Differential roles of geminivirus Rep and AC4 (C4) in the induction of necrosis in Nicotiana benthamiana.

SUMMARY The replication-associated protein (Rep) of two distinct begomoviruses, the bipartite African cassava mosaic virus (ACMV) and the monopartite Tomato yellow leaf curl virus-China (TYLCV-C), elicits a reaction resembling a hypersensitive response (HR), associated with the induction of local necrosis and a systemic burst of hydrogen peroxide production, when expressed from a potato virus X vector in Nicotiana benthamiana. Transient expression of the ACMV Rep after Agrobacterium infiltration of N. benthamiana also triggered an HR-like response. We have identified a region of the ACMV Rep, referred to as the HR-like determinant domain (HRD, amino acids 119-179) that is essential for induction of the phenotype. Two additional regions have been identified (amino acids 1-85 and 86-118) that have various effects on the Rep-mediated phenotype, suggesting that structural constraints are imposed on the functional HRD. The co-expression of Rep with either AC4 or C4, expressed from overlapping open reading frames, triggers systemic necrosis in infected-tissues, but AC4 or C4 alone is neither an inducer nor enhancer of the HR-like phenotype. We propose that ACMV AC4 and TYLCV-C C4 may counter the plant defence mechanism that is initiated by the Rep-mediated local HR-like phenotype.