Transduction of Light Signals in Plants

Abstract

The phytochromes are the best studied plant photoreceptors, controlling a wide variety of responses at both whole plant and single cell levels. Three signal transduction pathways, dependent on cGMP and/or calcium, are used by phytochrome to control the expression of genes required for chloroplast development and anthocyanin biosynthesis. Control of the relative inputs into these different transduction pathways may be a means by which plant cells can adjust their biochemical status, in particular for controlling the levels of photoprotectants (such as anthocyanins) and the relative amounts of the different photosynthetic complexes. If this were the case one would predict the existence of cross-talk mechanisms between the different phytochrome pathways, which would allow the activities of each pathway to be known by the others and which would allow regulatory signals to pass between them. Experiments will be presented that have elucidated some of these negative regulatory mechanisms. In addition to these studies of light-activated processes, we have also performed some experiments to understand how phytochrome can down-regulate other events. For example, the expression of several genes is known to be negatively-regulated by light. We have determined how such genes are regulated by phytochrome in the context of calcium and cGMP and have identified an 11 bp cis-element present within the promoters of such genes that is both necessary and sufficient to mediate light down-regulation.