Abscisic acid‐activated SNRK2 protein kinases function in the gene‐regulation pathway of ABA signal transduction by phosphorylating ABA response element‐binding factors

Abstract

SummaryThe plant hormone abscisic acid (ABA) induces gene expression via the ABA-response element (ABRE) present in the promoters of ABA-regulated genes. A group of bZIP proteins have been identified as ABRE-binding factors (ABFs) that activate transcription through this cis element. A rice ABF, TRAB1, has been shown to be activated via ABA-dependent phosphorylation. While a large number of signalling factors have been identified that are involved in stomatal regulation by ABA, relatively less is known about the ABA-signalling pathway that leads to gene expression. We have shown recently that three members of the rice SnRK2 protein kinase family, SAPK8, SAPK9 and SAPK10, are activated by ABA signal as well as by hyperosmotic stress. Here we show that transient overexpression in cultured cell protoplasts of these ABA-activated SnRK2 protein kinases leads to the activation of an ABRE-regulated promoter, suggesting that these kinases are involved in the generegulation pathway of ABA signalling. We further show several lines of evidence that these ABA-activated SnRK2 protein kinases directly phosphorylate TRAB1 in response to ABA. Kinetic analysis of SAPK10 activation and TRAB1 phosphorylation indicated that the latter immediately followed the former. TRAB1 was found to be phosphorylated not only in response to ABA, but also in response to hyperosmotic stress, which was interpreted as the consequence of phosphorylation of TRAB1 by hyperosmotically activated SAPKs. Physical interaction between TRAB1 and SAPK10 in vivo was demonstrated by a co-immunoprecipitation experiment. Finally, TRAB1 was phosphorylated in vitro by the ABA-activated SnRK2 protein kinases at Ser102, which is phosphorylated in vivo in response to ABA and is critical for the activation function.

SUCROSE NONFERMENTING1-RELATED PROTEIN KINASE2.6, an Ortholog of OPEN STOMATA1, Is a Negative Regulator of Strawberry Fruit Development and Ripening

“…Given that FaSnRK2.6 is a negative regulator of strawberry fruit development and ripening, whereas OST1 is a positive regulator of stomatal movement, FaSnRK2.6 must operate via distinct mechanisms in these biological processes. Although SnRK2 protein kinases have been suggested to be downstream signal transducers of ABA (Kobayashi et al, 2005;Wang et al, 2013b) and although we found that SnRK2.6 interacts physically with FaABI1 (Fig. 6), the FaSnRK2.6-mediated regulation of strawberry fruit development and ripening likely does not Figure 9.…”

“…For example, Kobayashi et al (2005) identified 10 SnRK2 protein kinases encoded in rice (Oryza sativa), and all of these SnRK2s were specifically activated by osmotic stress at both the transcriptional and the posttranscriptional levels. The involvement of the stress signal, FaSnRK2.6, in the regulation of fruit development and ripening may reasonably be explained from a plant ecological perspective.…”

“…The SnRK2 family has been suggested to have evolved specifically for hyperosmotic stress signaling (Boudsocq et al, 2004;Umezawa et al, 2004;Kobayashi et al, 2005;Fujii et al, 2007;Shukla and Mattoo, 2008;Fujita et al, 2009). For example, Kobayashi et al (2005) identified 10 SnRK2 protein kinases encoded in rice (Oryza sativa), and all of these SnRK2s were specifically activated by osmotic stress at both the transcriptional and the posttranscriptional levels.…”

“…fleshy fruit arose as a mechanism to facilitate seed dispersal and promote the survival of the plant in everchanging environments (Coombe, 1976), the regulation of fruit ripening by external cues is of great importance in the plant's adaptation to environmental stresses (Shulaev et al, 2008). Given that OST1 is a critical mediator of ABA signaling, and that SnRK2s are tightly associated with plant stress signaling (Yoshida et al, 2002;Boudsocq et al, 2004;Umezawa et al, 2004;Kobayashi et al, 2005;Fujii et al, 2007;Shukla and Mattoo, 2008;Fujita et al, 2009;Hirayama et al, 2010;Kulik et al, 2011;Hrabak et al, 2013), we sought to identify the ortholog of OST1 in strawberry and to evaluate whether this protein contributes to the regulation of strawberry fruit development and ripening, and particularly whether it modulates fruit ripening in response to temperature.…”

“…SUCROSE NONFERMENTING1-RELATED PROTEIN KINASES2 (SnRK2s) are a family of plant-specific protein kinases implicated in stress and abscisic acid (ABA) signaling (Yoshida et al, 2002;Boudsocq et al, 2004;Kobayashi et al, 2005;Umezawa et al, 2004;Fujii et al, 2007;Shukla and Mattoo, 2008;Fujita et al, 2009;Hirayama et al, 2010;Kulik et al, Figure 1. Phylogenetic analysis of SnRK2.6 proteins.…”