Mycorrhizal Symbiosis Alleviates Mn Toxicity and Downregulates Mn Transporter Genes in Eucalyptus~Tereticornis Under Low Soil Phosphorus

Background and aims Tropical soils often present two issues that can hinder plant growth: low phosphorus (P) and high manganese (Mn) availability. Eucalyptus tereticornis is frequently cultivated in such soils. We investigated the effects of Mn in E. tereticornis under contrasting soil P availability and hypothesized that arbuscular mycorrhizal (AM) symbiosis could alleviate Mn toxicity by improving P nutrition and altering the expression of Mn transporter genes. Methods Inoculated (AM) and non-inoculated (NM) seedlings grew for seven months in a soil with low or sufficient P availability, under three Mn doses: control, 75 and 150 mg kg−1 Mn. We assessed growth, AM colonization, nutrient concentrations, and the expression of eight genes related to Mn transport and homeostasis in roots. Mn distribution at one-leaf level was determined by μ-XRF. Results Low P exacerbated Mn toxicity and hindered plant growth. Mycorrhizal symbiosis did not influence Mn accumulation, but improved growth and Mn tolerance at low P, partly by improving P nutrition. At sufficient P, foliar Mn reached 3500 mg kg−1, and μ-XRF patterns suggest preferential accumulation in the leaf lamina compared to margins or midribs. In NM plants, the vacuolar transporters EtVIT1 and EtMTP8 and the Mn-nicotianamine influx transporter EtYSL6 were the most responsive genes to Mn, while in AM roots most were downregulated. Conclusion Vacuolar sequestration and transport of complexed Mn are important mechanisms behind Mn tolerance in E. tereticornis. We propose that Mn is transported via the mycorrhizal pathway, explaining why it does not elicit the same molecular response observed in NM roots.