The phytochrome-interacting factor PIF3 has been proposed to act as a positive regulator of chloroplast development. Here, we show that the pif3 mutant has a phenotype that is similar to the pif1 mutant, lacking the repressor of chloroplast development PIF1, and that a pif1pif3 double mutant has an additive phenotype in all respects. The pif mutants showed elevated protochlorophyllide levels in the dark, and etioplasts of pif mutants contained smaller prolamellar bodies and more prothylakoid membranes than corresponding wild-type seedlings, similar to previous reports of constitutive photomorphogenic mutants. Consistent with this observation, pif1, pif3, and pif1pif3 showed reduced hypocotyl elongation and increased cotyledon opening in the dark. Transfer of 4-d-old dark-grown seedlings to white light resulted in more chlorophyll synthesis in pif mutants over the first 2 h, and analysis of gene expression in dark-grown pif mutants indicated that key tetrapyrrole regulatory genes such as HEMA1 encoding the ratelimiting step in tetrapyrrole synthesis were already elevated 2 d after germination. Circadian regulation of HEMA1 in the dark also showed reduced amplitude and a shorter, variable period in the pif mutants, whereas expression of the core clock components TOC1, CCA1, and LHY was largely unaffected. Expression of both PIF1 and PIF3 was circadian regulated in dark-grown seedlings. PIF1 and PIF3 are proposed to be negative regulators that function to integrate light and circadian control in the regulation of chloroplast development.chlorophyll synthesis ͉ circadian regulation ͉ phytochrome ͉ etioplast ͉ light signaling

No abstract

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.

No abstract