Abstract
Xanthomonas oryzae pv.oryzae (Xoo) causes the serious bacterial blight disease of rice. Xoo secretes a repertoire of plant cell wall degrading enzymes (CWDEs) like cellulases, xylanases, esterases etc., which act on various components of the rice cell wall. The major cellulases and xylanases secreted by Xoo have been identified and their role in virulence has been determined. In this study, we have identified some of the pectin degrading enzymes of Xoo and assessed their role in virulence. Bioinformatics analysis indicated the presence of four pectin homogalacturonan (HG) degrading genes in the genome of Xoo. The four HG degrading genes include one polygalacturonase (pglA), one pectin methyl esterase (pmt) and two pectate lyases (pel and pelL). There was no difference in the expression of pglA, pmt and pel genes by laboratory wild type Xoo strain (BXO43) grown in either nutrient rich PS medium or in plant mimic XOM2 medium whereas the expression of pelL gene was induced in XOM2 medium as indicated by qRT-PCR experiments. Gene disruption mutations were generated in each of these four genes. The polygalacturonase mutant pglA- was completely deficient in degrading the substrate Na-polygalacturonicacid (PGA). Strains carrying mutations in the pmt, pel and pelL genes were as efficient as wild type Xoo (BXO43) in cleaving PGA. These observations clearly indicate that PglA is the major pectin degrading enzyme produced by Xoo. The pectin methyl esterase, Pmt, is the pectin de-esterifying enzyme secreted by Xoo as evident from the enzymatic activity assay performed using pectin as the substrate. Mutations in the pglA, pmt, pel and pelL genes have minimal effects on virulence. This suggests that, as compared to cellulases and xylanases, the HG degrading enzymes may not have a major role in the pathogenicity of Xoo.
Highlights
The plant cell wall is a structural barrier that is mostly composed of polysaccharides such as cellulose, various hemicelluloses and pectin
We have identified a pectin methyl esterase (Pmt) and a polygalacturonase (PglA) as the pectin de-esterifying and pectin degrading enzymes secreted by Xanthomonas oryzae pv.oryzae (Xoo)
Bioinformatics analysis indicated that there are 7 genes encoding pectin degrading enzymes in the Xoo genome including two polygalacturonases, one pectin methyl esterase, two pectate lyases, one rhamnogalacturonase and one rhamnogalacturonan acetyl esterase. These were identified based on similarity searches to pectin degrading genes of Xanthomonas campestris pv. campestris (Xcc) and Xanthomonas axonopodis pv. citri (Xac)
Summary
The plant cell wall is a structural barrier that is mostly composed of polysaccharides such as cellulose, various hemicelluloses and pectin. Pectin Degrading Enzymes Secreted by Xanthomonas oryzae pv. The de-esterified pectin is available for degradation by enzymes that degrade polygalacturonic acid (PGA) These enzymes are polygalacturonases and pectate lyases. Pectin degrading enzymes are secreted by many bacterial and fungal pathogens of plants In several such pathogens they are demonstrated to be important for full virulence[3,4,5]. Three polygalacturonases (namely PehA, PehB & PehC) and a pectin methyl esterase (Pme) are encoded in the genome of Ralstonia solanacearum. We have identified a pectin methyl esterase (Pmt) and a polygalacturonase (PglA) as the pectin de-esterifying and pectin degrading enzymes secreted by Xoo. We show that pectinase deficient mutants are minimally affected for virulence on rice
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