Combination of RNA-Seq transcriptomics and iTRAQ proteomics reveal the mechanism involved in fresh-cut yam yellowing

The aim of this study was to examine the regulation of transcriptomics and proteomics related to the yellowing of fresh-cut yams after storage. The comparison of yellow fresh-cut yam (YFY) vs. white fresh-cut yam (control) revealed 6894 upregulated and 6800 downregulated differentially expressed genes along with 1277 upregulated and 677 downregulated differentially expressed proteins. The results showed that the total carotenoids, flavonoids, and bisdemethoxycurcumin in YFY were higher than in the control due to the significant up-regulation of critical genes in the carotenoid biosynthesis pathway, flavonoid biosynthesis pathway, and stilbenoid, diarylheptanoid, and gingerol biosynthesis pathway. In addition, the tricarboxylic acid cycle and phenylpropanoid biosynthesis were both enhanced in YFY compared to the control, providing energy and precursors for the formation of yellow pigments. The results suggest that the synthesis of yellow pigments is regulated by critical genes, which might explain the yellowing of fresh-cut yam after storage.

Agro-ecological suitability assessment of Chinese Medicinal Yam under future climate change

Chinese Medicinal Yam (CMY) has been prescribed as medicinal food for thousand years in China by Traditional Chinese Medicine (TCM) practitioners. Its medical benefits include nourishing the stomach and spleen to improve digestion, replenishing lung and kidney, etc., according to the TCM literature. As living standard rises and public health awareness improves in recent years, the potential medicinal benefits of CMY have attracted increasing attention in China. It has been found that the observed climate change in last several decades, together with the change in economic structure, has driven significant shift in the pattern of the traditional CMY planting areas. To identify suitable planting area for CMY in the near future is critical for ensuring the quality and supply quantity of CMY, guiding the layout of CMY industry, and safeguarding the sustainable development of CMY resources for public health. In this study, we first collect 30-year records of CMY varieties and their corresponding phenology and agro-meteorological observations. We then consolidate these data and use them to enrich and update the eco-physiological parameters of CMY in the agro-ecological zone (AEZ) model. The updated CMY varieties and AEZ model are validated using the historical planting area and production under observed climate conditions. After the successful validation, we use the updated AEZ model to simulate the potential yield of CMY and identify the suitable planting regions under future climate projections in China. This study shows that regions with high ecological similarity to the genuine and core producing areas of CMY mainly distribute in eastern Henan, southeastern Hebei, and western Shandong. The climate suitability of these areas will be improved due to global warming in the next 50 years, and therefore, they will continue to be the most suitable CMY planting regions.

Cloning and overexpression of the ascorbate peroxidase gene from the yam (Dioscorea alata) enhances chilling and flood tolerance in transgenic Arabidopsis

Minghuai 1 (MH1) is a yam (Dioscorea alata) cultivar with high tolerance to flooding but sensitivity to chilling. MH1 responded differently to chilling and flooding according to various physiological parameters and antioxidant enzymes. Flooding led to an increase in ascorbate peroxidase (APX) activity in both roots and leaves, while chilling did not affect APX activity. The full length DaAPX ORF sequence from MH1 (750 bp) was then cloned. Phylogenetic analysis showed that plant cytosolic APXs into four major clusters and DaAPX was closely related to Oncidium. The DaAPX gene driven by a 35S promoter was transferred into Arabidopsis. The gene expression and enzyme activity of APX in the DaAPX transgenic lines 1-3 were significantly higher than in wild type (WT) plants. Compared to WT plants, seedling growth characteristics were significantly better in all transgenic lines under chilling, flooding, and oxidative stresses, indicating that the overexpression of DaAPX in Arabidopsis enhanced tolerance to several abiotic stresses. MH1 plants supplied with H₂O₂ presented an increase in the activity of APX leading to enhanced tolerance to chilling. Functional characterization of the APX gene should improve our understanding of the chilling- and flood-response mechanism in the yam.

