Genetic control mechanisms regulating the initiation of germination

Genetic analysis of the transition from embryogenesis to germination has sh own that these processes are under strict sequential, mutually exclusive, c ontrol. Understanding the regulation of this phase transition should provid e important approaches to new technologies that can be used to improve seed quality traits in crop plants. The developmental disorder pre-harvest sprouting (PHS) in cereals and pre-g ermination of seeds in the pods of oilseed rape are significant agronomic p roblems, that occur when seeds develop under cool moist environmental condi tions. Phenotypically, pre-germination of seeds on the mother plant is simi lar to the severe maize and Arabidopsis mutations viviparous 1 (vp 1), and abscisic acid insensitive3 (abi3). The corresponding loci encode homologous transcription factors that simultaneously activate embryo maturation and r epress germination. We have analysed gene expression programmes in wheat em bryos under conditions that induce PHS, and have analysed in detail the str ucture and expression of wheat Vp 1 homeologues. These analyses show that b oth maturation and germination genes are expressed simultaneously in embryo s grown under cool moist conditions, and that the majority of VP 1 transcri pts expressed in the cytoplasm during normal embryo maturation are not corr ectly spliced. These results suggest that under perturbed environmental con ditions wheat may not express enough functional VP 1 activity to repress ge rmination. This hypothesis is currently being tested using transgenic appro aches. Repression of germination by ABI3 and other loci in Arabidopsis indicates t hat these factors interact with loci that enhance germination potential. Us ing a novel genetic screen to search for regulators of germination, we have identified the COMATOSE(CTS) locus. Genetic and physiological analyses sho w that CTS regulates germination potential by enhancing after ripening, sen sitivity to gibberellins and pre-chilling, and by repressing the activities of loci that activate embryo maturation.