A gibberellin-regulated xyloglucan endotransglycosylase gene is expressed in the endosperm cap during tomato seed germination

Xyloglucan endotransglycosylases (XETs) modify xyloglucans, major components of primary cell walls in dicots. A cDNA encoding an XET (LeXET4) was isolated from a germinating tomato (Lycopersicon esculentum Mill.) seed cDNA library. DNA gel blot analysis showed that LeXET4 is a single-copy gene in the tomato genome. LeXET4 mRNA was strongly expressed in germinating seeds, was much less abundant in stems, and was not detected in roots, leaves or flower tissues. During germination, LeXET4 mRNA was detected in seeds within 12 h of imbibition with maximum mRNA abundance at 24 h. Tissue prints showed that LeXET4 mRNA was localized exclusively to the endosperm cap region. Expression of LeXET4 was dependent on exogenous gibberellin (GA) in GA-deficient (gib-1 mutant) tomato seeds, while abscisic acid, a seed germination inhibitor, had no effect on LeXET4 mRNA expression in wild-type seeds. LeXET4 mRNA disappeared after radicle emergence, even though degradation of the lateral endosperm cell walls continued. The temporal, spatial and hormonal regulation pattern of LeXET4 gene expression suggests that XET has a role in endosperm cap weakening, a key process regulating tomato seed germination.