Agromorphologic, genetic and methylation profiling of Dioscorea and Musa species multiplied under three micropropagation systems

Plant in vitro vegetative propagation using classical semi-solid culture medium is limited due to the low degree of automation, suboptimal nutrient availability and induced physiological stress which often reduce its efficiency. Temporary Immersion System (TIS) emerged as an innovative approach to optimize and eliminate the drawbacks associated with the conventional system of micropropagation. In this study, both Dioscorea and Musa spp. were subjected to conventional semi-solid culture media, complete immersion in shaking liquid culture media and TIS using RITA bioreactor. In vitro grown plantlets were screened for possible vegetative changes using agro-morphological descriptors while genetic and methylation differences were assessed using amplified fragment length polymorphism (AFLP) and methylation-sensitive amplification polymorphism (MSAP). In vitro results showed that the number of shoots produced in Musa spp. varied significantly (P≤0.001) with the type of culture system. The highest mean shoot produced was observed with TIS (28.40) and the least using semi-solid culture medium (1.13). For Dioscorea spp., there was no significant interaction between the hormone combination and the culture system. However, the lowest mean shoot value (1.55) was observed in the semi-solid culture medium. Genetic analysis via AFLP using 15 primer pair combinations revealed that the 3 culture systems maintained genetic variation for Musa and Dioscorea spp. under in vitro and field conditions. Results showed 99% and 91% of the total bands were polymorphic under in vitro and field conditions respectively for Musa and 100% polymorphism for Dioscorea under in vitro and field conditions. Methylation investigation via MSAP using 12 primer pair combinations showed 25% and 46% polymorphic methylated-sensitive loci, 100% and 78% of non-methylated loci of the total bands generated under in vitro and field conditions respectively. Unmethylated (HPA+/MSP+) levels were highest in TIS (0.0842) as compared to CI (0.0227) and SS (0.0161) while full methylation or absence of target (HPA-/MSP-) was lowest in TIS (0.5890) and highest in SS (0.7138). For Dioscorea, 52% and 53% methylated sensitive loci and 100% non-methylated loci were polymorphic under in vitro and field conditions respectively. Although in vitro plant tissue culture techniques led to methylation at some loci of both species, there were no observable changes in the phenotype of both crops under field conditions. This also confirmed that not all methylation events lead to phenotypic changes.

Morphological and functional seed traits of the wild medicinal plant Dioscorea strydomiana, the most threatened yam in the world

Morphological and functional seed traits have important roles in characterising the species regeneration niche and help to understand the reproductive biology of rare and threatened plants, which can thus support appropriate plant conservation measures. Seed morphometric and dispersal kinetics of the critically endangered Dioscorea strydomiana were measured and compared with those of four other Dioscorea species, and seed germination response under constant temperatures (5-35 °C) was compared with that of the congeneric and widespread D. sylvatica. Seed mass of D. strydomiana (ca. 14 mg) was twice that of D. sylvatica, but similar to or smaller than the other species examined. Seeds of D. strydomiana have the lowest speed of descent and lowest variability in most of the morphological traits considered, suggesting lower phenotypic plasticity but higher variance in the wing-loading value. Seeds of D. strydomiana reached maximum germination at 15 °C (ca. 47%), which decreased slightly to ca. 37% at 25 °C and was completely inhibited at 35 °C. D. sylvatica seeds started to germinate at 10 °C (ca. 3%), reached 75-80% germination at 15-20 °C and maximum (ca. 90%) at 25-30 °C. Base temperatures for germination (T_b ) were 9.3 and 5.7 °C, for D. strydomiana and D. sylvatica, respectively. Due to the higher germination percentages of D. sylvatica, ceiling and optimum temperatures could also be modelled for this species, suggesting higher sensitivity to high temperature for seeds of D. strydomiana. The detected poor seed lot quality of D. strydomiana suggests difficulties in reproduction from seed, highlighting the need for further investigation and conservation actions for this threatened yam species.

Characterization of the Dioscorin Gene Family in Dioscorea alata Reveals a Role in Tuber Development and Environmental Response

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.

Dioscorea alata tuber proteome analysis shows over thirty dioscorin isoforms and novel tuber proteins

In Dioscorea, dioscorin (31 kDa) is the major storage protein constituting 85% of the total tuber proteins. An integrated proteomic and biochemical approach was used to understand the physiological role of dioscorin in the two contrasting growth stages (germinating and mature tuber). HPLC analysis showed 3 fold reduction in mannitol and 12.88 and 1.24 fold increase in sucrose and maltose in the germinating tuber. A 1.8 and 3 fold increase in sucrose phosphate synthase and mannitol dehydrogenase activity respectively was observed in the germinating tuber while a 2 fold higher invertase probably lowers the sucrose accumulation in the mature tuber. SDS-PAGE and 2-D maps of the mature and germinating tubers confirmed depletion (more than 50%) of dioscorin on germination. Dioscorin was purified using ion exchange and gel filtration chromatography with 43.32 fold purification and 38.16 yield. Out of a trail of 35 spots at 31 kDa only 12 spots (identified as dioscorin isoforms) were present in the 2D gel of the purified fraction. To search for other tuber proteins besides dioscorin, the unbound fractions of DEAE column were analysed by 2DGE. DREB 1A, caffeic acid 3-O-methyltransferase and Rab-1 small GTP binding protein were identified perhaps for the first time in the Dioscorea proteome. The interactome analysis revealed these to be involved in oxidative stress, carotenoid synthesis and vesicular transport. This is perhaps the first attempt to identify tuber proteome (although limited) and to understand the physiological significance of these proteins.

