Sweet Potato Plants Research Articles

Studies on NO functions in plant defense responses

The late Emeritus Professor I. Uritani in Nagoya University (1919–2010), one of the professors who contributed to the initial understanding of plant defense physiology, identified ipomeamarone as a phytoalexin produced in sweet potato infected with Ceratocystis fimbriata in 1956. He then primarily investigated phytoalexin biosynthetic pathways in sweet potato plants (Ipomoea batatas). He coined the catch phrase ‘‘From usual to unusual, from unusual to usual’’, defining that ordinary plant physiological phenomena and their targets and mechanisms comprise ‘‘usual’’ plant development to sustain life, while plant defense physiology deals with de novo physiological phenomena generated under ‘‘unusual’’ stress conditions, especially during infection by plant pathogens. He suggested that plant defense physiology focuses on secondary metabolism, which is fully expressed under stress conditions, and a total understanding of both plant physiology and plant defense physiology should enable us to reveal the entire life of the plant. His suggestion that multiple viewpoints are required to elucidate the spatial and temporal changes and the resultant complex phenomena in plant–pathogen interactions is still valuable. The late Emeritus Professor K. Tomiyama in Nagoya University (1917–1998) began detailed studies of hypersensitive cell death at the early stage of resistance reactions in potato tuber slices (Solanum tuberosum) infected with Phytophthora infestans, the potato late blight pathogen. Appressorium formation by the pathogen on a host cell causes cytoplasmic aggregation in the cell, defined as the rapid translocation of cytoplasm and the nucleus to the penetration site of the pathogen. The host cell itself finally dies, causing pathogen to die. The hypersensitive response (HR) that results in hypersensitive cell death is closely related to the production of reactive oxygen species (ROS). In 1983, Emeritus Professor N. Doke in Nagoya University found that superoxide (O2 ), a kind of ROS, was necessary to trigger hypersensitive responses in potato tubers inoculated with an avirulent race of P. infestans. The rapid production of O2 (oxidative burst) can inhibit pathogen infection, by inducing various other defense responses, and is observed in various plant tissues infected with various pathogens. Since then, ROS is considered to be a signal that initiates various other defense responses. However, it is not the only molecule required for inducing defense responses. O2 alone cannot induce hypersensitive cell death in plants; nitric oxide (NO) was thought to be another molecule involved in inducing hypersensitive cell death. In this article, I describe NO function in plant defense responses, the NO-producing system and resistant reactions induced by NO-producing elicitors.

Effects of drying on caffeoylquinic acid derivative content and antioxidant capacity of sweet potato leaves

Caffeoylquinic acid (CQA) derivatives are known to possess antioxidative potential and have many beneficial effects on human health. The present study compared the CQA contents and antioxidant activities of aerial parts of sweet potato plants. The effects of drying methods (freeze drying, and drying at 30°C, 70°C, and 100°C) on these two parameters of the first fully expanded leaves were also assessed. The results indicated that the CQA derivatives were detectable in leaves, stem, and flowers of sweet potato plants (varied from 39.34 mg/g dry weight to 154.05 mg/g dry weight), with the leaves (particularly expanding and first fully expanded leaves) containing more CQA derivatives than other aerial plant parts. The expanding and first fully expanded leaves also exhibited greater antioxidant activities than other aerial plant parts, possibly due to their higher contents of CQA derivatives. Drying method significantly affected the content of CQA derivatives in dried sweet potato leaf tissues. Drying treatments at both 70°C and 100°C significantly reduced the CQA derivative content and antioxidant activity in the first fully expanded leaves. Among the tested drying methods, the freeze-drying method demonstrated the preservation of the highest amount of CQA derivatives (147.84 mg/g) and antioxidant property. However, 30°C cool air drying was also a desirable choice (total CQA derivative content was reduced to only 129.52 mg/g), compared to 70°C and 100°C hot air drying, for commercial-scale processing of sweet potato leaves, if the higher operation cost of freeze drying was a major concern.

