Steering the solar panel: plastids influence development

As if they were industrial factories, plant cells reveal a multitude of interactions between compartments, with many activities requiring the traffic of components and products between them. Thirty years ago researchers made what seemed, at first, a surprising observation: a mutant of barley that, through maternal inheritance of ribosome-deficient plastids, possessed contiguous stripes of green and white tissue, showed, in the white tissue, not only very low levels of plastid-based enzymatic activities but also very low levels of synthesis template for them in the cytoplasm (Bradbeer et al., 1979). The deficient state of the plastids was providing information to the nucleus of those cells, leading to low mRNA levels of the nuclear-encoded genes for those enzymes in the cytoplasm where they were synthesized. The compartments were sharing information, including commands from seemingly subordinate organelles (namely the chloroplasts), to control expression of genes in the nucleus. This specific flow of interorganellar information is known as plastid–nuclear communication or plastid retrograde signalling. In this issue of New Phytologist, Ruckle & Larkin (pp. 367–379) provide evidence showing that this retrograde signalling controls not just the nuclear processes involved in the biogenesis and function of the organelle, but also aspects of the differentiation of cells and the development of the plant overall in its response to light, at least at its crucial seedling stage. Light is a key environmental cue without whose presence neither chloroplast biogenesis nor normal (photomorphogenic) seedling development take place. Ruckle et al. (2007) recently showed that plastid signals are capable of ‘rewiring’ the light signalling network controlling a gene (Lhcb1) for a major chloroplast protein; the cryptochrome1 (cry1*) photoreceptor and a bZIP downstream transcription factor, HY5, seemed to reverse their roles in the expression of this gene if plastids were damaged. In this issue, Ruckle & Larkin show that this reversal, as judged by the consequences of the absence of cry1, applies also to several other aspects of seedling photomorphogenesis: for example, the expansion of the first photosynthetic organs (the cotyledons) and, under some conditions, the elongation of the seedling stem (the hypocotyl), although it does not affect responses related to the production of sunscreens in nonphotosynthetic cells.