Abstract
Syngomycin (SR) and syringotoxin (ST), two toxigenic peptides produced by ecotypic strains of Pseuomonas syringae pv. syringae, caused several changes in oxidative metabolism of mithochodria isolated from etiolated shoots of ACCO maize lines WF9 and B37 which differed in susceptibility to holus spot disease. Evidence was obtained that both toxins, in a concentration range of 6 to 24 μg mg−1 mitochondrial protein, depending on the toxin and on the substrate being oxidized, caused uncoupling of oxidative phosphorylation, stimulation of succinate, malate/glutamate and NADH respiration, stimulation of ATPase activity, inhibition of ATP formation, increased swelling, and a decrease in mitochoondrial membrane potential. Generally the effect induced by ST was greater than that of SR. ST at 6 μg mg−1 protein caused complete uncoupling with all three substrates tested, while SR had the same effect at 12 μg mg−1 protein with NADH and succinate and at 24 μg mg−1 protein with malate/glutamate. This difference between SR and ST was probably due to a difference in specificity for their sites of action. Electron micrographs of B37 mithochodria indicated that SR and ST caused swelling with fragmentation of the inner membrane.The effects of the two toxins on WF9 and B37 mithochondria were the same, although, based on number of lesions and population dynamics of the pathogen in leaf tissues, pathogenicity tests indicated that leaves of WF9 were more susceeptible to P. syringae pv. syringae than those of B37. However, when leaf tissues were treated with SR or ST, B37 was more sensitive to the toxins than WF9. Both toxins caused typical holcus spot symptoms. Based on assays of bacterial populations in leaves, susceptibility of WF9 in contrast to the resistance of B37 to holcus spot was apparently due to the effects of host substrate on the extent and rate of growth of pathogenic bacteria in relation to production of SR.
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