Diversification into novel habitats in the Africa clade of Dioscorea (Dioscoreaceae): erect habit and elephant's foot tubers

BACKGROUND

Dioscorea is a widely distributed and highly diversified genus in tropical regions where it is represented by ten main clades, one of which diversified exclusively in Africa. In southern Africa it is characterised by a distinct group of species with a pachycaul or "elephant's foot" structure that is partially to fully exposed above the substrate. In contrast to African representatives of the genus from other clades, occurring mainly in forest or woodland, the pachycaul taxa and their southern African relatives occur in diverse habitats ranging from woodland to open vegetation. Here we investigate patterns of diversification in the African clade, time of transition from forest to more open habitat, and morphological traits associated with each habitat and evaluate if such transitions have led to modification of reproductive organs and mode of dispersal.

RESULTS

The Africa clade originated in the Oligocene and comprises four subclades. The Dioscorea buchananii subclade (southeastern tropical Africa and South Africa) is sister to the East African subclade, which is respectively sister to the recently evolved sister South African (e. g., Cape and Pachycaul) subclades. The Cape and Pachycaul subclades diversified in the east of the Cape Peninsula in the mid Miocene, in an area with complex geomorphology and climate, where the fynbos, thicket, succulent karoo and forest biomes meet.

CONCLUSIONS

Diversification out of forest is associated with major shifts in morphology of the perennial tuber (specifically an increase in size and orientation which presumably led them to become pachycaul) and rotation of stem (from twining to non-twining). The iconic elephant's foot morphology, observed in grasslands and thicket biomes, where its corky bark may offer protection against fire and herbivory, evolved since mid Miocene. A shift in pollination trait is observed within the forest, but entry into open habitat does not show association with reproductive morphology, except in the seed wing, which has switched from winged all round the seed margin to just at the base or at the apex of it, or has been even replaced by an elaiosome.

Melatonin-loaded alginate beads improve cryopreservation of yam (Dioscorea alata and D. cayenensis)

BACKGROUND

The cryopreservation of yam is constrained with many challenges.

OBJECTIVE

This study tested the effects of melatonin on shoot tips of D. alata and D. cayenensis accessions exposed to water and liquid nitrogen (LN) stresses.

MATERIALS AND METHODS

Sucrose pretreatment (0.3 M) was applied for 48 h before cryopreservation. Shoot tips were encapsulated in beads loaded with 0.75 M sucrose, with and without melatonin and desiccated over sterile dry silica gel for 0 - 9 h.

RESULTS

The beads moisture content declined from 100% to ~ 13% after 9 h. The 3 h desiccation period without melatonin produced a significantly higher regeneration compared to 6 h and 9 h. Shoot tips with melatonin had significantly higher regeneration after 3 - 6 h desiccation compared to 9 h and the regeneration of all accessions after 6 h was >80%. Regeneration following 6 h desiccation and LN was significantly greater for melatonin-treated shoot tips compared to non-treated ones.

CONCLUSION

The results indicate that melatonin significantly increased regeneration from 15% to 35%.

Evolution and Phylogenetic Diversity of Yam Species (Dioscorea spp.): Implication for Conservation and Agricultural Practices

Yams (Dioscorea spp.) consist of approximately 600 species. Presently, these species are threatened by genetic erosion due to many factors such as pest attacks and farming practices. In parallel, complex taxonomic boundaries in this genus makes it more challenging to properly address the genetic diversity of yam and manage its germplasm. As a first step toward evaluating and preserving the genetic diversity yam species, we use a phylogenetic diversity (PD) approach that has the advantage to investigate phylogenetic relationships and test hypotheses of species monophyly while alleviating to the problem of ploidy variation within and among species. The Bayesian phylogenetic analysis of 62 accessions from 7 species from three regions of Cameroon showed that most Dioscorea sections were monophyletic, but species within sections were generally non-monophyletic. The wild species D. praehensilis and cultivated D. cayenensis were the species with the highest PD. At the opposite, D. esculenta has a low PD and future studies should focus on this species to properly address its conservation status. We also show that wild species show a stronger genetic structure than cultivated species, which potentially reflects the management of the yam germplasm by farmers. These findings show that phylogenetic diversity is a promising approach for an initial investigation of genetic diversity in a crop consisting of closely related species.

