Elicitors and suppressors of hydroxyproline-rich glycoprotein accumulation are solubilized from plant cell walls by endopolygalacturonase.

Treatment of bean cell walls with a pure endopolygalacturonase of the bean pathogen Colletotrichum lindemuthianum race beta released oligogalacturonides and pectic fragments which were separated according to their charge and size. Among galacturonic-acid-containing components, elicitors and suppressors of the plant cell wall hydroxyproline-rich glycoprotein (HRGP) were recovered. Two active small oligogalacturonides with degrees of polymerization of 2 and 3 were characterized by high-performance anion-exchange-chromatography pulsed amperometric detection and fast-atom-bombardment mass spectrometry; they elicited 40-70% hydroxyproline increase within 48 hours at 450 nmol/bean cutting. In contrast, pectic fragments of higher molecular mass, predominantly composed of galacturonic acid and containing sugars typical of the rhamnogalacturonan II domain of pectic polysaccharides, had the ability to substantially suppress hydroxyproline deposition. Maximum suppressor activity, 30-40% below the activity of the control, occurred in 48 hours. In view of the low one-cycle turnover of these proteins in the cell wall and of their structural role, these changes might significantly affect cell wall properties. Elicitation and/or suppression of hydroxyproline were correlated to modifications of HRGP-extensin gene expression. Northern-blot analysis of RNA showed that changes in the transcript intensity became clearly visible within the first 12 hours after the start of either treatment. The results show that pectic components of the plant extracellular matrix have the potential to regulate wall matrix biogenesis. Implications of this finding in plant defense and development are discussed.

Antibacterial Activity of Sirodesmin PL Phytotoxin: Application to the Selection of Phytotoxin-Deficient Mutants.

Sirodesmin PL, a phytotoxin and mycotoxin produced by Leptosphaeria maculans, the causal agent of stem-canker disease of crucifers, exhibited antibacterial activity against gram-positive bacteria and particularly Bacillus subtilis. The importance of the disulfide bridge of the molecule in antibacterial activity was demonstrated. A simple and reliable bioassay based on the antibacterial activity of the toxin was performed for screening sirodesmin PL-deficient mutants when grown on solid culture medium. A mutant was selected and found to produce 3,700-fold less toxin than did the wild-type strain. A sensitive procedure for quantification of the toxin by high-pressure liquid chromatography was developed. Levels of product as low as 100 ng could be detected by this procedure.