REGULATION OF PECTIN METHYLESTERASE AND POLYGALACTURONATE LYASE ACTIVITY DURING DIFFERENTIATION OF INFECTION STRUCTURES IN UROMYCES-VICIAE-FABAE

The broad bean rust fungus Uromyces viciae-fabae differentiates infect ion structures up to the haustorial mother cell stage on thigmotropica lly inductive membranes in the absence of its host plant. Formation of pectin methylesterase (PME) and polygalacturonate lyase (PL), potenti ally involved in host cell wall degradation, was studied during infect ion structure differentiation by this biotrophic fungus. PME was first detectable when substomatal vesicles were formed and reached a maximu m when infection hyphae and haustorial mother cells were differentiate d. Four isoenzymes, exhibiting pls of 8.2, 5.6, 5.2 and 4.5, were sepa rated by chromatofocusing, and the kinetics of their synthesis and the K(m)s of the three major isoenzymes were determined. The enzyme activ ity 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 concentratio ns lower than 0.025 mg ml(-1) induced enzyme synthesis, and at 0.25 mg ml(-1) the induction process appeared to be saturated. Enzyme formati on in the presence of 50 mM glucose, fructose or sucrose suggested tha t neither pectic enzyme was subject to catabolite repression. Signific ant proportions of PME (approx. 57 %) and PL (approx. 76 %) activity w ere located extracellularly in 24-h-old differentiated infection struc tures and could contribute to the establishment of the parasite, Physi co-chemical and kinetic properties of the enzymes and associated alter ations of the apoplastic ph of infected host plants appeared to be imp ortant factors in the success of infection and could explain the restr iction of cell wall damage at the penetration site usually observed in interactions involving obligately biotrophic fungi.