AUTHOR=Zhang Chao , Du Peixiu , Yan Hailin , Zhu Zongcai , Wang Xuefeng , Li Weimin 

TITLE=A Sec-Dependent Secretory Protein of the Huanglongbing-Associated Pathogen Suppresses Hypersensitive Cell Death in Nicotiana benthamiana

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.594669

DOI=10.3389/fmicb.2020.594669

ISSN=1664-302X

ABSTRACT=<p>“<italic>Candidatus</italic> Liberibacter asiaticus” (CLas) is a phloem-restricted Gram-negative bacterium that is the causal agent of citrus huanglongbing (HLB). In this study, we identified a CLas-encoded Sec-dependent secretory protein CLIBASIA_04405 that could contribute to the pathogenicity of this bacterium. The gene expression level of CLIBASIA_04405 was significantly higher in citrus than in psyllids. Transient overexpression of the mature CLIBASIA_04405 protein (m4405) in <italic>Nicotiana benthamiana</italic> leaves could suppress hypersensitive response (HR)-based cell death and H<sub>2</sub>O<sub>2</sub> accumulation triggered by the mouse BAX and the <italic>Phytophthora infestans</italic> INF1. An alanine-substitution mutagenesis assay revealed the essential of amino acid clusters EKR<sup>45–47</sup> and DE<sup>64–65</sup> in cell death suppression. Challenge inoculation of the transgenic <italic>N. benthamiana</italic>-expressing m4405 with <italic>Pseudomonas syringae</italic> DC3000Δ<italic>hopQ1-1</italic> demonstrated the greatly reduced bacterial proliferation. Remarkably, transcriptome profiling and RT-qPCR analysis disclosed that the gene expression of six small heat shock proteins (sHSPs), a set of plant defense regulators, were significantly elevated in the transgenic m4405 lines compared with those in wild-type <italic>N. benthamiana</italic>. In addition, the transgenic m4405 lines displayed phenotypes of dwarfism and leaf deformation. Altogether, these data indicated that m4405 was a virulence factor of CLas.</p>