Abstract
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.
Highlights
The citrus industry is facing an unprecedented challenge from Huanglongbing (HLB)
It is found in all sequenced Candidatus Liberibacter asiaticus’ (CLas) isolates from various geographic regions and its expression was detected from CLas-infected citrus varieties including limes, sweet oranges, and grapefruits[15,16]
The mechanisms underlying papain-like cysteine proteases (PLCPs)-mediated defense could work on multiple levels
Summary
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. PLCPs are defense-inducible and exhibit increased protein accumulation in CLas-infected trees, suggesting a role in citrus defense responses. SDE1-expressing transgenic citrus exhibit reduced PLCP activity. These data demonstrate that SDE1 inhibits citrus PLCPs, which are immune-related proteases that enhance defense responses in plants. Insecttransmitted bacteria, such as CLas, often lack this specialized delivery machinery, but can utilize the general Sec secretion system to release effectors[11]. 86 proteins were confirmed to possess a functional Sec-secretion signal, indicating that they could potentially be released by CLas into the phloem during infection[15]. The expression of SDE1 is ~10-fold higher in citrus than in ACP16, indicating a role in CLas colonization of plant hosts. This research advances our understanding of HLB pathogenesis by identifying citrus targets of a conserved CLas effector, which could be exploited for HLB management
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