The Relative Sensitivity of Marigold vs. Tomato to Iron (Fe) Toxicity Is Associated with Root Traits: Root-to-Shoot Mass Ratio, Failure to Sequester Fe in Roots, and Levels of the Major Fe-Uptake Protein, IRT

Abstract

Some plants are susceptible to iron (Fe) toxicity, resulting in excess leaf Fe, followed by photo-oxidative damage and necrosis. Avoidance of Fe toxicity should relate to root Fe-uptake properties, including the concentration of Fe-uptake proteins, which are rarely measured. Therefore, we compared responses to Fe toxicity in marigold (Tagetes erecta), a species prone to Fe toxicity, with tomato (Solanum lycopersicum) to determine which root responses were related to Fe-toxicity sensitivity. Plants were grown at 5, 50, and 500 µM Fe. Biomass decreased at 50 and/or 500 vs. 5 µM Fe in both species, with leaf necrosis occurring in marigold. Fe uptake per g root increased with increasing Fe in both species. Root and shoot Fe increased with Fe availability, with Fe much higher in roots than in shoots. Root Fe was higher in tomato, but shoot Fe was higher in marigold. Root protein levels of IRT, the main Fe transporter, decreased from 5 to 50 µM Fe in both species, but further decreased at 500 µM Fe only in tomato. In contrast, responses of secondary Fe-uptake proteins, FRO and H+-ATPase, were similar in the two species. Marigold was likely more susceptible to Fe toxicity because it could not prevent Fe transport to, and accumulation in, leaves; and its sensitivity to high Fe corresponded to a limited capacity to down-regulate IRT as available Fe increased.