Huanglongbing (HLB or greening) is currently one of the most serious diseases of citrus and has caused substantial economic losses, both in Brazil and the United States. Center APTA Citrus Sylvio Moreira / IAC has been conducting a broad program of genetic improvement of citrus by controlled crosses. Populations of plants obtained from controlled crosses between different species and varieties of citrus have been an extensive network of potential new genotypes. In previous study with citrus HLB pathosystem performed in our group, using a population obtained from crosses between Citrus sunki and Poncirus trifoliata, we observed differences in the multiplication of bacteria in both parental and progeny, which the infection rate was higher in Sunki mandarin in relation to Poncirus trifoliata and a wide variation between the hybrids. Thus, it would be important to deepen the studies in this genus and their hybrids to understand what mechanisms might be involved in tolerance to HLB. In this sense, the project aims to determine the anatomic and biochemical changes and plant responses to this pathogen in leaf tissue infected with the bacteria, using the parental and hybrids contrasting within the tolerance to disease. It will be observed formation and deposition of callose and accumulation of sucrose likely resulting from disorders of transport in the phloem vessels. These changes will be associated with the quantification of bacteria within the tissues and with analysis of global gene expression by RNA-Seq technology. From the results of global gene analysis, it will be selected genes possibly involved in the process of plant defense responses to pathogen attack. We will be based on the hypothesis that genes differentially expressed between the tolerant and susceptible parental and between tolerant and susceptible hybrids will be promising candidates involved in tolerance to HLB. This information will also enable to map genomic regions associated with tolerance to HLB by QTL analysis (phenotypic data) and expression QTLs (gene expression data) using previously constructed linkage maps of Citrus sunki Hort. Ex Tan. and Poncirus trifoliata (L.) Raf. Cv. Rubidoux and a population of 80 F1 individuals. The phenotypic data are based on results of quantification of the multiplication of bacteria. For identification of expression QTL (eQTL) the selected genes will be quantified using the technique of real-time quantitative PCR (qPCR). The eQTL will be classified as cis or trans-controlled by the co-location of the peak LOD score of eQTL to the region associated with the resistance gene based on the evaluation of the phenotype, which will explore and combine information in order to conduct further study on the genetics of citrus and knowledge of metabolic pathways involved in tolerance to HLB.
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