First Report of Sweet potato golden vein associated virus Infecting Sweet Potato in Korea.

Sweet potato (Ipomoea batatas) is one of the most important crops in eastern Asia, including Korea. Consumption of sweet potato is increasing gradually because of its growing reputation as a health food. Recently, outbreaks of viruses infecting sweet potatoes have increased all over the world, probably because sweet potatoes are produced via vegetative propagation (1,2). In Korea, most sweet potatoes in fields have been infected by a begomovirus, Sweet potato leaf curl virus (SPLCV), and other viruses such as Sweet potato feathery mottle virus, Sweet potato virus G, and Sweet potato latent virus (3). Many countries have monitored sweet potato virus infections in fields as well as in germplasm collections to select virus-free stocks. In 2013, 20 sweet potato plants showing leaf roll symptoms in Muan, South Korea, were collected and analyzed. Total DNA was isolated from sweet potato leaves (Viral Gene-spin Viral DNA/RNA Extraction Kit, iNtRON Biotechnology, Seongnam, Korea) and viral DNA was amplified by rolling circle amplification (RCA, TempliPhi Amplification Kit, GE Healthcare Life Sciences, Uppsala, Sweden) following the manufacturer's instructions. Amplicons were digested by restriction enzyme SacI (TaKaRa Bio, Shiga, Japan) and products were run on a 1.5% agarose gel. A 2.8-kb DNA fragment was purified from a gel, ligated into a pGEM-T easy vector (Promega, Madison, WI), and sequenced (Macrogen, Seoul, Korea). Based on a BLAST search, most of the sequences (36/38) were identified as SPLCV, but two independent clones 2,824 nt in length from sweet potato cv. Sincheonmi were similar to Sweet potato golden vein associated virus (SPGVaV) isolate US:MS:1B-3 (94.38%, GenBank Accession No. HQ333143). The complete genome sequence of the SPGVaV-Korea isolate contained six ORFs, as expected for a typical monopartite begomovirus. The sequence was deposited in GenBank under accession number KF803170. SPGVaV is a whitefly (Bemisia tabaci)-transmitted virus (genus Begomovirus, family Geminiviridae). A phylogenetic analysis that included other begomoviruses that infect sweet potato showed SPGVaV-Korea to segregate with other SPGVaV isolates. SPGVaV has previously only been reported in Brazil and the United States (1). This is the first report of SPGVaV in sweet potato outside of the Americas.

Development of a Mechanical Undercutting System to Minimize SweetpotatoSkinning during Harvest

<abstract><title><italic>Abstract. </italic></title> Sweetpotatoes have been an important high-value crop in Mississippi and future market growth is expected. Industry demand has created the need for a continuous supply of sweetpotatoes throughout the year. Therefore, managing the harvest process and postharvest storage environment is critical to maintaining a year-round supply of quality sweetpotato roots. This has been a challenge in Mississippi and growers have been experiencing postharvest losses due to excessive root shrinkage (weight loss) and bacterial and fungal rots. These losses are directly related to damage during harvest procedures that cause cuts and abrasions (skinning) to the delicate skin of the sweetpotato root. De-vining sweetpotatoes prior to harvest is a commonly used method to halt root growth and begin toughening of the skin. This method is viable for producers using manually-assisted harvesting for the fresh market. Producers using bulk harvesting prefer to leave vines on to reduce the amount of foreign material going into storage. A new method of halting plant growth and allowing the root to cure in the ground prior to harvest is needed. The objective of this study was to design and test a mechanical root pruning blade to halt plant growth and initiate skin set prior to harvest of sweetpotatoes and to quantify the effects of undercutting sweetpotatoes on skin strength relative to de-vining. The hypothesis was that cutting the deep root of the sweetpotato plant would allow this process to begin. Therefore, two different undercutting implements were designed and fabricated. These implements were tested in experimental plots and the skin strength was directly measured. Root skin strength was measured at three days and six days after treatment. Results indicated that at three days after treatment, undercutting had no significant effect on skin strength for both vine conditions (vine-on and de-vined). At six days after treatment, undercutting with the newly developed implement significantly increased skin strength for roots in which the vine had been left on.

