Research Progress on Pathogenic Mechanism of Effectors of Candidatus Liberibacter asiaticus

Citrus Huanglongbing (HLB) is one of the most serious citrus diseases worldwide, causing huge economic losses every year and seriously affecting the development of the global citrus industry. In Lingnan region of China, HLB threatens the growth of important medicinal plants, such as Citrus reticulata 'Chachi', C. grandis 'Tomentosa', and C. medica 'Fingered'. Candidatus Liberibacter asiaticus (CLas) is the most common pathogen, which colonizes the phloem of plants and mainly infects rutaceae citrus species. At present, there is a lack of reports on the successful cultivation of CLas in vitro, making it difficult to study the pathogenic mechanism and control of the disease. Effector (also called effector protein) is a kind of pathogenic factor secreted by pathogens to the host, serving as an important weapon for pathogens to infect plants. The effectors are capable of targeting the key immune signal transduction and regulatory components of host cells, thereby inhibiting plant defense responses and enhancing the pathogenicity of pathogens. CLas can secrete over a hundred effectors, mainly divided into Secdependent effectors and non-classical secretory effectors according to different secretion pathways. Among them, there are 90 Sec-dependent effectors and 15 non-classical secretory effectors. They play a key role in the pathogenic process of infecting the host. Studies have shown that HLB effectors could target multiple cellular processes including plant immune signal transduction, autophagy, salicylic acid signaling pathway, and seriously disrupt the host's defense response to achieve colonization. Combined with recent researches on the pathogenic mechanism of HLB effectors, this article elaborates on the research overview of HLB and the classification of HLB effectors, and summarizes the pathogenic mechanism of effectors that affect citrus defense response. Finally, it offers a prospect on the future research directions, discusses the possibility of searching for important resistance genes to HLB based on identification of effector targets, with a aim to provide guidance for the pathogenic mechanism and disease control of HLB through the study of the effectors.