Over-fertilization and agrochemicals adversely affect soil quality and agricultural ecosystem sustainability. Tomato (Solanum lycopersicum) is ranked as an important crop due to its high profitability and nutritional value. In Argentina, tomato is mainly produced in horticultural belts at peri-urban areas, whose soils frequently are contaminated by heavy metals and/or agrochemicals. To explore safer alternatives, we investigated the effects of seed inoculation with a common plant-growth-promoting rhizobacteria (PGPR), i.e., Bacillus subtilis subsp. spizizenii, on development at various growth stages of two tomato varieties, “Platense” and “Río Grande”, and on production and fruit quality at harvest time of the “Río Grande” variety. The experimental design consisted of three treatments per variety: a control versus traditional planktonic or biofilm inoculation, with three replicates per treatment. Germination at 10 days and seedling agronomical parameters showed that the response to seed inoculation was superior in the “Río Grande” variety. At harvest time, and irrespective of the inoculant, several agronomical parameters of the “Río Grande” variety were significantly enhanced with respect to the control. The biofilm significantly increased tomato production, as quantified by fruit number and weight, compared to the planktonic inoculum. This case study demonstrates that the incorporation of bio-inoculants is relevant in sustainable agriculture to promote crop growth and quality.

Biofilm obtained from Bacillus subtilis subsp. spizizenii inoculated on vegetable seeds has been shown to have plant growth-promoting capacity. Seed inoculation with biofilm produced by this strain could also reduce the adverse effects on plant growth caused by soil or substrate heavy metal overabundance. Therefore, the objective of this work was to evaluate the impact of biofilm inoculated on tomato (Solanum lycopersicum L.) seeds, which were planted on a substrate with artificially added zinc. First, seeds of the Río Grande tomato variety were exposed to increasing zinc concentrations, namely: 50, 100, 200, and 400 ppm, with and without bacterial biofilm inoculation. Zinc addition and seed inoculation affected germination parameters. For example, an extra 200 and 400 ppm of zinc led to high toxicity. Biofilm inoculation, however, reduced the noxious effects of excess zinc, bringing acute toxicity down to moderate. Then, tomato plants growing from inoculated and non-inoculated seeds were cropped for 4 months in both substrates with 400 ppm zinc and without added zinc. Extra zinc addition significantly (p < 0.05) reduced tomato root and shoot biomass, plant height, and fruit number at harvest time. However, seed biofilm inoculation avoided the harmful effect of zinc on plant growth parameters, fruit yield, and fruit quality. The roots and shoots of plants growing on contaminated substrates showed very noticeable increases in zinc levels compared to the control, while fruits only showed a much weaker zinc gain, even if this was significant (p < 0.05). Moreover, root shoot and fruit concentrations of elements other than zinc, (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, lead, and cadmium) were not or only weakly affected by the addition of this metal to the substrate. In summary, the biofilm of B. subtilis proved to be effective as a bioinoculant to alleviate negative effects on tomatoes cropped in a substrate with excess zinc.

The plant growth-promoting bacterium, B. subtilis subsp. Spizizizenii, has been proven to develop a biofilm under certain culture conditions, which can be applied as an efficient bioinoculant. Biofilm can be produced cost-effectively using biodiesel byproduct glycerol as a carbon source. Soils from urban peripheries may contain very high lead (Pb) levels. The main aim of this study was to assess the impact of biofilm seed inoculation on plant development and fruit quality of tomatoes growing on a Pb-contaminated substrate. Also, effects of excess Pb on biofilm production, stability, and seed germination were analyzed. B. subtilis biofilm was produced with Pb concentrations ranging from 0 to 300 ppm. Biofilm stability was tested at 4 °C and 25 °C. The impacts of Pb and inoculation on seed germination were evaluated in laboratory conditions, while the impacts on plant agronomic parameters were assessed via a greenhouse assay. Adding Pb to the culture medium increased biofilm production by about 20%. Regardless of Pb level, biofilms were more stable at 4 °C than at 25 °C. Beneficial effects of biofilm on germination were greater on seeds exposed to 200 and 300 ppm Pb. Excess Pb significantly reduced plant biomass and tomato yield. However, biofilm inoculation significantly increased plant aboveground and root biomass, plant height, leaf area, fruit number, and fruit size, regardless of substrate Pb excess. Tomato fruits of plants grown in the metal-contaminated substrate showed no significant increases in Pb concentration with respect to the control. In summary, the biofilm produced by B. subtilis subsp. spizizenii proved to be an effective bioinoculant to counteract the negative effects of substrate excess Pb on tomato germination, growth, and production.

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.