Protein kinases and phosphatases and their role in cellular signaling in 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 subsequently activate stress or defense-related functions. Lately, progress 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 signal and changes in the gene expression. It is now realized that protein phosphorylation/dephosphorylation has a very important role in signal transduction and also in regulating metabolic activities. During the 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, cell division controlling kinases, whereas others are of very novel kind, like calcium-dependent protein kinases, Ca2+ and calmodulin-stimulated protein kinases with visinin-like domain and some specific receptor kinases with defined functions in self-incompatibility, pathogen response, and in regulating growth and differentiation. In many cases, genes coding for these kinases have also been cloned and characterized. Similarly, protein phosphatases, also of different types, have been reported, purified, and their genes cloned. In this article, we have reviewed the biochemical properties, the protein, and the gene structure and the biological role of all known important protein kinases and phosphatases. In addition, we have also reviewed, when known, the changes and role of protein phosphorylation in response to exogenous factors like light, hormones, stress, and pathogen attack. In the end, a description of transacting factors that have been shown to bind to specific cis-elements in a phosphorylation/dephosphorylation manner has been given to show the importance of protein kinases and phosphatases in modulating specific gene expression.