품종 간 고구마 흰비단병(Sclerotium rolfsii) 발생 정도 비교

caused a stem rot on seedling of sweet potato plants and causes a crown rot on lower stems near or at the soil line at favorable environmental conditions. White mycelial mats and sclerotia were formed at the infection sites. Plants severely infected were fell over or died because lower stems near soil sur-face were rotten. The degree of disease incidence was varied according to cultivars. Two cultivars, Bio-mi and Deayumi, were very resistant, while five cultivars, Shinjami, Shingeonmi, Hongmi, Yeonjami, and Shinhung-3 were highly susceptible.

Small RNA deep sequencing‐based detection and further evidence of DNA viruses infecting sweetpotato plants in Tanzania

AbstractSmall interfering RNA deep sequencing (SRDS) was used to detect viruses in 23 sweetpotato plants, collected from various locations in Tanzania. Alignment of small RNA reads using a MAQ program recovered genomes of viruses from five families, namely Geminiviridae (2), Closteroviridae (1), Betaflexiviridae (1), Caulimoviridae (1) and Potyviridae (1). This was in agreement with the variation of symptoms observed on sweetpotato plants in fields and screen house, which included leaf curl, vein yellowing, chlorosis, stunted growth and brown blotches. PCR was also used to confirm the occurrence of viruses associated with leaf curl and symptomless infections. A complete genome (2768 nucleotides) was obtained for a sweepovirus that was 89.9% identical to the strain of Sweet potato leaf curl Sao Paulo virus (SPLCSPV; Begomovirus) reported in South Africa. Sweepoviruses are known to undergo frequent recombinations and evidence for this was found in the SPLCSPV sequence studied. The SRDS‐based results indicated occurrence of the poorly studied Sweet potato badnavirus B (SPBV‐B) and Sweet potato badnavirus A (collectively known as Sweet potato pakakuy virus; SPPV; Caulimoviridae) in sweetpotato plants in Tanzania. A 5′‐end partial sequence (3065 nucleotides), encoding hypothetical, movement and coat proteins, was obtained and found to be 86.3% and 73.1% identical to SPBV‐B and SPBV‐A, respectively. A survey for the distribution of SPPV and Sweet potato symptomless mastrevirus 1 (SPSMV‐1) showed that these viruses were wide spread and co‐infecting sweetpotato plants in Tanzania. The importance of East Africa as a hot spot for the diversity and evolution of sweet potato viruses is discussed.

Agroinfection of sweet potato by vacuum infiltration of an infectious sweepovirus.

Sweepovirus is an important monopartite begomovirus that infects plants of the genus Ipomoea worldwide. Development of artificial infection methods for sweepovirus using agroinoculation is a highly efficient means of studying infectivity in sweet potato. Unlike other begomoviruses, it has proven difficult to infect sweet potato plants with sweepoviruses using infectious clones. A novel sweepovirus, called Sweet potato leaf curl virus-Jiangsu (SPLCV-JS), was recently identified in China. In addition, the infectivity of the SPLCV-JS clone has been demonstrated in Nicotiana benthamiana. Here we describe the agroinfection of the sweet potato cultivar Xushu 22 with the SPLCV-JS infectious clone using vacuum infiltration. Yellowing symptoms were observed in newly emerged leaves. Molecular analysis confirmed successful inoculation by the detection of viral DNA. A synergistic effect of SPLCV-JS and the heterologous betasatellite DNA-β of Tomato yellow leaf curl China virus isolate Y10 (TYLCCNV-Y10) on enhanced symptom severity and viral DNA accumulation was confirmed. The development of a routine agroinoculation system in sweet potato with SPLCV-JS using vacuum infiltration should facilitate the molecular study of sweepovirus in this host and permit the evaluation of virus resistance of sweet potato plants in breeding programs.

