The physiological and molecular reasons for manganese (Mn) toxicity and Mn leaf-tissue tolerance in cowpea (Vigna unguiculata (L.) Walp.) are still not well understood. The available information suggests that Mn excess leads to clear changes in the proteome of the leaf apoplast. Therefore, in the presently running project Ho 931/17-1 an in-depth characterization of apoplastic protein fractions is in the centre of the interest. However, parallel determinations of metabolites in the apoplast indicate that these metabolites play a major role in the expression of Mn toxicity. In this proposal we suggest to complement the extended proteomic studies toward the leaf symplast by a characterization of the Mn toxicity-induced changes of the transcriptome and genome, because the identification of the related genes is a prerquisire for the elucidation of the physiological relationships. After the successful establishment of the methodology we want to build and analyse substractive cDNA libraries from leaf tissues differing in Mn tolerance (genotype, Si nutrition, leaf age). After identification of the most strongly and sensitively expressed genes through sequencing of the selected clones and data bank searches the differentially transcribed genes will be functionally characterised through further molecular analysis. We expect that complementing the present studies through this project will substantially advance the physiological and molecular understanding of Mn toxicity and Mn tolerance.