Light signalling pathways regulating the Mg-chelatase branchpoint of chlorophyll synthesis during de-etiolation in Arabidopsis thaliana.

Precise regulation of tetrapyrrole synthesis is critical for plant survival when seedlings first emerge into the light. At this time there is a massive increase in demand for chlorophyll to drive the assembly of the photosynthetic apparatus. To understand how this demand is met we have followed the expression of genes encoding the chelatase enzymes at the branchpoint between chlorophyll and heme synthesis. Dark-grown Arabidopsis thaliana seedlings were transferred to continuous white, red, far-red or blue light and the expression of eight tetrapyrrole pathway genes was followed using real-time RT-PCR. Our results show that the CHLH gene encoding the H subunit of Mg-chelatase was induced by light under all conditions with an initial peak after 2-4 h light. The other Mg-chelatase subunit genes CHLI and CHLD and the ferrochelatase genes FC1 and FC2 were not strongly regulated at the level of transcript abundance, but the Mg-chelatase regulator GUN4 had an expression profile almost identical to that observed for CHLH. The CHLM gene encoding Mg-protoporphyrin IX methyltransferase, the next enzyme in the pathway, was also light regulated, but showed a very different pattern of expression. Using photoreceptor mutants it was demonstrated that regulation of CHLH and GUN4 is primarily under the control of phytochromes A and B with some input from the cryptochromes. Induction of CHLH and GUN4 under red and far-red light was also compromised in the phytochrome-signalling mutants, fhy1 and fhy3. These results establish GUN4 as a major target of photoreceptor regulation during the earliest stages of de-etiolation.