Production, storage and costs of inoculum of arbuscular mycorrhizal fungi (AMF)

Arbuscular mycorrhizal fungi (AMF) inoculation may contribute to the development of sustainable agriculture. Inoculation requires production and storage of inocula in a commercial scale, and few protocols have been developed for this purpose. AMF inocula were produced in an aeroponic system, and their infectivities were tested after storage for 3, 6, and 10 months, at room temperature (28 °C) or at 4 °C, in substrates composed of sand, sand + expanded clay, or sand + vermiculite. Sweet potato plants, previously grown in soil + sand for 90 days and mycorrhized by Claroideoglomus etunicatum and Glomus clarum, were transplanted to the aeroponic system and harvested after 120 days. The density of glomerospores of both AMF per gram of roots in the aeroponic system was much higher than per gram of soil in pots, facilitating handling and storage of the inocula. Storage at room temperature resulted in decreased infectivity of the inocula in all substrates after 6 months, while storage at 4 °C in sand + vermiculite and sand + clay maintained the viability (>59 % infectivity) even after 10 months. The costs of production to inoculate one seedling pot with C. etunicatum and with G. clarum were R$ 0.98 (US$ 0.41) and R$ 3.07 (US$ 1.30), respectively, and should be reduced in a commercial system. With an adequate infectivity (>25 %) and relatively low production costs these inocula can be recommended as biofertilizers.

An Ipomoea batatas iron-sulfur cluster scaffold protein gene, IbNFU1, is involved in salt tolerance.

Iron-sulfur cluster biosynthesis involving the nitrogen fixation (Nif) proteins has been proposed as a general mechanism acting in various organisms. NifU-like protein may play an important role in protecting plants against abiotic and biotic stresses. An iron-sulfur cluster scaffold protein gene, IbNFU1, was isolated from a salt-tolerant sweetpotato (Ipomoea batatas (L.) Lam.) line LM79 in our previous study, but its role in sweetpotato stress tolerance was not investigated. In the present study, the IbNFU1 gene was introduced into a salt-sensitive sweetpotato cv. Lizixiang to characterize its function in salt tolerance. The IbNFU1-overexpressing sweetpotato plants exhibited significantly higher salt tolerance compared with the wild-type. Proline and reduced ascorbate content were significantly increased, whereas malonaldehyde (MDA) content was significantly decreased in the transgenic plants. The activities of superoxide dismutase (SOD) and photosynthesis were significantly enhanced in the transgenic plants. H2O2 was also found to be significantly less accumulated in the transgenic plants than in the wild-type. Overexpression of IbNFU1 up-regulated pyrroline-5-carboxylate synthase (P5CS) and pyrroline-5-carboxylate reductase (P5CR) genes under salt stress. The systemic up-regulation of reactive oxygen species (ROS) scavenging genes was found in the transgenic plants under salt stress. These findings suggest that IbNFU1gene is involved in sweetpotato salt tolerance and enhances salt tolerance of the transgenic sweetpotato plants by regulating osmotic balance, protecting membrane integrity and photosynthesis and activating ROS scavenging system.

Plant age and genotype affect the bacterial community composition in the tuber rhizosphere of field-grown sweet potato plants

The hypothesis that sweet potato genotypes containing different starch yields in their tuberous roots can affect the bacterial communities present in the rhizosphere (soil adhering to tubers) was tested in this study. Tuberous roots of field-grown sweet potato of genotypes IPB-149 (commercial genotype), IPB-052, and IPB-137 were sampled three and six months after planting and analyzed by denaturing gradient gel electrophoresis (DGGE) and pyrosequencing analysis of 16S rRNA genes PCR-amplified from total community DNA. The statistical analysis of the DGGE fingerprints showed that both plant age and genotypes influenced the bacterial community structure in the tuber rhizosphere. Pyrosequencing analysis showed that the IPB-149 and IPB-052 (both with high starch content) displayed similar bacterial composition in the tuber rhizosphere, while IPB-137 with the lowest starch content was distinct. In comparison with bulk soil, higher 16S rRNA gene copy numbers (qPCR) and numerous genera with significantly increased abundance in the tuber rhizosphere of IPB-137 (Sphingobium, Pseudomonas, Acinetobacter, Stenotrophomonas, Chryseobacterium) indicated a stronger rhizosphere effect. The genus Bacillus was strongly enriched in the tuber rhizosphere samples of all sweet potato genotypes studied, while other genera showed a plant genotype-dependent abundance. This is the first report on the molecular identification of bacteria being associated with the tuber rhizosphere of different sweet potato genotypes.

