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Background  Very little is known about manganese (Mn)-toxicity-responsive genes in citrus plants. Seedlings of ‘Xuegan’ ( Citrus sinensis ) and ‘Sour pummelo’ ( Citrus grandis ) were irrigated for 17 weeks with nutrient solution containing 2 μM (control) or 600 μM (Mn-toxicity) MnSO 4 . The objectives of this study were to understand the mechanisms of citrus Mn-tolerance and to identify differentially expressed genes, which might be involved in Mn-tolerance.    Results  Under Mn-toxicity, the majority of Mn in seedlings was retained in the roots;  C. sinensis  seedlings accumulated more Mn in roots and less Mn in shoots (leaves) than  C. grandis  ones and Mn concentration was lower in Mn-toxicity  C. sinensis  leaves compared to Mn-toxicity  C. grandis  ones. Mn-toxicity affected  C. grandis  seedling growth, leaf CO 2  assimilation, total soluble concentration, phosphorus (P) and magenisum (Mg) more than  C. sinensis . Using cDNA-AFLP, we isolated 42 up-regulated and 80 down-regulated genes in Mn-toxicity  C. grandis  leaves. They were grouped into the following functional categories: biological regulation and signal transduction, carbohydrate and energy metabolism, nucleic acid metabolism, protein metabolism, lipid metabolism, cell wall metabolism, stress responses and cell transport. However, only 7 up-regulated and 8 down-regulated genes were identified in Mn-toxicity  C. sinensis  ones. The responses of  C. grandis  leaves to Mn-toxicity might include following several aspects: (1) accelerating leaf senescence; (2) activating the metabolic pathway related to ATPase synthesis and reducing power production; (3) decreasing cell transport; (4) inhibiting protein and nucleic acid metabolisms; (5) impairing the formation of cell wall; and (6) triggering multiple signal transduction pathways. We also identified many new Mn-toxicity-responsive genes involved in biological and signal transduction, carbohydrate and protein metabolisms, stress responses and cell transport.    Conclusions  Our results demonstrated that  C. sinensis  was more tolerant to Mn-toxicity than  C. grandis , and that Mn-toxicity affected gene expression far less in  C. sinensis  leaves. This might be associated with more Mn accumulation in roots and less Mn accumulation in leaves of Mn-toxicity  C. sinensis  seedlings than those of  C. grandis  seedlings. Our findings increase our understanding of the molecular mechanisms involved in the responses of plants to Mn-toxicity.

Leaf cDNA-AFLP analysis of two citrus species differing in manganese tolerance in response to long-term manganese-toxicity