Global Regulation of Genes in Citrus Fruit in Response to the Postharvest Pathogen Penicillium digitatum

Large-scale EST sequencing projects and microarray hybridization nowadays constitute two major approaches to analyse biological systems at a molecular level. Although the use of these methodologies is becoming commonplace in plant pathology, their application to postharvest pathology has not yet been reported. In this chapter we analyze the overall response of citrus fruit to Penicillium digitatum infection from a genomic perspective. We have constructed both subtracted and regular cDNA libraries from infected fruits. Analysis of the non-subtracted library gives us a picture of what genes are being transcribed, whereas the subtracted library provides more direct information on what citrus genes are induced upon P digitatum infection. A cDNA macroarray generated from the subtracted library has been used to identify what genes are up-regulated in response to pathogen infection, and to determine the influence of ethylene on their expression. Under the framework of the Spanish 'Citrus Functional Genomic Project, CFGP' more than fifty cDNA libraries have been generated encompassing more than 90,000 ESTs. These clones have been used to develop a cDNA microarray representing roughly 7,000 unigenes. With this tool we have analyzed the differences and commonalities of the citrus fruit responses to ethylene, wounding, P digitatum infection and induced resistance. We will present the results of such analyses emphasizing not only what genes are affected to a larger extent but also describing what are the major changes in biological processes and metabolic pathways.