A PECTATE LYASE HOMOLOGUE PEL1 FROM XANTHOMONAS AXONOPODIS pv. CITRIIS ASSOCIATED WITH THE WATER-SOAKED MARGINS OF CANKER LESIONS

The typical citrus canker lesions produced by Xanthomonas axonopodis pv. citri (Xac) have water-soaked margins. Three new variants of Xac producing atypical symptoms were recently described in Taiwan. The variant designated A(r) type produces lesions without water-soaked margins on the leaves of citrus species and does not possess pectolytic activity on polypectate media. Five pectolytic genes pel1, pel2, pel3, peh1, and peh2 from strains XW19 (a typical canker lesion-producing strain) and XW121 (a strain of A(r) type) were PCR-amplified, cloned and characterized. Only the 1.2-kb pel1 gene from XW19 expressed pectolytic activity. Sequence analyses revealed over 99% homology in nucleotide and deduced amino acid sequences among pel1 genes from XW19 and XW121, and a pel homolog from strain 306 (GenBank accession No. NC 003919.1, XAC3562). The pel1 gene from XW19 and XW121 encoded a Pel1 protein of 377 amino acids. However, there was a stop codon located at the 350(th) amino acid residue of the gene from XW121. When the pel1 gene from XW19 was transformed into XW121, the transformant exhibited pectate lyase activity and also induced water-soaked margins surrounding the lesions on grapefruit leaves. The results suggest that expression of the pel1 gene of Xac is associated with formation of the water-soaked margin of the bacterial canker lesion.