Crop derived food security is threatened by severe weather events such as flooding, the frequency of which has increased with climate change, costing the agricultural industry billions of USD per year in lost yield. Flood related deterioration is largely the result of reduced oxygen availability (hypoxia), which impairs normal metabolism, causing crop damage and eventual death.
Submergence-induced hypoxia in plants results in stabilisation of group VII ETHYLENE RESPONSE FACTORS (ERF-VIIs), which aid survival under these adverse conditions. ERF-VII stability is controlled by the N-end rule pathway, which proposes that ERF-VII N-terminal cysteine oxidation in normoxia enables their sequential arginylation, polyubiquitination and proteasomal degradation. We have shown that this oxidation is catalysed Fe2+ and, crucially, O2-dependent enzymes called the PLANT CYSTEINE OXIDASEs (PCOs). ERF-VII stabilisation in hypoxia presumably arises from reduced PCO activity, suggesting that they have an O2 sensing capability. Accordingly, the PCOs may be viable intervention targets to effectively and sensitively stabilise N-end rule substrates, including the ERF-VIIs, to enhance flood tolerance in agriculture.
We have investigated the kinetic behaviour of all five Arabidopsis thaliana PCO homologs to ascertain their biochemical ability to act in an O2-sesnitive manner, helping to establish whether they constitute a multifaceted regulatory system with divergent biological roles. We find that the calculated PCO dissociation constants endorse a potential O2 sensing function and that they are susceptible to decreases in pH. There is also a strong correlation between PCO expression behaviour, kinetic properties and ERF-VII substrate preferences, which supports the conclusion that they have different responsibilities during the hypoxic response, allowing individual enzymes to be targeted. The crystal structures of the two most catalytically efficient PCO homologs have been solved, identifying important active site residues, which could be rationally altered to modify enzyme activity should modulation enhance crop survival under low oxygen (i.e. flood) conditions.