Cryopreservation for the 'in perpetuity' conservation of yam and cassava genetic resources

Cryopreservation via droplet vitrification showed high efficiency for cassava meristems (79 pecent average recovery) when these were excised from in vitro seedlings. The efficiency of the process dropped considerably (to > 23 percent) when meristems were excised from field-grown plants, thus precluding the use of such explants for routine cryobanking. In yam, large disparities were observed in the ability of meristems to produce a shoot after cryopreservation ranging from 0 to 60 percent, depending on the accession. Overall, better recovery was observed for Dioscorea rotundata than for D. alata, the two main species tested. Using a probabilistic decision support tool and taking into consideration our cryoprocessing capacity, we conclude that processing 100 meristems per accession and retrieving 30 to estimate the recovery rate of the batch are a good compromise for the cryobanking routine.

Production of yam mosaic virus (ymv)-free Dioscorea opposita plants by cryotherapy of shoot-tips

In the present study, Yam mosaic virus (YMV) could be efficiently eliminated by cryotherapy in Dioscorea opposita. Shoot apices were precultured for 16 h with 0.3 M sucrose, encapsulated in sodium alginate and dehydrated for 4 h prior to direct immersion in liquid nitrogen. Up to 90 percent of the plants regenerated from cryopreserved shoot tips were YMV-free, whereas only 40% of those regenerated using meristem culture were YMV-free. YMV-free yam plantlets could be propagated in vitro through nodal stem culture, with sequential subculturing at 6-week intervals on medium containing 0.5 mg per liter kinetin. The microtubers formed at the bottom and axil of the explants, incubated at 30 degreeC after being chilled (4 degree C) for 3 months, could be sprouted successfully under in vivo conditions. Healthy plants were established without any damaging symptoms of the virus. Thus, cryotherapy provides an alternative method for efficient elimination of yam viruses, and could be simultaneously used for long-term storage of yam germplasm and for the production of virus-free plants.

Understanding the germination of bulbils from an ecological perspective: a case study on Chinese yam (Dioscorea polystachya)

BACKGROUND AND AIMS

Bulbils serve as a means of vegetative reproduction and of dispersal for many plants; this latter aspect making them analogous to seeds. However, germination of bulbils may differ considerably from seeds due to dissimilar anatomical structures and perhaps environmental cue perception. The few laboratory studies done on bulbils suggest that their germination is similar to that of seeds in the same habitats and to vegetative buds of winter-dormant plants. The present study is the first to examine how bulbil germination is controlled in nature in relation to dispersal (before vs. after winter of the same cohort) and to ambient temperatures.

METHODS

Under laboratory conditions, temperature and light requirements for root and shoot emergences from bulbils of Dioscorea polystachya collected in September, 2005, February, 2006 (produced in 2005) and July, 2006 were determined. Effects of cold stratification and dry storage for releasing dormancy were tested on September and July bulbils. The phenology of dormancy release and of root and shoot emergences and the persistence of bulbils in soil were followed over time under field conditions.

KEY RESULTS

Although a low percentage of bulbils collected in July or in September produced roots, but no shoots, in the laboratory and field, these roots died within approx. 1 month. Regardless of collection date, cold stratification markedly increased root and shoot emergences. Bulbils sown outdoors in October produced roots and shoots the following March and April, respectively. The soil bulbil bank is short lived.

CONCLUSIONS

Bulbils of D. polystachya are similar to seeds of many temperate plants being mostly dormant when dispersed in summer or autumn and overcoming dormancy with cold stratification during winter. Adaptively, bulbil germination primarily occurs in spring at the beginning of a favourable period for survivorship and growth.

Differential effect of hot water treatment on whole tubers versus cut setts of yam (Dioscorea spp.)

BACKGROUND

The use of thermotherapy or hot water treatment (HWT) is recommended for the management of plant-parasitic nematodes and other pathogens for a range of planting material, especially vegetatively propagated crops including yams, Dioscorea spp. The sprouting (germination) and consequent viability of yam following HWT, however, appear to be influenced by the post-treatment method of planting (whole or cut setts) and cultivar. The present study was established to evaluate the sensitivity of the most popular yam cultivars in Benin and Nigeria, West Africa, to HWT at 50-53 degrees C for 20 min.

