Sweet Potato Chlorotic Stunt Crinivirus Research Articles

Suppression of RNAi by dsRNA-degrading RNaseIII enzymes of viruses in animals and plants.

Certain RNA and DNA viruses that infect plants, insects, fish or poikilothermic animals encode Class 1 RNaseIII endoribonuclease-like proteins. dsRNA-specific endoribonuclease activity of the RNaseIII of rock bream iridovirus infecting fish and Sweet potato chlorotic stunt crinivirus (SPCSV) infecting plants has been shown. Suppression of the host antiviral RNA interference (RNAi) pathway has been documented with the RNaseIII of SPCSV and Heliothis virescens ascovirus infecting insects. Suppression of RNAi by the viral RNaseIIIs in non-host organisms of different kingdoms is not known. Here we expressed PPR3, the RNaseIII of Pike-perch iridovirus, in the non-hosts Nicotiana benthamiana (plant) and Caenorhabditis elegans (nematode) and found that it cleaves double-stranded small interfering RNA (ds-siRNA) molecules that are pivotal in the host RNA interference (RNAi) pathway and thereby suppresses RNAi in non-host tissues. In N. benthamiana, PPR3 enhanced accumulation of Tobacco rattle tobravirus RNA1 replicon lacking the 16K RNAi suppressor. Furthermore, PPR3 suppressed single-stranded RNA (ssRNA)—mediated RNAi and rescued replication of Flock House virus RNA1 replicon lacking the B2 RNAi suppressor in C. elegans. Suppression of RNAi was debilitated with the catalytically compromised mutant PPR3-Ala. However, the RNaseIII (CSR3) produced by SPCSV, which cleaves ds-siRNA and counteracts antiviral RNAi in plants, failed to suppress ssRNA-mediated RNAi in C. elegans. In leaves of N. benthamiana, PPR3 suppressed RNAi induced by ssRNA and dsRNA and reversed silencing; CSR3, however, suppressed only RNAi induced by ssRNA and was unable to reverse silencing. Neither PPR3 nor CSR3 suppressed antisense-mediated RNAi in Drosophila melanogaster. These results show that the RNaseIII enzymes of RNA and DNA viruses suppress RNAi, which requires catalytic activities of RNaseIII. In contrast to other viral silencing suppression proteins, the RNaseIII enzymes are homologous in unrelated RNA and DNA viruses and can be detected in viral genomes using gene modeling and protein structure prediction programs.

CHARACTERISTICS AND DIVERSITY IN SWEETPOTATO-INFECTING VIRUSES IN AFRICA

In Africa, seven viruses occurring singly or in combination have been identified in sweetpotato. The viruses include Sweet potato feathery mottle potyvirus (SPFMV), Sweet potato chlorotic stunt crinivirus (SPCSV), Sweet potato mild mottle ipoitiovirus (SPMMV), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato virus II (SPVII), Sweet potato caulimo-like virus (SPCaLV) and Sweet potato virus G (SPVG). Out of these, the presence of SPFMV was detected in most countries surveyed, while SPCSV occurred in Nigeria, Gabon, Kenya, Uganda, Tanzania, Zambia, Madagascar and Egypt. SPCFV and SPMMV had limited distribution, mainly in East Africa. SPCaLV was only found in Uganda, SPVII in South Africa and SPVG in Ethiopia and Egypt. Molecular characterisation showed that most of the virus isolates have wide genetic variations, although some such as SPCSV are highly conserved. SPCSV is unique among these viruses because it has the ability to synergise others, therefore, is considered to be the most important virus responsible for the development of severe symptoms characteristic of sweetpotato virus disease, hence, it should be a major target for control.

