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Calcium-dependent pectate lyase production in the soft-rotting bacterium Pseudomonas fluorescens.

Liao, C.H., Mc Callus, D. E. and Wells, J. M.

Phytopathology. Volume 83, Number 3, Aug 1993. Pages 883-887.

1993

บทคัดย่อ

Calcium-dependent pectate lyase production in the soft-rotting bacterium Pseudomonas fluorescens.

Pectate lyase (PL) is the principal or sole enzyme responsible for maceration of plant tissue caused by most strains of soft-rotting pseudomonads. Production of PL in four out of 25 Pseudomonas fluorescens (or P. marginalis) strains examined was not induced by the enzyme substrate, polygalacturonate (PGA), but was induced by Ca2+. These four strains produced 10 times more PL in medium containing 1 mM CaCl2 than in one containing no CaCl2 supplement. Over 86% of total PL produced by these strains in CaCl2-supplemented medium was excreted into the culture fluid. Only a small portion (13%) of total PL produced by these strains in CaCl2-deficient medium was detected in the extracellular fraction. Ca2+ thus affected not only the amount but also the final destination of PL produced by these pseudomonads. Additionally, all four strains were unable to use PGA as a nutritional source when cultured in Ca2+-deficient medium, which indicates that the initial step of PGA degradation was mediated by Ca2+-dependent PL and not by Ca2+-independent polygalacturonase. The optimal Ca2+ concentration required for PL production in one of these strains, CY091, was determined to be 0.2 mM. A linear correlation was observed between the amounts of PL produced and the concentrations of Ca2+ included in the medium. Furthermore, the requirement of Ca2+ for PL induction could be replaced by Sr2+ but not by other divalent cations, such as Zn2+, Fe2+, Mn2+, Mg2+ and Ba2+. Because of the indispensable role of Ca2+ in PGA degradation and in PL production, the possibility of using the ion-chelating agent ethylenediaminetetraacetic acid (EDTA) for control of Pseudomonas rot was evaluated. EDTA exhibited bactericidal activity against P. fluorescens at a minimal inhibitory concentration of 4 mM.

When assayed on potato tuber disks, EDTA at a concentration of 0.13 mM (40 ppm), which is 30-fold lower than the minimal inhibitory concentration, was effective in preventing P. fluorescens from growing and causing maceration in potato tuber tissue.