Molecular Responses to Osmotic Stresses in Soybean

Molecular Responses to Osmotic Stresses in Soybean 217 phosphatidylinositol 3-and 4-phosphate (PI3P and PI4P), cyclic adenosine 5'diphosphoribose (cADPR), and sphingosine-1-phosphate (S1P) are implicated to control the activities of diverse members of Ca 2+ channels and Ca 2+ transporters which are evolved to regulate the specificity of Ca 2+ signatures (period, frequency, and amplitude) (Ng and McAinsh, 2003;McAinsh and Pittman, 2009).Different strength of environmental stimuli can also generate differential spatial-temporal Ca 2+ waves (Goddard et al., 2000). Ca 2+ sensorsThe Ca 2+ signatures are detected and decoded by Ca 2+ sensor proteins (Dodd et al., 2010), which exhibit different Ca 2+ -binding characteristics, subcellular localizations, and downstream signalling interactions.The molecular features enable the sensor proteins to decode and process the information embedded within Ca 2+ signatures into alterations of cell functions (Dodd et al., 2010).Ca 2+ sensor proteins can be classified into sensor responders and sensor relays (Dodd et al., 2010).Sensor responder proteins combine the sensing function (mediated by Ca 2+ -binding domains) and the response activity (e.g.kinase activity) within a single protein (Dodd et al., 2010).In contrast, sensor relay proteins (e.g.most calmodulins) only possess Ca 2+ -binding domains that can undergo Ca 2+ -induced conformational changes to interact and regulate the activity of target proteins (Dodd et al., 2010).Calcineurin B-like protein (CBLs) are sensor relay proteins, sensing Ca 2+ by four Ca 2+binding EF-hands.They form complex with CBL-interacting protein kinases (CIPKs) in the conserved NAF (Asn-Ala-Phe) domain to release the C-terminal (autoinhibitory) domain from the kinase domain; thereby transforming the CIPKs into their active state.CBLs can form independent complex with CIPKs for transmitting the Ca 2+ signal to activate different subset of stress-responsive genes.Cacium-dependent protein kinases (CDPKs) are typical sensor responders that are activated after binding of Ca 2+ to the C-terminal EF-hand-containing regulatory domain, causing conformational changes that relieve the active site of the kinase domain from masking by an autoinhibitory domain.They are then fully activated by autophosphorylation.Activated CDPKs will phosphorylate downstream kinase and phosphatase components and transmit the signals via phosphorylation.The roles and regulation of CDPKs in higher plants were reviewed previously (Ludwig et al., 2004).Calmodulins (CaMs), forming a large protein family in higher plants (McCormack and Braam, 2003), are another group of Ca 2+ signature decoders.In response to osmotic stress, this calcium sensor may transmit the calcium signal by functioning as a transcription factor to regulate gene expression directly (as sensor responders) (Kushwaha et al., 2008), working as sensor relays through the interaction with transcription factors and transcription factor-binding protein, or modulating phosphorylation status of transcription factors (Kim et al., 2009). Phosphorylation cascadeProtein phosphorylation cascade, regulated by kinases and phosphatases, plays central role to link Ca 2+ sensors to cellular responses.The mitogen-activated protein kinase (MAPK) pathway that has received much attention is composed of three kinase modules: MAPK, MAPKK and MAPKKK (Jonak et al., 2002) .MAPKs are serine/theonine kinases that modulate a variety of downstream gene expression and physiological responses (Jonak et al., 2002;Zhu, 2002).The substrates include transcription factors, protein kinases, and cytoskeletal proteins.MAPKKs are dual-specificity kinases which