tale-Based Genetic Diversity of Chinese Isolates of the Citrus Canker Pathogen Xanthomonas citri subsp. citri

Abstract

Pathotype A of Xanthomonas citri subsp. citri, the cause of citrus bacterial canker (CBC), is assumed to have originated in southern China. PthA, a type III secreted transcriptional activator-like effector (TALE), is a major pathogenicity determinant in X. citri subsp. citri. To investigate the diversity of X. citri subsp. citri in China, genomic and plasmid DNA of 105 X. citri subsp. citri isolates, collected from nine citrus-growing provinces of China, were digested by BamHI and hybridized with an internal repeat region of pthA. Strains were classified into 14 different genotypes (designated A to N) based on the number and size of pthA homologues. Genotypes B and G represented 19 and 62% of the isolate collection, respectively. Genotypes J and L lacked pthA or a pthA-hybridizing fragment and were less virulent on grapefruit (C. paradisi) and sweet orange (C. sinensis) compared with strains containing pthA or a pthA homologue. The virulence of genotypes J and L was increased when the wild-type pthA was introduced. Genotype I, which was isolated from sweet orange in Jiangxi province, caused typical canker symptoms and may contain a novel pthA-like gene. To our knowledge, this is the first description of genetic diversity in Chinese CBC strains based on tale gene analysis.