Plastid photodamage and Cab gene expression in barley leaves

The effects of norflurazon (NF) and amitrole (AM), two bleaching herbicides which inhibit carotenogenesis, were compared in leaves of 7-day-old barley (Hordeum vulgare L. cv Express) plants grown in damaging light. The herbicide effects were analysed with respect to chloroplast organization, photosynthetic functionality and nuclear photodependent expression of the Lhcb1 gene, which codes for the Lhcb1 light-harvesting chlorophyll a/b binding protein of photosystem II. Both herbicides caused dramatic photooxidation of organelles, which were photosynthetically unfunctional. Plastids of NF-treated plants lacked thylakoids and pigments. Plastids of AM-treated plants had some strikingly altered membranes and contained only very small quantities of chlorophylls. Despite the presence of severely photodamaged plastids, cells of AM-treated leaves contained high levels of Lhcb1 transcript. This transcript, on the contrary, was completely absent in the cells of NF-treated plants. These findings suggest that in order to block expression of nuclear genes coding for plastid-resident proteins, photodamage leading to the complete dismantling of thylakoids and to the total absence of any form of photosynthetic pigment is required.