GLK transcription factors regulate chloroplast development in a cell-autonomous manner

In a number of land plants, Golden2-like (GLK) genes encode a pair of partially redundant nuclear transcription factors that are required for the expression of nuclear photosynthetic genes and for chloroplast development. As chloroplast biogenesis depends on close co-operation between the nuclear and plastid compartments, GLK gene function must be dependent on tight intracellular control. However, the extent to which GLK-mediated chloroplast development depends on intercellular communication is not known. Here we used sector analysis to show that GLK proteins operate cell-autonomously in leaf mesophyll cells. To establish whether GLK proteins are able to influence adjacent cell layers, we used tissue-specific promoters to restrict GLK gene expression to the epidermis and to the phloem. GLK genes driven by the Arabidopsis epidermal FIDDLEHEAD (FDH) and MERISTEM LAYER1 (AtML1) promoters failed to rescue the pale-green Atglk1 Atglk2 mutant phenotype, confirming the suggestion that GLK proteins can only influence chloroplast development intracellularly. An exception to this rule was seen in lines in which GLK genes were expressed in the phloem. However, we believe that the partial complementation of the mutant phenotype that was observed resulted from phloem unloading, as opposed to inherent properties of GLK proteins. We conclude that GLK proteins act in a cell-autonomous manner to coordinate and maintain the photosynthetic apparatus within individual cells. Significantly, this suggests that GLK proteins provide a means to fine-tune photosynthesis according to the differential requirements of cells within the leaf.