Selective inhibition of HEMA gene expression by photooxidation in Arabidopsis thaliana

Norfiurazon (NF), a photobleaching herbicide, inhibits carotenoid biosynthesis. Lack of carotenoid pigments leads to photoxidative damage of chloroplasts. In this study of Arabidopsis thaliana we demonstrate that NF-treated photobleached plants are still able to make 5-aminoevulinic acid (ALA) the first precursor of porphyrins and tetrapyrroles. ALA is formed in the tRNA-dependent two-step C-5-pathway in the chloroplast of plants. The expression of glutamyl-tRNA reductase (GluTR), the first enzyme in the pathway, was severely inhibited by NF, while treatment with this compound did not significantly reduce the levels of the other enzyme, glutamate-1-semialdehyde aminomutase, or of tRNA(Gln), the initial metabolite of the pathway, Extracts of these plants retained the capacity, albeit reduced, to convert exogenously added glutamate to ALA. Thus, the much-reduced level of ALA formation in photobleached plants is dire to selective inhibition of GluTR expression. (C) 1999 Elsevier Science Ltd. All rights reserved.