Light-signalling pathways leading to the co-ordinated expression of HEMA1 and Lhcb during chloroplast development in Arabidopsis thaliana.

During de-etiolation, the co-ordinated synthesis of chlorophyll and the chlorophyll a/b-binding proteins is critical to the development of functional light-harvesting complexes. To understand how this co-ordination is achieved, we have made a detailed study of the light-regulated signalling pathways mediating the expression of the HEMA1 and Lhcb genes encoding glutamyl-tRNA reductase, the first committed enzyme of 5-aminolaevulinic acid formation, and chlorophyll a/b-binding proteins, respectively. To do this, we have screened 7 photoreceptor and 12 light-signalling mutants of Arabidopsis thaliana L. for induction of HEMA1 and Lhcb expression in continuous red, far-red and blue light and following a red pulse. We have categorised these mutants into two groups. The phyA, phyB, phyAphyB, cry1, cry2, cop1, det1, poc1, eid1, and far1 mutations lead to diverse effects on the light regulation of HEMA1, but affect Lhcb expression to a similar degree. The hy1, hy2, hy5, fin219, fhy1, fhy3, spa1, ndpk2, and pat1 mutants also affect light regulation of both HEMA1 and Lhcb expression, but with differences in the relative magnitude of the two responses. The fhy1 and fhy3 mutants show the most significant differences in light regulation between the two genes, with both showing a strong inhibition of HEMA1 expression under continuous red light. These results demonstrate that co-ordinated regulation of HEMA1 and Lhcb is largely achieved through parallel light regulation mediated by shared phytochrome- and cryptochrome-signalling pathways. However, glutamyl-tRNA reductase is also required for the synthesis of other tetrapyrroles and this dual role may account for the observed differences in these light-signalling pathways.