Abstract Sucrose nonfermenting-1 (SNF1)-related protein kinases (SnRKs) form a major family of signalling proteins in plants and have been associated with metabolic regulation and stress responses. They comprise three subfamilies: SnRK1, SnRK2, and SnRK3. SnRK1 plays a major role in the regulation of carbon metabolism and energy status, while SnRKs 2 and 3 have been implicated in stress and abscisic acid (ABA)-mediated signalling pathways. The burgeoning and divergence of this family of protein kinases in plants may have occurred to enable cross-talk between metabolic and stress signalling, and ABA-response-element-binding proteins (AREBPs), a family of transcription factors, have been shown to be substrates for members of all three subfamilies. In this study, levels of SnRK1 protein were shown to decline dramatically in wheat roots in response to ABA treatment, although the amount of phosphorylated (active) SnRK1 remained constant. Multiple SnRK2-type protein kinases were detectable in the root extracts and showed differential responses to ABA treatment.
They included a 42 kDa protein that appeared to reduce in response to 3 h of ABA treatment but to recover after longer treatment. There was a clear increase in phosphorylation of this SnRK2 in response to the ABA treatment. Fractions containing this 42 kDa SnRK2 were shown to phosphorylate synthetic peptides with amino acid sequences based on those of conserved phosphorylation sites in AREBPs. The activity increased 8-fold with the addition of calcium chloride, indicating that it is calcium-dependent. The activity assigned to the 42 kDa SnRK2 also phosphorylated a heterologously expressed wheat AREBP. ,,, Introduction Sucrose nonfermenting-1 (SNF1)-related protein kinases (SnRKs) form a major family of signalling proteins in plants and have been associated with metabolic regulation and stress responses (; ).
SnRKs have been grouped into three subfamilies: SnRK1, SnRK2, and SnRK3 (). SnRK1, the homologue of adenosine monophosphate-activated protein kinase (AMPK) from mammals and SNF1 from yeast, has been implicated in the regulation of carbon metabolism and energy status (; ). It controls metabolism at multiple levels; for example, it phosphorylates enzymes such as 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) and sucrose phosphate synthase, leading to their inactivation. It also phosphorylates nitrate reductase, trehalose-phosphate synthase, and 6-phosphofructo-2-kinase/fructose-2,6- bisphosphatase, but these enzymes also require the binding of a 14-3-3 protein for inactivation (reviewed by ).
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Another key metabolic enzyme in plants, adenosine diphosphate (ADP)-glucose pyrophosphorylase, is regulated by SnRK1 through modulation of its redox state (). In addition, SnRK1 causes changes in gene expression in response to nutrient starvation () and, paradoxically, sucrose (; ). The other two SnRK subfamilies, SnRK2 and SnRK3, do not have any counterpart in fungal or animal cells (). They both comprise relatively large gene families, with 10 SnRK2 genes described in Arabidopsis ( Arabidopsis thaliana) and rice ( Oryza sativa) (; ) and 25 SnRK3 genes described in Arabidopsis ().
There is now convincing evidence to associate SnRK2 and SnRK3 with responses to abiotic stresses such as drought, salinity, cold, and osmotic stress (). For example, a SnRK3 family member, SOS2 (salt overly sensitive 2), is involved in phosphorylating and activating SOS1, a Na +/H + antiporter, in order to maintain ion homeostasis (; ). A number of SnRK2 family members have been shown to be directly up-regulated and activated by osmotic stress and some but not all of these are also activated by abscisic acid (ABA). The fact that not all osmotic stress-regulated SnRK2s are also activated by ABA indicates that activation of SnRK2 in response to osmotic stress and ABA involves different mechanisms (,; ).
Recent evidence from Arabidopsis shows that these mechanisms involve different patterns of phosphorylation at two serine residues in the activation loop. SnRK2.6, for example, which is an ABA- and osmotic stress-induced kinase, is phosphorylated independently on both of these residues, while for SnRK2.10, which is induced by osmotic stress but not ABA, phosphorylation of one site is required for phosphorylation of the other (). Recent studies have shown SnRK2s to be integral components of ABA signalling. Firstly, protein phosphatases type 2C (PP2C) were shown to be negative regulators of ABA signalling and to be involved in SnRK2 inactivation (; ). Yeast two-hybrid interactions were described between different PP2Cs and SnRK2 () and further work demonstrated that, in the absence of ABA, PP2C inactivated SnRK2 by direct dephosphorylation of one of the serine residues in the activation loop (). The elucidation of ABA signalling mechanisms then advanced significantly with the identification of a family of proteins named PYR/PYL/RCAR as ABA receptors ().