植物の防御応答におけるNO機能に関する研究

The late Emeritus Professor I. Uritani in Nagoya University (1919–2010), one of the professors who contributed to the initial understanding of plant defense physiology, identified ipomeamarone as a phytoalexin produced in sweet potato infected with Ceratocystis fimbriata in 1956. He then primarily investigated phytoalexin biosynthetic pathways in sweet potato plants (Ipomoea batatas). He coined the catch phrase ‘‘From usual to unusual, from unusual to usual’’, defining that ordinary plant physiological phenomena and their targets and mechanisms comprise ‘‘usual’’ plant development to sustain life, while plant defense physiology deals with de novo physiological phenomena generated under ‘‘unusual’’ stress conditions, especially during infection by plant pathogens. He suggested that plant defense physiology focuses on secondary metabolism, which is fully expressed under stress conditions, and a total understanding of both plant physiology and plant defense physiology should enable us to reveal the entire life of the plant. His suggestion that multiple viewpoints are required to elucidate the spatial and temporal changes and the resultant complex phenomena in plant–pathogen interactions is still valuable. The late Emeritus Professor K. Tomiyama in Nagoya University (1917–1998) began detailed studies of hypersensitive cell death at the early stage of resistance reactions in potato tuber slices (Solanum tuberosum) infected with Phytophthora infestans, the potato late blight pathogen. Appressorium formation by the pathogen on a host cell causes cytoplasmic aggregation in the cell, defined as the rapid translocation of cytoplasm and the nucleus to the penetration site of the pathogen. The host cell itself finally dies, causing pathogen to die. The hypersensitive response (HR) that results in hypersensitive cell death is closely related to the production of reactive oxygen species (ROS). In 1983, Emeritus Professor N. Doke in Nagoya University found that superoxide (O2 ), a kind of ROS, was necessary to trigger hypersensitive responses in potato tubers inoculated with an avirulent race of P. infestans. The rapid production of O2 (oxidative burst) can inhibit pathogen infection, by inducing various other defense responses, and is observed in various plant tissues infected with various pathogens. Since then, ROS is considered to be a signal that initiates various other defense responses. However, it is not the only molecule required for inducing defense responses. O2 alone cannot induce hypersensitive cell death in plants; nitric oxide (NO) was thought to be another molecule involved in inducing hypersensitive cell death. In this article, I describe NO function in plant defense responses, the NO-producing system and resistant reactions induced by NO-producing elicitors.

奄美群島におけるアリモドキゾウムシおよびイモゾウムシの分布（2010-2011年）

The geographical distribution of the sweetpotato weevil, Cylas formicarius and the West Indian sweetpotato weevil, Euscepes postfasciatus were investigated on sweetpotato, Ipomoea batatas and wild host plants, I. india and I. pes-caprae on the Amami Islands except Kikai Island during 2010 to 2011. The number of C. formicarius male adults was determined using simple sticky traps with synthetic sex pheromone at 235 sites during November 2010 to February 2011. The occurrence of the two weevils were investigated at 109 sweetpotato fields and 97 creeping colonies of wild host plants, I. india and I. pes-caprae, sites during November 2010 to February 2011. Both weevil species were found in all islands of the Amami Islands. This was consistent with the result of 1997-1998, and the distribution of both weevils species have not changed on the Amami Islands. The occurrence of E. postfasciatus was on the increase from 1997. On Okinoerabu Island, the number of male adults of C. formicarius captured using pheromone traps were fewer than the other islands.

