PHYTOCHROME SIGNAL-TRANSDUCTION - CHARACTERIZATION OF PATHWAYS AND ISOLATION OF MUTANTS

The study of phytochrome signalling has yielded a wealth of data descr ibing both the perception of light by the receptor, and the terminal s teps in phytochrome-regulated gene expression by a number of transcrip tion factors. We are now focusing on establishing the intervening step s linking phytochrome photoactivation to gene expression, and the regu lation and interactions of these signalling pathways. Recent work has utilized both a pharmacological approach in phototrophic soybean suspe nsion cultures and microinjection techniques in tomato to establish th ree distinct phytochrome signal-transduction pathways: (i) a calcium-d ependent pathway that regulates the expression of genes encoding the c hlorophyll a/b binding protein (CAB) and other components of photosyst em II; (ii) a cGMP-dependent pathway that regulates the expression of the gene encoding chalcone synthase (CHS) and the production of anthoc yanin pigments; and (iii) a pathway dependent upon both calcium and cG MP that regulates the expression of genes encoding components of photo system I and is necessary for the production of mature chloroplasts. T o study the components and the regulation of phytochrome signal-transd uction pathways, mutants with altered photomorphogenic responses have been isolated by a number of laboratories. However, with several possi ble exceptions, Little real progress has been made towards the isolati on of mutants in positive regulatory elements of the phytochrome signa l-transduction pathway. We have characterized a novel phytochrome A (p hyA)-mediated far-red light (FR) response in Arabidopsis seedlings whi ch we are currently using to screen for specific phyA signal-transduct ion mutants.