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.

Transketolase Is Identified as a Target of Herbicidal Substance α-Terthienyl by Proteomics

“…The activity of α -terthienyl was also tested in dark and light for C. reinhardti , as shown in the Figure S1 , where the result shows that α -terthienyl still retained activity in the darkness. Furthermore, Campbell [ 16 ] and Brennan [ 20 ] also reported the activity of α -terthienyl on plants in dark, and Gommers [ 21 ] reported that it had a high activity on nematodes. It does seem that α -terthienyl is capable of exerting toxic effects in the absence of light.…”

Participation of the Photosensitizer Alpha-terthienyl in thePeroxidase-catalyzed Oxidation of Indole-3-acetic acid†

“…Nevertheless, it does seem that ␣T is capable of exerting toxic effects on both plants (4,6) and animals (7) in the absence of light. Thiarubrines, related UVA-absorbing compounds produced in the roots of some plants, show lightindependent fungal toxicity that is proposed to result from the reactivity of their 1,2-dithiin ring, possibly with a specific enzyme in the fungus (22,23).…”

“…Some of the toxic effects of ␣T cannot be explained in terms of its normal photosensitizing properties. For example, the ␣T-induced inhibition of seedling growth noted above was significantly more pronounced in the presence of UVA, but was also clearly observable in the dark (approximately 30% inhibition with 10 M ␣T ϩ UVA versus 15% with ␣T alone) (6). Also, the germination and growth of several weed species are inhibited by ␣T concentrations in the range of 1-10 ppm in the absence of UVA treatment (4).…”

“…For example, we have previously shown that low concentrations of ␣T, in combination with UVA radiation, are capable of damaging chloroplast membranes, interfering with light-activation of Calvin Cycle enzymes and inhibiting photosynthetic CO 2 fixation (5). We have also reported the decarboxylation of the hormone indole-3-acetic acid (IAA) and inhibition of Avena seedling growth by ␣T and a variety of other naturally occurring photosensitizers (6). These findings raise the possibility that such compounds might play an allelopathic role in nature, and that they or similar synthetic phototoxins may have potential herbicidal applications.…”

Plant Growth Regulators I: Introduction; Auxins, their Analogues and Inhibitors

“…It was shown to participate in the mechanism of colonization of alfalfa roots by Sinorhizobium meliloti (Yang et al, 2002), to promote induction of embryogenic callus in Zoysia japonica (Asano et al, 1996) and to protect auxin from oxidation (Brennan, 1996).…”