Light quantity controls leaf-cell and chloroplast development in Arabidopsis thaliana wild type and blue-light-perception mutants

Plants acclimate to changes in light quantity by altering leaf-cell develop ment and the accumulation of chloroplast components, such that light absorp tion is favoured under limiting illumination, and light utilisation under n on-limiting conditions. Previous evidence suggests an involvement of a high -light photosynthetic redox signal in the down-regulation of the accumulati on of the light-harvesting complexes of photosystem II (Lhcb) and the expre ssion of the Lhcb genes, and of a blue-light signal in the control of leaf development and in the increase in photosynthetic capacity, as affected by the accumulation of ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisc o). We examined the internal anatomy of leaves, the ultrastructure of chlor oplasts and accumulation of light-hal vesting complexes and Rubisco in wild -type Arabidopsis thaliana (L.) Heynh. and in mutants in each of the three known blue-light photoreceptors, cryptochrome 1, cryptochrome 2 and phototr opin, as well as a mutant in both cryptochromes. Our results indicate an ex tensive capacity of the Arabidopsis mesophyll cells to adapt to high light fluence rate with an increase in palisade elongation. Under high light, chl oroplasts showed increased starch accumulation and reductions in the amount of granal thylakoids per chloroplast, in the proportion of chlorophyll b r elative to chlorophyll a, and in the accumulation of the major Lhcb polypep tides. The responses were similar for all four mutants, with respect to the ir wild types. The results are consistent with either a complete redundancy in function between cryptochromes and phototropin, or their absence of inv olvement in the light-quantity responses tested. We observed minimal effect s of light quantity on Rubisco accumulation over the range of fluence rates used, and conclude that elongation of palisade mesophyll cells and accumul ation of Rubisco are controlled separately. This indicates that light accli mation must be the result of a number of individual elementary responses. Q uantitative differences in the acclimatory responses were observed between the Landsberg erecta and Columbia wild-type ecotypes used.