Biofilm of B. subtilis as a Growth Promoter of Lettuce (Lactuca sativa L.) in the Presence of Heavy Metals

The negative effects of soil heavy metal contamination on food production could be mitigated using nature-based solutions, i.e., plant-growth-promoting rhizobacteria (PGPR). Yield of Lactuca sativa L. has been shown to increase by seed inoculation with biofilm of Bacillus subtilis subsp. spizizenii. The aim of this work was to assess whether this promoting effect occurs even in the presence of toxic concentrations of copper (Cu) and zinc (Zn). First, germination rates of lettuce seeds with increasing Cu and Zn concentrations were assessed. Then, lettuce plants growing from inoculated and non-inoculated seeds were cropped on substrates with excess Cu and Zn. Above- and below-ground lettuce variables were measured, and leaf macro- and microelements were determined. Germination was more severely affected by Cu overload than by Zn overload, while this trend was reversal for plant growth. Seed inoculation enhanced germination and increased plant growth assessed by root and shoot biomass, plant height and leaf area. For example, seed inoculation increased lettuce root and aerial biomass of lettuce growing on a metal- free substrate by 68% and 62%, respectively. This practice also promoted lettuce growth in metal-overloaded substrates, increasing aerial and root biomass by 32% and 29%, respectively, in connection with Cu, and by 260% and 183% when it came to Zn. Both Cu or Zn accumulated in the edible parts of lettuce growing on contaminated substrates, but seed inoculation did not mitigate metal uptake in any case. Except for Cu and Zn, macronutrients, micronutrients and heavy metal levels in lettuce leaves were affected neither by excess metal nor by seed inoculation. Altogether, B. subtilis biofilm has been proven to be an effective seed inoculant promoting seed germination and plant growth even in the presence of heavy metals. Adverse health effects due to metal accumulation in the lettuce edible parts are not expected to increase following seed inoculation.