Arbuscular mycorrhizal fungi inoculation inhibits plant Zn toxicity and promotes Zn biofortification for lettuce (Lactuca sativa L.)

Insufficient dietary intake of zinc (Zn) is a significant public health concern globally, as it is closely linked to impaired immune function and pregnancy complications. Addressing this issue may include strategies such as agronomic biofortification of globally important vegetables. For example, lettuce (Lactuca sativa L.) contributes to the dietary intake of millions of people and may be well used for agronomic biofortification. Arbuscular mycorrhizal fungi (AMF), symbiotic with 80% of terrestrial plants, also facilitate nutrient uptake, contributing to improving the nutritional value of crops. Our research aimed to assess the effects of AMF inoculation and Zn fertilization on plant growth and biofortification of lettuce leaves. A greenhouse pot experiment was conducted in a completely randomized block design, in a double factorial arrangement (5 × 2), composed of five rates of Zn (0, 8, 32, 64, and 96 mg dm⁻³) and two levels of mycorrhizal inoculation (presence and absence) with 10 replicates. Plant growth, AMF colonization, and plant Zn uptake were measured. AMF inoculation substantially increased AMF root colonization across all Zn levels, while non-inoculated plants presented an 89% decrease in root colonization at the highest Zn rate (96 mg Zn dm⁻³). Without inoculation, high Zn fertilization reduced lettuce yield by 16% at the highest Zn rate (96 mg Zn dm⁻³), with no negative effects in inoculated plants. Inoculated plants produced 37% more fresh biomass without Zn fertilization (0 mg Zn dm⁻³) and 68% more at the highest Zn rate (96 mg Zn dm⁻³) compared to non-inoculated plants. Fertilized plants were successfully biofortified, reaching Zn concentrations eightfold (inoculated plants) to 10-fold (non-inoculated plants) higher than controls. AMF inoculation promoted superior Zn absorption under toxic Zn levels while inhibiting detrimental effects of Zn toxicity on plant growth. Therefore, our data provide new evidence that AMF inoculation enables the application of high Zn rates in lettuce biofortification programs while enhancing plant growth.