Abstract
Abscisic acid (ABA) signal transduction has been proposed to utilize cytosolic Ca2+ in guard cell ion channel regulation. However, genetic mutants in Ca2+ sensors that impair guard cell or plant ion channel signaling responses have not been identified, and whether Ca2+-independent ABA signaling mechanisms suffice for a full response remains unclear. Calcium-dependent protein kinases (CDPKs) have been proposed to contribute to central signal transduction responses in plants. However, no Arabidopsis CDPK gene disruption mutant phenotype has been reported to date, likely due to overlapping redundancies in CDPKs. Two Arabidopsis guard cell–expressed CDPK genes, CPK3 and CPK6, showed gene disruption phenotypes. ABA and Ca2+ activation of slow-type anion channels and, interestingly, ABA activation of plasma membrane Ca2+-permeable channels were impaired in independent alleles of single and double cpk3cpk6 mutant guard cells. Furthermore, ABA- and Ca2+-induced stomatal closing were partially impaired in these cpk3cpk6 mutant alleles. However, rapid-type anion channel current activity was not affected, consistent with the partial stomatal closing response in double mutants via a proposed branched signaling network. Imposed Ca2+ oscillation experiments revealed that Ca2+-reactive stomatal closure was reduced in CDPK double mutant plants. However, long-lasting Ca2+-programmed stomatal closure was not impaired, providing genetic evidence for a functional separation of these two modes of Ca2+-induced stomatal closing. Our findings show important functions of the CPK6 and CPK3 CDPKs in guard cell ion channel regulation and provide genetic evidence for calcium sensors that transduce stomatal ABA signaling.
Highlights
Stomatal pores in the epidermis of aerial parts of plants facilitate gas exchange between plants and the atmosphere
If so, where calcium-dependent protein kinase (CDPK) may function in ion channel regulation and guard cell signal transduction branches, we first identified CDPK genes expressed in guard cells using a guard cell–enriched cDNA library and RT-PCR with degenerate oligomers [55]
Guard cell preparations were further examined for contamination of mesophyll cells by analyzing mRNA abundance of a putative calmodulin-binding protein (CBP) (AGI Arabidopsis gene identifier number (No).: At4g33050), which was identified as being highly expressed in mesophyll cells but absent in guard cells [54]
Summary
Stomatal pores in the epidermis of aerial parts of plants facilitate gas exchange between plants and the atmosphere. Stomatal pores are surrounded by pairs of guard cells that mediate stomatal pore opening and closing. Stomatal movements are mediated by ion transport across the plasma membrane and vacuolar membrane of guard cells and by organic solute content changes [1,4]. Guard cell ion channels and proton pumps are regulated by the cytosolic free Ca2þ concentration ([Ca2þ]cyt) such that [Ca2þ]cyt elevation activates stomatal closing mechanisms [5,6,7,8,9]. These findings correlate with the Ca2þ dependence of ABA-induced stomatal closing [10,11]
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