HORT640 - Metabolic Plant Physiology
Aromatic amino acid biosynthesis
Inhibitors of histidine biosynthesis
Amitrole is an effective herbicide on annual and perennial monocots and dicots. It rapidly translocates throughout the plant via the phloem and is probably one of the most mobile herbicides known.
There is controversy about the mode of action of amitrole in plants. The most obvious response of plants treated with amitrole is albinism of the emerging leaves. However, amitrole causes cessation of growth before these leaves emerge. Amitrole has also been shown to decrease the levels of soluble protein and increase the free amino acid levels. In addition this herbicide causes a rapid decrease in the growth of roots, inhibits the incorporation of radiolabeled precursors into RNA, DNA, and acid-soluble nucleotides, and disrupts the development of chloroplasts.
Amitrole interferes with histidine biosynthesis in yeast and Salmonella by inhibiting the enzyme imidazoleglycerol-phosphate dehydratase (IGPD) [EC 188.8.131.52] which leads to the accumulation of imidazoleglycerol (IG) in the growth medium. The toxicity of amitrole to these organisms can be reversed through exogenous application of histidine.
The site of action of amitrole in plants has not been established. IGPD from plants has been reported to be sensitive to amitrole, and IG accumulates in Paul's Scarlet Rose suspension cells treated with the herbicide. Furthermore, toxicity of amitrole can be prevented by exogenous applications of histidine at the cell culture level. However, in whole plant studies, the herbicidal effects of amitrole could not be reversed by exogenous applications of histidine. It was found that in young maize seedlings, the endogenous levels of histidine were higher in amitrole-treated than in control plants, and that amitrole inhibited growth of seedlings during this period of high histidine levels. Amitrole may have sites of action in addition to histidine biosynthesis (Shaner, 1989)?
May and Leaver (1993) have shown that aminotriazole is a catalase [EC 184.108.40.206] (and aconitase [EC 220.127.116.11]) inhibitor and causes a large increase in the pool of GSH (reduced glutathione). Aminotriazole inhibition of catalase has recently been used to demonstrate that catalase plays a major role, probably with other antioxidant enzymes, in inducing chilling tolerance in pre-emergent maize seedlings. Aminotriazole abolished cold acclimation (Prasad, 1997).
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May MJ, Leaver CJ 1993 Oxidative stimulation of glutathione synthesis in Arabidopsis thaliana suspension cultures. Plant Physiol. 103: 621-627.
Prasad TK 1997 Role of catalase in inducing chilling tolerance in pre-emergent maize seedlings. Plant Physiol. 114: 1369-1376.
Shaner DL 1989 Sites of action of herbicides in amino acid metabolism: primary and secondary physiological effects. In "Plant Nitrogen Metabolism" (JE Poulton, JT Romeo, EE Conn eds) Recent Advances in Phytochemistry, Vol. 23, Academic Press, New York, pp. 227-261.