A mutation in the Arabidopsis MAP kinase kinase 9 gene results in enhanced seedling stress tolerance

MAP kinase signal transduction pathways are important conduits for stress signalling in plants. There are 10 genes encoding possible MAP kinase kinase proteins in Arabidopsis thaliana. In this work, a T-DNA insertion mutation of the Arabidopsis MKK9 gene (At1g73500) has been isolated and characterised. The T-DNA is inserted towards the 3' end of the coding region of MKK9, resulting in a truncated transcript and the mutant is predicted to produce an impaired or non-functional protein. The mkk9 plants appear morphologically normal, but are insensitive to low levels of ABA (0.5 µM) during germination. The response to other plant growth substances appears normal, as is the response to freezing temperatures and the systemic wound response. The mkk9 mutant is however salt insensitive, it will germinate on media containing up to 150 mM NaCl. Similarly, mkk9 is insensitive to mannitol in the germination medium. Induction of expression of the stress-induced genes RD29 and RD22 after salt treatment is faster in the mutant, and for RD22, the expression is prolonged. Enhanced stress-related gene expression in mkk9 is hypothesised to be the reason for the enhanced ABA and salt tolerance, which suggests that MKK9 acts as a negative regulator of the abiotic stress response. © 2007 Elsevier Ireland Ltd. All rights reserved.