Overexpression of IbP5CR enhances salt tolerance in transgenic sweetpotato

Pyrroline-5-carboxylate reductase (P5CR) lies at the converging point of the glutamate and ornithine pathways and is the last and critical enzyme in proline biosynthesis. In the present study, a P5CR gene, named IbP5CR, was isolated from salt-tolerant sweetpotato line ND98. Expression of IbP5CR was up-regulated in sweetpotato under salt stress. The IbP5CR-overexpressing sweetpotato (cv. Kokei No. 14) plants exhibited significantly higher salt tolerance compared with the wild-type. Proline content and superoxide dismutase and photosynthetic activities were significantly increased, whereas malonaldehyde content was significantly decreased in the transgenic plants. H2O2 was also found to be significantly less accumulated in the transgenic plants than in the wild-type. Overexpression of IbP5CR up-regulated pyrroline-5-carboxylate synthase gene and down-regulated proline dehydrogenase and P5C dehydrogenase genes under salt stress. The systemic up-regulation of reactive oxygen species (ROS) scavenging genes was found in the transgenic plants under salt stress. These findings suggest that overexpression of IbP5CR increases proline accumulation, which enhances salt tolerance of the transgenic sweetpotato plants by regulating osmotic balance, protecting membrane integrity and photosynthesis and activating ROS scavenging system. This study indicates that IbP5CR gene has the potential to be used for improving salt tolerance of plants.

The ability of cultivars of sweetpotato in East Africa to ‘revert’ from Sweet potato feathery mottle virus infection

Asymptomatic field plants are the normal source of the vine cuttings used as sweetpotato planting material in Africa. Previous and new tests of such East African material, mostly using the very sensitive method of graft inoculation to the indicator plant Ipomoea setosa, showed that a majority tested virus-negative. This was despite their never having undergone any science-based therapy. To investigate how this occurs, in a replicated greenhouse experiment, plants of susceptible cultivars from the USA and Peru and three resistant Ugandan cultivars were graft-inoculated with Sweet potato feathery mottle virus (SPFMV), the commonest virus infecting sweetpotato. When the grafts were established, cuttings were taken, rooted and proved to be infected. The health status of each of these new plants was then followed over a 10-week period using a quantitative polymerase chain reaction assay. Most of the plants of the Ugandan cultivars eventually tested SPFMV-negative whereas those of the USA and Peru seldom did. Furthermore, in subsequent graft-inoculations of scions from the tip, top, middle and base of the vine of every plant to I. setosa plants, again, most of the scions of the Ugandan cultivars tested SPFMV-negative whereas those of the USA and Peru seldom did. These tests demonstrate the phenomenon of reversion in the Ugandan cultivars and can explain how most unprotected Ugandan sweetpotato field plants tested SPFMV-negative.

Infection of Sweetpotato by Fusarium solani and Macrophomina phaseolina Prior to Harvest.

The end rot disease complex, caused mainly by Fusarium solani and Macrophomina phaseolina, can be an important postharvest problem in sweetpotato. The disease develops a few weeks after storage roots are harvested and stored. Isolations attempted after harvest showed that the pathogens can be present inside the storage roots before symptoms appear. To determine how and when end rot pathogens enter sweetpotato storage roots, two greenhouse experiments were designed using tissue culture-derived plants free of F. solani and M. phaseolina. In one experiment, plants were grown in autoclaved soil, and 1 month after transplanting, plants were inoculated at the soil line with either noninfested toothpicks or toothpicks infested with each fungus alone or combined. In the other experiment, plants were grown in noninfested soil or in soil infested with each fungus alone or combined. F. solani and M. phaseolina were isolated from roots, storage roots, and plant stems below the soil line, at the soil line, and 5 cm above the soil line in both experiments. This suggests these fungi are capable of invading sweetpotato plants and storage roots from infested soil, and can systemically colonize the plant from infected plant propagation material.

