A gymnosperm ABI3 gene functions in a severe abscisic acid-insensitive mutant of Arabidopsis (abi3-6) to restore the wild-type phenotype and demonstrates a strong synergistic effect with sugar in the inhibition of post-germinative growth

The CnABI3 gene of yellow-cedar is an orthologue of the ABI3/VP1 gene of angiosperms; it shares many common characteristics with other ABI3/VP1 genes, yet has unique characteristics as well. We examined whether this gymnosperm transcription factor can functionally complement an angiosperm species with a defective ABI3 gene. A severe Arabidopsis abi3 null mutant abi3-6 was stably transformed with the CnABI3 gene coding-region driven by a modified CaMV 35S promoter. Several of the visible mutant phenotypes ( e. g., production of green seeds due to a lack of chlorophyll breakdown) were fully restored to those of the wild-type and the transformed seeds acquired desiccation tolerance. The functional complementation of the mutant also extended to the accumulation of several seed proteins ( including seed-storage-proteins, alpha-tonoplast intrinsic protein, dehydrin-related polypeptides and oleosin), which were restored to wild-type levels. However, not all phenotypes were fully restored; sensitivities of transgenic seeds to exogenous ABA ( as far as germination is concerned) were lower than that of the wild-type seeds, and flowering times were intermediate of those characteristic of wild-type and abi3-6 plants. A novel function for CnABI3, potentially related to a direct or indirect role in ER homeostasis was revealed. Two proteins with a molecular chaperone function in the ER (BiP and protein disulphide isomerase) were elevated in mutant seeds ( indicative of ER stress); expression of the CnABI3 gene decreased the accumulation of these proteins to levels characteristic of the wild-type. These studies reveal the degree of conservation of ABI3 functions between gymnosperms and angiosperms as well as some novel functions of ABI3-related genes.