CYCLIC-GMP AND CALCIUM MEDIATE PHYTOCHROME PHOTOTRANSDUCTION

We have previously used single-cell assays in a phytochrome-deficient tomato mutant to demonstrate that phytochrome signaling involves heter otrimeric G proteins, calcium, and calmodulin. While G protein activat ion could stimulate full chloroplast development and anthocyanin pigme nt biosynthesis, calcium and calmodulin could not induce anthocyanins and were only able to stimulate the development of immature chloroplas ts tacking cytochrome b(6)f and photosystem I core components. We now report that cyclic GMP is able to trigger the production of anthocyani ns, and that a combination of cyclic GMP with calcium can induce the d evelopment of fully mature chloroplasts containing all the photosynthe tic machinery. Furthermore, using reporter genes for these different p athways (cab-gus, chs-gus and fnr-gus) we demonstrate that cGMP and ca lcium act primarily by modulating gene expression.