The role of the ripening-specific expansin Exp1 protein in fruit softening and cell wall metabolism was investigated by suppression and overexpression of Exp1 in transgenic tomato plants. Fruit in which Exp1 protein accumulation was suppressed to 3% that of wild-type levels were firmer than controls throughout ripening. Suppression of Exp1 protein also substantially inhibited polyuronide depolymerization late in ripening but did not prevent the breakdown of structurally important hemicelluloses, a major contributor to softening. In contrast, fruit overexpressing high levels of recombinant Exp1 protein were much softer than controls, even in mature green fruit before ripening commenced. This softening was correlated with the precocious and extensive depolymerization of structural hemicelluloses, whereas polyuronide depolymerization was not altered. These data are consistent with there being at least three components to fruit softening and textural changes. One component is a relaxation of the wall directly mediated by Exp1, which indirectly limits part of a second component due to polyuronide depolymerization late in ripening, perhaps by controlling access of a pectinase to its substrate. The third component is caused by depolymerization of hemicelluloses, which occurs independently of or requires only very small amounts of Exp1 protein.
INTRODUCTIONPlant cell growth is constrained by the cell wall, whose rigid cellulose microfibrils are held together by three structurally independent but interacting matrices (Carpita and Gibeaut, 1993). The matrix glycans, or hemicelluloses, are polysaccharides composed of neutral sugars, a major constituent of which in dicotyledonous species is xyloglucan. Xyloglucan molecules coat and cross-link cellulose microfibrils and, by extensive noncovalent bonding, anchor the microfibrils relative to one another. The xyloglucan-cellulose framework is itself embedded in a pectin matrix composed of polyuronide molecules, together with a domain of structural glycoproteins. Precisely how cell wall components and their intermolecular interactions are changed to allow wall loosening during growth or remodeling during cell development is not known, but a group of highly conserved cell wall proteins called expansins have been implicated in many aspects (Cosgrove, 1998).Expansins are encoded by large multigene families (Shcherban et al., 1995; Cho and Kende, 1997; Cosgrove et al., 1997;Brummell et al., 1999a), and the expression of expansin mRNA and protein is correlated with growth in many tissues of the plant, including hypocotyls (McQueen-Mason et al., 1992), coleoptiles (Li et al., 1993), internodes (Cho and Kende, 1997), leaves (Keller and Cosgrove, 1995), roots (Wu et al., 1996), and green fruit (Brummell et al., 1999a). However, expansins also may play a role in the cell wall modification involved in other aspects of plant development. In tomato, expansin Exp1 mRNA is expressed at high levels specifically during fruit ripening (Rose et al., 1997) and is the major expansin gene family ...
