Resistance Mechanism To Amicarthiazol In Xanthomonas Axonopodis Pv. Citri

Amicarthiazol (2-amino-4-methyl-carboxanilithiazol) is a bactericide containing both groups of carboxylamide and thiazole, which has not only the biological characterizations of carboxylamide fungicides to control basidiomycete diseases, but also those of thiazole bactericides to control bacterial diseases, especially those caused by Xanthomonas spp. In this research we studied the physiological, biochemical and molecular biological properties of the wild-type sensitive strain Xcc and its UV-induction mutants of X. axonopodis pv. citri to reveal and confirm the mode of resistance in X. axonopodis pv. citri to amicarthiazol in molecular genetics.Three amicarthiazol-resistant mutants (Xuv10, Xuv20 and Xuv40) were obtained by UV induction from Xcc and used in this research. Minimal inhibition concentrations (MICs) of amicarthiazol against the growth of mutants and wild-type isolate were 400μg/mL and 100μg/mL, respectively; and the medium effective concentration (EC50) of Xcc, Xuv10, Xuv20 and Xuv40 were 33.3473, 24.6278, 20.4681 and 1.4638μg/mL, respectively. The bacterial growth and pathogenicity of these mutants was not affected by the UV-induction compared with their parent strain Xcc, since even a bacterial suspension with concentrations as low as 10² cfu/mL could cause typical canker symptom on citrus leaves in vivo.A pathogenicity gene, pthA and two hrp gens, hrpF and hrpB₂, which were necessary for the secretion of PthA protein, pathogenicity and recognition between pathogen and host, were cloned and sequenced. The entire gene sequences of pthA, hrpF and hrpB₂ were 3492bp, 2475bp and 393bp in length, respectively; G+C% content were 66.87%, 58.42% and 60.05%; the deduced polypeptides contained 1063, 824 and 130 amino acid residues, which were 122.33504, 88.4609 and 13.799 kDa in size; isoelectric point were 8.21, 5.52 and 8.29,respectively. By sequence analysis, 14.5 102-bp tandem repeats and three nuclear localization signals were found in the sequence of pthA. But no mutation were found in all the sequence of pthA, hrpF and hrpB₂ from Xcc and its mutants, which suggested that there was no direct relationship between the amicarthiazol-resistance and pathogenicity gene, pthA.