RESULTS

Sprouting of both setts and whole tubers of most cultivars was affected by HWT. Across experiments, 47% of HWT material, compared with 61% of non-HWT material, sprouted over 8 weeks. When cut into setts, 41% of HWT or untreated tubers sprouted, compared with 72% of whole tubers. Whole, untreated tubers had highest sprouting rates (84%), and setts following HWT had the lowest (38%). Yam planting material was also not completely free of parasitic nematodes following HWT. The reaction to HWT or cutting was highly cultivar specific.

CONCLUSION

Yam cultivars vary in their sensitivity to hot water therapy. Care is therefore advised in selecting yam cultivars for HWT, especially when using cut setts.

Promiscuous arbuscular mycorrhizal symbiosis of yam (Dioscorea spp.), a key staple crop in West Africa

Yam (Dioscorea spp.) is a tuberous staple food crop of major importance in the sub-Saharan savannas of West Africa. Optimal yields commonly are obtained only in the first year following slash-and-burn in the shifting cultivation systems. It appears that the yield decline in subsequent years is not merely caused by soil nutrient depletion but might be due to a loss of the beneficial soil microflora, including arbuscular mycorrhizal fungi (AMF), associated with tropical "tree-aspect" savannas and dry forests that are the natural habitats of the wild relatives of yam. Our objective was to study the AMF communities of natural savannas and adjacent yam fields in the Southern Guinea savanna of Benin. AMF were identified by morphotyping spores in the soil from the field sites and in AMF trap cultures with Sorghum bicolor and yam (Dioscorea rotundata and Dioscorea cayenensis) as bait plants. AMF species richness was higher in the savanna than in the yam-field soils (18-25 vs. 11-16 spp.), but similar for both ecosystems (29-36 spp.) according to the observations in trap cultures. Inoculation of trap cultures with soil sampled during the dry season led to high AMF root colonization, spore production, and species richness (overall 45 spp.) whereas inoculation with wet-season soil was inefficient (two spp. only). The use of D. cayenensis and D. rotundata as baits yielded 28 and 29 AMF species, respectively, and S. bicolor 37 species. AMF root colonization, however, was higher in yam than in sorghum (70-95 vs. 11-20%). After 8 months of trap culturing, the mycorrhizal yam had a higher tuber biomass than the nonmycorrhizal controls. The AMF actually colonizing D. rotundata roots in the field were also studied using a novel field sampling procedure for molecular analyses. Multiple phylotaxa were detected that corresponded with the spore morphotypes observed. It is, therefore, likely that the legacy of indigenous AMF from the natural savanna plays a crucial role for yam productivity, particularly in the low-input traditional farming systems prevailing in West Africa.

[Seedling botanical characters and determination of effective components on Dioscorea zingerbrensis by space mutation]

OBJECTIVE

In order to find the new varieties with different horticultural characters, and investigate the mutation effects of seeds of Dioscorea zingerbrensis.

METHOD

The seeds were carried by a satellite into space and recovered. The space mutation effect on the germination, seedling growth, chromosomes and rhizome diosgenin content of SP, populations of D. zingerbrensis were investigated.

RESULT

Stimulated by space environment, the seed possessed the fast germinating characteristics. Germination rate showed no change. It was also found that a few plants were aneuploid or tetraploid. Fresh rhizome weight and rhizome diosgenin content in the second year plant were remarkably higher than those of the control. However, the increasing of third year plant was slow down, and rhizome diosgenin content in the third year plant declined simultaneously.

CONCLUSION

The space environment showed stimulating effects on seed germination, fresh rhizome weight and rhizome diosgenin content.

Sucrose treatment and explant water content: critical factors to consider in development of successful cryopreservation protocols for shoot tip explants of the tropical species Dioscorea rotundata (YAM)

A study was conducted to determine the optimum methods for conditioning explants to be used in the development of a simple protocol for long-term conservation of the germplasm of Dioscorea rotundata via cryopreservation. Shoot tips from cultures maintained in vitro were exposed to high concentrations of sucrose prior to silica gel-based dehydration and vitrification solution-based cryopreservation protocols. Explant water contents were determined, and ultrastructural studies were also carried out. Initially, culturing explants on medium supplemented with 0.3 M sucrose for 3-5 d considerably reduced tissue water content from about 12.2 g/g dry mass to between 4.8 and 5.5 g/g dry mass before cryoprotection with modified PVS2 (MPVS2) or silica gel dehydration. Ultrastructural studies indicated that cells had deposits of starch in plastids following sucrose treatments. Survival for D. rotundata shoot tips treated with MPVS2 vitrification solution, unloaded with 1.0 M sucrose medium and cooled to -7 degree C, was 16 percent for 15 min treatment and 44 percent for 40 min. After the 40 min MPVS2 treatment the TTZ test indicated 88 percent viability retention of explants cooled to -70 degree C, and 44 percent at -196 degree C. Plantlet development was obtained for -70 degree C-cooled shoot tips, whereas only callus development occurred from tissues exposed to liquid nitrogen. Explant regeneration was not obtained with air-dehydration techniques. It was concluded that vitrification-solution based cryopreservation presently offers the best option for conservation of this Dioscorea species.

