A germination-specific endo-beta-mannanase gene is expressed in the micropylar endosperm cap of tomato seeds

Endo-beta-mannanase (EC 3.2.1.78) is involved in hydrolysis of the mannan-r ich cell walls of the tomato (Lycopersicon esculentum Mill.) endosperm duri ng germination and post-germinative seedling growth. Different electrophore tic isoforms of endo-beta-mannanase are expressed sequentially in different parts of the endosperm, initially in the micropylar endosperm cap covering the radicle tip and subsequently in the remaining lateral endosperm surrou nding the rest of the embryo. We have isolated a cDNA from imbibed tomato s eeds (LeMAN2) that shares 77% deduced amino acid sequence similarity with a post-germinative tomato mannanase (LeMAN1). When expressed in Escherichia coli, the protein encoded by LeMAN2 cDNA was recognized by anti-mannanase a ntibody and exhibited endo-beta-mannanase activity, confirming the identity of the gene. LeMAN2 was expressed exclusively in the endosperm cap tissue of tomato seeds prior to radicle emergence, whereas LeMAN1 was expressed on ly in the lateral endosperm after radicle emergence. LeMAN2 mRNA accumulati on and mannanase activity were induced by gibberellin in gibberellin-defici ent gib-1 mutant seeds but were not inhibited by abscisic acid in wild-type seeds. Distinct mannanases are involved in germination and post-germinativ e growth, with LeMAN2 being associated with endosperm cap weakening prior t o radicle emergence, whereas LeMAN1 mobilizes galactomannan reserves in the lateral endosperm.