Resistencia frente a la infección por Penicillium digitatum durante la postcosecha de los frutos cítricos. Bases moleculares y metabolismo de fenilpropanoides.

Penicillium digitatum is the causal agent of green mould and constitutes the major postharvest pathogen of citrus fruit, accounting for up to 80 % of losses due to fungal decay. Although control of this pathogen is achieved by chemical fungicides, there is a trend to adopt new and safer control alternatives. Induction of natural resistance in fruit constitutes one of these alternatives. The aim of this work was to gain an insight into the biological and molecular bases of the induction of resistance in citrus fruit against P. digitatum. Different methods to induce resistance against P. digitatum have been evaluated in order to optimize and to select one of them for further studies. Various chemical (methyl jasmonate, β-aminobutyric acid and Brotomax®), biological (non pathogenic fungi of citrus and cellular walls of P. digitatum) treatments, as well as the possible resistance induced by a previous infection with P. digitatum followed by its inactivation by a heat treatment were compared. The latter system was selected because of the high resistance achieved (the incidence was reduced more than 70 % and the severity more than 85 %) and its reproducibility. Global changes in gene expression induced by the selected treatment were analyzed in the flavedo and albedo of citrus fruit. To that end, two cDNA microarrays developed by the Citrus Functional Genomics Project, named 7k and 12k, which contain about 7.000 and 12.000 genes, respectively, were used. The highest inductions were detected in genes that code for enzymes involved in phenylpropanoid metabolism, mainly methyltransferases (OMTs), ethylene biosynthesis, among which there are several ACO genes (1-aminocyclopropane 1-carboxylic acid oxidase), pathogenesis-related proteins (PR) and oxidoreductases. An important reduction in the expression of genes regulated by either cold or light, together with others of unknown function, was also observed. Gene ontology comparisons showed that processes related to amino acids and phenylpropanoid metabolism were upregulated in treated fruits, being these changes more marked in the internal tissue. An increase in ethylene production was observed after the heat treatment, coincident with the resistance acquisition, reaching a concentration similar to that observed in fruits infected by P. digitatum. Nevertheless, in the treated fruits the fungus does not progress, so the rise in ethylene must be due to the fruit. Northern blot analysis showed that this increase in ethylene paralleled a high induction on CsACO expression, as it occurs in infected fruits, and that the highest induction takes place into the albedo. In order to get a deeper insight into the role of the phenylpropanoid metabolism in the induction of resistance, the expression of genes encoding PAL, 4CL, F3H, IRL1, CADs, SAD and OMTs was analyzed by Northern blot. The expression of most of these genes increased in response to the induction treatment, showing a similar pattern in both tissues. Although the overall expression levels were higher in the flavedo, the highest inductions were detected in the albedo. In addition, changes in the expression of these genes in response to the infection by P. digitatum were similar to those originated by the induction of resistance treatment. The putative involvement of this pathway was further studied by examining changes in the metabolic profile by means of HPLC. In the treated fruits a greater abundance and a higher concentration of phenolic compounds in the flavedo with respect to the albedo were observed. Flavanones were the only phenolics whose concentration was higher in the internal tissue. Although the coumarin scoparone was the phenolic compound showing the highest increase in response to the treatment, it has to be pointed out that, among the different phenylpropanoids, benzoic acid derivatives were the most affected. Similar changes were detected during the infection of fruits by P. digitatum. The possible antifungal activity of diverse coumarins, flavanones and alkaloids was tested both in vitro and in vivo against different pathogenic fungi. The coumarins scoparone and umbelliferone and the flavanone hesperidin showed the highest in vitro antifungal activity, although none of them inhibited completely the growth of the citrus fungal pathogens analyzed. However, only the coumarins had an effect in vivo delaying the development of P. digitatum in inoculated citrus fruit. On the other hand, addition of some alkaloids detected in citrus tissues to the culture medium significantly reduced the growth of P. digitatum, being caffeine and 1, 2, 3, 4-tetrahydroharmane 3-carboxylic acid (THHCA) the most effective. This is the first work undertaking both a transcriptomic and a metabolomic approach to study changes associated with relevant processes in postharvest pathology. The global results indicate that both the secondary metabolism, mainly phenylpropanoids, and ethylene play important roles in the induction of resistance in citrus fruit against P. digitatum infection.