Expression of an expansin is associated with endosperm weakening during tomato seed germination

Expansins are extracellular proteins that facilitate cell wall extension, p ossibly by disrupting hydrogen bonding between hemicellulosic wall componen ts and cellulose microfibrils. In addition, some expansins are expressed in non-growing tissues such as ripening fruits, where they may contribute to cell wall disassembly associated with tissue softening. We have identified at least three expansin genes that are expressed in tomato (Lycopersicon es culentum Mill.) seeds during germination. Among these, LeEXP4 mRNA is speci fically localized to the micropylar endosperm cap region, suggesting that t he protein might contribute to tissue weakening that is required for radicl e emergence. In gibberellin (CA)-deficient (gib-1) mutant seeds, which germ inate only in the presence of exogenous GA, GA induces the expression of Le EXP4 within 12 hours of imbibition. When gib-1 seeds were imbibed in CA sol ution combined with 100 mum abscisic acid, the expression of LeEXP4 was not reduced, although radicle emergence was inhibited. In wild-type seeds, LeE XP4 mRNA accumulation was blocked by far-red light and decreased by low wat er potential but was not affected by abscisic acid. The presence of LeEXP4 mRNA during seed germination parallels endosperm cap weakening determined b y puncture force analysis. We hypothesize that LeEXP4 is involved in the re gulation of seed germination by contributing to cell wall disassembly assoc iated with endosperm cap weakening.