Ionome In Citrus Leaf And Ionomic Responses To Infection Of Huanglongbing Pathogen

The term’ionome’refers to all the mineral nutrients and trace elements in an organism, tissue or cell. Mineral elements are essential for plant life. The imbalanced mineral nutrients within a plant can cause physiological disturbance, and can also induce infective disease by pathogens. Plant-pathogen interaction is a complex process including variations in morphology, physiology, biochemistry and signaling. In this process, more than one mineral elements play vital roles directly or indirectly. Therefore, through ionomic tools, to investigate the relationship between mineral elements and plant pathogen-induced infective disease, the specificity of plant-pathogen interaction and plant defense strategies against pathogen infection, is helpful for understanding plant pathological mechanism and finding effective control measures.The interaction between citrus plants and Huanglongbing (HLB) disease is a challenging and leading issue to international citrus community. The mechanisms regarding citrus HLB still remain unknown by now, and therefore future research from different aspects is warranted. There is a paucity of ionomic information regarding citrus HLB disease, relative to that on transcriptome, proteome and metabolome. Furthermore, the relevant research has focused only on one citrus species (mainly on Citrus paradisi or C. sinensis) graft-inoculated with HLB pathogen under the standard environment in greenhouse or screen-house. However, the interactions between plant immune system and pathogen are very complex, and different plants differentially respond to pathogen infection. This study investigated ionome profiles in citrus leaves and their responses to the infection of Candidatus Liberibacter asiaticus (one of HLB-pathogen species), using high-throughput ionomic profiling (ICP-MS and ICP-OES), and gave an insight into the effects of key factors including genetic variability (three species:C. reticulata, C. limon and C. maxima), leaf growth (live leaves, senescing leaves and senesced leaves) and environmental aspects (seasons: summer and autumn; sites:three orchards). The main results are as follows.1) Biodiversity is found in ionomes within a citrus plant and among different species as well. Ionome within a citrus species exhibits diversity, and this diversity changes upon HLB-pathogen infection. The ionomes among different citrus species studied present differential responses to HLB-pathogen infection.2) The’Ca. L. asiaticus’infection has significant effects on nitrogen (N) and phosphorus (P) concentrations in live leaves of citrus plants. The N and P concentrations in live leaves of infected C. reticulata plants in June drop to the deficient and low levels, respectively. However, the N and P concentrations in infected C. maxima plants still remain at optimum or high levels. Different species as well as seasons also have significant effects on concentrations of N and P in live leaves, and there are interactive effects of species and health status on live-leaf N and P. Concentrations of N and P in senesced leaves decline in infected C. reticulata, C. limon and C.maxima plants, relative to those in the healthy plants. The’Ca. L. asiaticus’ infection has significant effects on N:P ratios in live leaves of C. reticulata and C. limon, but the effects of the infection on N:P ratios in senesced leaves are not observed in C. maxima plants. The variations in N:P ratio in live leaves may reflect the difference in HLB tolerance among different citrus species to some extent.3) The patterns in potassium are consistent with those in N and P, in response to HLB-pathogen infection. Calcium concentrations decrease, increase and remain unaltered in HLB-pathogen infected C. reticulata, C.limon and C. maxima plants, respectively, compared with those in the healthy plants. HLB-pathogen infection induces significant decreases in zinc (Zn) concentrations in live leaves of both C. reticulata and C.limon plants. Although it is often considered that symptoms of HLB-affected plants is similar to those of zinc deficient plants, it is particularly noteworthy that Zn concentrations in live leaves of the HLB-pathogen infeted C. maxima plants record significant increases compared with those in the healthy plants. In addition, copper concentrations in live leaves are not significantly affected by HLB-pathogen infection.4) The correlations of all elements studied are highly variable among species and between healthy and HLB-pathogen infected citrus plants as well. HLB-pathogen infection induces significant variations in element correlations in both C. reticulata and C. limon, but it does not affect element correlations in C. maxima. Some element correlations that are genetically conservative are broken by the infection of HLB pathogen; however, some element correlations are not only genetically conservative but also robust even under the disease stress.5) The Principal Component Analysis (PCA) indicates that the effects of HLB-pathogen infection are not significant on C. maxima, and however the effects on C. reticidata and C. limon are complex.In this study, ionomic variations in response to HLB-pathogen infection present significant diversity in citrus leaves. Citrus plants improve the defense ability against HLB pathogen by regulating the balance of elements. The defense ability varies in different plant species under disease stress, and thus plant hosts show different tolerance to the disease. The study on plant ionomes during pathogen infection is helpful for understanding changes in ionomic diversity in host plants under pathogen infection and the mechanisms underlying element regulating.