Arabidopsis Raf‐like kinases act as positive regulators of subclass III SnRK2 in osmostress signaling

Abstract

SUMMARY Given their sessile nature, land plants must use various mechanisms to manage dehydration under water‐deficit conditions. Osmostress‐induced activation of the SNF1‐related protein kinase 2 (SnRK2) family elicits physiological responses such as stomatal closure to protect plants during drought conditions. With the plant hormone ABA receptors [PYR (pyrabactin resistance)/PYL (pyrabactin resistance‐like)/RCAR (regulatory component of ABA receptors) proteins] and group A protein phosphatases, subclass III SnRK2 also constitutes a core signaling module for ABA, and osmostress triggers ABA accumulation. How SnRK2 is activated through ABA has been clarified, although its activation through osmostress remains unclear. Here, we show that Arabidopsis ABA and abiotic stress‐responsive Raf‐like kinases (AtARKs) of the B3 clade of the mitogen‐activated kinase kinase kinase (MAPKKK) family are crucial in SnRK2‐mediated osmostress responses. Disruption of AtARKs in Arabidopsis results in increased water loss from detached leaves because of impaired stomatal closure in response to osmostress. Our findings obtained in vitro and in planta have shown that AtARKs interact physically with SRK2E, a core factor for stomatal closure in response to drought. Furthermore, we show that AtARK phosphorylates S171 and S175 in the activation loop of SRK2E in vitro and that Atark mutants have defects in osmostress‐induced subclass III SnRK2 activity. Our findings identify a specific type of B3‐MAPKKKs as upstream kinases of subclass III SnRK2 in Arabidopsis. Taken together with earlier reports that ARK is an upstream kinase of SnRK2 in moss, an existing member of a basal land plant lineage, we propose that ARK/SnRK2 module is evolutionarily conserved across 400 million years of land plant evolution for conferring protection against drought.

Evolution of Abscisic Acid Signaling for Stress Responses to Toxic Metals and Metalloids

“…ABA also enables the activation of guard cell membrane-localized transporters through phosphorylation mediated by calcium dependent kinases (CDPKs) and other kinases, indicating alternative stomatal regulatory pathways independent of SnRK2s (Cai et al, 2017;Pornsiriwong et al, 2017). Most recently, subgroup B Raf-like kinases have been identified as upstream regulators of SnRK2s for ABA signal transduction and response to osmotic and drought stresses in Arabidopsis (Saruhashi et al, 2015;Katsuta et al, 2020;Lin et al, 2020;Soma et al, 2020;Takahashi et al, 2020). Furthermore, it's revealed that this regulatory module is evolutionarily conserved in land plants, at least for conferring protection against drought (Katsuta et al, 2020).…”

“…Most recently, subgroup B Raf-like kinases have been identified as upstream regulators of SnRK2s for ABA signal transduction and response to osmotic and drought stresses in Arabidopsis (Saruhashi et al, 2015;Katsuta et al, 2020;Lin et al, 2020;Soma et al, 2020;Takahashi et al, 2020). Furthermore, it's revealed that this regulatory module is evolutionarily conserved in land plants, at least for conferring protection against drought (Katsuta et al, 2020). The other biological regulations induced by ABA are mostly implemented through transcriptional processes mediated by ABA Insensitive 3/4 (ABI3/4) and 9 ABA-response element (ABRE) binding factors (ABFs) (Chen et al, 2020).…”

Decoding ABA and osmostress signalling in plants from an evolutionary point of view

“…The three Arabidopsis B3-MAPKKKs, At1G18160, At1G73660 and At4G24480, complement the function of P. patens ARK when ectopically expressed in P. patens ark mutants; therefore, we have designated these genes as AtARK1, AtARK2 and AtARK3, respectively (Katsuta et al, 2020). Furthermore, an Arabidopsis subclass III SnRK2 could not be activated in the ark mutant of P. patens despite com- The B2, B3 and B4 Raf-like kinases are characterized by quick activation in response to osmostress.…”

“…This model of ABA signalling can be reconstituted in vitro (Brandt et al, 2012;. A recent update to the model is the addition of B group Raf-like (B-Raf) kinases as activators of SnRK2 (Katsuta et al, 2020;Lin et al, 2020;Saruhashi et al, 2015;Soma, Takahashi, Suzuki, Shinozaki, & Yamaguchi-Shinozaki, 2020;Takahashi et al, 2020) (Figure 1).…”

Mediator Complex: A Pivotal Regulator of ABA Signaling Pathway and Abiotic Stress Response in Plants

“…The core ABA signaling pathway has been reconstituted successfully with those key components in vitro [ 35 ]. Interestingly, several recent studies simultaneously showed that Raf-like kinases (RAFs) could quickly activate SnRK2s to respond to ABA, osmotic and drought stress by direct phosphorylation [ 36 , 37 , 38 , 39 ].…”