Cab gene expression in bleached leaves of carotenoid-deficient maize

The chloroplast photo-oxidation and the expression of the Cab gene Lhcb1, e ncoding the Lhcb1 light-harvesting chlorophyll a/b-protein of PS II, have b een studied in leaf cells of maize treated with the two bleaching herbicide s norflurazon and amitrole and of the two carotenoid-free mutants vp9 and v p2 grown under high photodamaging light. Both herbicides and mutations caus ed severe photo-oxidation of organelles. However, the plastids of norfluraz on-treated and vp2 leaves were totally devoid of thylakoids and did not con tain any chlorophyll, while the organelles of amitrole-treated and vp9 leav es still had a few altered and photosynthetically unfunctional membranes an d very small quantities of chlorophylls. Despite the dramatic photodamage u ndergone by the plastids over several days, the cells of amitrole-treated a nd vp9 leaves maintained a certain expression of the Lhcb1 gene which, on t he contrary, was completely blocked in the cells of norflurazon-treated and vp2 leaves. The experimental results, obtained by integrating biochemical and molecular analyses with ultrastructural observations, show that the mai ntainance of Cab gene expression does not strictly depend on intact and fun ctional chloroplasts. The transcription of these genes, still maintained in cells with greatly photo-oxidized organelles, seems to be inversely relate d to the degree of thylakoid demolition, which can affect the last steps of chlorophyll biosynthesis.