Expression of a divergent expansin gene is fruit-specific and ripening-regulated

Abstract

Expansins are proteins that induce extension in isolated plant cell walls in vitro and have been proposed to disrupt noncovalent interactions between hemicellulose and cellulose microfibrils. Because the plant primary cell wall acts as a constraint to cell enlargement, this process may be integral to plant cell expansion, and studies of expansins have focused on their role in growth. We report the identification of an expansin (LeExp1) from tomato that exhibits high levels of mRNA abundance 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 also identified in ripening melons and strawberries, suggesting that they are a common feature of fruit undergoing rapid softening. Furthermore, the sequence of LeExp1 and its homologs from other ripening fruit define 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 inf luenced directly by ethylene and by a developmentally modulated transduction pathway. The identification of a ripening-regulated expansin gene in tomato and other fruit suggests that, in addition to their role in facilitating the expansion of plant cells, expansins may also contribute to cell wall disassembly in nongrowing tissues, possibly by enhancing the accessibility of noncovalently bound polymers to endogenous enzymic action.

Temporal Sequence of Cell Wall Disassembly in Rapidly Ripening Melon Fruit1

“…Expansins are proteins that are thought to disrupt the noncovalent association between cellulose and matrix polysaccharides and could act to regulate noncovalent cell wall polymer associations in ripening fruit. Rose et al (1997) recently identified and characterized a highly abundant, fruit-ripening-specific expansin gene from tomato, as well as homologs from strawberry and melon. The role of expansins in fruit softening is not clear and is currently under investigation.…”

Map position and functional allelic diversity of Md-Exp7, a new putative expansin gene associated with fruit softening in apple (Malus × domestica Borkh.) and pear (Pyrus communis)

“…Pc-Exp7 (Hiwasa et al 2003) and the novel Md-Exp7 are phylogenetically grouped close to Le-Exp1 (Fig. 8), a tomato expansin gene that is fundamental for fruit softening (Rose et al 1997). …”

“…Expansins, a novel class of proteins, have been proposed as the major enzymatic agent in cell-wall remodelling (Cosgrove 2000b) and are thought to be involved in the disruption of the non-covalent bonds between the hemicellulose matrix and the cellulose microfibril (Cosgrove 1997), thus exposing the structural polymer to the action of other cell-wall enzymes. Recently, a two-step fruit softening model has been defined in which expansin acts in the early stage and the latter is regulated by hydrolases like polygalacturonases (Rose et al 1997;Powell et al 2003).…”

Expression of an Expansin Gene Is Correlated with Root Elongation in Soybean

“…A recent study showed that the LeEXP18 expression does not correlate with elongation growth and that some of the tomato expansin genes are differentially expressed during fruit development, suggesting that not all the expansins are involved in the expansion process (Brummell et al, 1999a(Brummell et al, , 1999bCaderas et al, 2000;Rose et al, 2000). Therefore, the complexity of the expansin family and the tissue-specific expression patterns of some of its members suggest that different expansins may play different roles in various cell types during organ development in plants (Rose et al, 1997;Reinhardt et al, 1998).…”