Cryopreservation of embryogenic callus of Dioscorea bulbifera by vitrification

Cryopreservation of callus of Dioscorea bulbifera by vitrification was optimized. Calli of Dioscorea bulbifera were pretreated in liquid Murashige and Skoog (MS) medium supplemented with 2 mg L(-1) kinetin (KT), 0.5 mg L(-1) NAA, 0.5 mg L(-1) 2,4-D and 0.2 M sucrose for 5 d under continuous light (36 microM m(-2) s(-1)) at 25 + or - 1 degree C. The material was then loaded with 60 percent vitrification solution (PVS2) for 20 min at room temperature and dehydrated with 100 percent PVS2 for 30 min at 0 degree C. After changing the solution with fresh PVS2, the calli were directly immersed in liquid nitrogen and conserved for 1- 360 d. After rapid thawing in a water-bath at 35 degree C, the calli were washed three times with liquid MS medium supplemented with 2 mg L(-1) KT, 0.5 mg L(-1) NAA, 0.5 mg L(-1) 2, 4-D and 1.2 M sucrose and then transferred onto solid MS medium supplemented with KT 2 mg L(-1), NAA 0.5 mg per liter, 0.09 M sucrose and 0.75 percent agar. The cultures were kept in the dark for 2 days prior to exposure to the light (12 h light-dark cycle). The TTC test showed that 80-90 percent of the calli survived this cryoprocedure and there was a 60-70 percent regeneration of plantlets from the calli. The regenerated material did not exhibit any morphological variations.

Cryopreservation of plantlet nodes of Dioscorea opposita Thunb. using a vitrification method

A cryopreservation method by vitrification was developed for long-term storage of Dioscorea opposita Thunb., a valuable native medicinal plant species in Henan Province of China. The cryopreservation protocol was established with cultivar B and evaluated with another four cultivars, Tiegun, 47, Taigu and Huaiqing 1. The results showed that nodes with a bud excised from 60 d plantlets were desirable for the cryopreservation. The optimum procedure was established as: 1) the plantlets were cultured on the Murashige and Skoog (MS) medium supplemented with 2 mg L(-1) KT and 0.02 mg L(-1) NAA at 4 degree C for 7 d before nodes with length of 1-1.5 cm were excised; 2) the nodes were precultured at 4 degree C for 7 d on the MS supplemented with 10 percent dimethyl sulfoxide (DMSO) followed by loading with 60 percent Dioscorea vitrification solution 1 (DVS1): 22 percent (w/v) glycerol + 13 percent (w/v) ethylene glycol + 13 percent (w/v) polyethylene glycol + 10 percent (w/v) DMSO for 60 min at 0 degree C and dehydrated with 100 percent DVS1 for 60 min at 0 degree C; 3) the nodes were then immersed into liquid nitrogen (LN) directly and conserved for 180 d; 4) after rapid thawing in a water-bath at 37 degree C, the nodes were rinsed four times with MS medium supplemented with 5 percent sucrose, then transferred to the MS medium supplemented with 2 mg L(-1) kinetin (KT) and 0.02 mg L(-1) NAA for regeneration. In the present research the regeneration rate of cv. B was about 77.1 percent, those of cvs. Tiegun and Huaiqing 1 were 67.2 percent and 54.0 percent respectively, while cvs. Taigu and 47 were about 40 percent. There were no visual changes observed between the plantlets regenerated from nodes with and without cryopreservation in terms of the morphology indices, indicating that the method established could be applicable to D. opposita with optimized protocol.

