Papain Inhibitor Contributes to Phytophthora palmivora Infection in Papaya

Papaya trees are rich in papain, a cysteine protease that has been shown to mediate plant defense against pathogens and insects. However, the pathogen Phytophthora palmivora infects all parts of papaya plants, suggesting that it has evolved cysteine protease inhibitors to counter papain and enable infection.

Out of several cystatin-like cysteine protease inhibitors from P. palmivora, PpalEPIC8 appeared to be unique to P. palmivora and was highly induced during infection of papaya. Analysis revealed that PpalEPIC8 strongly inhibited papain enzyme activity, suggesting that it is a cysteine protease inhibitor. Phytophthora palmivora mutants with knocked-out PpalEPIC8 were the generated using CRISPR-Cas9-mediated gene editing.

The mutant strains exhibited increased papain sensitivity in vitro and had reduced pathogenicity during infection of papaya fruits compared to the wild-type strain. This strongly suggests that PpalEPIC8 indeed plays a role in P. palmivora virulence by inhibiting papain.

This study provides evidence of pathogens secreting cystatins to invade plants. It also established a gene-editing system for P. palmivora using CRISPR-Cas9.

For more on this study, read the article in Molecular Plant-Microbe Interactions.