Activation of SnRK2 by Raf-like kinase ARK represents a primary mechanism of ABA and abiotic stress responses

Abstract

The Raf-like protein kinase abscisic acid (ABA) and abiotic stress-responsive Raf-like kinase (ARK) previously identified in the moss Physcomitrium (Physcomitrella) patens acts as an upstream regulator of subgroup III SNF1-related protein kinase2 (SnRK2), the key regulator of ABA and abiotic stress responses. However, the mechanisms underlying activation of ARK by ABA and abiotic stress for the regulation of SnRK2, including the role of ABA receptor-associated group A PP2C (PP2C-A), are not understood. We identified Ser1029 as the phosphorylation site in the activation loop of ARK, which provided a possible mechanism for regulation of its activity. Analysis of transgenic P. patens ark lines expressing ARK-GFP with Ser1029-to-Ala mutation indicated that this replacement causes reductions in ABA-induced gene expression, stress tolerance, and SnRK2 activity. Immunoblot analysis using an anti-phosphopeptide antibody indicated that ABA treatments rapidly stimulate Ser1029 phosphorylation in the wild type (WT). The phosphorylation profile of Ser1029 in ABA-hypersensitive ppabi1 lacking protein phosphatase 2C-A (PP2C-A) was similar to that in the WT, whereas little Ser1029 phosphorylation was observed in ABA-insensitive ark missense mutant lines. Furthermore, newly isolated ppabi1 ark lines showed ABA-insensitive phenotypes similar to those of ark lines. Therefore, ARK is a primary activator of SnRK2, preceding negative regulation by PP2C-A in bryophytes, which provides a prototype mechanism for ABA and abiotic stress responses in plants.

ZmHDT103 Negatively Regulates Drought Stress Tolerance in Maize Seedlings

“…ABA accumulates rapidly upon drought stress, causing stomatal closure to limit transpiration water loss [42]. Additionally, ABA stimulates the expression of a number of stress-responsive genes [43]. In this study, ABA content was higher in ZmHDT103 mutant seedlings compared to that of the respective wild type under drought stress (Figure 3B), and the leaf WLR of maize seedlings in ZmHDT103 mutants was significantly lower than that of WT (Figure 3A), indicating that HDT103 acts upstream of genes controlling ABA content and ABA-mediated stomatal movement.…”

A framework for improving wheat spike development and yield based on the master regulatory TOR and SnRK gene systems

“…SnRK2s stimulate stress resistance responses through complex networks of signal transduction via both ABA-dependent and ABA-independent resistance mechanisms. [ABA is a hormone formed in response to many stresses, especially drought ( Ma et al , 2018 ; Islam et al , 2021 )]. The regulatory linkages between ABA, SnRKs, and downstream activities are multiple and complex, involving ABA receptors and phosphatases.…”

The Multifaceted Regulation of SnRK2 Kinases

“…In Arabidopsis, the RAFs engaged in SnRK2 pathways are activated in response to osmotic stress but rather not in response to ABA [ 60 ]; their basal activity is sufficient for the phosphorylation of the ABA-dependent SnRK2s. RAFs activity relies on phosphorylation within their activation loop [ 55 , 61 ], with some exceptions [ 60 ]. The elements upstream of the RAFs remain unknown.…”