A pthA homolog from a variant of Xanthomonas axonopodis pv. citri enhances virulence without inducing canker symptom

Three novel atypical symptom-producing types of Xanthomonas axonopodis pv. citri were described recently. The variants designated as Xac-A^p type produce large flat necrotic with water-soaked margin lesions instead of canker lesions on leaves of Mexican lime (Citrus aurantifolia) and grapefruit (C. paradisi). Random mutagenesis using transposon Tn5 in Xac-A^p type strain XL38 was used to isolate a virulence-deficient mutant T38, which induced small flat necrotic with water-soaked margin lesions on leaves. PthAp being inactivated by the transposon was cloned and characterized. Our result demonstrated that pthAp is responsible for XL38 to cause large flat necrotic with water-soaked margin lesions and bacterial multiplication but fails to confer the ability of inducing canker lesions. Sequence analysis reveled pthAp is a new member belong to the pthA homologs. The sequence is almost identical to the other homologs except for the number of direct tandem repeats in the central region of the gene. The pthAp contained an intact promoter and a full-length reading frame but with 18.5 direct repeats. Moreover, the amino acid at position 3rd in 1st, 2nd, 3rd repeats and at 13th position in 11th, 12th, 13th, 14th repeats were varied. Mutant T38 is fully complemented by pthA carried 17.5 direct repeats for the pathogenicity of X. axonopodis pv. citri to elicit canker lesions. The pthAp affected in planta bacterial growth and lesions in size but no ability of inducing canker symptom. These results suggested that pthAp is a new distinct virulence effector responsible for the enhancement of virulence.