Expansins

Expansins are cell wall proteins that have been proposed to participate in disruption of hydrogen bonding between hemicellulose and cellulose polymers at the surface of the cellulose micro fibril. These polymeric associations are particularly important for cell expansion and other developmental events in which cell wall disassembly occurs, such as tissue softening during fruit ripening (Rose et al., 1997). Several expansin genes are expressed during tomato fruit development and ripening (Brummell et al, 1999b). The over-expression and suppression of one expansin gene, LeExp1, was examined in tomato plants expressing a sense full-length or truncated LeExp1 gene (Brummell et al., 1999c). Transgenic fruit from plants over-expressing LeExp1 were significantly less firm at the mature green and breaker stages and analysis of the cell wall material demonstrated that precocious depolymerization of the hemicellulose structure of the wall correlated with constitutive over-expression of LeExp1. However, expression in tomato of a cucumber hypocotyl expansin CsExp1, did not result in phenotypic alterations of transgenic tomato fruit, suggesting that divergent expansin proteins may have distinct functions or substrates in vivo (Rochange and McQueen-Mason, 2000). Suppression of LeExp1 in transgenic tomato resulted in increased fruit firmness, especially at the early (e.g. Breaker) stage of ripening (Brummell et al., 1999c). Surprisingly, in the LeExp1 suppressed fruit, polyuronide depolymerization but not hemicellulose depolymerization was reduced in the later stages of ripening in these fruit.