Expression of a polygalacturonase associated with tomato seed germination

Radicle protrusion from tomato (Lycopersicon esculentum Mill.) seeds to com plete germination requires weakening of the endosperm tissue opposite the r adicle tip. In common with other cell wall disassembly processes in plants, polygalacturonases (PGs) may be involved. Only calcium-dependent exo-PG ac tivity was detected in tomato seed protein extracts. Chromatographic profil es of a partially acid-hydrolyzed fraction of polygalacturonic acid further digested with seed extract were consistent with the presence of only calci um-dependent exo-PG activity. In addition, a transcript encoding a previous ly unknown PC was detected prior to the completion of germination. The mRNA , produced from a gene (LeXPG1) estimated by Southern analysis to be repres ented once in the genome, was also present in flowers (anthers) and in lowe r amounts in roots and stems. LeXPG1 mRNA abundance was low during seed dev elopment, increased during imbibition, and was even greater in seeds that h ad completed germination. Expression of LeXPG1 during germination predomina tes in the endosperm cap and radicle tip, and in the radicle appears as a d istinct band possibly associated with vascular tissue differentiation. We s uggest that PG is involved in cell wall loosening of the endosperm necessar y for radicle protrusion from tomato seeds and in subsequent embryo and see dling growth.