Decarboxylation of lndole‐3‐Acetic Acid and Inhibition of Growth in Avena sativa Seedlings by Plant‐Derived Photosensitizers*

Abstract

Abstract— A number of plant phototoxins, when supplemented with UVA (320–400 nm) radiation, are capable of sensitizing the decomposition of indole‐3‐acetic acid (IAA), as measured by release of 14CO2 from carboxyl‐labeled IAA. Alpha‐terthienyl (αT) and harmine caused significant rates of IAA decarboxylation at concentrations as low as 1 nM and were approximately 80% as effective as riboflavin and flavin mononucleotide. Partial inhibition by sodium azide indicates that the aT‐induced decarboxylation of IAA is predominately, but not entirely, a type II reaction mediated by singlet oxygen. Based on changes in UV ab sorption spectra, it appears that the hormones gibberellic acid, abscisic acid and 6‐benzylaminopurine (a cytokinin) are less susceptible to photosensitized decomposition than is IAA. Alpha‐terthienyl plus UVA also inhibited elongation growth and reduced endogenous IAA levels in Avena sativa L. coleoptile sections and promoted senescence in intact Avena seedlings. These results confirm the allelopathic potential of plant photosensitizers such as αT and indicate that the phytohormone IAA may represent an additional target for the action of photosensitizers.