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 ex tension in vitro. The identification and characterization of six expansin c DNAs from strawberry fruit, termed FaExp3 to FaExp7, as well as the previou sly identified FaExp2 is reported here. Analysis of expansin mRNAs during f ruit 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 highes t expression in stolon tissue. During fruit development FaExp6 had the high est expression at the white, turning and orange stages whereas expression o f FaExp7 was highest in white fruit. The expression profiles of FaExp2 and FaExp5 in developing fruit were similar except that FaExp2 was induced at a n earlier stage. Analysis of expansin protein by Western blotting using an antibody raised against CsExp1 from cucumber hypocotyls identified two band s 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 obser ved with protein from unripe green fruit. These results demonstrate the pre sence of at least six expansin genes in strawberry fruit and that during ri pening the fruit acquires the ability to cause extension in vitro, characte ristic of expansin action.