Integrated proteomic and transcriptomic analyses of Citrus australasica in response to Candidatus Liberibacter asiaticus infection

Candidatus Liberibacter asiaticus (CLas) is the most prevalent pathogen causing the globally prevalent citrus Huanglongbing (HLB). The citrus relative Citrus australasica F. Muell. shows tolerance to HLB. In this study, we grafted HLB-tolerant Ca (C. australasica) and HLB-susceptible Cs (C. sinensis) Osbeck branches onto healthy C. reticulate Blanco ‘Shatangju’ (SH) and CLas-infected (SI) rootstocks. To understand HLB tolerance mechanisms in C. australasica, a comprehensive analysis of transcriptomic and proteomic data of leaves from CaSH, CaSI, CsSH, and CsSI was conducted. Differentially expressed genes (DEGs) between CaSI and CaSH were enriched in acridone alkaloid biosynthesis. In contrast, some DEGs between CsSI and CsSH were enriched in plant hormone signal transduction, and MAPK signaling pathway. Several differentially expressed proteins (DEPs) between CaSI and CaSH were enriched in phenylpropanoid biosynthesis, whereas several DEPs between CsSI and CsSH were enriched in amino acids biosynthesis. In response to CLas infection, several genes involved in amino acid metabolism pathway and citrate cycle pathway showed opposite trends in C. australasica and C. sinensis. Among these genes with opposite expression trends, it is noteworthy that in response to HLB, arogenate/prephenate dehydratase and phenylalanine-4-hydroxylase in phenylalanine and tyrosine biosynthesis remained stable in C. australasica. In addition, glutamate dehydrogenase and amino acid N-acetyltransferase, which are involved in arginine metabolism, were up-regulated in CLas-infected C. australasica. The expression levels of isocitrate dehydrogenase, malate dehydrogenase, and aconitate hydratase, which are involved in the citrate cycle, were higher in CaSI than in CsSI. These data suggest that the stabilization of phenylalanine biosynthesis and the up-regulation of arginine biosynthesis contribute to HLB-tolerance of C. australasica, and that the changes in enzymes in the citrate cycle pathway and flow of carbon sources to arginine biosynthesis reduce the energy supply for HLB pathogens. These results provide new insights into the mechanism of HLB-tolerance in C. australasica, and provide a theoretical molecular basis for breeding HLB-tolerant citrus varieties.