Resistance to Sweetpotato Chlorotic Stunt Virus and Sweetpotato Feathery Mottle Virus Is Mediated by Two Separate Recessive Genes in Sweetpotato

When sweetpotato chlorotic stunt crinivirus (SPCSV) and sweetpotato feathery mottle potyvirus (SPFMV) infect sweetpotato [Ipomoea batatas (L.) Lam.], they interact synergistically and cause sweetpotato virus disease (SPVD), a major constraint to food productivity in east Africa. The genetic basis of resistance to these diseases was investigated in 15 sweetpotato diallel families (1352 genotypes) in Uganda, and in two families of the same diallel at the International Potato Center (CIP), Lima, Peru. Graft inoculation with SPCSV and SPFMV resulted in severe SPVD symptoms in all the families in Uganda. The distribution of SPVD scores was skewed toward highly susceptible categories (SPVD scores 4 and 5), eliminating almost all the resistant genotypes (scores 1 and 2). Likewise, when two promising diallel families (`Tanzania' × `Bikilamaliya' and `Tanzania' × `Wagabolige') were graft inoculated with SPCSV and SPFMV at CIP, severe SPVD was observed in most of the progenies. Individual inoculation of these two families with SPCSV or SPFMV, and Mendelian segregation analysis for resistant vs. susceptible categories led us to hypothesize that resistance to SPCSV and SPFMV was conditioned by two separate recessive genes inherited in a hexasomic or tetradisomic manner. Subsequent molecular marker studies yielded two genetic markers associated with resistance to SPCSV and SPFMV. The AFLP and RAPD markers linked to SPCSV and SPFMV resistance explained 70% and 72% of the variation in resistance, respectively. We propose naming these genes as spcsv1 and spfmv1. Our results also suggest that, in the presence of both of these viruses, additional genes mediate oligogenic or multigenic horizontal (quantitative) effects in the progenies studied for resistance to SPVD.

NATURE OF RESISTANCE OF SWEETPOTATO TO SWEETPOTATO VIRUS DISEASE

Sweetpotato virus disease (SPVD) is a disease syndrome due to the dual infection and synergistic interaction of sweetpotato chlorotic stunt crinivirus (SPCSV) and sweetpotato feathery mottle potyvirus (SPFMV). SPVD causes up to 98% yield loss in East Africa. Uganda’s sweetpotato breeding program released six sweetpotato cultivars with moderate to high levels of field SPVD resistance in 1995. A multidisciplinary partnership involving Namulonge Agricultural and Animal Production Research Institute in Uganda, North Carolina State University, the US Vegetable laboratory (USV, USDA-ARS) and Clemson University, Charleston, South Carolina, and the International Potato Center (CIP), Lima, Peru, has investigated the genetic basis of SPVD resistance. This paper highlights research findings of the response of sweetpotato to sweetpotato virus disease. General combining ability (GCA) and specific combining ability of (SCA) of 45 sweetpotato diallel families, and the GCA to SCA variance component ratios were high, indicating that additive gene effects are predominant in the inheritance of SPVD and recovery. The distribution of SPVD scores in the promising families was skewed toward highly susceptible categories in Uganda and Peru. In the proposed model for inheritance, two genes are unlinked and they are inherited in a hexosamic or tetradisomic manner. Based on amplified fragment length polymorphism (AFLP) and quantitative trait loci (QTL) analyses, two unlinked AFLP markers were associated with the loci conferring resistance to SPCSV and SPFMV.

Diallel analysis of sweetpotatoes for resistance to sweetpotato virus disease

Sweetpotato virus disease (SPVD) is due to the dual infection and synergistic interaction of Sweetpotato feathery mottle potyvirus (SPFMV) and Sweetpotato chlorotic stunt crinivirus(SPCSV), and causes up to 98% yield loss in sweetpotato in East Africa. This study was conducted to determine the inheritance of resistance to SPVD in sweetpotato and to estimate the nature of genetic variance. Ten parental clones varying in reaction to SPVD were crossed in a half diallel mating design to generate 45 full-sib families. The families were graft-inoculated with SPCSV and SPFMV to induce SPVD and evaluated for resistance in a randomized complete block design at two sites in Namulonge, Uganda during 1998–2000. In serological assays for SPFMV and SPCSV,resistance to symptom development and recovery from initial systemic SPVD symptoms, characterised resistant genotypes. Genetic component analysis showed significant effects for both general combining ability (GCA) and specific combining ability (SCA) for resistance to SPVD. GCA to SCA variance component ratios were large (0.51–0.87), hence GCA effects were more important than SCA effects. Resistant parents exhibited high GCA indicating that additive gene effects were predominant in the inheritance of resistance to SPVD and recovery. Narrow-sense heritability (31–41%) and broad-sense heritability (73–98%) were moderate to high, indicating that rapid genetic gains for SPVD resistance could be accomplished by mass selection breeding techniques. Two genotypes, New Kawogo and Sowola, had high negative GCA effects and had several families in specific crosses,which exhibited rapid recovery from SPVD,and are promising parents for enhancement of SPVD resistance and recovery.

