Manganese is a widely distributed plant disorder appearing on acidic and insufficiently drained soils of the (sub)tropics. In the tropical legume cowpea increased manganese (Mn) availability leads to typical toxicity symptoms in form of visible brown apoplastic depositions consisting of Mn oxides and polyphenols followed by chlorosis and leaf shedding. Silicon (Si) supply alleviates toxicity symptoms in Mn-sensitive cultivars. Here, Mn and Si-mediated apoplastic and symplastic modulations in the metabolome and proteome of two genotypes differing in Mn tolerance are described. The key role of apoplastic peroxidases in Mn toxicity was confirmed by the characterization of isoenzymes. The decisive role of specific phenols as NADH-peroxidase activity-modulators was substantiated. Modifications of photosynthesis were greater in the Mn-sensitive than in the Mn-tolerant cultivar. These changes also affected the primary carbohydrate and nitrogen metabolism reflecting its disturbance and need for rebalancing. Proteomic but also metabolomic results particularly after short-term Mn supply indicated coordinated stress-perception and signal-transduction processes to be triggered earlier more in the Mn-sensitive cultivar. Differences and changes in the abundance of other metabolites and enzymes of the primary and secondary metabolism and the antioxidative regeneration system not only in the symplast but also the apoplast may partly explain and contribute to genotypic and Si-mediated Mn tolerance. In conclusion, Mn toxicity seems to involve a coordinated interplay of apoplastic and symplastic metabolic pathways, whereas Mn tolerance seems to be based primarily on a constitutive higher antioxidative capacity.

The apoplast is known to play a predominant role in the expression of manganese (Mn) toxicity in cowpea (Vigna unguiculata [L.] Walp). The initial rapid symplastic molecular changes leading to Mn tolerance or triggering proteomic and metabolomic apoplastic processes leading to Mn toxicity in cowpea are currently poorly understood. Therefore, the focus was on the identification of early excess Mn-induced genotypic differences in the leaf-gene expression of the Mn-tolerant cowpea cv. TVu 1987 and the Mn-sensitive cowpea cv. TVu 91. Furthermore, the beneficial mineral element silicon (Si), known to improve Mn tolerance in plants, was included in the experiments in order to detect modifications in gene expression leading to enhanced Si-mediated Mn tolerance in the Mn-sensitive cowpea genotype. Thus, the PCR-based Suppression Subtractive Hybridization technique (SSH) enabled a creation of cultivar and treatment specific subtractive cDNA libraries. The results showed that short-term elevated Mn supply induced large-scale changes in the transcriptome of both cowpea cultivars. Furthermore, the coexistence of a Mn-combined but also broad constitutive impact of Si on the transcriptome level could be demonstrated for the Mn-sensitive cowpea gentoype. These results call for a more in-depth analysis of genotypic differences in the excess Mn response on the transcriptome level including a comprehensive selection of further candidate genes. Therefore, an expanded transcriptomic assay, including the selection of differentially expressed candidate genes and the investigation of their genotype specific expression pattern by quantitative Real-Time PCR (qRT-PCR), might reveal further insights into the symplastic processes triggering Mn tolerance through apoplastic processes in cowpea.