Germination—Still a mystery

Abstract Germination is a complex process during which the seed must quickly recover physically from maturation drying, resume a sustained intensity of metabolism, complete essential cellular events to allow for the embryo to emerge, and prepare for subsequent seedling growth. Early following the start of imbibition of the dry seed there is re-establishment of metabolism; restitution of the chemical and structural integrity of cells requires the co-participation of synthetic and protective events. Protein synthesis and respiratory activity initially involve components stored within the mature dry seed, although transcription and translation commence early during imbibition, as shown by transciptome and metabolome analyses. Increases or modifications to hormones, especially GA, play an important role in achieving the completion of germination, at least in intact seeds. Removal or deactivation of ABA is also important; interactions between this and GA play a regulatory role. A restraint on the completion of germination in seeds of some species is imposed by the surrounding structures, e.g. the endosperm, and thus there is a requirement either for it to be enzymically weakened to allow the radicle to emerge, or for sufficient force to be generated within the embryo axis to physically break through, or both. While there is much information with respect to changes in gene expression during germination, no key event(s) has been identified that results in its completion. The downstream effects of the observed hormone changes are not known, and given the multipart nature of the seed, the requirements imposed upon it (repair, maintenance, preparation for seedling growth) in addition to completing germination (which involves only a limited number of cells), the challenge to identify ‘germination-completion’ genes is large. Hence there are limited opportunities at present for improving germination through genetic manipulation.