Accumulation of copper (Cu) in soils devoted to intensive agriculture due to anthropogenic additions is becoming a significant threat to plant productivity. Biological inoculants may play an important role in alleviating toxic effects of heavy metals on plants. The plant-growth-promoting rhizobacteria (PGPR) Bacillus subtilis subsp. spizizenii has demonstrated the ability to reduce harmful impacts of heavy metals on crops. This study aimed to evaluate the suitability of seed inoculation with biofilm produced by this bacterium to mitigate the severity of Cu toxicity on tomato. In the laboratory, first, B. subtilis was cultivated under increased Cu concentrations. Then, germination of inoculated and non-inoculated tomato seeds was tested for Cu concentrations of 0, 50, 100, 150, and 200 ppm. Next, a greenhouse experiment was conducted for four months to assess the effects of both inoculation and excess 150 ppm Cu in the substrate. The studied treatments included control, no inoculation and Cu surplus, inoculation and no Cu surplus, and inoculation plus Cu surplus. In the laboratory, first, the bacterium’s ability to grow in a liquid medium containing Cu was confirmed. Thereafter, we verified that the germination of non-inoculated seeds was negatively affected by Cu, with higher concentrations leading to a more detrimental effect. However, seed inoculation with biofilm mitigated the adverse impact of Cu on germination. Under greenhouse conditions, excess Cu significantly reduced root dry weight, tomato number, and tomato yield compared with the control, whereas shoot dry weight, plant height, leaf area, and soluble solid concentration (Brix index) did not experience significant changes (p < 0.05). However, seed inoculation mitigated the toxic effects of excess Cu, significantly enhancing all the aforementioned plant parameters, except plant height. Seed inoculation also significantly reduced the Cu contents in the fruits of tomato plants growing in the metal contaminated substrate. The biofilm of the B. subtilis strain used demonstrated its effectiveness as a bioinoculant, attenuating the detrimental effects induced by a substrate with excess Cu.

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.