Functional analyses of a Conifer AB13 gene, an ABA signalling component regulating seed dormancy and germination

Seed dormancy is an adaptive trait improving survival of the next generation by controlling germination until favorable conditions ensue. While beneficial to the plant itself, seed dormancy is not always viewed as desirable by agricultural and forest industries, which rely upon the rapid and synchronous germination of seeds. Yellow-cedar (Chamaecyparis nootkatensis D. Don Spach), an important conifer species of the Pacific northwest, produces seeds that are deeply dormant at maturity; it is thus an important model species to study mechanisms of dormancy and germination. ABI3 (Abscisic Acid Insensitive3) is a transcription factor that regulates ABAresponsive genes during seed development, including those involved in reserve deposition, dormancy inception and acquisition of desiccation tolerance. The yellow-cedar CnABI3 gene was functionally characterized. CnABI3 encodes a protein of 794 amino acids and, under normal conditions, its expression is only detected in seeds. A role in dormancy maintenance of seeds of conifer species was established by monitoring mRNA and protein levels before, during and after dormancy termination. Dormancy termination and germination is associated with a downregulation of the expression of this gene, but posttranscriptional controls are also likely operative. CnABI3 also plays a potential role in oxidative stress responses during early post-germinative growth. CnABI3 is a true orthologue of ABI3 genes of angiosperms; it shares many common features with other ABI3 genes, yet displays unique characteristics as well. Like other AB13 orthologues, ectopic expression of the CnABI3 gene in tobacco vegetative tissues activates genes driven by seed storage-protein gene promoters and ABA has a synergistic effect on activation. The CnABI3 gene, when transformed into a severe Arabidopsis abi3 null mutant (abi3-6) functions nearly perfectly. This functional complementation reveals the degree of conservation of ABI3 functions between gymnosperms and angiosperms. Three yellow-cedar proteins that physically interact with CnABI3 were identified using a yeast two-hybrid approach; their biological hnctions in relation to CnABI3 have been investigated in yellow-cedar, and in Arabidopsis using homologous genes. The results suggest a global role for ABI3 in the control of many key transitions in seeds and plants, and a potential role for the protein in the regulation of flowering.