Technical Abstract: Huanglongbing (HLB) is a highly destructive citrus disease which threatens citrus production worldwide. Among three known ‘Candidatus Liberibacter’ species associated with HLB, ‘Ca. Liberibacter asiaticus’ (Las) is the most prevalent. To better understand the physiological and molecular processes involved in host responses to Las infection, proteomic analyses via 2-DE and mass spectrometry as well as ICP spectroscopy analysis were employed to elucidate the global protein expression profiles and nutrient concentrations in leaves of grapefruit plants that were infected with Las at pre-symptomatic or symptomatic stages for HLB. A Las-mediated down-regulation of 71 protein spots that matched to 56 proteins including those associated with photosynthesis, protein synthesis, and metabolism was correlated with significant reductions in the concentrations of Ca, Mg, Fe, Zn, Mn, and Cu, especially in symptomatic plants. Interestingly, a Las-mediated up-regulation of 26 protein spots that matched to 13 proteins including those associated with pathogen response, redox-homeostasis and starch anabolism was correlated with an increase in K concentration in pre-symptomatic or symptomatic plants. Since starch synthase requires K for activation, this study is the first to highlight a coordinated accumulation of granule-bound starch synthase and K in Las-infected plants. This study describes biochemical and physiological responses of citrus plants to HLB, a key step in the development of host-based biomarker(s) for early detection and improved nutritional management of HLB.