Genetic Control and Integration of Maturation and Germination Pathways in Seed Development

Abstract

The viviparous and germination mutants of maize and Arabidopsis thaliana illuminate the mechanism that integrates control of morphogenetic, maturation, dormancy, and germination pathways in seed development. Key elements of this mechanism include (a) developmental control of abscisic acid and gibberellin hormone synthesis and perception, (b) integration of maturation and anthocyanin pathways in the maize seed, (c) functions of the VP1 and ABI3 factors in abscisic acid-regulated gene expression, and (d) intrinsic developmental genes that couple seed maturation to the program of embryo morphogenesis. The scarcity of mutants that affect timing or tissue specificity of hormone synthesis in the seed is an important constraint to progress in understanding the role of hormone signals. The interactions among the abscisic acid-insensitive abil, abi2, abi3, abi4, and abi5 mutants of A. thaliana are consistent with multiple pathways of abscisic acid signal transduction in the seed. The maize Vp1 and A. thaliana Abi3 genes are functional homologs that mediate a seed-specific abscisic acid response necessary for maturation. The specific roles of these genes in controlling dormancy and anthocyanin synthesis in the seed have diverged since the evolutionary separation of maize and A. thaliana. The coupling of anthocyanin synthesis to maturation in maize may have resulted from changes in the downstream cl regulatory gene rather than a functional change in VPl. Functional analysis indicates that VP1 is a transcriptional activator of the Em and C1 genes in maize, although its specific role in abscisic acid signal transduction remains poorly understood. The lecl and fus3 mutants of A. thaliana and pleiotropic viviparous mutants of maize may identify intrinsic factors that couple the maturation pathway to embryo morphogenesis.