Mitogen-activated protein kinase (MAPK) cascades play important roles in plant responses to various environmental stimuli, including high salt or drought levels. Arabidopsis MAP3Kδ4 with PAS (period circadian protein, arylhydrocarbon receptor nuclear translocator protein and single-minded protein) domain is one of the Raftype MAPKKKs whose function has not been identified to date. Previous studies have shown that the MAP3Kδ4 overexpressing transformant exhibits vigorous growth. In this study, RT-PCR analysis showed that MAP3Kδ4 transcripts are increased through stress treatments, such as high salt, osmosis, drought and cold and the plant hormone, abscisic acid (ABA). The precipitation of MAP3Kδ4 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 MAP3Kδ4. These results suggested that MAP3Kδ4 plays an important role in ABA signalling. Transgenic Arabidopsis over-expressing MAP3Kδ4 also exhibited enhanced tolerance to salt stress. The results obtained in this study demonstrated that MAP3Kδ4 was active during ABA-related responses and is involved in both stress tolerance and increased biomass. Therefore, MAP3Kδ4 and its counterpart genes are important with respect to agricultural developments.

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