Zinc Translocation from Zn-Sufficient to Zn-Deficient Roots as an Adaptation to Heterogeneous Zn Availability

Zinc is essential for plant development and human health. While the effects of soil nutrient heterogeneity on plant growth were studied for macronutrients, adaptive mechanisms for micronutrients like Zn remain largely unexplored. We investigate Zn homeostasis in plants grown in a transparent soil medium mimicking natural soil conditions with spatially heterogeneous Zn availability. Our findings suggests that Zn is translocated between lateral roots, moving from Zn-sufficient to Zn-deficient ones, mitigating Zn deficiency responses and reducing Zn uptake. Under heterogeneous Zn conditions, the expression of key Zn homeostasis-related genes (Zn importer - NtZIP4B, Zn exporter - NtHMA4a/b and Zn chelator – NtNAS) was significantly altered. NtHMA4a/b expression was influenced by the vertical positioning of the Zn-sufficient medium, while NtZIP4B and NtNAS showed suppressed expression in roots under heterogeneous conditions compared to homogeneous Zn-sufficient conditions. This suggests a systemic regulatory mechanism coordinating Zn allocation depending on whole root system Zn access. Elemental analysis revealed reduced overall Zn concentrations in plants grown in heterogeneous Zn media, with elevated Zn levels in leaf veins. This study uncovers a novel mechanism of Zn translocation within the root system in response to heterogeneous Zn supply, highlighting the complexity of micronutrient homeostasis and its adaptive regulation.