Mutation of SAD2, an importin β‐domain protein in Arabidopsis, alters abscisic acid sensitivity
2006
A number of protein and RNA‐processing mutants have been shown to affect ABA sensitivity. A new mutant, sad2‐1, was isolated from a T‐DNA mutagenized population of RD29A:LUC plants and shown to have increased luminescence after ABA, salt, cold or polyethylene glycol treatments. Expression of several ABA‐ and stress‐responsive genes was higher in the mutant than in the wild type. sad2‐1 also exhibited ABA hypersensitivity in seed germination and seedling growth. SAD2 was found to encode an importin β‐domain family protein likely to be involved in nuclear transport. SAD2 was expressed at a low level in all tissues examined except flowers, but SAD2 expression was not inducible by ABA or stress. Subcellular localization of GFP‐tagged SAD2 showed a predominantly nuclear localization, consistent with a role for SAD2 in nuclear transport. Knockout of the closest importin β homolog of SAD2 in Arabidopsis did not duplicate the sad2 phenotype, indicating that SAD2 plays a specific role in ABA signaling. Analysis of RD29A:LUC luminescence and ABA and stress sensitivity in double mutants of sad2‐1 and sad1 or abh1‐7, a newly isolated allele of ABH1 also in the RD29A:LUC background, suggested that SAD2 acts upstream of or has additive effects with these two genes. The results suggest a role for nuclear transport in ABA signal transduction, and the possible roles of SAD2 in relation to that of SAD1 and ABH1 are discussed.
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