Possible consequences of an inability of plants to control manganese uptake

Background This commentary presents several thoughts elicited by the observation of Lambers et al. (Plant Soil, 2021) in this Special Issue that the release of carboxylates by roots increases manganese (Mn) uptake by plants. Manganese is a plant nutrient, but is toxic in excess. Root cells take up Mn from the rhizosphere solution through non-specific transporters, whose activities are regulated by elements other than Mn, and Mn phytoavailability in soil is also impacted by plant nutritional status of elements other than Mn. These complications could result in a plant being unable to respond appropriately to vagaries in Mn phytoavailability. Scope The release of carboxylates by roots increases Mn phytoavailability and Mn uptake by plants. Lambers et al. (Plant Soil, 2021) suggest that this phenomenon might be used to identify plant species that release carboxylates into the rhizosphere in response to P deficiency. We suggest that, for the approach of Lambers and colleagues to be successful, it is necessary for all plants being compared (1) to be capable of increasing root Mn uptake and leaf Mn concentration should carboxylates be released, and increase these to a similar extent for the approach to be quantitative, and (2) to tolerate the greater tissue Mn concentrations resulting from increased Mn phytoavailability. Conclusions We observe (1) that the leaf Mn requirement, critical leaf Mn concentration for toxicity and capacity for Mn accumulation when grown hydroponically in a nutrient-replete solution are all positively correlated among plant species, which suggests that they might have evolved in parallel, and (2) that, although some orders containing species accumulating large shoot Mn concentrations are typically non-mycorrhizal and release carboxylates into the rhizosphere, such as the Proteales, many orders containing species with this trait are characterised by conventional mycorrhizal associations.