| Summary: | Plant-growth-promoting bacteria (PGPB) are gaining attention as a sustainable alternative to current agrochemicals. This study evaluated the impact of three <i>Bacillus</i> spp. (5PB1, 1PB1, FV46) and one <i>Brevibacillus</i> sp. (C9F) on the important crop tomato (<i>Solanum lycopersicum</i>) using the model cv. ‘MicroTom’. The effects of these isolates were assessed on (a) seedlings’ growth and vigor, and (b) adult potted plants. In potted plants, several photosynthetic parameters (chlorophylls (<i>a</i> and <i>b</i>), carotenoids and anthocyanins contents, transpiration rate, stomatal conductance, net CO<sub>2</sub> photosynthetic rate, and intercellular CO<sub>2</sub> concentration, and on chlorophyll fluorescence yields of light- and dark-adapted leaves)), as well as soluble sugars and starch contents, were quantified. Additionally, the effects on redox status were evaluated. While the growth of seedlings was, overall, not influenced by the strains, some effects were observed on adult plants. The <i>Bacillus safensis</i> FV46 stimulated the content of pigments, compared to C9F. <i>Bacillus zhangzhouensis</i> 5PB1 increased starch levels and was positively correlated with some parameters of the photophosphorylation and the gas exchange phases. Interestingly, <i>Bacillus megaterium</i> 1PB1 decreased superoxide (O<sub>2</sub><sup>−</sup>) content, and <i>B. safensis</i> FV46 promoted non-enzymatic antioxidant defenses, increasing total phenol content levels. These results, conducted on a model cultivar, support the theory that these isolates differently act on tomato plant physiology, and that their activity depends on the age of the plant, and may differently influence photosynthesis. It would now be interesting to analyze the influence of these bacteria using commercial cultivars.
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