Proteomics analysis reveals novel host molecular mechanisms associated with thermotherapy of ‘Ca. Liberibacter asiaticus’-infected citrus plants

Authors: Nwugo, Chika; Doud, Melissa; Duan, Ping; Lin, Hong

Submitted to: BMC Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2016
Publication Date: 11/14/2016
Citation: Nwugo, C.C., Doud, M.S., Duan, Y., Lin, H. 2016. Proteomics analysis reveals novel host molecular mechanisms associated with thermotherapy of ‘Ca. Liberibacter asiaticus’-infected citrus plants. Biomed Central (BMC) Plant Biology. 16:253. doi:10.1186/s12870-016-0942-x.

Interpretive Summary: Huanglongbing (HLB) is the most devastating disease of citrus plants. Existing HLB control practices include removal of infected plants from the population and the use of insecticides against the psyllid vector. Longer-term measures to combat HLB via breeding or genetic engineering methods are hampered by the fact that all cultivated citrus species are susceptible to the disease. However, the degree of HLB susceptibility varies among citrus species, which has prompted efforts to identify potential 'Ca. Liberibacter asiaticus' (Las) resistance tolerance-related genes in citrus plants for application in breeding or genetic engineering programs. Furthermore, a recent study showed that continuous heat treatment of 40°C to 42°C for a minimum of 48 h was sufficient to significantly reduce HLB pathogen titer or completely eliminate the HLB pathogen in HLB-affected citrus seedlings. A preliminary proteomics analysis of heat-treated HLB-affected lemon plants, detected proteins that were markedly up-regulated only in plants that were simultaneously exposed to heat and HLB. This suggests that heat treatment induces proteins in Las-infected citrus plants that could play an active role in the suppression of Las growth.

Technical Abstract: Citrus huanglongbing (HLB) is the most devastating disease of citrus plants, and longer-term measures for controlling the disease via breeding or genetic engineering are hampered by the fact that all cultivated citrus species are susceptible to the disease. However, the degree of HLB susceptibility varies among citrus species which has prompted efforts to identify potential Las resistance tolerance-related genes in citrus plants for application in breeding or genetic engineering programs. Plant exposure to one form of stress has been shown to serendipitously induce innate resistance to other forms of stress, and a recent study showed that continuous heat treatment (40 to 42°C) significantly reduced pathogen titer in HLB-affected citrus seedlings. The goal of the present study was to apply comparative proteomics analysis via 2-DE and mass spectrometry to elucidate the molecular processes associated with heat-induced mitigation of HLB in citrus plants. Healthy or Las-infected citrus grapefruit plants were exposed to room temperature or to continuous heat treatment of 40°C for 6 days. An exhaustive total protein extraction process facilitated the identification of 107 differentially-expressed proteins in response to Las and/or heat treatment, which included a strong up-regulation of chaperones including small (23.6, 18.5 and 17.9 kDa) heat shock proteins, a HSP70-like protein and a ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO)-binding 60 kDa chaperonin, particularly in response to heat treatment. Other proteins that were generally down-regulated due to Las infection but up-regulated in response to heat treatment include RuBisCO activase, chlorophyll a-b binding protein, glucosidase II beta subunit-like protein, a putative lipoxygenase protein, a ferritin-like protein, and a glutathione S-transferase. The differential expression of these proteins highlights molecular mechanisms potentially involved in the reversal of Las-induced pathogenicity processes in citrus plants and are hence proposed targets for application towards the development of cisgenic HLB-resistant/tolerant citrus plants.