The lignification of strawberry fruit: molecular basis and effects on fruit quality
Final Report Abstract
Lignin is the second most abundant polymer found in nature after cellulose. Among the many roles lignin plays in plant growth and development are those providing structural support for land plants. But until now only carbohydrate polymers like pectin and cellulose and to a lesser extent also proteins like expansin have been regarded as structure forming and stabilizing components in fruits. However, results obtained from microarray analyses have provided evidence that expression of specific genes involved in lignin formation affect strawberry fruit firmness. We have isolated and cloned cinnamoyl-CoA reductase (CCR), cinnamyl alcohol dehydrogenase (CAD), and class III peroxidase (POD) genes from strawberry (Fragaria x ananassa cv. Elsanta) supposed to be involved in lignin formation. In order to identify allelic variation of these genes, 20 random clones of each gene were selected for sequencing. Up to 18 highly similar homologues were detected for each of the genes. One CCR and CAD gene and two POD genes were chosen for the production of recombinant proteins. Enzyme assays confirmed their catalytic activities. CAD, CCR and POD27 showed high transcript levels in fully ripe fruit while POD was rarely expressed in red fruit. Wounding of the fruit did not change the expression levels of all four genes but transcript levels of POD27 dramatically increased due to infection by Agrobacterium tumefaciens resulting in enhanced firmness and lignin content. POD could be successfully transiently overexpressed whereas CCR and CAD were transiently downregulated in ripening strawberry fruits as confirmed by qPCR analysis. Agroinfiltration of different constructs consistently resulted in firmer fruits when compared with the untreated controls. Firmness always correlated with lignin content and increased levels of phenylpropenoyl glucose esters as determined by LC-MS analysis. When overexpression and downregulation of CCR, CAD and POD was performed in a chalcone synthase negative background (CHS-) similar results were obtained. Overall, the results suggest that POD27 but not POD should be considered as a key gene for improving firmness of the strawberry fruit and probably enhances resistance against pathogens.
