Novel light-activated protein kinases as key regulators of plant growth and development

Plants have evolved highly sensitive sensory photoreceptor systems to regulate various aspects of their growth and development. Many responses such as seed germination, flowering and dormancy are controlled by red and far-red regions of the solar spectrum through the phytochrome family of photoreceptors. However, several other responses such as stem growth inhibition, phototropism and opening of stomata are controlled by blue and/or ultraviolet light absorbing photoreceptors called cryptochromes and phototropin. Despite their central role in plant biology, the mode of action of these photoreceptors has been shrouded in mystery. Even the biochemical isolation of a photoreceptor, as in the case of phytochrome was accomplished decades ago, did not help in elucidating the mechanism of action. Nevertheless, due to advances in recombinant DNA technology, generation of extensive databanks and the capability to predict function by base sequence analysis, a breakthrough has now come about. It is clear that certain phytochromes, at least in the cyanobacteria and algae which represent the simplest plants, are hybrid photoreceptor-cum-kinases. These novel kinases utilize captured photons rather than conventional ligands to trigger conformational change and in consequence enzyme activity. The kinases apparently, then, cause phosphorylation of many other types of target molecules, leading eventually to various developmental changes. There is suggestive evidence that in higher plants, too, at least some phytochromes may operate as kinases. As compared to work on phytochromes, the blue light photoreceptors have begun to be studied only recently. However, the exciting discovery has been made of at least one photoactive kinase that is critically required for phototropism. This article summarizes the above discoveries from the perspective of general biology. Dedicated to the memory of Drs Harry Borthwick, Sterling Hendricks and James Bonner whose classical studies paved the way for modern researches on mechanism of action of plant photoreceptors and whom the senior author was previleged to know.