Effect of Rubber Petiole Cell Walls and Virulence of Strains on Secretion of Pectic Enzymes by Pytophthora Meadii

The characteristics of the pectic enzyme polygalacturonase (PG) of Phytophthora meadii isolates from petioles of rubber (Hevea brasiliensis) genotypes were compared in relation to (i) aggressiveness (virulence) in infecting detached petioles of a susceptible rubber genotype (PB86) or (ii) in relation to the availability of purified rubber petiole cell walls (obtained from a Phytophthora-resistant clone, RRIC100) in the growth medium, as the sole carbon source. The four most highly aggressive isolates produced PG forms varying within mw 48-60 kDa, while, the highly aggressive MAD86 isolate produced two PG forms of mw 48 and 70 kDa. Moderately or weakly aggressive isolates produced PG forms within the range of 62-66 kDa. With rubber petiole cell walls in the growth medium, the four most highly aggressive isolates produced PG forms of mw 42-62 kDa, while, MAD86 produced a much smaller PG form of 21 kDa. Weak or moderately aggressive isolates produced similar PG forms in both cell wall and pectin media. Quite smaller PG molecules produced in rubber cell wall medium could be due to elicitor molecular fragments bound to rubber cell walls, which may induce P. meadii to secret specific PG forms via signal transduction pathway. None of the isolates produced pectin lyase (PL) in any growth medium. Neither PG nor PL activity was detected in P. meadii-infected petioles.