Proposed strategy for robust identification of serine/threonine phosphorylation in plants. In case of low stoichiometry as for the in vivo situation enrichment of phosphoproteins is necessary. To cope with this a novel enrichment procedure called metaloxide affinity chromatography (MOAC) is used in the strategy . The whole approach is applicable to identify phosphorylation sites out of complex samples. The library of in vivo sites may be used for screening of protein phosphorylation dynamics using triple-quadruple mass spectrometry .