AUTHOR=Shen Pan , Li Xueyi , Fu Shimin , Zhou Changyong , Wang Xuefeng 

TITLE=A “Candidatus Liberibacter asiaticus”-secreted polypeptide suppresses plant immune responses in Nicotiana benthamiana and Citrus sinensis

JOURNAL=Frontiers in Plant Science

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.997825

DOI=10.3389/fpls.2022.997825

ISSN=1664-462X

ABSTRACT=<p>Citrus Huanglongbing (HLB), known as the most economically devastating disease in citrus industry, is mainly caused by phloem-restricted Gram-negative bacterium “<italic>Candidatus</italic> Liberibacter asiaticus” (<italic>C</italic>Las). To date, <italic>C</italic>Las is still unculturable <italic>in vitro</italic>, which has been dramatically delaying the research on its pathogenesis, and only few Sec-dependent effectors (SDEs) have been identified to elucidate the pathogenesis of <italic>C</italic>Las. Here, we confirmed that a <italic>C</italic>Las-secreted Sec-dependent polypeptide, namely SECP8 (CLIBASIA_05330), localized in nucleus, cytoplasm and cytoplasmic membrane, and showed remarkably higher transcript abundance in citrus than in psyllids. <italic>Potato virus X</italic> (PVX)-mediated transient expression assays indicated that mSECP8 (the mature form of SECP8) suppressed pro-apoptotic mouse protein BAX and <italic>Phytophthora infestans</italic> elicitin INF1-triggered hypersensitive response (HR) associated phenotypes, including cell death, H<sub>2</sub>O<sub>2</sub> accumulation and callose deposition. Intriguingly, mSECP8 also inhibited SDE1 (CLIBASIA_05315)-induced water-soaked and dwarfing symptoms in <italic>Nicotiana benthamiana</italic>. In addition, mSECP8 can promote the susceptibility of transgenic Wanjincheng orange (<italic>Citrus sinensis</italic>) to <italic>C</italic>Las invasion and further HLB symptom development, and it contributes to the proliferation of <italic>Xanthomonas citri</italic> subsp. <italic>citri</italic> (<italic>Xcc</italic>). Moreover, the expression of ten immunity-related genes were significantly down-regulated in mSECP8 transgenic citrus than those in wide-type (WT) plants. Overall, we propose that mSECP8 may serve as a novel broad-spectrum suppressor of plant immunity, and provide the first evidence counteractive effect among <italic>C</italic>Las effectors. This study will enrich and provide new evidences for elucidating the pathogenic mechanisms of <italic>C</italic>Las in citrus host.</p>