Evaluation of new hybrids of citrus for resistance to citrus variegated chlorosis

Abstract

Although the citriculture is one of the most important economic activities in Brazil, it is based on a small number of varieties. This fact has contributed for the vulnerability of the culture regarding the phytosanitary problems. A higher number of varieties/genotypes with potential for commercial growing, either for the industry or fresh market, has been one of the main objectives of citrus breeding programs. The genetic breeding of citrus has improved, in the last decades, due to the possibility of an association between biotechnological tools and classical methods of breeding. The use of molecular markers for early selection of zygotic seedlings from controlled crosses resulted in the possibility of selection of a high number of new combination and, as a consequence, the establishment of a great number of hybrids in field experiments. The Centro APTA Citros Sylvio Moreira / IAC has launched, since 1997, a wide program of genetic breeding of citrus through controlled crosses. Plant populations obtained from hybridization between several varieties, selected by molecular markers, were recently established in several regions of the São Paulo State resulting in an experimental network for new genotypes. Moreover, other project regarding ESTs sequencing of different genotypes and challenging conditions. The existence of linkage maps and QTLs already mapped, the development of EST library, altogether make very demanding and urgent the exploration of such data to launch a wider genetic study of citrus. Accordingly, besides the field experiments for phenotypic evaluation, experiments with differential genes expression will be conducted. The validation of genes differentially expressed and their correlation with phenotypic traits will permit the understanding of disease resistance mechanisms and increase the accuracy during hybrids selection. The integration of molecular-genetic information with those derived from evaluations of field certainly enable the association of gene expression with phenotype. This project has as its main objective the evaluation of 94 of the hybrid cross between tangor Murcott (C. sinensis x C. paradisi) vs Pêra Sweet orange (C. sinensis) to the selection of plants resistant to Citrus Variegated Chlorosis. The results will be used in the work of mapping of QTLs associated with resistance to Citrus Variegated Chlorosis in genetic maps already established and correlation of phenotypic characteristics with the differential expression of genes evaluated by microarrays and validated by RT qPCR (real-time quantitative polymerase chain reaction). They are part of a project entitled "Resistance to disease in citrus: functional genomics and genetic mapping of new hybrids" (Fapesp - Case n æ 2007/08435-5). (AU)