Genetic control mechanisms regulating the initiation of germination

Abstract

Genetic analysis of the transition from embryogenesis to germination has shown that these processes are under strict sequential, mutually exclusive, control. Understanding the regulation of this phase transition should provide important approaches to new technologies that can be usedto improve seed quality traits in crop plants. The developmental disorder pre-harvest sprouting (PHS) in cereals and pre-germination of seeds in the pods of oilseed rape are significant agronomic problems, that occur when seeds develop under cool moist environmental conditions. Phenotypically, pre-germination of seeds on the mother plant is similar to the severe maize andArabidopsismutationsviviparous1(vp1), andabscisic acid insensitive3(abi3).The corresponding loci encode homologous transcription factors thatsimultaneously activate embryo maturation and repress germination. We have analysed gene expression programmes in wheat embryos under conditions that induce PHS, and have analysed in detail the structure and expression of wheat Vp1 homeologues. These analyses show that both maturation and germination genes are expressed simultaneously in embryos grown under cool moist conditions, and that the majority ofVP1 transcripts expressed in the cytoplasm during normal embryo maturation are not correctly spliced. These results suggest that under perturbed environmental conditions wheat may not express enough functional VP1 activity to repress germination. This hypothesis is currently being tested using transgenic approaches. Repression of germination byABI3 andother loci inArabidopsis indicates that these factors interact with loci that enhance germination potential. Using a novel genetic screen to search for regulators of germination, we have identified theCOMATOSE(CTS) locus. Genetic and physiological analyses show thatCTS regulates germination potential by enhancing after ripening, sensitivity to gibberellins and pre-chilling, and by repressing the activities of loci that activate embryo maturation.