An effector from the Huanglongbing-associated pathogen targets citrus proteases.
Clark K., Franco JY., Schwizer S., Pang Z., Hawara E., Liebrand TWH., Pagliaccia D., Zeng L., Gurung FB., Wang P., Shi J., Wang Y., Ancona V., van der Hoorn RAL., Wang N., Coaker G., Ma W.
The citrus industry is facing an unprecedented challenge from Huanglongbing (HLB). All cultivars can be affected by the HLB-associated bacterium 'Candidatus Liberibacter asiaticus' (CLas) and there is no known resistance. Insight into HLB pathogenesis is urgently needed in order to develop effective management strategies. Here, we use Sec-delivered effector 1 (SDE1), which is conserved in all CLas isolates, as a molecular probe to understand CLas virulence. We show that SDE1 directly interacts with citrus papain-like cysteine proteases (PLCPs) and inhibits protease activity. PLCPs are defense-inducible and exhibit increased protein accumulation in CLas-infected trees, suggesting a role in citrus defense responses. We analyzed PLCP activity in field samples, revealing specific members that increase in abundance but remain unchanged in activity during infection. SDE1-expressing transgenic citrus also exhibit reduced PLCP activity. These data demonstrate that SDE1 inhibits citrus PLCPs, which are immune-related proteases that enhance defense responses in plants.