|dc.description.abstract||The strawberry (Fragaria × ananassa) belongs to the family Rosaceae in the genus Fragaria. This soft fruit is cultivated in different regions of the world and is part of the diet of millions of people. Spain is the first producer of strawberries in Europe and the second one in the world after United States (FAO, 2012). The main octoploid variety Fragaria × ananassa cultivated is the result of crossing two native American species, F. virginiana and F. chiloensis (Hancock, 1999; Mabberley, 2002; Eriksson et al., 2003). The wild diploid species Fragaria vesca is also considered as an ancestor of the cultivated octoploid variety. Recently, the genome of the wild species Fragaria vesca has been sequenced (Shulaev et al., 2011). This information, together with the ESTs (expressed sequence tag) availability from cultivated species and the development of efficient transformation techniques of these varieties, will allow the development of genomics and recombinant DNA studies between different species of Rosaceae in the future (Bombarely et al., 2010).
Strawberry fruit has been classified as non-climacteric, since there is no concomitant burst of respiration and ethylene production that triggers to the ripening process. Thus, all changes related with the fruit ripening occur without a significant increase in ethylene production, which suggests that this process is independent of this hormone (Iwata et al., 1969a and 1969b; Villareal et al., 2010). The strawberry fruit has a maximum respiration at the transition between stages ripe to overripe.
Strawberries are much appreciated for their flavor, aroma and nutritional value. The mature fruit is composed of approximately 90 % water and 10% total soluble solids. Moreover, it contains many important dietary components such as vitamin C, soluble sugars such as glucose and fructose (which constitute over 80 % of total sugars), organic acids such as citric acid (88 % of total acids) and ellagic acid, which has anticancer properties (Green, 1971; Wrolstad and Shallenberger, 1981; Maas et al., 1991; Hemphill and Martin, 1992; Maas et al. 1996; Hancock, 1999).
Soft fruits have an initial phase of growth and elongation, followed by a phase of maturity. The growth of the strawberry receptacle depends of the cortex and medulla cells development while the fruit size is mainly determined for the medulla cells development and the fruit position in the inflorescence (Hancock, 1999). Moreover, the fruit development is determined by the number and distribution of achenes, the receptacle area around each achene and the percentage of fertilized carpels. In this sense, the synthesis of auxin, fundamentally indole-3-acetic acid (IAA), which takes place in the achenes, is considerate the main responsible of the receptacle growth while gibberellins, cytokinins and abscisic acid have a limited role in the fruit growth (Nitsch,1950; Perkins-Veazie, 1995).
Along the development and ripening processes, the strawberry fruit suffers important molecular changes such the removal of existing polypeptides and the synthesis of new proteins (Manning, 1994). In this sense, three evolution models of the transcripts have been described: mRNA whose concentration increases along the ripening, mRNA whose levels decrease over the ripening, and mRNA whose components exceed their maximum concentration in the intermediate stage, which then declined in stages of maturation (Veluthambi and Poovaiah, 1984; Reddy and Poovaiah, 1990; Reddy et al., 1990; Manning, 1994).||en