UC San Diego

To ensure survival, plants are equipped to adjust and respond to many environmental conditions. Plants have specialized pairs of epidermal cells, guard cells, which are centers of control, adaptation and immediate action in response to abiotic stresses. These guard cells are constantly maintaining internal homeostasis in response to shifts in light, salt stress, ozone, C0₂ concentrations, humidity, soil saturation, temperature, and pathogens. The opening and closing of a myriad of macroscopic pores (stomata) is a tightly controlled mechanism mediated by guard cells. ABA is an endogenous hormone responsible for signaling responses to many of these environmental conditions. The three major roles of ABA are minimizing water loss, inhibiting seed germination, and regulating root growth during drought conditions. Known as a drought hormone, ABA accumulation in guard cells causes stomatal pore closure, through changes in cell water potentials. The guard cell ABA signaling pathway has become a model for its role in plant calcium signaling and for its regulation of agriculturally-relevant abiotic stresses. Calcium-dependent-protein kinases (CPK's) are a plant- specific family of kinases that are important sensors of calcium and are involved in ABA signaling. In guard cells, they transduce these upstream signals into anion channel activity that control stomatal aperture. However, many aspects of the role of CPKs in guard cell ABA-signaling are still unknown. Loss-of-function CPK mutants have shown varying degrees of impairment in ABA- and Ca²⁺-induced anion currents and stomatal closure, raising the question whether this is due to functional redundancy or due to the involvement of other Ca²⁺-sensing proteins. A quadruple mutant, cpk5/6/11/23 was found to have strongly abrogated anion currents. ABA-induced stomatal bioassays were conducted to test the effect that this impairment has on gross stomatal movements. In addition to cpk5/6/11/23, a cpk3/4/5/6/11 quintuple mutant was established and phenotypically characterized for its drought responses