Involvement of phaseolotoxin in halo blight of beans: transport and conversion to functional toxin

Abstract

Phaseolotoxin ([N(delta)-phosphosulfamyl]ornithylalanylhomoarginine) is produced by Pseudomonas phaseolicola (Burkh.) Dows. in liquid culture. When phaseolotoxin was applied to leaves of bean (Phaseolus vulgaris L.) at 0.1 to 1 nmoles/g fresh weight of leaf by a prick-assay procedure, the characteristic "halo" symptom of bean halo blight disease developed after 24 to 48 hours. At higher concentrations (10-100 nmoles/g fresh weight) the systemic symptoms, which are commonly a feature of diseased plants, also developed after 24 to 48 hours.When applied to bean leaves, phaseolotoxin was rapidly broken down by the sequential removal of homoarginine and alanine. N(delta)-Phosphosulfamylornithine was the major product formed, although phosphosulfamate and unreacted phaseolotoxin were also present. When P. phaseolicola infected bean plants, very little phaseolotoxin was detected within the plant, but the amount of N(delta)-phosphosulfamylornithine formed was sufficient to account for the observed chlorosis, the ornithine accumulation, and the systemic symptoms. N(delta)-Phosphosulfamylornithine therefore seemed to be the main functional phytotoxin of bean halo blight disease.When (35)S-phaseolotoxin was applied to primary leaves, (35)S (assumed to be a mixture of phaseolotoxin, N(delta)-phosphosulfamylornithine, and phosphosulfamate) was actively loaded into the fine veins of the leaf and moved through the plant in the vascular system at a speed greater than 3 cm/hour, particularly toward the apical and lateral buds and the root tips. Certain factors which affect pholem transport (arsenate, cold) affected toxin movement and the expression of systemic symptoms. Autoradiography suggested that the (35)S was transported in the phloem.A model for the involvement of phaseolotoxin in halo blight disease is presented.