Inhibition of ornithine decarboxylase activity by phaseolotoxin: Implications for symptom production in halo blight of French bean

Abstract

The plant pathogenic bacteriumPseudomonas syringaepv.phaseolicolasecretes the toxic tripeptide phaseolotoxin (PT) which is known to irreversibly inhibit ornithine carbamoyltransferase (OCT; E.C. 2.1.3.3). Crude toxin-containing preparations from virulent isolates of the bacterium, as well as purified PT, were demonstrated to inhibit another enzyme of ornithine metabolism, ornithine decarboxylase (ODC; E.C. 4.1.1.17) from both plant and bacterial sources. ODC is a key enzyme in the polyamine biosynthetic pathway and is important in the control of the cell cycle and senescence.Infection of mature bean leaves withP.s.pv.phaseolicolacauses a bright yellow halo surrounding the site of inoculation. It has been suggested that this symptom is due to inhibition of chlorophyll synthesis resulting from OCT inhibition by the toxin. However, we show here that the halo contains chlorophyll catabolites similar to those seen in senescence-associated yellowing of leaves; thus indicating that the local chlorosis symptom is also associated with chlorophyll destruction and not just an inhibition of chlorophyll synthesis. Exogenous putrescine supplied to infected plants prevented chlorotic halo formation; suggesting thatin vivoODC inhibition leading to changes in polyamine metabolism might be a contributing factor to the chlorosis symptom. Putrescine itself does not accumulate in large amounts in bean but it is a direct precursor of the quantitatively more significant spermine and spermidine. However, levels of these polyamines did not differ significantly from controls in the course of an infection. The role of PT in the pathogenesis of halo blight in French bean is discussed.