Ralstonia solanacearum pectin methylesterase is required for growth on methylated pectin but not for bacterial wilt virulence

Ralstonia (Pseudomonas) solanacearum causes bacterial wilt, a serious disease of many crop plants. The pathogen produces several extracellular plant cell wall-degrading enzymes, including polygalacturonases (PGs) and pectin methylesterase (Pme). Pme removes methyl groups from pectin, thereby facilitating subsequent breakdown of this cell wall component by PGs, which are known bacterial wilt virulence factors. R. solanacearum PGs could not degrade 93% methylated pectin unless the substrate was first demethylated by Pme, but as the degree of methylation of the pectin substrate decreased, PG activity increased. Primers derived from a published pme sequence generated an 800-bp DNA probe fragment, which identified Pme-encoding plasmids from a R. solanacearum genomic library. A pme chromosomal mutant had no detectable Pme activity in vitro and no longer grew on 93% methylated pectin as a carbon source. Curiously, the pme mutant, which had no detectable PG activity on highly methylated pectin, was just as virulent as the wild-type strain on tomato, eggplant (aubergine), and tobacco. Since PG activity is required for full virulence, this result suggests that the pectin in these particular hosts may not be highly methylated, or that the breakdown of highly methylated pectin is not a significant factor in the disease process in general. A positive response regulator of PG production called PehR was not required for wild-type Pme production. However, a mutant strain lacking PhcA, which is a global regulator of several virulence genes, produced no detectable Pme activity. Thus, pme expression is directly or indirectly regulated by PhcA but not by PehR.