The physiology of parasitism by Phymatotrichum omnivorum (Shear) Duggar

Abstract

The physiology of parasitism of the cotton root rot fungus, Phymatotrichum omnivorum (Shear) Duggar, was investigated. Murashige and Skoog (MS) liquid medium was developed as a defined medium for axenic growth of P. omnivorum (PO). It is a highly useful medium for fungal growth studies, for isolation of cell organelles, and for the determination of excreted fungal metabolites. Stationary liquid cultures of PO grew exponentially over a period of 12 to 14 days when grown on MS medium with 31 sucrose as carbon source. L-Ascorbic acid and ascorbate-2-sulfate accumulated in the growth medium when PO was grown in stationary liquid culture. When PO was incubated in [U -^14C] glucose, these two organic acids represent approximately 60% of the isotope incorporated into excreted organic acids. Oxalic acid did not accumulate in fungal growth-media and in significant amounts of [^14C]oxalic acid were recovered from fungal cultures that had been incubated in [^14C]glucose. Microbodies isolated from PO did not contain glyoxylate DH, isocitrate lyase, or malate synthetase activities, the enzymes relevant to oxalic acid biosynthesis in fungal microbodies. Studies with in vitro cell cultures of cotton (TM-1) suggest that the synthesis of ascorbic acid by PO plays an intermediate role in the initiation of cellular necrosis in host-root tissues. When incubated in increasing concentrations of ascorbic acid (1 to 10,000 μmol/culture) cotton suspension cultures exhibited both a loss of cell viability and an accumulation of oxalic acid. When incubated in a variety of other organic acids, the cotton cells failed to accumulate oxalic acid and showed no loss of viability. These data indicate that the relative pathogenicity of PO on host-root tissues is critically related to the host's ability to tolerate increased leveles of oxalic acid or to remove toxic levels of oxalic acid (by oxidation to CO2 or by precipitation as calcium oxalate) produced by the catabolism of ascorbic acid and ascorbic acid metabolites excreted by PO.