PROTEIN-KINASES AND PHOSPHATASES AND THEIR ROLE IN CELLULAR SIGNALINGIN PLANTS

Plants respond very efficiently to changes in the external environment and accordingly regulate their growth and development. They also have the ability to sense stress conditions and pathogen presence and subs equently activate stress or defense-related functions. Lately, progres s has been made in identification of cis-and trans-acting factors that mediate gene expression induced by variable external factors. However , in most of the cases our understanding is limited in relation to the molecular signal transduction events that couple perception of the si gnal and changes in the gene expression. It is now realized that prote in phosphorylation/dephosphorylation has a very important role in sign al transduction and also in regulating metabolic activities. During th e last decade, a large number of serine/threonine protein kinases have been isolated from plants. Some of these belong to the same category as reported in animal systems, like MAP kinases, receptor kinases, cel l division controlling kinases, whereas others are of very novel kind, like calcium-dependent protein kinases, Ca2+ and calmodulin-stimulate d protein kinases with visinin-like domain and some specific receptor kinases with defined functions in self-incompatibility, pathogen respo nse, and in regulating growth and differentiation. In many cases, gene s coding for these kinases have also been cloned and characterized. Si milarly, protein phosphatases, also of different types, have been repo rted, purified, and their genes cloned. In this article, we have revie wed the biochemical properties, the protein, and the gene structure an d the biological role of all known important protein kinases and phosp hatases. In addition, we have also reviewed, when known, the changes a nd role of protein phosphorylation in response to exogenous factors li ke light, hormones, stress, and pathogen attack. In the end, a descrip tion of transacting factors that have been shown to bind to specific c is-elements in a phosphorylation/dephosphorylation manner has been giv en to show the importance of protein kinases and phosphatases in modul ating specific gene expression.