The Arabidopsis kinase-associated protein phosphatase regulates adaptation to Na+ stress

The kinase-associated protein phosphatase (KAPP) is a regulator of the receptor-like kinase (RLK) signaling pathway. Loss-of-function mutations rag1-1 (root attenuated growth1-1) and rag1-2, in the locus encoding KAPP, cause NaCl hypersensitivity in Arabidopsis thaliana. The NaCl hypersensitive phenotype exhibited by rag1 seedlings includes reduced shoot and primary root growth, root tip swelling, and increased lateral root formation. The phenotype exhibited by rag1-1 seedlings is associated with a specific response to Na+ toxicity. The sensitivity to Na+ is Ca2+ independent and is not due to altered intracellular K+/Na+. Analysis of the genetic interaction between rag1-1 and salt overly sensitive1 (sos1-14) revealed that KAPP is not a component of the SOS signal transduction pathway, the only Na+ homeostasis signaling pathway identified so far in plants. All together, these results implicate KAPP as a functional component of the RLK signaling pathway, which also mediates adaptation to Na+ stress. RLK pathway components, known to be modulated by NaCl at the messenger RNA level, are constitutively down-regulated in rag1-1 mutant plants. The effect of NaCl on their expression is not altered by the rag1-1 mutation.