Genetic variability and evolutionary implications of RNA silencing suppressor genes in RNA1 of sweet potato chlorotic stunt virus isolates infecting sweetpotato and related wild species.

BackgroundThe bipartite single-stranded RNA genome of Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus; Closteroviridae) encodes a Class 1 RNase III (RNase3), a putative hydrophobic protein (p7) and a 22-kDa protein (p22) from genes located in RNA1. RNase3 and p22 suppress RNA silencing, the basal antiviral defence mechanism in plants. RNase3 is sufficient to render sweetpotato (Ipomoea batatas) virus-susceptible and predisposes it to development of severe diseases following infection with unrelated virus. The incidence, strains and gene content of SPCSV infecting wild plant species have not been studied.Methodology/Principal FindingsThirty SPCSV isolates were characterized from 10 wild Ipomoea species, Hewittia sublobata or Lepistemon owariensis (family Convolvulaceae) in Uganda and compared with 34 local SPCSV isolates infecting sweetpotatoes. All isolates belonged to the East African (EA) strain of SPCSV and contained RNase3 and p7, but p22 was not detected in six isolates. The three genes showed only limited genetic variability and the proteins were under purifying selection. SPCSV isolates lacking p22 synergized with Sweet potato feathery mottle virus (SPFMV, genus potyvirus; Potyviridae) and caused severe symptoms in co-infected sweetpotato plants. One SPCSV isolate enhanced accumulation of SPFMV, but no severe symptoms developed. A new whitefly-transmitted virus (KML33b) encoding an RNase3 homolog (<56% identity to SPCSV RNase3) able to suppresses sense-mediated RNA silencing was detected in I. sinensis. Conclusions/SignificanceSPCSV isolates infecting wild species and sweetpotato in Uganda were genetically undifferentiated, suggesting inter-species transmission of SPCSV. Most isolates in Uganda contained p22, unlike SPCSV isolates characterized from other countries and continents. Enhanced accumulation of SPFMV and increased disease severity were found to be uncoupled phenotypic outcomes of RNase3-mediated viral synergism in sweetpotato. A second virus encoding an RNase3-like RNA silencing suppressor was detected. Overall, results provided many novel and important insights into evolutionary biology of SPCSV.

Binding and processing of small dsRNA molecules by the class 1 RNase III protein encoded by sweet potato chlorotic stunt virus

Sweet potato chlorotic stunt virus (SPCSV; genus Crinivirus, family Closteroviridae) causes heavy yield losses in sweet potato plants co-infected with other viruses. The dsRNA-specific class 1 RNase III-like endoribonuclease (RNase3) encoded by SPCSV suppresses post-transcriptional gene silencing and eliminates antiviral defence in sweet potato plants in an endoribonuclease activity-dependent manner. RNase3 can cleave long dsRNA molecules, synthetic small interfering RNAs (siRNAs), and plant- and virus-derived siRNAs extracted from sweet potato plants. In this study, conditions for efficient expression and purification of enzymically active recombinant RNase3 were established. Similar to bacterial class 1 RNase III enzymes, RNase3-Ala (a dsRNA cleavage-deficient mutant) bound to and processed double-stranded siRNA (ds-siRNA) as a dimer. The results support the classification of SPCSV RNase3 as a class 1 RNase III enzyme. There is little information about the specificity of RNase III enzymes on small dsRNAs. In vitro assays indicated that ds-siRNAs and microRNAs (miRNAs) with a regular A-form conformation were cleaved by RNase3, but asymmetrical bulges, extensive mismatches and 2'-O-methylation of ds-siRNA and miRNA interfered with processing. Whereas Mg(2+) was the cation that best supported the catalytic activity of RNase3, binding of 21 nt small dsRNA molecules was most efficient in the presence of Mn(2+). Processing of long dsRNA by RNase3 was efficient at pH 7.5 and 8.5, whereas ds-siRNA was processed more efficiently at pH 8.5. The results revealed factors that influence binding and processing of small dsRNA substrates by class 1 RNase III in vitro or make them unsuitable for processing by the enzyme.

