title: Two Novel DNAs That Enhance Symptoms and Overcome CMD2 Resistance to Cassava Mosaic Disease.
abstract: Cassava mosaic begomoviruses (CMBs) cause cassava mosaic disease (CMD) across 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 SEGS-1 and SEGS-2 (forsequencesenhancinggeminivirussymptoms), that enhance symptoms and break resistance to CMD. The SEGS are characterized by GC-rich regions and the absence of long open reading frames. Both SEGS enhanced CMD symptoms in cassava (Manihot esculentaCrantz) when coinoculated withAfrican cassava mosaic virus(ACMV),East African cassava mosaic Cameroon virus(EACMCV), orEast African cassava mosaic virus-Uganda(EACMV-UG). SEGS-1 also overcame resistance of a cassava landrace carrying the CMD2 resistance locus when coinoculated with EACMV-UG. 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 to 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 replication origins. These results suggest that SEGS-1 is derived from the cassava genome and facilitates CMB infection as an integrated copy and/or an episome, while SEGS-2 was originally from the cassava genome but now is encapsidated into virions and transmitted as an episome by whiteflies.
  
  
  OBJECTIVE
  Cassava is a major crop in the developing world, with its production in Africa being second only to maize. CMD is one of the most important diseases of cassava and a serious constraint to production across Africa. CMD2 is a major CMD resistance locus that has been deployed in many cassava cultivars through large-scale breeding programs. In recent years, severe, atypical CMD symptoms have been observed occasionally on resistant cultivars, some of which carry the CMD2 locus, in African fields. In this report, we identified and characterized two DNA sequences, SEGS-1 and SEGS-2, which produce similar symptoms when coinoculated with cassava mosaic begomoviruses onto a susceptible cultivar or a CMD2-resistant landrace. The ability of SEGS-1 to overcome CMD2 resistance and the transmission of SEGS-2 by whiteflies has major implications for the long-term durability of CMD2 resistance and underscore the need for alternative sources of resistance in cassava.
authors: Mikocheni Agricultural Research Institute, Dar es Salaam, Tanzania jndunguru2003@yahoo.co.uk linda_hanley-bowdoin@ncsu.edu.
authors: Joseph
authors: J
authors: Ndunguru
authors: Department of Molecular and Structural Biochemistry, North Carolina State University, Raleigh, North Carolina, USA.
authors: Leandro
authors: L
authors: De León
authors: Department of Plant and Microbial Biology, North Carolina State University, Raleigh, North Carolina, USA.
authors: Catherine D
authors: CD
authors: Doyle
authors: Mikocheni Agricultural Research Institute, Dar es Salaam, Tanzania.
authors: Peter
authors: P
authors: Sseruwagi
authors: Center for Computational Biology and Bioinformatics, Columbia University, New York, New York, USA.
authors: German
authors: G
authors: Plata
authors: International Institute of Tropical Agriculture-Tanzania, Dar es Salaam, Tanzania.
authors: James P
authors: JP
authors: Legg
authors: ARC-Institute for Industrial Crops, Rusternburg, South Africa.
authors: Graham
authors: G
authors: Thompson
authors: International Center for Tropical Agriculture, Cali, Colombia.
authors: Joe
authors: J
authors: Tohme
authors: University of Pretoria, Department of Microbiology and Plant Pathology, Pretoria, South Africa.
authors: Theresa
authors: T
authors: Aveling
authors: Department of Molecular and Structural Biochemistry, North Carolina State University, Raleigh, North Carolina, USA.
authors: Jose T
authors: JT
authors: Ascencio-Ibáñez
authors: Department of Plant and Microbial Biology, North Carolina State University, Raleigh, North Carolina, USA jndunguru2003@yahoo.co.uk linda_hanley-bowdoin@ncsu.edu.
authors: Linda
authors: L
authors: Hanley-Bowdoin
mesh_terms: D001483:Base Sequence; D053839:Begomovirus / Q000235:genetics* / Q000472:pathogenicity; D003001:Cloning, Molecular; D004279:DNA, Viral; D016679:Genome, Viral; D002365:Manihot / Q000821:virology*; D009029:Mosaic Viruses / Q000235:genetics / Q000472:pathogenicity; D010935:Plant Diseases / Q000276:immunology / Q000821:virology*; D010957:Plasmids / Q000235:genetics; D013636:Tanzania; D014026:Nicotiana
chemical_list: D004279:DNA, Viral