PHYSIOLOGICAL-RESPONSES OF PHYTOPHTHORA-CITROPHTHORA TO A SUBINHIBITORY CONCENTRATION OF PHOSPHONATE

Various physiological processes were measured in the mycelium of Phytophthora citrophthora treated with 0.5 mM phosphonate, a concentration which caused approximately 20% inhibition of growth. DNA syntheses decreased by 30% 24 hr after treatment, while RNA and protein syntheses were not affected 48 hr after application of phosphonate. Incorporation of [U-14C]acetate into neutral and polar lipid as well as into other metabolites was inhibited 33 and 25% 5 hr after phosphonate application, respectively. Phosphonate affects the respiration rate and the total adenylate pool of P. citrophthora. The residual respiratory activity of phosphonate-treated mycelium was more resistant to potassium cyanide than untreated mycelium. The glyceraldehyde-3-phosphate dehydrogenase of P. citrophthora is able to use phosphonate as a physiological substrate with the same affinity as phosphate, but with a maximal velocity that is six times lower than that of phosphate. The intracellular acid phosphatase activity was not modified by phosphonate application, but this anion enhances the intracellular alkaline phosphatase activity by a factor of 4.8. Phosphonate increases the intracellular phosphate pool of P. citrophthora. The activities of both key enzymes of the pentose phosphate pathway, glucose-6-phosphate and 6-phosphogluconate dehydrogenase, were enhanced by a factor of 1000 after 24 hr of incubation in the presence of phosphonate. After the same period of incubation, phosphonate enhanced the activity of UDP-glucose pyrophosphorylase, which is implicated in β-glucan biosynthesis, by a factor of 10. These results support the hypothesis that phosphonate may interact with phosphate for the catalytic site of phosphorylating enzymes, but also at the transcriptional and/or post-transcriptional level for the regulation of several enzymes. Thus, the activity of phosphonate produced a physiological state similar to that produced by a limitation of phosphate. © 1992.