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.