Yang, Zuhui

Citrus Huanglongbing (HLB), a devastating disease caused by the Gram-negative bacterium ‘ Candidatus Liberibacter asiaticus’ ( C Las), poses serious threats to global citrus production and lacks effective control strategies. Previously, SDE2470 (CLIBASIA_02470) was identified as a Sec-dependent effector that contributes to C Las pathogenesis, although its underlying molecular mechanisms were not fully elucidated. In this study, SDE2470 was found to target a citrus vascular one-zinc-finger transcription factor CsVOZ2. CsVOZ2 overexpression ( CsVOZ2- OE) in transgenic citrus plants significantly suppressed C Las colonization, whereas its RNA interference (RNAi) in citrus hairy roots enhanced susceptibility to C Las. Additionally, CsVOZ2 -OE significantly increased reactive oxygen species (ROS) and abscisic acid (ABA) contents accumulation and activated related genes expression. Further investigation revealed that the E3 ligase CsBTS1 directly interacts with CsVOZ2 and promotes its degradation via the 26S proteasome pathway. CsBTS1E3 -OE in citrus hairy roots markedly enhanced C Las proliferation. Importantly, SDE2470 directly interacts with CsBTS1E3 and strengthen CsBTS1E3-CsVOZ2 interaction. Meanwhile, SDE2470 strengthened the E3 ligase activity of CsBTS1, promoting CsVOZ2 degradation. Taken together, these findings support a model in which SDE2470 hijacks CsBTS1 to destabilize CsVOZ2, thereby disrupting ROS- and ABA-dependent immunity and promoting C Las infection in citrus.

Abstract Autophagy functions in plant host immunity responses to pathogen infection. The molecular mechanisms and functions used by the citrus Huanglongbing (HLB)-associated intracellular bacterium ‘Candidatus Liberibacter asiaticus’ (CLas) to manipulate autophagy are unknown. We identified a CLas effector, SDE4405 (CLIBASIA_04405), which contributes to HLB progression. ‘Wanjincheng’ orange (Citrus sinensis) transgenic plants expressing SDE4405 promotes CLas proliferation and symptom expression via suppressing host immunity responses. SDE4405 interacts with the ATG8-family of proteins (ATG8s), and their interactions activate autophagy in Nicotiana benthamiana. The occurrence of autophagy is also significantly enhanced in SDE4405-transgenic citrus plants. Interrupting NbATG8s-SDE4405 interaction by silencing of NbATG8c reduces Pseudomonas syringae pv. tomato strain DC3000ΔhopQ1-1 (Pst DC3000ΔhopQ1-1) proliferation in N. benthamiana, and transient overexpression of CsATG8c and SDE4405 in citrus promotes Xanthomonas citri subsp. citri (Xcc) multiplication, suggesting that SDE4405-ATG8s interaction negatively regulates plant defense. These results demonstrate the role of the CLas effector protein in manipulating autophagy, and provide new molecular insights into the interaction between CLas and citrus hosts.