Cryopreservation of in vitro shoot tips of Dioscorea deltoidea Wall., an endangered medicinal plant: effect of cryogenic procedure and storage duration

In vitro shoot tips of Dioscorea deltoidea Wall., an endangered medicinal plant, were successfully cryopreserved using the vitrification and the encapsulation-dehydration techniques with subsequent high frequency plant regeneration. Using vitrification, post-liquid nitrogen (LN) shoot regeneration up to 83% was recorded when excised shoot tips were pretreated overnight on MS medium containing 0.3 M sucrose followed by loading with MS containing 2 M glycerol plus 0.4 M sucrose for 20 min at 25 degree C, dehydration with PVS2 for 90 min at 0 degree C and quenching in LN. After 1 h of storage in LN, the shoot tips were rewarmed in a water-bath at 40 degrees C, unloaded with 1.2 M sucrose solution for 20 min and cultured on recovery growth medium. While using encapsulation-dehydration, the highest regeneration frequency recorded was 76% when sucrose-pretreated shoot tips were encapsulated with 3% calcium alginate, precultured in 0.75 M sucrose for 3 days, dehydrated to 25% moisture content (FW basis) under the laminar air flow, stored in LN for 1h and rewarmed at 40 degree C. The cryopreserved shoot tips maintained their viability and an unaltered level of regeneration capability after up to one year of storage in LN.

Regeneration of Dioscorea floribunda plants from cryopreserved encapsulated shoot tips: effect of plant growth regulators

The encapsulation-dehydration protocol for the cryopreservation of in vitro shoot tips of Dioscorea floribunda was optimized. Maximum survival of 87% was obtained when overnight pretreatment with 0.3 M sucrose was followed by encapsulation, preculture in 0.75 M sucrose for 4 d, dehydration in a laminar air flow for 5.5 h, quenching in liquid nitrogen and thawing at 40 degrees C. During recovery growth, 29% shoot formation was obtained when cryopreserved shoot tips were initially cultured for 25 d on a medium with 1.5 mg per liter (-1) BAP, 0.2 mg per liter(-1) NAA and 0.2 mg per liter(-1) GA3 followed by culturing for 15 d on a medium with reduced BAP (1 mg per liter(-1)) but increased NAA (0.5 mg per liter(-1)) and GA3 (0.3 mg per liter(-1)). Finally, transfer on to a medium with further reduced doses of BAP (0.05 mg per liter(-1)) and NAA (0.15 mg per liter(-1)) but without GA3 stimulated production of fully grown plantlets. All plants regenerated without callus formation. Modification of post-thaw culture media with plant growth regulators was essential for regrowth of shoot tips to plantlets.

Genetic fidelity of in vitro regenerants, encapsulation of shoot tips and high diosgenin content in Dioscorea bulbifera L., a potential alternative source of diosgenin

Dioscorea bulbifera L. containing the pharmaceutically important compound, diosgenin, was regenerated in vitro through nodal segments on supplemented Murashige and Skoog medium (MS). Diosgenin was at 12 mg g(-1)dry wt in 12-week-old plantlets raised on MS with various growth hormones. Random amplified polymorphic DNA (RAPD) analysis showed genetic fidelity of regenerants. Encapsulation of shoot tips in 3% (w/v) calcium alginate for storage and germplasm exchange was achieved.

Farmers' use of wild relative and sexual reproduction in a vegetatively propagated crop. The case of yam in Benin

The impact of traditional farmers' management on genetic diversity of vegetatively propagated crops is poorly documented. In this study, we analysed the impact of ennoblement of spontaneous yams, an original traditional farmers' practice, on the genetic diversity of yam (Dioscorea sp.) in Benin. We used 11 microsatellite markers on yam tubers from a small village in northern Benin and demonstrated that wild x cultivated hybrids are spontaneously formed. Many of the spontaneous yams collected by farmers from surrounding savannah areas for ennoblement were shown to be of wild and hybrid genotypes. Moreover, we demonstrated that some yam varieties have a wild or hybrid signature. Lastly, we performed a broader ranging genetic analysis on yam material from throughout Benin and showed that this practice is used in different ecological and ethno-linguistic regions. Through this practice, farmers create new varieties with new genetic combinations via sexual reproduction of wild and cultivated yams. This system, whereby a sexual cycle and asexual propagation are mixed, ensures potential large-scale cultivation of the best genotypes while preserving the potential for future adaptation.

Phases of dormancy in Yam tubers (Dioscorea rotundata)

BACKGROUND AND AIMS

The control of dormancy in yam (Disocorea spp.) tubers is poorly understood and attempts to shorten the long dormant period (i.e. cause tubers to sprout or germinate much earlier) have been unsuccessful. The aim of this study was to identify and define the phases of dormancy in Dioscorea rotundata tubers, and to produce a framework within which dormancy can be more effectively studied.

