The invention provides methods for assaying for the presence or activity of a MAP kinase in a sample, the methods comprising: (a) contacting the sample under conditions necessary for a phosphorylation reaction with a peptide substrate comprising the following target motif: HP(x)SPR, wherein: (x) represents any amino acid; (b) assessing any resulting phosphorylation of the peptide substrate; (c) optionally correlating the result from step (b) with the presence or activity of MAP kinase in the sample. Examples substrates include the Arabidopsis protein AtPhos34. In another aspect the invention provides methods and materials for influencing the salt andor drought tolerance phenotype of a plant, for example by use of nucleic acid encoding AtPhos34 or variants thereof.