Extensive transcriptomic studies on the roles played by abscisic acid and auxins in the development and ripening of strawberry fruits

Strawberry is an ideal model for studying the molecular biology of the development and ripening of non-climacteric fruits. Hormonal regulation of gene expression along all these processes in strawberries is still to be fully elucidated. Although auxins and ABA have been pointed out as the major regulatory hormones, few high-throughput analyses have been carried out to date. The role for ethylene and gibberellins as regulatory hormones during the development and ripening of the strawberry fruit remain still elusive. By using a custom-made and high-quality oligo microarray platform done with over 32,000 probes including all of the genes actually described in the strawberry genome, we have analysed the expression of genes during the development and ripening in the receptacles of these fruits. We classify these genes into two major groups depending upon their temporal and developmental expression. First group are genes induced during the initial development stages. The second group encompasses genes induced during the final maturation and ripening processes. Each of these two groups has been also divided into four sub-groups according their pattern of hormonal regulation. By analyzing gene expression, we clearly show that auxins and ABA are the main and key hormones that combined or independently are responsible of the development and ripening process. Auxins are responsible for the receptacle fruit development and, at the same time¸ prevent ripening by repressing crucial genes. ABA regulates the expression of the vast majority of genes involved in the ripening. The main genes expressed under the control of these hormones are presented and their physiological rule discussed. We also conclude that ethylene and gibberellins do not seem to play a prominent role during these processes.

FaGAST2, a strawberry ripening-related gene, acts together with FaGAST1 to determine cell size of the fruit receptacle

Numerous GAST-like genes have been reported in higher plants, but only one GAST-like gene (FaGAST1) has been described in strawberry so far. Herein, we have identified a novel strawberry FaGAST gene (FaGAST2) whose expression showed an increase throughout fruit receptacle development and ripening, coinciding with those stages where a decrease in fruit expansion processes (G3-W and R-OR stages) occurs. FaGAST2 only shares 31% and 15.7% amino acid and nucleotide sequence homology, respectively, with the previously reported FaGAST1 gene, but both genes contain a signal peptide and a highly conserved GASA domain (cysteine-rich domain) in the C-terminal region. FaGAST2 expression is mainly confined to the fruit receptacle and is not regulated by auxins, GA(3) or ABA, but is regulated by ethephon, an intracellular generator of ethylene. In addition, the expression of the FaGAST2 gene also increased under oxidative stress conditions (H(2)O(2) or Colletotrichum acutatum infection), suggesting a direct role for FaGAST2 protein in reactive oxygen species scavenging during fruit growth and ripening and during fungal infection. On the other hand, the overexpression of the FaGAST2 gene in different transgenic lines analyzed caused a delay in the growth of strawberry plants and a reduction in the size of the transgenic fruits. The histological studies performed in these fruits showed that their parenchymal cells were smaller than those of the controls, supporting a relationship between FaGAST2 gene expression, strawberry fruit cell elongation and fruit size. However, transitory silencing of FaGAST2 gene expression through RNA interference approaches revealed an increase in FaGAST1 expression, but no changes in fruit cell size were observed. These results support the hypothesis that both genes must act synergistically to determine fruit cell size during fruit development and ripening.