Sucrose metabolizing genes are critical for growth and development of maize seed

Seed development is intimately dependent upon the metabolic utilization of sucrose. Our long-term studies have shown that two seed mutants in maize, m iniature1 (mn1) and shrunken1 (sh1), are caused by mutations in seed-specif ic genes of sucrose metabolism. A deficiency of the Mn1-encoded cell wall i nvertase (CWI) enzyme leads to a drastic reduction in seed size identified as the mn1 seed mutation. We suggest that CWI plays a critical role in prov iding hexose sugars for mitotic divisions in the early stages of endosperm development. The CWI also controls endosperm sink strength and developmenta l stability of the maternal cells in pedicel. The sh1 seed is characterized by a collapsed crown of the mutant endosperm. The causal basis for the sh1 phenotype is a loss of the Sh1-encoded endosperm-specific sucrose synthase -l (SSI) which cleaves sucrose to yield precursors for both cellulose and s tarch biosynthesis. Our recent studies show that the SS1 enzyme plays a pre dominant role of providing the substrate for cellulose biosynthesis; wherea s, the second enzyme, SS2, appears to yield precursors for starch biosynthe sis in a developing endosperm.