A prophage‐encoded nonclassical secretory protein of “Candidatus Liberibacter asiaticus” induces a strong immune response in Nicotiana benthamiana and citrus

Abstract

Huanglongbing (HLB), associated with “Candidatus Liberibacter asiaticus” (CLas), is a globally devastating plant disease. The highly reduced genome of CLas encodes a number of secretory proteins. The conserved prophage‐encoded protein AGH17470 is herein identified as a nonclassical secretory protein. We confirmed that the N‐terminal and C‐terminal sequences jointly determine the secretion of AGH17470. The transient expression of AGH17470 protein in Nicotiana benthamiana caused hypersensitive response (HR) cell death in infiltrated leaves and systemically infected leaves as well as the dwarfing of the entire plant, suggesting that AGH17470 is involved in the plant immune response, growth, and development. Overexpression of AGH17470 in N. benthamiana and citrus plants up‐regulated the transcription of pathogenesis‐related and salicylic acid (SA)‐signalling pathway genes and promoted SA accumulation. Furthermore, transient expression of AGH17470 enhanced the resistance of sweet orange to Xanthomonas citri subsp. citri. To our knowledge, AGH17470 is the first prophage‐encoded secretory protein demonstrated to elicit an HR and induce a strong plant immune response. The findings have increased our understanding of prophage‐encoded secretory protein genes, and the results provide clues as to the plant defence response against CLas.

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

“…By contrast, the pathogens secrete effectors to suppress cell death to facilitate their successful infection. C Las encodes a large number of Sec-dependent secretory proteins with unknown functions and a few have been characterized, but with only three, SDE1, m460, and AGH17470 identified as PCD elicitors, while others functioning as suppressors ( Pitino et al, 2016 , 2018 ; Liu et al, 2019 ; Du J. et al, 2022 ). In our study, mSECP8 was also identified as a suppressor by suppressing BAX- and INF-triggered PCD syndromes.…”

“…Previous studies have confirmed that many C Las-associated Sec-dependent effectors (SDEs) as virulence factors can suppress or trigger plant immune responses ( Jain et al, 2015 , 2018 ; Pitino et al, 2016 , 2018 ; Clark et al, 2018 ; Liu et al, 2019 ; Zhang et al, 2019 , 2020 ; Pang et al, 2020 ; Du et al, 2021 ; Du J. et al, 2022 ; Du M. et al, 2022 ). For instance, non-classical secretory proteins (ncSecPs) SC2_gp095 and Lasbcp (CLIBASIA_RS00445), down-regulation the expression of RbohB gene associated with H 2 O 2 -mediated defense signaling in plants ( Jain et al, 2015 , 2018 ).…”

“…Prophage-encoded ncSecP AGH17470 triggers PR-associated and salicylic acid (SA)-signaling pathway genes expression and then enhances the resistance of citrus to Xanthomonas citri subsp. citri ( Xcc ) ( Du J. et al, 2022 ).…”

Biological Features and In Planta Transcriptomic Analyses of a Microviridae Phage (CLasMV1) in “Candidatus Liberibacter asiaticus”

“…The discovery of CLas phages/prophages has opened a new venue for CLas biology research. In particular, the prophage, an integrated form of phage genome in the CLas genome, has played critical roles in the genomic evolution, pathogenicity, adaptability and survival of CLas [ 8 , 12 , 13 , 14 , 15 , 16 , 17 ]. A peroxidase gene (SC2_gp095) encoded by Type 2 prophage (SC2) was found to be involved in the fitness and survival of CLas in the host plant by the suppression of the reactive oxygen-mediated host defenses [ 13 ].…”

“…Transgenic citrus expressing a prophage-encoded protein (LasP235) showed the HLB-like symptoms and altered biosynthesis of secondary metabolites pathways in Carrizo citrange ( Citrus sinensis × Poncirus trifoliata ) [ 18 ]. In a recent study, the transient expression of a nonclassical secretory prophage-encoded protein (AGH17470) caused the hypersensitive response and dwarfing of Nicotiana benthamiana Domin, and also up-regulated the pathogenesis-related genes and promoted SA accumulation in citrus plants, indicating its important role in influencing plant immune response and growth [ 17 ]. Unlike SC1, SC2 and P-JXGC-3, which contained a large genome (>30 kb) with a prophage form [ 8 ], the prophage form of CLasMV1 has not been found [ 10 ].…”