‘<i>Candidatus</i> Liberibacter asiaticus’ secretory protein SDE3 inhibits host autophagy to promote Huanglongbing disease in citrus

Antimicrobial acroautophagy/autophagy plays a vital role in degrading intracellular pathogens or microbial molecules in host-microbe interactions. However, microbes evolved various mechanisms to hijack or modulate autophagy to escape elimination. Vector-transmitted phloem-limited bacteria, <i>Candidatus</i> Liberibacter (<i>Ca</i>. Liberibacter) species, cause Huanglongbing (HLB), one of the most catastrophic citrus diseases worldwide, yet contributions of autophagy to HLB disease proliferation remain poorly defined. Here, we report the identification of a virulence effector in "<i>Ca</i>. Liberibacter asiaticus" (Las), SDE3, which is highly conserved among the "<i>Ca</i>. Liberibacter". <i>SDE3</i> expression not only promotes the disease development of HLB and canker in sweet orange (<i>Citrus sinensis</i>) plants but also facilitates <i>Phytophthora</i> and viral infections in <i>Arabidopsis</i>, and <i>Nicotiana benthamiana</i> (<i>N.</i> <i>benthamiana</i>). SDE3 directly associates with citrus cytosolic glyceraldehyde-3-phosphate dehydrogenases (CsGAPCs), which negatively regulates plant immunity. Overexpression of <i>CsGAPCs</i> and <i>SDE3</i> significantly inhibits autophagy in citrus, <i>Arabidopsis</i>, and <i>N.</i> <i>benthamiana</i>. Intriguingly, SDE3 undermines autophagy-mediated immunity by the specific degradation of CsATG8 family proteins in a CsGAPC1-dependent manner. CsATG8 degradation is largely rescued by treatment with an inhibitor of the late autophagic pathway, E64d. Furthermore, ectopic expression of <i>CsATG8s</i> enhances <i>Phytophthora</i> resistance. Collectively, these results suggest that SDE3-CsGAPC interactions modulate <i>CsATG8</i>-mediated autophagy to enhance Las progression in citrus.<b>Abbreviations:</b> ACP: asian citrus psyllid; ACD2: ACCELERATED CELL DEATH 2; ATG: autophagy related; <i>Ca</i>. Liberibacter: <i>Candidatus</i> Liberibacter; CaMV: cauliflower mosaic virus; CMV: cucumber mosaic virus; Cs: <i>Citrus sinensis</i>; EV: empty vector; GAPC: cytosolic glyceraldehyde-3-phosphate dehydrogenase; HLB: huanglongbing; H₂O₂: hydrogen peroxide; Las: liberibacter asiaticus; Laf: liberibacter africanus; Lam: liberibacter americanus; <i>Pst</i>: <i>Pseudomonas syringae</i> pv. tomato; PVX: potato virus X; ROS: reactive oxygen species; SDE3: sec-delivered effector 3; TEM: transmission electron microscopy; VIVE : virus-induced virulence effector; WT: wild-type; <i>Xcc</i>: <i>Xanthomonas citri</i> subsp. <i>citri</i>.