Ethylene in seed formation and germination

In seed formation the role of ethylene has received little attention. The d ata available on zygotic embryogenesis suggest an association of the ethyle ne biosynthetic pathway and seed maturation. Over the course of dicot embry ogenesis, ACC-oxidase mRNA can be expressed in the cotyledons and embryonic axis. However, as maturation proceeds, cotyledonary ACC-oxidase expression disappears. In some seeds that develop primary dormancy, ethylene synthesi s can be among the prerequisites for breaking dormancy. Moreover, the persi stence of dormancy may be related to the difficulty of the embryonic axis t o produce the necessary ethylene levels or to low tissue sensitivity. The u se of inhibitors of ethylene biosynthesis or its action has provided data i mplicating an ethylene requirement for seed dormancy or germination in some species. However, the role of ethylene in germination remains controversia l. Some authors hold that gas production is a consequence of the germinatio n process, while others contend that ethylene production is a requirement f or germination. Furthermore, among seeds that require ethylene, some are ex tremely sensitive to the gas, while others require relatively high levels t o trigger germination. Recent studies with Xanthium pennsylvanicum seeds su ggest that beta-cyanoalanine-synthase is involved in ethylene-dependent ger mination. In addition, regulation of the partitioning of S-adenosyl-L-methi onine (AdoMet) between the ethylene vs polyamine biosynthetic pathways may be a way of controlling germination in some seeds. Such regulation may also apply to the reversal of seed thermoinhibition, which can occur when polya mine synthesis is inhibited, thereby strongly channelling AdoMet towards et hylene. The biological models and approaches that may shed additional light on the role of ethylene during seed germination are presented.