Ralstonia solanacearum Pectin Methylesterase Is Required for Growth on Methylated Pectin but Not for Bacterial Wilt Virulence

Abstract

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 pmechromosomal 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.

Evaluation of antibacterial activity of Stenotrophomonas maltophilia against Ralstonia solanacearum under different application conditions

“…In addition to EPS, R. solanacearum secretes several plant cell wall-degrading enzymes. These includes three polygalacturonases (PglA, PehB and PehC) Huang and Allen 1997), an endoglucanase (Egl) ) and a pectin methylesterase (Tans-Kersten et al 1998). PhcA represses expression of hrpB, which encode T3SS.…”

Two Ralstonia species associated with bacterial wilt of Eucalyptus

“…The trial was repeated once. The inoculation methods used were adapted from those used by Tans-Kersten et al (1998) and Williamson et al (2002). The roots were wounded by placing a scalpel into the soil 1 cm away from the stems.…”

Ralstonia solanacearum genes induced during growth in tomato: an inside view of bacterial wilt

“…The genes pehR, vsrB, and vsrD encode known virulence factor regulators (Schell, 2000;Fig. 1); rpoS encodes a sigma factor required for wild-type virulence (Flavier et al, 1998); hrcC encodes a structural protein of the type III secretion system (Arlat et al, 1992); pme encodes a plant cell wall degrading pectinmethylesterase which is regulated by the global virulence regulator, PhcA (Tans-Kersten et al, 1998); and gspK encodes a component of the general secretary pathway, which is required for virulence in many pathogenic bacteria, including R. solanacearum (Kang et al, 1994). Additionally, our screen identified seven genes predicted by Salanoubat et al (2002) to encode candidate pathogenesis-related proteins based on homology to proteins from other pathogenic bacteria: ipx143 (hypothetical protein; similar to SspH 1 in S. typhimurium), ipx77 (ankyrin repeat-containing signal peptide), ipx150 and ipx84 (haemagglutinin-like proteins), ipx34 (transmembrane protein), ipx137 (peptide synthetase), and ipx53 (transmembrane protein).…”