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Tomato Seed Research Tomato seed has also become a model system of seed germination research. Tomato seed provides an excellent system for seed germination research, because it has the embryo and the endosperm, which is essential for analyzing physical and chemical interactions between these two tissues. Its size is relatively larger than seeds in other plant species, such as tobacco and Arabidopsis seeds, making it feasible to dissect into different seed parts, and small enough for population analyses such as germination tests and biochemical assays.
In germinating seeds, radicle emergence is determined by the balance between the embryo growth potential and the mechanical resistance of the endosperm. This basic concept of seed germination has been greatly advanced by studies on tomato seeds. In tomato seed, the embryo is surrounded by the rigid endosperm. Although the endosperm plays an important role in supplying nutrition for growing embryo after germination, the tissue is an obstacle to radicle protrusion.
The micropylar region of the endosperm (called “endosperm cap”) which is adjacent to the radicle tip provides a mechanical resistance. For germination to occur in tomato seed, the endosperm cap must be weakened. Our research focuses on the mechanisms of endosperm weakening.
The rigidity of the endosperm cap is due to the relatively thick cell walls. The cell walls of this tissue mainly consist of mannan polymers, possibly galactomannans or galactoglucomannans.
Galactomannans consist of linear mannose backbone and side branches of galactose. The complete degradation of galactomannans requires concerted action of three major enzymes, b-mannanase, b-mannosidase and a-galactosidase. Our research focuses on the gene expression and function of endo-b-mannanase.
Different forms of mannanase appear during and after germination. In the endosperm of tomato seed, the cell wall galactomanans are major carbohydrate reserves. Galactomannans are degraded in the endosperm of germinated seeds to support seedling growth. This stage is called the post-germinative stage. The post-germinative stage-specific endo-b-mannanase (LeMAN1) is expressed in the whole endosperm of germinated tomato seeds. In contrast, the germination-specific mannanase (LeMAN2) is expressed exclusively in the endosperm cap region which is adjacent to the radicle tip. The physiological role of the geminative mannanase is to degrade the cell walls and subsequently weaken this tissue for the radicle to penetrate it. Tissue print and RNA hybridization experiments clearly show that these two mannanase genes are expressed in spatially and temporally different manners.
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