Background
The core ABA signaling components functioning in stomatal closure/opening, namely ABA receptors, phosphatases, SnRK2s and SLAC1, are well characterized in Arabidopsis, but their functions in guard cells of rice have not been extensively studied.
Results
In this study, we confirmed that
OsSLAC1
, the rice homolog of
AtSLAC1
, is specifically expressed in rice guard cells. Among the rice
SAPKs
,
SAPK10
was specifically expressed in guard cells. In addition, SAPK10 phosphorylated OsSLAC1 in vitro and transgenic rice overexpressing
SAPK10
or
OsSLAC1
showed significantly less water loss than control. Thus, those might be major positive signaling components to close stomata in rice. We identified that only OsPP2C50 and OsPP2C53 among 9 OsPP2CAs might be related with stomatal closure/opening signaling based on guard cell specific expression and subcellular localization. Transgenic rice overexpressing
OsPP2C50
and
OsPP2C53
showed significantly higher water loss than control. We also characterized the interaction networks between OsPP2C50 and OsPP2C53, SAPK10 and OsSLAC1 and found two interaction pathways among those signaling components: a hierarchical interaction pathway that consisted of OsPP2C50 and OsPP2C53, SAPK10 and OsSLAC1; and a branched interaction pathway wherein OsPP2C50 and OsPP2C53 interacted directly with OsSLAC1.
Conclusion
OsPP2C50 and OsPP2C53 is major negative regulators of ABA signaling regarding stomata closing in rice. Those can regulate the OsSLAC1 directly or indirectly thorough SAPK10.
Electronic supplementary material
The online version of this article (10.1186/s12284-019-0297-7) contains supplementary material, which is available to authorized users.
The abscisic acid (ABA) signaling pathway is regulated by clade A type 2C protein phosphatases (PP2CAs) in plants. In the presence of ABA, PP2Cs release stress/ABA-activated protein kinases by binding to ABA-bound receptors (PYL/RCARs) for activation. Although the wedging tryptophan in PP2Cs is critical in the interaction with PYL/RCARs in Arabidopsis and rice, it remains elusive as to how other interface regions are involved in the interaction. Here, we report the identification of a conserved region on PP2Cs, termed the VxGΦL motif, which modulates the interaction with PYL/RCARs through its second and fourth residues. The effects of the second and fourth residues on the interaction of OsPP2C50 with several OsPYL/RCAR proteins were investigated by systematic mutagenesis. One OsPP2C50 mutant, VFGML ("FM") mutant, lowered the affinity to OsPYL/RCAR3 by ∼15-fold in comparison with the wild-type. Comparison of the crystal structures of wild-type OsPP2C50:ABA:OsPYL/RCAR3 with those composed of FM mutant revealed local conformational changes near the VxGΦL motif, further supported by hydrogen-deuterium exchange mass spectrometry. In rice protoplasts, ABA signaling was altered by mutations in the VxGΦL motif. Transgenic Arabidopsis plants overexpressing OsPP2C50 and OsPP2C50FM showed altered ABA sensitivity. Taken together, the VxGΦL motif of PP2Cs appears to modulate the affinity of PP2Cs with PYL/RCARs and thus likely to alter the ABA signaling, leading to the differential sensitivity to ABA in planta.
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