ABA Signaling in Guard Cells Entails a Dynamic Protein-Protein Interaction Relay from the PYL-RCAR Family Receptors to Ion Channels

Plant hormone abscisic acid （ABA） serves as an integrator of environmental stresses such as drought to trig-ger stomatal closure by regulating specific ion channels in guard cells. We previously reported that SLAC1, an outward anion channel required for stomatal closure, was regulated via reversible protein phosphorylation events involving ABA signaling components, including protein phosphatase 2C members and a SnRK2-type kinase （OST1）. In this study, we reconstituted the ABA signaling pathway as a protein-protein interaction relay from the PYL/RCAR-type receptors, to the PP2C-SnRK2 phosphatase-kinase pairs, to the ion channel SLAC1. The ABA receptors interacted with and inhibited PP2C phosphatase activity against the SnRK2-type kinase, releasing active SnRK2 kinase to phosphorylate, and activate the SLAC1 channel, leading to reduced guard cell turgor and stomatal closure. Both yeast two-hybrid and bimolecular fluorescence complementation assays were used to verify the interactions among the components in the pathway. These biochemical assays demonstrated activity modifications of phosphatases and kinases by their interaction partners. The SLAC1 channel activity was used as an endpoint readout for the strength of the signaling pathway, depending on the presence of different combinations of signaling components. Further study using transgenic plants overexpressing one of the ABA receptors demonstrated that changing the relative level of interacting partners would change ABA sensitivity.

Loss-of-function mutation of rice SLAC7 decreases chloroplast stability and induces a photoprotection mechanism in rice

Plants absorb sunlight to power the photochem- ical reactions of photosynthesis, which can potentially damage the photosynthetic machinery. However, the mech- anism that protects chloroplasts from the damage remains unclear. In this work, we demonstrated that rice （Oryza sativa L.） SLAC7 is a generally expressed membrane protein. Loss- of-function of SLAC7 caused continuous damage to the chloroplasts of mutant leaves under normal light conditions. Ion leakage indicators related to leaf damage such as H^O2 and abscisic acid levels were significantly higher in slac7-1 than in the wild type. Consistently, the photosynthesis efficiency and Fv/Fm ratio of slac7-1 were significantly decreased （similar to photoinhibition）. In response to chloroplast damage, slat7- 1 altered its leaf morphology （curled or fused leaf） by the synergy between plant hormones and transcriptional factors to decrease the absorption of light, suggesting that a photoprotection mechanism for chloroplast damage was activated in slac7-1. When grown in dark conditions, slac7-1 displayed a normal phenotype. 5LAC7 under the control of the AtSLAC1 promoter could partially complement thephenotypes of Arabidopsis slacl mutants, indicating a partial conservation of SLAC protein functions. These results suggest that SLAC7 is essential for maintaining the chloroplast stability in rice.