Two novel DNAs that enhance symptoms and overcome CMD2 resistance to cassava mosaic disease

Abstract

Cassava mosaic begomoviruses (CMBs)causecassavamosaicdisease (CMD)acrossAfricaandtheIndiansubcontinent. Like all membersofthegeminivirusfamily, CMBshavesmall,circular single-stranded DNA genomes. Wereportherethediscovery of twonovelDNAsequences,designatedSEGS-1andSEGS-2(forsequencesenhancinggeminivirussymptoms),thatenhance symptomsandbreakresistance toCMD.TheSEGSarecharacterizedbyGC-richregionsandtheabsenceoflongopenreading frames. Both SEGS enhanced CMDsymptomsincassava(Manihot esculenta Crantz) when coinoculated with African cassava mosaic virus (ACMV), East African cassava mosaic Cameroon virus (EACMCV), or East African cassava mosaic virus-Uganda (EACMV-UG).SEGS-1alsoovercameresistanceofacassavalandracecarrying the CMD2resistancelocus whencoinoculated with EACMV-UG.EpisomalformsofbothSEGSweredetectedinCMB-infectedcassavabutnotinhealthycassava.SEGS-2episomeswerealsofoundinvirions andwhiteflies. SEGS-1 hasnohomologytogeminivirusesortheirassociated satellites, but the cassava genomecontains a sequence that is 99%identical to full-length SEGS-1. The cassava genome also includes three sequences with 84to89%identitytoSEGS-2thattogetherencompassallofSEGS-2exceptfora52-bpregion,whichincludesthe episomal junction and a 26-bpsequencerelated to alphasatellite replication origins. These results suggest that SEGS-1 is derived fromthecassava genomeandfacilitates CMBinfection as an integrated copy and/or an episome, while SEGS-2 wasoriginally fromthecassava genomebutnowisencapsidatedintovirions andtransmitted as anepisomebywhiteflies.