Two Novel DNAs Responsible for Enhanced Symptoms and Overcome Resistance to CMD in Plants

Cassava mosaic begomoviruses (CMBs) cause cassava mosaic disease (CMD) in Africa and the Indian subcontinent. Like all members of the geminivirus family, CMBs have small, circular single-stranded DNA genomes. We report here the discovery of two novel DNA sequences designated as SEGS-1 and SEGS-2 (sequences enhancing geminivirus symptoms) that enhance symptoms and break resistance to CMD. Both SEGS enhanced CMD symptoms in cassava (Manihot esculenta Crantz) when co-inoculated with a CMB. SEGS-1 also overcame resistance of a cassava landrace carrying the CMD2 resistance locus in co-inoculation studies with a CMB. Episomal forms of both SEGS were detected in CMB-infected cassava, but not in healthy cassava. SEGS-2 episomes were also found in virions and whiteflies. SEGS-1 has no homology to geminiviruses or their associated satellites, but the cassava genome contains a sequence that is 99% identical to full-length SEGS-1. The cassava genome also includes three sequences with 84-89% identity to SEGS-2 that together encompass all of SEGS-2 except for a 52-bp region, which includes the episomal junction and a 26-bp sequence related to alphasatellite sequence. These results suggest that SEGS-1 is derived from the cassava genome and facilitates CMB infection either as an integrated copy and/or an episome, while SEGS-2 was originally from the cassava genome but is now encapsidated into virions and transmitted as an episome by whiteflies. The SEGS also enhance symptoms and break resistance in Arabidopsis co-inoculated with a CMB. We are using Arabidopsis to determine how the SEGS impact infection and host defenses.