Dallas, M. M. | Search bioRxiv

bioRxiv2024

SEGS-1 episomes generated during cassava mosaic disease influence disease severity

Cassava is an important root crop that is produced by smallholder farmers across Sub-Saharan Africa. Cassava mosaic disease (CMD), which is caused by a complex of cassava mosaic begomoviruses (CMBs), is one of the most devastating diseases of cassava. A previous study showed that SEGS-1 (sequences enhancing geminivirus symptoms), which occurs in both the cassava genome and as an episome during CMD, can increase CMD disease severity and overcome host resistance. In this report, we examined the effects of exogenously applied SEGS-1 on the incidence of CMB infection, symptom severity, and viral DNA copy number in five cassava cultivars that ranged from highly susceptible to highly resistant to CMD. These studies revealed that the effect of SEGS-1 is cultivar dependent. Susceptible cultivars developed severe CMD in the absence or presence of exogenous SEGS-1, while exogenous SEGS-1 increased disease severity in cultivars carrying CMD2 but not CMD1 resistance. Analysis of infected plants in the absence of exogenous SEGS-1 revealed that some, but not all cultivars, form SEGS-1 episomes during CMD. The presence of endogenous SEGS-1 episomes in TME14, a CMD2 resistant cultivar, correlated with CMD severity. In contrast, TME3, a closely related CMD2 cultivar, did not produce endogenous SEGS-1 episomes and showed more resistance than TME14. DNA sequence analysis indicated that the different capacities of TME3 and TME14 to form SEGS-1 episomes is unlikely due to sequence differences in and around their genomic SEGS-1 loci. Because of its inability to form episomes, TME3 was used to map the functional regions of SEGS- 1 to sequences flanking the epsiome junction, but junction itself was not required for activity. Together, these experiments provided insight into the functional form of SEGS-1 in cassava and the effect of cassava genotype on SEGS-1 activity.

molecular biology↗

bioRxiv2021

A New Type of Satellite associated with Cassava Mosaic Begomoviruses

Cassava mosaic disease (CMD), which is caused by single-stranded DNA begomoviruses, severely limits cassava production across Africa. A previous study showed that CMD symptom severity and viral DNA accumulation increase in cassava in the presence of a DNA sequence designated as SEGS-2 (sequence enhancing geminivirus symptoms). We report here that when SEGS-2 is co-inoculated with African cassava mosaic virus (ACMV) onto Arabidopsis thaliana, viral symptoms increase. Transgenic Arabidopsis with an integrated copy of SEGS-2 inoculated with ACMV also display increased symptom severity and viral DNA levels. Moreover, SEGS-2 enables Cabbage leaf curl virus (CaLCuV) to infect a geminivirus resistant Arabidopsis accession. Although SEGS-2 is related to cassava genomic sequences, an earlier study showed that it occurs as episomes and is packaged into virions in CMD-infected cassava and viruliferous whiteflies. We identified SEGS-2 episomes in SEGS-2 transgenic Arabidopsis. The episomes occur as both double-stranded and single-stranded DNA, with the single-stranded form packaged into virions. In addition, SEGS-2 episomes replicate in tobacco protoplasts in the presence, but not the absence, of ACMV DNA-A. SEGS-2 episomes contain a SEGS-2 derived promoter and an open reading frame with the potential to encode a 75-amino acid protein. An ATG mutation at the beginning of the SEGS-2 coding region does not enhance ACMV infection in Arabidopsis. Together, the results established that SEGS-2 is a new type of begomovirus satellite that enhances viral disease through the action of a SEGS-2 encoded protein that may also be encoded in the cassava genome. IMPORTANCECassava is an important root crop in the developing world and a food and income crop for more than 300 million African farmers. Cassava is rising in global importance and trade as the demands for biofuels and commercial starch increase. More than half of the worlds cassava is produced in Africa, where it is primarily grown by smallholder farmers, many of whom are from the poorest villages. Although cassava can grow under high temperature, drought and poor soil conditions, its production is severely limited by viral diseases. Cassava mosaic disease (CMD) is one of the most important viral diseases of cassava and can cause up to 100% yield losses. We provide evidence that SEGS-2, which was originally isolated from cassava crops displaying severe and atypical CMD symptoms in Tanzanian fields, is a novel begomovirus satellite that can compromise the development of durable CMD resistance.

plant biology↗

bioRxiv2020

Deeply sequenced infectious clones of key cassava begomovirus isolates from Cameroon

We deeply sequenced two pairs of widely used infectious clones (4 plasmids) of the bipartite begomoviruses African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV). The sequences of the ACMV clones were quite divergent from our expectations. We have made raw reads, consensus plasmid sequences, and the infectious clones themselves publicly available.

microbiology↗