“Changes in gene expression in citrus fruits in response to the infection by fungi from the genus Penicillium ”
      Infection produced by P. digitatum and P. italicum is one of the main causes of losses during postharvest of citrus fruit. Problems derived from the application of chemical fungicides for its control justify the search of efficient alternatives. Knowledge of the basis of plant-pathogen interaction and plant defence is essential for the development of alternatives to control plant pathologies. So far, there are few studies dealing with the processes involved in the defence response of fruits to infection by phytopathogenic fungi.
      During the development of this work we have used different functional genomics approaches to analyse the response of the citrus fruit to the infection by fungi from the genus Penicillium. We have obtained two cDNA libraries: RindPdig24 was made from ‘Clemenules’ mandarin fruits 24 hours after being wounded and inoculated with P. digitatum, whereas PostharvP1 was made from wounded ‘Clemenules’ mandarin fruits, and from fruits that had been wounded and inoculated with P. digitatum, at different times after inoculation, and it is enriched in full-length cDNAs. In addition, a previous cDNA subtractive library, RindPdigS, which is enriched in infection-responsive genes of ‘Navelina’ oranges, was also studied. Massive sequencing of clones from these three libraries, sequence annotation and functional categorization, offer a global representation of the genes expressed in the citrus fruit during the infection. A total of 2.505 different clones have been sequenced, and 1.941 unigenes have been identified. Data have been integrated within the Spanish Citrus Functional Genomic Project (CFGP). Twenty five percent of the clones do not have homology in public databases, and twenty six percent of the unigenes are not present in any other cDNA library of the CFGP. We conclude that the response of citrus fruit to infection has distinctive features that deserve detailed analyses, being a significant part of the response relatively unknown.
      In a second part, we have carried out gene expression studies through array technology using a cDNA macroarray containing 1.436 clones randomly selected from RindPdigS, and a 7k cDNA microarray constructed by the CFGP consortium that contains 12.671 clones grouped into 6.333 unigenes. Hybridization of these arrays allowed the characterization of the global expression profile in different treatments related to infection. Individual changes in gene expression in response to wounding or infection by different fungi from the genus Penicillium have also been studied in detail. Taken together, the results show noteworthy changes during infection of the fruit metabolism towards specific secondary metabolism pathways involved in the synthesis of defence related compounds, specially of the phenilpropanoid and alkaloid families, identifying significant genes from their abundance or expression level. These represent an enrichment of the repertoire of genes involved in the biosynthesis of secondary metabolites belonging to different families.
      Finally, we carried out studies towards the elucidation of the involvement of the plant hormone ethylene in the response of citrus fruits to the infection. The results obtained suggest that although ethylene is an important mediator of this response, there are probably additional mechanisms/regulators involved.