Collectively, effectors aid infection by suppressing plant immunity and creating environments favorable for colonization and proliferation 8, 9. Secreted proteins of pathogens, called effectors, play an essential role in bacterial pathogenesis.
In the US, Florida has lost over $7 billion in total industry output due to HLB since it was first detected in 2005 till 2014 6, 7. Infected trees exhibit leaf mottling, deformed/discolored fruits, premature fruit drop, and premature mortality 2.
CLas is transmitted to citrus by the Asian citrus psyllid (ACP) during sap feeding, where it then colonizes the phloem sieve elements, eventually leading to disease symptoms. In the major citrus-growing areas including the US and Asia, the presumed causal agent of HLB is a Gram-negative bacterium, ‘Candidatus Liberibacter asiaticus’ ( CLas). Huanglongbing (HLB), or citrus greening disease, is currently considered the most destructive disease of citrus worldwide 1, 2, 3, 4, 5. These data demonstrate that SDE1 inhibits citrus PLCPs, which are immune-related proteases that enhance defense responses in plants. SDE1-expressing transgenic citrus also exhibit reduced PLCP activity.
We analyzed PLCP activity in field samples, revealing specific members that increase in abundance but remain unchanged in activity during infection. PLCPs are defense-inducible and exhibit increased protein accumulation in CLas-infected trees, suggesting a role in citrus defense responses. We show that SDE1 directly interacts with citrus papain-like cysteine proteases (PLCPs) and inhibits protease activity. Here, we use Sec-delivered effector 1 (SDE1), which is conserved in all CLas isolates, as a molecular probe to understand CLas virulence. Insight into HLB pathogenesis is urgently needed in order to develop effective management strategies. All cultivars can be affected by the HLB-associated bacterium ‘Candidatus Liberibacter asiaticus’ ( CLas) and there is no known resistance. The citrus industry is facing an unprecedented challenge from Huanglongbing (HLB).
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