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How did Yejin Shim (@yeahgene.bsky.social) engineer osmotic stress tolerance in rice 🌾? The story starts in 1995 when Guoliang Wang and Wenyuan Song led the discovery of the immune receptor XA21.

The sequence of the predicted XA21 protein, which carries both a leucine-rich repeat motif and a serine-threonine kinase-like domain, suggests a role in cell surface recognition of a pathogen molecule and subsequent activation of an intracellular defense response. escholarship.org/content/qt4x...

What is the molecule recognized by XA21? Rory Pruitt led the identification of a gene, called RaxX (required for activation of XA21-mediated immunity) encoding a small sulfated peptide.

RaxX triggers a robust and effective immune response in rice expressing XA21. A synthetic 21-amino acid sulfated derivative of RaxX is sufficient to activate XA21-mediated immune responses (Pruitt et al., 2015). www.science.org/doi/full/10....

It turns out that bacterial RaxX shows similarity to peptides in the PSY (plant peptide containing sulfated tyrosine) family. nph.onlinelibrary.wiley.com/doi/10.1111/...

A synthetic sulfated RaxX derivative comprising 13 residues, highly conserved between RaxX and PSY, induces root growth in Arabidopsis and rice in a manner similar to that triggered by PSY. A Xanthomonas strain lacking raxX is impaired in virulence.

Bacterial RaxX competes effectively with plant PSY in binding to the PSY receptors, PSYR2 and PSYR3, indicating that RaxX is also perceived by PSYRs in Arabidopsis.

In Arabidopsis, these receptors represses plant growth and activate stress-response genes when they are not repressed by their peptide hormone ligands.https://www.science.org/doi/10.1126/science.abq5735

These studies raised the question: what is the function of PSY in rice? Our new PNAS study shows that rice PSY8 promotes root growth under favorable conditions but is repressed during osmotic stress, allowing rice to shift from growth to stress adaptation.

Yejin made knockouts in rice PSY8 and found that two independent mutants displayed enhanced tolerance to osmotic stress. Survival rates exceeded 60% in both KO1 and KO2, compared with approximately 40% in WT plants

Yejin then showed that OsPSY8 is directly repressed by OsWRKY24, a transcription factor previously shown to positively regulate resistance against Magnaporthe grisea, the causal agent of rice blast disease.

To translate these mechanistic insights into crop improvement strategies, future studies should investigate how modulation of OsPSY8 signaling affects plant performance under soil-grown conditions, including greenhouse and field environments.