Beneficial effect of plant growth promoting bacteria isolated from the roots of potato plant

This study was conducted with a view to isolate bacteria associated with the roots of sweet potato plants (Ipomoea batatas (L.) Lam.) and to assess their functional potentialities in relation to plant growth promoting activities. Seven bacterial isolates namely P18, P19, P31, P32, P35, P39, and P42 were obtained from surface sterilized healthy roots of sweet potato. The isolates were tested for morphological and biochemical characteristics. The results of in vitro assays showed that all isolates can produce IAA, while four isolates i.e. P18, P31, P35, and P42) solubilize rock phosphate. These isolates having abilities for IAA production and phosphate solubilization were tested as bioinoculant to potato tubers. The results of inoculated plants showed significant differences in vegetative growth parameters as well as photosynthetic pigments and N, P, and K concentrations compared with control. Consequently, the more efficient isolates namely P31 and P35 were identified by 16S ribosomal DNA sequencing analysis as Bacillus cereus and Achromobacter xylosoxidans, respectively. These can be recommended as biofertilizers for reducing the dependence on chemical fertilizers and providing a step forward toward sustainable agriculture.

Sweetpotato plant regeneration via an improved somatic embryogenesis protocol

Genetic transformation holds considerable promise for sweetpotato enhancement, but is limited by the low regeneration efficiency of transformed cells. Somatic embryogenesis is the most efficient method for regenerating genetically transformed sweetpotato plants, but is still limited by low efficiency and strong genotype dependency. In this study, based on a review of previous work, various growth regulators and gelling agents were assessed for an optimized three-stage protocol. Lateral meristems of sweetpotato cultivars ‘Jonathan’ and ‘Jewel’ were used to investigate: (i) the influence of the auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) on the induction of sweetpotato somatic embryogenesis, (ii) the effect of the abscisic acid (ABA) and gibberellic acid (GA3) on the development and maturation of somatic embryos, (iii) the effect of GA3 on development of mature embryos into plantlets, and (iv) the influence of two gelling agents (agar and Gelrite) on different embryogenic phases during all stages of the protocol. Among the treatments evaluated, best results were obtained when growing the explants on Murashige and Skoog (MS) medium with 1.3mg/l 2,4,5-T (in the dark) during the first stage, transferring the calli to media with 1mg/l ABA at the second stage and then cultivating them on 0.424mg/l GA3 for plant development. However, no significant differences were found between gelling agents.

Effects of shading on the photosynthetic capacity, endogenous hormones and root yield in purple-fleshed sweetpotato (Ipomoea batatas (L.) Lam)

A field experiment was conducted to examine the effects of shading on the photosynthetic capacity, endogenous hormones and root yield in purple-fleshed sweetpotato [Ipomoea batatas L. cv. Jishu18 and Ayamuraski (Aya)]. Sweetpotato plants were treated with two shading levels, 40 and 70 % shading, with full radiation used as a control. The results showed that the photosynthetic rate, adenosine triphosphatase activity, Ribulose 1,5-bisphosphate carboxylase activity and soluble sugar content decreased under both shading treatments. Leaf indole-3-acetic acid (IAA) and abscisic acid content increased, whereas leaf gibberellic acid content, zeatin riboside (ZR) content, root IAA, and ZR content decreased in the plants under both shading treatments. Shading also altered the production of sweetpotato storage root, including reductions in the root yield and dry matter accumulation, increase in the top/root (T/R) ratio, and the difference between the treatments and control for the T/R value and storage root yield was significant. Therefore, the responses of the photosynthetic parameters and endogenous hormones to shading were closely correlated with the variation in the storage root yield of the different cultivars. In response to shading, the reduction of root ZR contents, the fresh dry weight of the above-ground parts and the root yield for Jishu18 were higher than that for cv. Aya, indicating that cv. Jishu18 might be more sensitive to weak light than cv. Aya.