Molecular studies on the sweet potato virus disease and its two causal agents

The studies presented in this thesis contribute to an increased understanding of the molecular aspects, variability and interaction of the two most important viral pathogens of sweet potato (Ipomoea batatas L): Sweet potato feathery mottle virus (SPFMV) and Sweet potato chlorotic stunt virus (SPCSV), which cause the severe sweet potato virus disease (SPVD) when co-infecting sweet potato plants. SPVD is the most important disease affecting sweet potato in Africa, and may be the most important virus disease of sweet potato globally. The coat protein gene sequences of several African SPFMV isolates were determined and compared by phylogenetic analyses. Results showed that East African SPFMV isolates were genetically distinct. They could furthermore be divided into two serotypes which differed in their ability to systemically infect the sweet potato cultivar Tanzania. The aetiology of SPVD was studied in sweet potato plants co-infected with SPFMV and SPCSV using nucleic acid hybridisation, bioassays, tissue printing and thin section immunohistochemistry. Resistance to SPFMV in East African sweet potato cultivars was found to be due to inhibition of virus replication rather than movement and resistance was suppressed by infection with SPCSV, resulting in a ca. 600-fold increase in titres of SPFMV. Furthermore, in SPVD affected plants SPFMV is detected outside of the phloem, whereas SPCSV is detected only inside the phloem, which suggests novel as yet unknown mechanisms how SPCSV synergises SPFMV. The genomic sequence of SPCSV was determined. It was composed of two RNA molecules (9407 and 8223 nucleotides), representing the second largest (+)ssRNA genome of plant viruses. The genomic organization of SPCSV revealed novel features for the genus Crinivirus, such as i) the presence of a gene putatively encoding an ribonuclease III-like protein, ii) near-identical, 208 nucleotides long 3’-sequences on both viral RNAs, and iii) the placement of the SHP gene at a new position on the genome of SPCSV relative to other closteroviridae. Northern analyses showed the presence of several sub-genomic RNAs, of which the accumulation was temporally regulated in infected tissues. The 5’-ends of seven sub-genomic RNAs were determined using a PCR based method, which indicated that the sgRNAs were capped

Horizontal gene transfer contributes to plant evolution : the case of Agrobacterium T-DNAs

Horizontal gene transfer (HGT) can be defined as the acquisition of genetic material from another organism without being its offspring. HGT is common in the microbial world including archaea and bacteria, where HGT mechanisms are widely understood and recognized as an important force in evolution. In eukaryotes, HGT now appears to occur more frequently than originally thought. Many studies are currently detecting novel HGT events among distinct lineages using next-generation sequencing. Most examples to date include gene transfers from bacterial donors to recipient organisms including fungi, plants, and animals. In plants, one well-studied example of HGT is the transfer of the tumor-inducing genes (T-DNAs) from some Agrobacterium species into their host plant genomes. Evidence of T-DNAs from Agrobacterium spp. into plant genomes, and their subsequent maintenance in the germline, has been reported in Nicotiana, Linaria and, more recently, in Ipomoea species. The transferred genes do not produce the usual disease phenotype, and appear to have a role in evolution of these plants. In this paper, we review previous reported cases of HGT from Agrobacterium, including the transfer of T-DNA regions from Agrobacterium spp. to the sweetpotato [ Ipomoea batatas (L.) Lam.] genome which is, to date, the sole documented example of a naturally-occurring incidence of HGT from Agrobacterium to a domesticated crop plant. We also discuss the possible evolutionary impact of T-DNA acquisition on plants

Utilization of engineered resistance to viruses in crops of the developing world, with emphasis on sub-Saharan Africa

Engineering for viral resistance • Viruses and CancerViral diseases in crop plants constitute a major obstacle to food security in the developing world. Subsistence crops, including cassava, sweetpotato, potato, banana, papaya, common bean, rice and maize are often infected with RNA and/or DNA viruses that cannot be controlled with pesticides. Hence, healthy planting materials and virus-resistant cultivars are essential for high yields of good quality. However, resistance genes are not available for all viral diseases of crop plants. Therefore, virus resistance engineered in plants using modern biotechnology methods is an important addition to the crop production toolbox.Peer reviewe

The horizontal gene transfer of Agrobacterium T-DNAs into the series Batatas (genus Ipomoea) genome is not confined to hexaploid sweetpotato

The discovery of the insertion of IbT-DNA1 and IbT-DNA2 into the cultivated (hexaploid) sweetpotato [Ipomoea batatas (L.) Lam.] genome constitutes a clear example of an ancient event of Horizontal Gene Transfer (HGT). However, it remains unknown whether the acquisition of both IbT-DNAs by the cultivated sweetpotato occurred before or after its speciation. Therefore, this study aims to evaluate the presence of IbT-DNAs in the genomes of sweetpotato's wild relatives belonging to the taxonomic group series Batatas. Both IbT-DNA1 and IbT-DNA2 were found in tetraploid I. batatas (L.) Lam. and had highly similar sequences and at the same locus to those found in the cultivated sweetpotato. Moreover, IbT-DNA1 was also found in I. cordatotriloba and I. tenuissima while IbT-DNA2 was detected in I. trifida. This demonstrates that genome integrated IbT-DNAs are not restricted to the cultivated sweetpotato but are also present in tetraploid I. batatas and other related species

Challenge of virus disease threats to ensuring sustained uptake of vitamin-A-rich sweetpotato in Africa

Orange-fleshed sweetpotatoes (OFSP) are a rich source of pro-vitamin A and can alleviate vitamin A deficiency in the developing world. In Africa, traditional varieties have been almost exclusively white-fleshed and introduction and breeding of orange-fleshed varieties into Africa has been severely hampered by virus diseases to which many varieties are susceptible. Breeding progress to generate resistant varieties has been slow due to rare and recessive occurrence of resistance in breeding populations. Production of virus-free seed is complicated by the fact that most sweetpotato viruses show no or only limited symptoms and very low virus concentrations when infected by individual viruses, making them difficult to detect. Even single infections can lead to significant yield losses, but when they combine severe disease complexes are generated, which can lead to total crop failure. Significant efforts have been made in characterizing and understanding virus interactions in sweetpotato over the last two decades to address this challenge; they are reviewed in this chapter. We also review the state of the art in detection of viruses in support of seed systems and breeding. We conclude with recommendations for the most urgent future research directions needed to address virus problems in sweetpotatoes

Resultados Finales del Análisis de Riesgo y su Validación para el Complejo Polilla en Bolivia

Se puede observar que hay una relación directamente proporcional entre los parámetros de vida estimados y el total de capturas (de todas las localidades evaluadas), Si bien la relación es ligera dado las causas de error normal o ruido (error de lectura del sensor, registros sesgados en las capturas, etc.), se ve que en las 3 especies mientras que el Ro incrementa su valor las capturas aumentan. Por lo que ILCYM puede generar predicciones útiles para contrarrestar el riesgo de infestación de estas plagas de papa

Resultados Finales del Análisis de Riesgo y su Validación para el Complejo Polilla y Bactericera cockerelli en Cotopaxi e Imbabura

Se puede observar que hay una relación directamente proporcional entre los parámetros de vida estimados y el total de capturas (de todas las localidades evaluadas), Si bien la relación es ligera dado las causas de error normal o ruido (error de lectura del sensor, registros sesgados en las capturas, etc.), se ve que en las 3 especies mientras que el Ro incrementa su valor las capturas aumentan. Por lo que ILCYM puede generar predicciones útiles para contrarrestar el riesgo de infestación de estas plagas de papa