METHODS

Plants of 'TDr 131' derived from tissue culture were grown in a glasshouse simulating temperature and photoperiod at Ibadan (7 degrees N), Nigeria to produce tubers. Tubers were sampled on four occasions: 30 d before shoot senescence (149 days after planting, DAP), at shoot senescence (179 DAP), and twice during storage at a constant 25 degrees C (269 and 326 DAP). The development of the apical shoot bud was described from tissue sections. In addition, the responsiveness of shoot apical bud development to plant growth regulators (gibberellic acid, 2-chloroethanol and thiourea) applied to excised tuber sections was also examined 6 and 12 d after treatment.

KEY RESULTS AND CONCLUSIONS

Three phases of tuber dormancy are proposed: Phase I, from tuber initiation to the appearance of the tuber germinating meristem; Phase II, from the tuber germinating meristem to initiation of foliar primordium; and Phase III, from foliar primordium to appearance of the shoot bud on the surface of the tuber. Phase I is the longest phase (approx. 220 d in 'TDr 131'), is not affected by PGRs and is proposed to be an endo-dormant phase. Phases II and III are shorter (<70 d in total), are influenced by PGRs and environmental conditions, and are therefore endo-/eco-dormant phases. To manipulate dormancy to allow off-season planting and more than one generation per year requires that the duration of Phase I is shortened.

In vitro micropropagation, differentiation of aerial bulbils and tubers and diosgenin content in Dioscorea bulbifera

Dioscorea bulbifera could be micropropagated through nodal segments and bulbils. The best medium for regeneration and bulbil differentiation was MS + 0.5 microM IAA (indole-3-acetic acid) + 20.0 microM Kn (kinetin) + 500 mg/L CH (casein hydrolysate) + activated charcoal (20 %). Diosgenin content was maximum in regenerants grown on MS + 5.0 microM IAA + 20.0 microM Kn + 500 mg/L CH. T.s of bulbils could also be used for direct plantlet differentiation as well as bulbil differentiation on MS + 10.0 microM IAA + 20.0 microM Kn + (in mg/L) 30 each of Asp (asparagine) + Arg (arginine) + Gln (glutamine) + 10 Ad (adenine) + 500 CH + 10 Cyst hyd (cysteine hydrochloride). Diosgenin yield in plantlets reached a maximum after 20 weeks. The results indicate that micropropagation, bulbil formation and tuberisation can be achieved in vitro in D. bulbifera, hitherto a less exploited plant, and can further be used for obtaining enhanced levels of diosgenin.

Genetic stability assessment of plants regenerated from cryopreserved embryogenic tissues of Dioscorea bulbifera l. Using RAPD, biochemical and morphological analysis

Embryogenic tissues of Dioscorea bulbifera were cryopreserved using the encapsulation-dehydration technique. Genetic stability of plants regenerated from cryopreserved embryogenic tissues was assessed using molecular, biochemical and morphological analysis. The random amplified polymorphic DNA (RAPD) analysis of 60 cryopreserved-derived and 20 in vitro grown (control) plantlets showed that 10 primers produced 62 clear reproducible DNA fragment profiles. The amplification products were monomorphic for all the plantlets except one. A total of 4960 DNA fragments were obtained from this study showing no variation in RAPD profiles. The diosgenin content of cryopreserved-derived plants, analyzed using HPLC, was similar to that of control plants. Morphology and the ability to form microtuber were also found to be unaltered in cryopreserved embryo-derived plantlets. Thus, the D. bulbifera plants regenerated from cryopreserved embryogenic tissues were genetically stable at the molecular, biochemical and morphological levels.

Biotransformation of n-hexadecane by cell suspension cultures of Cinchona robusta and Dioscorea composita

This manuscript evaluates the phytotoxicity and biotransformation of n-hexadecane as well as peroxidase activity and cytochrome P450 concentration in microsomes for cell suspension cultures of Cinchona robusta and Dioscorea composita. Phytotoxicity was evaluated based on viability and growth. Cell cultures were exposed to a 2 and 4% (v/v) dose of n-hexadecane. The biotransformation of n-hexadecane was determined based on labeled recovery in polar, nonpolar, and cell residue fractions after cell culture extraction during exponential cell growth phase and stationary phase. Differences were observed in accumulation of label during cell growth phase and stationary phase for the cells of the two plants. Differences also were observed between phases for label in polar and nonpolar fractions. Thin-layer chromatography determined labeled intermediates and some were identified. The activity of peroxidase and concentration of cytochrome P450 was lower in C. robusta than in controls and greater in D. composita than in controls. In vitro biotransformation was not successful.