The interface between metabolic and stress signalling

Abstract

Background: It is becoming increasingly clear that stress and metabolic signalling networks interact and that this interaction is important in plant responses to herbivory, pathogen attack, drought, cold, heat and osmotic stresses including salinity. At the interface between these two major signalling systems are the hormone abscisic acid (ABA) and signalling factors including protein kinases and transcription factors.

Scope: This briefing reviews links between ABA, stress and sugar signalling, focusing on the roles of sucrose non-fermenting-1-related protein kinases (SnRKs), SnRK1-activating protein kinases (SnAKs), calcium-dependent protein kinases (CDPKs) and ABA response element binding proteins (AREBPs, which are transcription factors). Links between stress and nitrogen / amino acid signalling are also described, including the roles of a protein kinase called general control non-derepressible (GCN)-2 in regulating protein synthesis through phosphorylation of the alpha-subunit of translation initiation factor-2 (eIF2alpha) in response not only to decreases in amino acid levels but also to a range of stresses. Evidence of a link between sugar and amino acid signalling is explored, with nitrate reductase being a target for regulation by both SnRK1 and GCN2 through different mechanisms; possible links between SnRK1 and GCN2 via a pathway including the protein kinase target of rapamycin (TOR)-1 are described. The significance of these interactions to the concept of signalling networks as opposed to simple cascades and pathways, and the importance of the subject in the context of the predicted increase in severity and range of stresses that plants will have to withstand as a result of global climate change are discussed.

Fig. 1

ABA response element binding proteins (AREBPs) as hubs at the interface between metabolic and stress signalling networks, potentially targeted for phosphorylation by all three SNF1-related protein kinases (SnRK1 to 3) and with multiple roles in stress responses, germination and development.

Fig. 2

Interactions between carbon and nitrogen metabolic signalling networks. The link between SnRK1 and GCN2 and the effect of nitrogen availability on the GCN2 signalling pathway are hypothetical and therefore represented by dashed arrows. In animal and fungal systems the sugar signalling → GCN2 link comprises several factors, probably including the protein kinase TOR1.