Arabidopsis glycogen synthase kinase 3 (GSK3)-like kinases have versatile functions in plant development and in responding to abiotic stresses. Although physiological evidence suggested a potential role of GSK3-like kinases in abscisic acid (ABA) signaling, the underlying molecular mechanism was largely unknown. Here we identified members of Snf1-related kinase 2s (SnRK2s), SnRK2.2 and SnRK2.3, that can interact with and be phosphorylated by a GSK3-like kinase, brassinosteroid insensitive 2 (BIN2). bin2-3 bil1 bil2, a loss-of-function mutant of BIN2 and its two closest homologs, BIN2 like 1 (BIL1) and BIN2 like 2 (BIL2), was hyposensitive to ABA in primary root inhibition, ABA-responsive gene expression, and phosphorylating ABA Response Element Binding Factor (ABF) 2 fragment by in-gel kinase assays, whereas bin2-1, a gain-of-function mutation of BIN2, was hypersensitive to ABA, suggesting that these GSK3-like kinases function as positive regulators in ABA signaling. Furthermore, BIN2 phosphorylated SnRK2.3 on T180, and SnRK2.3^T180A had decreased kinase activity in both autophosphorylation and phosphorylating ABFs. Bikinin, a GSK3 kinase inhibitor, inhibited the SnRK2.3 kinase activity and its T180 phosphorylation in vivo. Our genetic analysis further demonstrated that BIN2 regulates ABA signaling downstream of the PYRABACTIN RESISTANCE1/PYR1-LIKE/REGULATORY COMPONENTS OF ABA RECEPTORS receptors and clade A protein phosphatase 2C but relies on SnRK2.2 and SnRK2.3. These findings provide significant insight into the modulation of ABA signaling by Arabidopsis GSK3-like kinases. Significance Abscisic acid (ABA) is a key stress-responsive hormone. Subgroup III Snf1-related kinase 2s (SnRK2s) are crucial positive regulators in ABA signaling pathway, but it is still not clear how SnRK2s are activated. In addition, besides ABA, some abiotic stresses can also activate SnRK2s with unknown mechanisms. Here we provide several lines of evidence to strongly support that brassinosteroid insensitive 2, a glycogen synthase kinase 3 (GSK3)-like kinase, interacts with and phosphorylates SnRK2s on specific novel sites to activate SnRK2s, which provides significant insights into the function of GSK3-like kinases in ABA signaling and transactivation of SnRK2s. Moreover, many GSK3-like kinases are regulated at transcriptional and/or posttranslational levels by many abiotic stresses, implying the presence of direct regulation of ABA signaling by other abiotic stresses.

The RLA1/SMOS1 transcription factor acts as an integrator of the transcriptional complex directly downstream of the kinase GSK2 and plays an essential role in B

Nitrogen fixation in legume roots: Soybeans and other legumes form symbiotic relationships with nitrogen-fixing bacteria in root nodules, thus ensuring the plant a ready supply of the necessary nutrient. Wang et al . worked out the signaling pathways that integrate above-ground light with below-ground root nodulation. Two mobile regulators move from shoot to root in response to light and, together, they form a signaling module in the root that induces the expression of nodulation factors. Thus, when the shoot sees enough light to support photosynthetic productivity, the root gets the signal to ramp up nitrogen fixation. —PJH