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A single amino acid change in a geminiviral Rep protein differentiates between triggering a plant defence response and initiating viral DNA replication

¹ Warwick HRI, University of Warwick, Wellesbourne, Warwick CV35 9EF, UK
² School of Life Science, East China Normal University, Shanghai 200062, PR China
³ Department of Plant Pathology and State Key Laboratory of Agrobiotechnology, China Agricultural University, Beijing 100094, PR China

Correspondence
Yiguo Hong
yiguo.hong{at}warwick.ac.uk

We have devised an in planta system for functional analysis of the replication-associated protein (Rep) of African cassava mosaic virus (ACMV). Using this assay and PCR-based random mutagenesis, we have identified an ACMV Rep mutant that failed to trigger the hypersensitive response (HR), but had an enhanced ability to initiate DNA replication. The mutant Rep–green fluorescent protein (GFP) fusion protein was localized to the nucleus. Sequence analysis showed that the mutated Rep gene had three nucleotide changes (A6T, T375G and G852A); only the A6T transversion resulted in an amino acid substitution (Arg to Ser), which is at the second residue in the 358 amino acid ACMV Rep protein. Our results indicate that a single amino acid can alter the differential ability of ACMV Rep to trigger the host-mediated HR defence mechanism and to initiate viral DNA replication. The implications of this finding are discussed in the context of plant–virus interactions.