Targeting plant cysteine oxidase activity for improved submergence tolerance

Plant cysteine oxidases (PCOs) are plant O-2-sensing enzymes. They catalyse the O-2-dependent step which initiates the proteasomal degradation of Group VII ethylene response transcription factors (ERF-VIIs) via the N-degron pathway. When submerged, plants experience a reduction in O-2 availability; PCO activity therefore decreases and the consequent ERF-VII stabilisation leads to upregulation of hypoxia-responsive genes which enable adaptation to low O-2 conditions. Resulting adaptations include entering an anaerobic quiescent state to maintain energy reserves and rapid growth to escape floodwater and allow O-2 transport to submerged tissues. Stabilisation of ERF-VIIs has been linked to improved survival post-submergence in Arabidopsis, rice (Oryza sativa) and barley (Hordeum vulgare). Due to climate change and increasing flooding events, there is an interest in manipulating the PCO/ERF-VII interaction as a method of improving yields in flood-intolerant crops. An effective way of achieving this may be through PCO inhibition; however, complete ablation of PCO activity is detrimental to growth and phenotype, likely due to other PCO-mediated roles. Targeting PCOs will therefore require either temporary chemical inhibition or careful engineering of the enzyme structure to manipulate their O-2 sensitivity and/or substrate specificity. Sufficient PCO structural and functional information should make this possible, given the potential to engineer site-directed mutagenesis in vivo using CRISPR-mediated base editing. Here, we discuss the knowledge still required for rational manipulation of PCOs to achieve ERF-VII stabilisation without a yield penalty. We also take inspiration from the biocatalysis field to consider how enzyme engineering could be accelerated as a wider strategy to improve plant stress tolerance and productivity.