BRI1-Associated Receptor Kinase 1 Regulates Guard Cell ABA Signaling Mediated by Open Stomata 1 in Arabidopsis

Abstract

Stomatal movements are critical in regulating gas exchange for photosynthesis and water balance between plant tissues and the atmosphere. The plant hormone abscisic acid (ABA) plays key roles in regulating stomatal closure under various abiotic stresses. In this study, we revealed a novel role of BAK1 in guard cell ABA signaling. We found that the brassinosteroid (BR) signaling mutant bak1 lost more water than wild-type plants and showed ABA insensitivity in stomatal closure. ABA-induced OST1 expression and reactive oxygen species (ROS) production were also impaired in bak1. Unlike direct treatment with H2O2, overexpression of OST1 did not completely rescue the insensitivity of bak1 to ABA. We demonstrated that BAK1 forms a complex with OST1 near the plasma membrane and that the BAK1/OST1 complex is increased in response to ABA in planta. Brassinolide, the most active BR, exerted a negative effect on ABA-induced formation of the BAK1/OST1 complex and OST1 expression. Moreover, we found that BAK1 and ABI1 oppositely regulate OST1 phosphorylation in vitro, and that ABI1 interacts with BAK1 and inhibits the interaction of BAK1 and OST1. Taken together, our results suggest that BAK1 regulates ABA-induced stomatal closure in guard cells.

Reactive Oxygen Species in the Regulation of Stomatal Movements

“…First, ABI1 has been implicated in dephosphorylation of Ser-175 (Vlad et al, 2009), which is autophosphorylated in OST1 activation process. Second, ABI1 dephosphorylated BAK1-mediated transphosphorylation sites of OST1 (Shang et al, 2016). Taken together, this suggests that OST1 and BAK1 are positive regulators of ABAinduced ROS production and ABI1-induced inhibition occurs at the level of OST1.…”

“…A role for BAK1 in ABA-induced stomatal closure was demonstrated by Shang et al (2016). ABA-induced ROS production and stomatal closure was absent in bak1, while application of H 2 O 2 rescued the phenotype, suggesting that BAK1 functions upstream of ROS production in stomatal closure.…”

“…OST1 is required for ABA-triggered ROS production in guard cells, indicating that it acts upstream of RBOHs (Mustilli et al, 2002;Shang et al, 2016). AtRBOHF is the main isoform involved in ABA-related stomatal responses (Kwak et al, 2003).…”

Aquaporins facilitate hydrogen peroxide entry into guard cells to mediate ABA- and pathogen-triggered stomatal closure

“…In agreement with earlier studies proposing a role for OST1 in guard cell responses to flg22, including activation of SLAC1 (12,20), the protein kinase was also required for flg22-dependent activation of AtPIP2;1. Knowing that OST1 is activated by BAK1 during guard cell response to ABA (41), it may be regulated in a similar way in response to flg22. This model fits with the idea that BAK1 acts as a relay between the flg22 receptor FLS2 and downstream components.…”

Bound by Fate: The Role of Reactive Oxygen Species in Receptor-Like Kinase Signaling

“…Interestingly, BAK1, a coreceptor for many RLKs, activates OST1 in response to ABA, but this activation is inhibited by ABI1 and by treatment with BR (Shang et al, 2016). However, the identity of RLKs associated with the BAK1-OST1 complex is currently unknown.…”