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.