Study on citrus response to huanglongbing highlights a down-regulation of defense-related proteins in lemon plants upon 'Ca Liberibacter asiaticus' infection

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

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2013
Publication Date: 6/19/2013
Citation: Nwugo, C.C., Duan, Y., Lin, H. 2013. Study on citrus response to huanglongbing highlights a down-regulation of defense-related proteins in lemon plants upon 'Ca Liberibacter asiaticus' infection. PLoS One. 8(6):e67442.

Interpretive Summary: Citrus huanglongbing (HLB) or citrus greening is a highly destructive disease and all cultivated citrus species are susceptible. The insect-transmissible bacterium, ‘Candidatus Liberibacter asiaticus’ (Las) is a presumed causal agent. The aim of this study was to identify a common biochemical response pattern of citrus plants to Las infection. To achieve this, molecular biology and analytical techniques were applied to identify differences in protein accumulation and concentrations of major nutrients in leaves of healthy or Las-infected lemon (Citrus limon) plants. Additional chemical analysis was performed on leaves of healthy or Las-infected pomelo (Citrus maxima), trifoliate orange (Poncirus trifoliata) and periwinkle (Catharanthus roseus) plants. Results showed that Las-infection caused significant changes in the accumulation of 27 proteins in lemon plants, including an increase in the accumulation of starch synthesis-related proteins but a decrease in the accumulation of photosynthesis- and protein synthesis-related proteins. Furthermore, there was a reduction in the concentrations of iron and copper in all plants analyzed and a 6% increase in potassium concentration in lemon plants. These results are supported by those from a previous study on grapefruit (Citrus paradisi) plants and might represent a common response of citrus to HLB. However, this study identified potential intra-species specific responses of citrus plants to Las infection including a reduction in the accumulation of pathogen resistance-related proteins and a 128% increase in zinc concentration in lemon plants. Additionally, Las infection increased the calcium concentrations in leaves of pomelo and trifoliate orange but decreased the concentrations of all analyzed nutrients in periwinkle plants. Taken together, this study provides new information regarding physiological and biochemical responses of citrus plants to HLB which will be useful for development of host-based biomarker(s) for early detection and improved nutritional management of the disease.

Technical Abstract: Huanglongbing (HLB) is a highly destructive citrus disease; the bacterium, ‘Candidatus Liberibacter asiaticus’ (Las) is a presumed causal agent. In this study, comparative 2-DE and mass spectrometry, in addition to Inductively Coupled Plasma (ICP) spectroscopy analyses, were applied to elucidate differences in protein accumulation and concentrations of cationic elements in leaves of healthy or Las-infected lemon (Citrus limon) plants. Additional elemental analysis was performed on healthy or Las-infected leaves of pomelo (Citrus maxima), trifoliate orange (Poncirus trifoliata) and periwinkle (Catharanthus roseus) plants. The aim was to identify a potential consensus pattern in the biochemical responses of citrus plants to Las infection particularly in comparison to results from a prior study on grapefruit (Citrus paradisi) plants. Results showed a Las-mediated differential accumulation of 27 proteins, which included increased-production of starch synthesis-related proteins but reduced-accumulation of photosynthesis- and protein synthesis-related proteins. Additionally, Las infection reduced the concentrations of Fe, Cu, Ca, Mg and Mn but increased K concentration by 6% (P > 0.05) in lemon plants, supporting previous observations on grapefruit plants. However, a general Las-mediated reduced-accumulation of pathogen resistance-related proteins accompanied by a 128% increase in Zn concentration in lemon plants was novel, and highlights potential intra-species differences in Las resistance-associated responses in citrus. Elemental analysis of leaves of other citrus or citrus-related plants, showed Las-mediated increases in Ca and Mg concentrations of pomelo plants as well as increases in Ca and Mn concentrations of trifoliate orange plants but a general reduction in concentrations of cationic elements in periwinkle plants.