Stress associated proteins (SAPs), novel A20/AN1 zinc-finger domain-containing proteins, are fast emerging as potential candidates for biotechnological approaches in order to improve abiotic stress tolerance in plants - the ultimate aim of which is crop-yield protection. Until relatively recently, such proteins had only been identified in humans, where they had been shown to be key regulators of innate immunity. Their phylogenetic relationship and recruitment of diverse protein domains reflect an architectural and mechanistic diversity. Emerging evidence suggests that SAPs may act as ubiquitin ligase, redox sensor, and regulator of gene expression during stress. Here, we evaluate the new knowledge on SAPs with a view to understand their mechanism of action. Furthermore, we set an agenda for investigating hitherto unexplored roles of these proteins.

The amino-terminal residue of a protein (or amino-terminus of a peptide following protease cleavage) can be an important determinant of its stability, through the Ubiquitin Proteasome System associated N-degron pathways. Plants contain a unique combination of N-degron pathways (previously called the N-end rule pathways) E3 ligases, PROTEOLYSIS (PRT)6 and PRT1, recognizing non-overlapping sets of amino-terminal residues, and others remain to be identified. Although only very few substrates of PRT1 or PRT6 have been identified, substrates of the oxygen and nitric oxide sensing branch of the PRT6 N-degron pathway include key nuclear-located transcription factors (ETHYLENE RESPONSE FACTOR VIIs and LITTLE ZIPPER 2) and the histone-modifying Polycomb Repressive Complex 2 component VERNALIZATION 2. In response to reduced oxygen or nitric oxide levels (and other mechanisms that reduce pathway activity) these stabilized substrates regulate diverse aspects of growth and development, including response to flooding, salinity, vernalization (cold-induced flowering) and shoot apical meristem function. The N-degron pathways show great promise for use in the improvement of crop performance and for biotechnological applications. Upstream proteases, components of the different pathways and associated substrates still remain to be identified and characterized to fully appreciate how regulation of protein stability through the amino-terminal residue impacts plant biology.

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