 |
 |
Hiro Nonogaki
Associate Professor, Horticulture
|
 |
||||||||||||||
RESEARCH:
Our research focuses on the biochemical and molecular mechanisms of seed germination, which is a first step in plant growth and development. Successful germination and seedling establishment are critical events to plant production whether in natural ecosystems and agriculture. Elucidating the mechanism of seed germination is an important theme in plant biology. Seed germination is completed when the radicle emerges from seed. Germination sensu stricto defines the physiological events that occur between seed imbibition and protrusion of radicle through the testa. Since the balance between the embryo growth potential and the resistance of the endosperm determines whether the radicle emerges, it is important to focus on embryo- and endosperm-specific physiological events and the interplay between these two tissues for comprehensive understanding of the mechanism of seed germination. We identify and characterize seed germination-associated genes that are expressed specifically in the embryo and the endosperm of germinating seeds using Arabidopsis and tomato seeds. The research projects are part of the NSF-funded Integrative Seed Biology Program.
Endosperm weakening: We are conducting research on the mechanism of endosperm weakening using tomato seed as a model system, in which the endosperm plays a significant role in controlling the timing of germination. The micropylar region of the endosperm of tomato seed (called 'endosperm cap'), which is adjacent to radicle tip, is a mechanical resistance to radicle elongation. The rigidity of this tissue is due to its thick cell walls, which consist mainly of galactomannans. One of the galactmannan-hydrolyzing enzymes, endo- b -mannanase plays an important role in endosperm weakening in tomato seeds by degrading the endosperm cell walls. We are characterizing a germination-specific endo- b -mannanase gene LeMAN2 which is expressed exclusively in the endosperm cap of germinating tomato seeds. Expression of endo- b -mannanase gene is also important for completion of embryogenesis in carrot seeds .
Embryo-specific factors: Compared with the intensive research on the mechanisms of endosperm weakening in tomato seeds in the past years, the embryo-specific events during germination have not been fully investigated. It is necessary to identify the key factors for germination in the embryo as well. Although tomato seed provides a good model system for studying endosperm weakening, Arabidopsis seed offers a more powerful tool for identification of the embryonic factors involved in seed germination. Arabidopsis provides an excellent model for identifying these embryonic factors given its seed morphology as well as the availability of its genomic sequence, a variety of suitable mutants and T-DNA knockout plants. We are currently using the Arabidopsis enhancer-trap lines for this study .