Over-expression of MAP3Kδ4, an ABA-inducible Raf-like MAP3K that confers salt tolerance in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades play important roles in plant responses to various environmental stimuli, including high salt or drought levels. Arabidopsis MAP3K delta 4 with PAS (period circadian protein, arylhydrocarbon receptor nuclear translocator protein and single-minded protein) domain is one of the Raf-type MAPKKKs whose function has not been identified to date. Previous studies have shown that the MAP3K delta 4 over-expressing transformant exhibits vigorous growth. In this study, RT-PCR analysis showed that MAP3K delta 4 transcripts are increased through stress treatments, such as high salt, osmosis, drought and cold and the plant hormone, abscisic acid (ABA). The precipitation of MAP3K delta 4 using its specific antibody showed that ABA treatment markedly induces the activity of this enzyme. Furthermore, the ABA-mediated inhibition of seed germination was relieved in transgenic Arabidopsis over-expressing MAP3K delta 4. These results suggested that MAP3K delta 4 plays an important role in ABA signalling. Transgenic Arabidopsis over-expressing MAP3K delta 4 also exhibited enhanced tolerance to salt stress. The results obtained in this study demonstrated that MAP3K delta 4 was active during ABA-related responses and is involved in both stress tolerance and increased biomass. Therefore, MAP3K delta 4 and its counterpart genes are important with respect to agricultural developments.