Two tomato expansin genes show divergent expression and localization in embryos during seed development and germination

Expansins are plant proteins that can induce extension of isolated cell wal ls and are proposed to mediate cell expansion. Three expansin genes were ex pressed in germinating tomato (Lycopersicon esculentum Mill.) seeds, one of which (LeEXP4) was expressed specifically in the endosperm cap tissue encl osing the radicle tip. The other two genes (LeEXP8 and LeEXP10) were expres sed in the embryo and are further characterized here. LeEXP8 mRNA was not d etected in developing or mature seeds but accumulated specifically in the r adicle cortex during and after germination. In contrast, LeEXP10 mRNA was a bundant at an early stage of seed development corresponding to the period o f rapid embryo expansion; it then decreased during seed maturation and incr eased again during germination. When gibberellin-deficient (gib-1) mutant s eeds were imbibed in water, LeEXP8 mRNA was not detected, but a low level o f LeEXP10 mRNA was present. Expression of both genes increased when gib-1 s eeds were imbibed in gibberellin. Abscisic acid did not prevent the initial expression of LeEXP8 and LeEXP10, but mRNA abundance of both genes subsequ ently decreased during extended incubation. The initial increase in LeEXP8, but not LeEXP10, mRNA accumulation was blocked by low water potential, but LeEXP10 mRNA amounts fell after longer incubation. When seeds were transfe rred from abscisic acid or low water potential solutions to water, abundanc e of both LeEXP8 and LeEXP10 mRNAs increased in association with germinatio n. The tissue localization and expression patterns of both LeEXP8 and LeEXP 10 suggest developmentally specific roles during embryo and seedling growth .