Comparative iTRAQ proteomic profiling of sweet orange fruit on sensitive and tolerant rootstocks infected by ‘Candidatus Liberibacter asiaticus’

Citrus Huanglongbing (HLB), which is also known as citrus greening, is a destructive disease continuing to devastate citrus production worldwide. Although all citrus varieties can be infected with ‘Candidatus Liberibacter asiaticus’ (CaLas), a certain level of HLB tolerance of scion varieties can be conferred by some rootstocks. To understand the effects of rootstock varieties on orange fruit under CaLas stress, comparative iTRAQ proteomic profilings were conducted, using fruit from ‘Valencia’ sweet orange grafted on the sensitive (‘Swingle’) and tolerant rootstocks (a new selection called ‘46x20-04-48’) infected by CaLas as experimental groups, and the same plant materials without CaLas infection as controls. The symptomatic fruit on ‘Swingle’ had 573 differentially-expressed (DE) proteins in comparison with their healthy fruit on the same rootstock, whereas the symptomatic fruit on ‘46x20-04-48’ had 263 DE proteins. Many defense-associated proteins were down-regulated in the symptomatic fruit on ‘Swingle’ rootstock that were seldom detected in the symptomatic fruit on the ‘46x20-04-48’ rootstock, especially the proteins involved in the jasmonate biosynthesis (AOC4), jasmonate signaling (ASK2, RUB1, SKP1, HSP70T-2, and HSP90.1), protein hydrolysis (RPN8A and RPT2a), and vesicle trafficking (SNAREs and Clathrin) pathways. Therefore, we predict that the down-regulated proteins involved in the jasmonate signaling pathway and vesicle trafficking are likely to be related to citrus sensitivity to the CaLas pathogen.