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Research Interests
Plant Reproduction, Calcium/Calmodulin-mediated Signaling
Research Summary
Changes in intracellular Ca2+ concentration can affect a number of physiological processes through the action of calmodulin (CaM), a ubiquitous Ca2+-binding protein. Ca2+/CaM-regulated protein phosphorylation plays a pivotal role in amplifying and diversifying the action of Ca2+-mediated signals. This laboratory has cloned a chimeric Ca2+/CaM-dependent protein kinase (CCaMK) gene with a neural visinin-like Ca2+-binding domain from developing anthers of lily (Patil et al., Proc. Natl. Acad. Sci. 92: 4897-4901, 1995). CCaMK is characterized by the presence of a catalytic domain, a CaM-binding domain, and a visinin-like Ca2+-binding domain in a single polypeptide; making it distinctly different from other known plant and animal kinases. The structural features of CCaMK suggest that it has evolved from a fusion of two genes that are functionally different and phylogenetically diverse in origin. CCaMK autophosphorylates at the threonine residue (T267) in a Ca2+-dependent manner. This autophosphorylation site is conserved both in monocotyledonous and dicotyledonous plants. Site-directed mutants of the visinin-like domain revealed that the EF-hands II and III are crucial for Ca2+-induced conformational changes in the visinin-like domain. CCaMK is expressed in anther in a stage-specific manner during microsporogenesis. Expression was detected at early stages of flower bud development (bud size < 0.5 cm), and it reached a peak when the bud size was between 0.6 cm and 1.0 cm. At later stages of flower bud development, the message decreased and became undetectable. These results indicate that CCaMK is expressed in a stage-specific manner coinciding with the stage of anther development when meiosis and microspore maturation occurs. Immunolocalization studies revealed a concomitant increase in the calmodulin level in pollen mother cells and tapetal cells. The presence of CCaMK was first detected in pollen mother cells and the message continued to increase during meiosis. No message was detected in the epidermis, endothecium, and connective tissues. As soon as the microspores were released, CCaMK expression decreased and at later stages of microspore development, expression was not detectable. Our results suggest that CCaMK phosphorylates proteins in pollen mother cells and tapetal cells in a Ca2+/CaM-dependent manner, and that phosphorylation could play a crucial role in regulating microsporogenesis and microspore maturation. The functional significance of CCaMK in plant reproduction is being investigated. Interestingly, CCaMK is also present in the roots where it is involved in plant:microbe interactions leading to symbiotic relations with bacteria (nitrogen fixation) and fungi (mychorrizae).
Research Publications
2005-2009
Du, L., Ali, G. S., Simons, K. A., Hou J., Yang, T., Reddy, A.S.N. and Poovaiah, B.W. 2009. Ca2+/calmodulin Regulates Salicylic Acid-mediated Immune Response in Plants through AtSR1/CAMTA3. Nature. 457:1154-1158
Gleason, C., Chaudhuri, S., Yang, T., Munoz-Gutierrez, A., Poovaiah, B.W. and Oldroyd, G.E.D. 2006. Nodulation independent of rhizobia induced by a calcium-activated kinase lacking autoinhibition. Nature. 441:1149-1152.
Du L., Poovaiah B.W. 2005 Ca2+/calmodulin
is critical for brassinosteroid biosynthesis and plant
growth. Nature. 437(7059):741-745.