Amitrole treatment of etiolated barley seedlings leads to deregulation of tetrapyrrole synthesis and to reduced expression of Lhc and RbcS genes

The effect of amitrole, known as an inhibitor of carotenoid biosynthesis, u pon tetrapyrrole biosynthesis and its regulation has been studied. Etiolate d barley (Hordeum vulgare L.) seedlings, grown in 125 muM amitrole, accumul ated high levels of 5-aminolevulinate, Mg-protoporphyrin, Mg-protoporphyrin monomethyl ester, and protochlorophyllide. The amitrole-treated seedlings did not form paracrystalline prolamellar bodies, and the induction of Lhc a nd RbcS gene expression was reduced by non-photooxidative, low-intensity li ght. None of these events was observed upon treatment of the seedlings with 100 muM norflurazon, another inhibitor of carotenoid biosynthesis. The eff ect of amitrole cannot. be explained solely by interaction with a presumed feedback inhibition of 5-aminolevulinate synthesis since incubation with am itrole and 5-aminolevulinate indicated that deregulation also occurs at lat er steps of tetrapyrrole biosynthesis. A possible relationship between this deregulation and ultrastructural changes is discussed. In connection with previously published data, we discuss Mg-protoporphyrin and its monomethyl ester as possible candidates for a "plastid signal" that operates as a nega tive factor, reducing the expression of Lhc and RbcS genes in this higher p lant.