Showing papers by "Jitendra P. Khurana published in 1999"

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

Abstract: 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.

Blue Light Perception and Signal Transduction in Higher Plants

Abstract: Blue light regulates a diverse range of plant responses that include phototropism, stem growth inhibition, leaf expansion, chloroplast development, stomatal opening, anthocyanin accumulation and changes in expression of various genes. Some of these responses are elicited by low fluence and others by a higher fluence of blue light. These responses are regulated by the photoreceptors absorbing principally in the UV-A (320–390 nm) and blue (390–500 nm) region of the electromagnetic spectrum. Until very recently, the biochemical identity of the blue light receptors was considered a major unsolved problem. The Combined application of physiological, biochemical and molecular genetic approaches has now generated new insight into not only the photoperception mechanism but also the signal transduction components that mediate the effects of blue light on plant development. Convincing evidence has now emerged for the existence of multiple blue light receptors which may harbor more than one chromophore. The long-standing controversy on whether flavins and/or carotenoids are the receptors for blue light still persists. The isolation and characterization of CRY genes has provided evidence for the involvement of a flavin, in addition to a pterin, in blue light-induced responses. There has been reasonable progress in deciphering some of the blue light signaling components, but the demonstration of the hierarchial role of these components in any single response pathway is presently lacking. Blue light also profoundly influences the expression of genes, and their light-responsive elements (cis-acting sequences) and the trans-acting (protein) factors are being identified. The wealth of information that has accumulated on these aspects mentioned above has been evaluated in the following text. A greater emphasis has been laid on the studies where the power of genetics has been exploited to provide answers to some of the unsolved mysteries that surround the perception of blue light and its mechanism of action.

Changes in Phosphorylation Status of Wheat Plastid Polypeptides as Influenced by Light, Calcium and Cyclic AMP

Abstract: The polypeptides of etioplast and chloroplast fractions, purified on Percoll discontinuous gradient, were phosphorylated in vitro using (γ-32P)ATP, resolved by SDS-PAGE and autoradiographed. In general, about 15-18 phosphopolypeptides in the range of 14-150 kD were distinctly visible in autoradiograms of both organelle fractions with varying degree of radiolabel incorporation. Although short-term irradiation with red or far-red light did not have any significant effect on phosphorylation status of etioplast polypeptides, in vivo irradiation with 1 h white light, followed by in vitro phosphorylation, decreased phosphorylation of a 116 kD polypeptide and increased the phosphorylation of polypeptides of 38 kD and a doublet around 20 kD. Strikingly, the phosphorylation status of 116 kD etioplast polypeptide was adversely affected by Ca2+ as well, and this phosphopolypeptlde was not distinctly visible in the autoradiogram of the chloroplast fraction proteins. However, in vitro phosphorylation of 98, 57 and 50 kD polypeptides of both etioplast and chloroplast fractions was found to be Ca2+ dependent. Unlike Ca2+, 3′,5′-cyclic AMP down-regulated the phosphorylation of several polypeptides of both etioplasts and chloroplasts, including 98 and 50 kD, and up-regulated the phosphorylation of 32 and 57 kD polypeptides. The significance of these observations on changes in phosphoprotein profile of etioplasts and chloroplasts, as influenced by light, Ca2+ and cyclic nucleotides, has been discussed.