Expression of six expansin genes in relation to extension activity in developing strawberry fruit

Expansins are proteins which have been demonstrated to induce cell wall extension in vitro. The identification and characterization of six expansin cDNAs from strawberry fruit, termed FaExp3 to FaExp7, as well as the previously identified FaExp2 is reported here. Analysis of expansin mRNAs during fruit development and in leaves, roots and stolons revealed a unique pattern of expression for each cDNA. FaExp3 mRNA was present at much lower levels than the other expansin mRNAs and was expressed in small green fruit and in ripe fruit. FaExp4 mRNA was present throughout fruit development, but was more strongly expressed during ripening. FaExp5 was the only clone to show fruit specific expression which was up-regulated at the onset of ripening. FaExp6 and FaExp7 mRNAs were present at low levels in the fruit with highest expression in stolon tissue. During fruit development FaExp6 had the highest expression at the white, turning and orange stages whereas expression of FaExp7 was highest in white fruit. The expression profiles of FaExp2 and FaExp5 in developing fruit were similar except that FaExp2 was induced at an earlier stage. Analysis of expansin protein by Western blotting using an antibody raised against CsExp1 from cucumber hypocotyls identified two bands of 29 and 31 kDa from developing fruit. Protein extracts from developing fruit were assayed for extension activity. Considerable rates of extension were observed with extracts from ripening fruit, but no extension was observed with protein from unripe green fruit. These results demonstrate the presence of at least six expansin genes in strawberry fruit and that during ripening the fruit acquires the ability to cause extension in vitro, characteristic of expansin action.