EXPRESSION OF A DIVERGENT EXPANSIN GENE IS FRUIT-SPECIFIC AND RIPENING-REGULATED

Expansins are proteins that induce extension in isolated plant cell wa lls in vitro and have been proposed to disrupt noncovalent interaction s between hemicellulose and cellulose microfibrils, Because the plant primary cell wall acts as a constraint to cell enlargement, this proce ss may be integral to plant cell expansion, and studies of expansins h ave focused on their role in growth, We report the identification of a n expansin (LeExp1) from tomato that exhibits high levels of mRNA abun dance and is specifically expressed in ripening fruit, a developmental period when growth has ceased but when selective disassembly of cell wall components is pronounced, cDNAs closely related to LeExp1 were al so identified in ripening melons and strawberries, suggesting that the y are a common feature of fruit undergoing rapid softening, Furthermor e, the sequence of LeExp1 and its homologs from other ripening fruit d efine a subclass of expansin genes, Expression of LeExp1 is regulated by ethylene, a hormone known to coordinate and induce ripening in many species, LeExp1 is differentially expressed in the ripening-impaired tomato mutants Nr, rin, and nor, and mRNA abundance appears to be infl uenced directly by ethylene and by a developmentally modulated transdu ction pathway, The identification of a ripening-regulated expansin gen e in tomato and other fruit suggests that, in addition to their role i n facilitating the expansion of plant cells, expansins may also contri bute to cell wall disassembly in nongrowing tissues, possibly by enhan cing the accessibility of noncovalently bound polymers to endogenous e nzymic action.