Light regulation of tetrapyrrole biosynthesis in Arabidopsis

The tetrapyrrole biosynthesis pathway results in the synthesis of both haem and chlorophyll and tight regulation of this pathway during de-etiolation is critical for the safe biogenesis of the photosynthetic apparatus. While much attention has been paid to light regulation of the total flux through this pathway less is known about the role of light in regulating the key, metal-insertion branchpoint leading either to haem or chlorophyll. Magnesium chelatase, responsible for channelling substrates into the chlorophyll branch, is composed of three subunits (CHLD, CHLH and CHLI), with a regulatory protein, GUN4, also necessary for full enzyme activity. To understand how light affects branchpoint regulation we have studied the transcriptional regulation of the four genes encoding these proteins in de-etiolating Arabidopsis thaliana seedlings using quantitative PCR. Analysis of gene expression in dark-grown seedlings transferred to far-red and red light has shown that CHLH and GUN4 are strongly induced, while the CHLD and CHLI1 genes show little change in expression. Comparison of expression profiles in wild-type and photoreceptor-deficient mutant of Arabidopsis indicate that regulation is primarily under the control of phytochromes A and B. These results suggest that phytochrome-mediated induction of GUN4 and CHLH is a major mechanism for redirecting tetrapyrrole synthesis to the chlorophyll branch of the pathway during de-etiolation. We are now looking to test this hypothesis further by examining protein levels and the flux of tetrapyrrole intermediates through the branch point at this critical stage of plant development.