Summary: The broad bean rust fungus differentiates infection structures up to the haustorial mother cell stage on thigmotropically inductive membranes in the absence of its host plant. Formation of pectin methylesterase (PME) and polygalacturonate lyase (PL), potentially involved in host cell wall degradation, was studied during infection structure differentiation by this biotrophic fungus. PME was first detectable when substomatal vesicles were formed and reached a maximum when infection hyphae and haustorial mother cells were differentiated. Four isoenzymes, exhibiting pls of 8.2, 5.6, 5.2 and 4.5, were separated by chromatofocusing, and the kinetics of their synthesis and the s of the three major isoenzymes were determined. The enzyme activity was formed independently of the presence of its substrate and its regulation was thus differentiation-specific. A single PL was induced when haustorial mother cells were formed and its synthesis appeared to be controlled by both the developmental stage of infection structures and the availability of its substrate. Polygalacturonate concentrations lower than 0.25 mg ml induced enzyme synthesis, and at 0.25 mg ml the induction process appeared to be saturated. Enzyme formation in the presence of 50 mM glucose, fructose or sucrose suggested that neither pectic enzyme was subject to catabolite repression. Significant proportions of PME (approx. 57%) and PL (approx. 76%) activity were located extracellularly in 24-h-old differentiated infection structures and could contribute to the establishment of the parasite. Physico-chemical and kinetic properties of the enzymes and associated alterations of the apoplastic pH of infected host plants appeared to be important factors in the success of infection and could explain the restriction of cell wall damage at the penetration site usually observed in interactions involving obligately biotrophic fungi.


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