Variability of sweetpotato feathery mottle virus in Africa

One sweetpotato breeding line resistant to sweetpotato feathery mottle virus (SPFMV) was infected by graft-inoculation with SPFMV and developed severe symptoms of sweetpotato virus disease, following co-inoculation with sweetpotato chlorotic stunt virus. Coat protein (CP) gene sequences from eight East African isolates of SPFMV and two West Africa isolates from Nigeria and Niger were determined. Phylogenetic analysis of sequences of these isolates and those of non-East African origin previously reported, showed that the East African SPFMV isolates form a separate cluster. The monoclonal antibody MAb 7H8 divided Uganda SPFMV isolates into serogroups, which differed in prevalence in different districts of Uganda and in two common sweetpotato cultivars. Key Words: Clustering, coat protein, East African SPFMV isolates, resistant sweetpotato cultivars, sweetpotato chlorotic stunt crinivirus, West African SPFMV isolates Mab + isolates (African Crop Science Journal 2001 9(1): 293-300)

Occurrence of two serotypes of sweet potato chlorotic stunt virus in East Africa and their associated differences in coat protein and HSP70 homologue gene sequences

Amongst samples of the sweet potato chlorotic stunt crinivirus (SPCSV) obtained from crops of sweet potato in Uganda, two serotypes (SEA1 and SEA2) were distinguished using a panel of monoclonal antibodies (Mabs) to a Kenyan isolate of SPCSV. SEA1 serologically resembled the Kenyan isolate of SPCSV whereas SEA2 has not previously been reported. SEA1 was predominant in eastern Uganda whereas SEA2 was predominant in southern and western Uganda. SEA2 tended to occur in more severely diseased sweet potato plants than SEA1. RNA was extracted from eight plants and replicate clones representing the heat shock protein 70 (HSP70) homologue and coat protein (CP) genes were generated by reverse transcription and PCR. Sequence analyses revealed substitutions at two nucleotide positions in the HSP70 homologue gene, although neither affected deduced amino‐acid sequences. Nucleotide substitutions in the CP gene region, which led to 11 amino‐acid substitutions, revealed two major groupings plus other minor variants.The EMBL accession numbers of the sequences reported in this paper are AJ010754 through to AJ01769 (coat protein sequences) and AJ010914 through to AJ010929 (partial HSP70 homologue gene sequences).

Effect of local inoculum on the spread of sweet potato virus disease: limited infection of susceptible cultivars following widespread cultivation of a resistant sweet potato cultivar

A study compared the spread of sweet potato virus disease (SPVD) into crops of two moderately resistant and initially SPVD‐free sweet potato cultivars in northern and southern Mpigi, Uganda. Whiteflies, the vector of sweet potato chlorotic stunt crini virus (SPCSV), a component cause of SPVD, were similarly abundant in farmers' sweet potato fields around Namulonge in northern Mpigi, and Kanoni in southern Mpigi. However, mean incidence of SPVD in farmers' crops neighbouring the trials was higher at Kanoni (13.3%) than at Namulonge (2.8%). Furthermore, spread of SPVD into initially SPVD‐free sweet potato plots of two only moderately resistant cultivars was greater in plots at Kanoni than in plots at Namulonge. The SPVD‐resistant New Kawogo was the most common cultivar grown in farmers' fields at Namulonge and had few diseased plants, whereas susceptible cultivars with relatively high incidences of disease predominated at Kanoni. Final SPVD incidence in each trial was positively correlated with a measure combining the proximity and level of inoculum in surrounding fields. The study demonstrates the importance of local SPVD inoculum in determining the rate of spread of the disease into fields and implies that the widespread cultivation of a resistant variety limits infection of susceptible cultivars grown nearby.

Identification of the East African Strain of Sweet Potato Chlorotic Stunt Virus as a Major Component of Sweet Potato Virus Disease in Southern Africa.

