using the DNAMAN (Lynnon Biosoft, Quebec, QC, Canada) program. Additionally, the neighbor-joining method of the MEGA6 program was used to construct a phylogenetic tree. The molecular weights and theoretical pI (isoelectric point) values of the dioscorins were calculated with the ProtParam online tool (Available online: http://www.expasy.ch/tools/protparam.html). Potential conserved domains and signal peptides were analyzed using the SMART program (Available online: http://smart.embl-heidelberg.de/).
Total RNA was isolated from frozen plant samples using the RNAplant Plus reagent (BioTeke, Beijing, China). First-strand cDNA was synthesized using the RevertAid™ First Strand cDNA Synthesis Kit (Fermentas, Burlington, ON, Canada).
qRT-PCR assay was used to analyze Da-dio gene expressions, with β-actin and α-tubulin-1 as internal controls for data normalization. The efficiency of each primer pair was determined to be 1.946–2.001. The qRT-PCR was conducted using SYBR® Premix Ex Taq™ II (Takara, Dalian, China) and the LightCycler 2.0 system (Roche Diagnostics, Basel, Switzerland). We used the LightCycler Relative Quantification Software 4.05 (Roche Diagnostics, Mannheim, Germany) to visualize and analyze the data, including the quantification cycle values, PCR efficiencies, and correlation coefficients. We calculated relative expression level fold changes using previously described methods .
For subcellular localization analysis, the Da-dio5 cDNA was amplified using the specific primer pairs of Da-dio5-GF and Da-dio5-GR (Table S3), and then cloned into the binary vector pCAMBIA-1300 at the restriction sites of Sal I and Kpn I to make the 35S::Da-dio5-GFP fusion construct. Fusion construct was transformed into Oryza sativa mesophyll protoplasts according to a previous study . The images were acquired with laser scanning confocal microscope (Olympus FV1000, Tokyo, Japan). The excitation and emission wavelengths were 480 nm and 510 nm for GFP, and 650 nm and 750 nm for chloroplast, respectively.
The 5′-flanking region of the Da-dio5 gene was isolated using the GenomeWalker Universal Kit (Takara, Dalian, China). The GenomeWalker libraries were constructed using genomic DNA extracted from yam leaves using the cetyltrimethylammonium bromide extraction method as previously described . Primary and nested PCRs were conducted using adapter primers provided by the kit and three gene-specific antisense primers (Table S3). The PCR products were ligated into the pMD18-T vector (Takara, Dalian, China), and then sequenced at Invitrogen, China. Regulatory elements in the promoter were analyzed using the Plantcare online program (Available online: http://bioinformatics.psb.ugent.be/webtools/plantcare/html/) .
To construct the Da-dio5 promoter-GUS fusion, the 1500-bp Da-dio5 promoter fragment was subcloned into the Sac I–Nco I site of pCAMBIA1301. The construct was transferred into Agrobacterium tumefaciens EHA105 cells using the liquid nitrogen freeze–thaw method  for the subsequent transformation of Arabidopsis plants using the floral dip method . Homozygous plants were selected from the T2 progenies and confirmed in the T3 generation according to hygromycin (50 μg/mL) resistance.
Histochemical localization of GUS activity in the homozygous transgenic plants was conducted as described . Quantitative analysis of GUS activity in transgenic seedlings was as described by Lü et al. . Each assay was repeated three times, with three technical replicates each time.
This research was supported by the National Natural Science Foundation of China (grant number 31660413, 31260346), the Department of Science and Technology of Hainan Province (grant number ZDZX2013023), the College Graduate Student Innovation Research Project of Hainan Province (grant number Hyb2014-04). We would like to thank Dr. Chaorong Tang for correcting the manuscript and valuable comments.
|ORF||Open reading frame|
|BLAST||Basic local alignment search tool|
|qRT-PCR||Quantitative real-time RT-PCR|
|DAP||Days after planting|
|SAM||Shoot apical meristem|
|DAG||Days after germination|
|GFP||Green fluorescent protein|
Supplementary materials can be found at www.mdpi.com/1422-0067/18/7/1579/s1.
Linya Liu and Yacheng Huang were responsible for all aspects of the research, including experimental design, data acquisition and analysis, and manuscript preparation. Xiaolong Huang and Jianghua Yang worked on primer design, technical and informatics’ analyses of these genes. Wenqiang Wu, Yun Xu, Ziwen Cong, Jun Xie and Wei Xia worked on the preparation of the studied materials. Dongyi Huang was responsibility for the programs and all experiments, revised the manuscript and provided the final approval of the article.
The authors declare no conflict of interest.