Sweet potato virus disease (SPVD) is the most damaging disease of sweet potato Ipomoea batatas (L.) Lam. in Africa. It is caused by sweet potato feathery mottle potyvirus (SPFMV) plus either the West African strain of sweet potato chlorotic stunt crinivirus (Closteroviridae) (SPCSV-WA) (2) or the serologically distinct and apparently more severe East African strain (SPCSV-EA) (1). Typical symptoms of SPVD include severe plant stunting, leaf distortion, chlorosis, mosaic, or vein clearing (1). During a survey done in February 1998 of 48 farmers' fields in Lusaka Province and North Western Province of Zambia, sweet potato plants with typical SPVD symptoms were observed. Incidence was generally 1 to 5% but occasionally >20%. To determine which viruses (SPFMV, SPCSV-EA, SPCSV-WA) were present in symptomatic plants, enzyme-linked immunosorbent assays (ELISAs) were done on leaf sap extracts. Twenty-two SPVD-affected plants from Lusaka Province and 15 from North Western Province were tested and SPFMV and SPCSV-EA (but not SPCSV-WA) were detected in all samples. SPCSV-EA by itself may cause purpling or yellowing of lower or middle leaves (1). Eight plants showing these symptoms were collected from North Western Province, and SPCSV-EA only was detected in six of the samples. SPVD was also observed in a 1997 survey of crops near Antsirable, Madagascar; incidence was generally <1% but occasionally >20%; SPFMV and SPCSV-EA, but not SPCSV-WA, were detected in two SPVD samples tested. Our results are the first report of SPCSV in southern Africa. SPVD in the regions surveyed appears to be due to SPFMV and SPCSV-EA; SPCSV-WA was not detected.

Resistance to sweet potato virus disease (SPVD) in wild East African Ipomoea

Summary.Sweet potato virus disease (SPVD), the most harmful disease of sweet potatoes in East Africa, is caused by mixed infection with sweet potato feathery mottle potyvirus (SPFMV) and sweet potato chlorotic stunt crinivirus (SPCSV). Wild Ipomoea spp. native to East Africa (J cairica, I. hildebrandtii, I. involucra and J wightii) were graft‐inoculated with SPVD‐affected sweet potato scions. Inoculated plants were monitored for symptom development and tested for SPFMV and SPCSV by grafting to the indicator plant J setosa, and by enzyme‐linked immunosorbent assay (ELISA). Virus‐free scions of sweet potato cv. Jersey were grafted onto these wild Ipomoea spp. in the field, and scions collected 3 wk later were rooted in the greenhouse and tested for viruses using serological tests and bioassays. In all virus tests, J cairica and J involucra were not infected with either SPFMV or SPCSV. J wightii was infected with SPFMV, but not SPCSV, in the field and following experimental inoculation; J hildebrandtii was infected with SPCSV, but not SPFMV, following experimental inoculation. These data provide the first evidence of East African wild Ipomoea germplasm resistant to the viruses causing SPVD.

The incidence of sweet potato virus disease and virus resistance of sweet potato grown in Uganda

SummarySweet potato virus disease (SPVD) was common (25–30% average incidences), and farmers recognised it as an important disease, in sweet potato crops in southern Mpigi, Masaka and Rakai Districts in Uganda, but SPVD was rare in Soroti and Tororo Districts. Whiteflies, which are the vector of sweet potato chlorotic stunt crinivirus (SPCSV) a component cause of SPVD, were correspondingly common on sweet potato crops in Mpigi and rare on crops in Tororo. However, aphids, which are the vectors of sweet potato feathery mottle potyvirus (SPFMV), the other component cause of SPVD, were not found colonising sweet potato crops, and itinerant alate aphids may be the means of transmission. Different sweet potato cultivars were predominant in the different districts surveyed and four local cultivars obtained from Kanoni in S. Mpigi, where whiteflies and SPVD were common, were more resistant to SPVD than four cultivars from Busia in Tororo District, where whiteflies and SPVD were rare. However, nationally released cultivars were even more resistant than the local cultivars from Kanoni. Yield results and interviews with farmers indicated that farmers in S. Mpigi were making compromises in their choice of cultivars to grow, some key factors being SPVD susceptibility, and the yield, taste, and marketability, duration of harvest and in‐ground storability of the storage roots. These compromises need to be included in an assessment of yield losses attributable to SPVD.