The Negative Effect of Coinoculation of Plant Growth-Promoting Bacteria Is Not Related to Indole-3-Acetic Acid Synthesis

The use of coinoculations of indole-3-acetic acid-synthesizing bacteria has become increasingly common in agriculture. Different bacterial isolates are capable of synthesizing IAA. Depending on the amount, IAA can promote, inhibit, or modify the growth and development of plants. However, little is known about the effect of auxins on plants under coinoculation conditions. The present study aimed to verify whether coinoculation with indole-3-acetic acid (IAA)-synthesizing bacteria may or may not harm plant development in three tomato genotypes with different sensitivities to auxins: Micro-Tom (MT), diageotropica (dgt) (low auxin sensitivity) and Entire (high auxin biosynthesis). The experiment was conducted in a completely randomized design with six replicates and eight treatments: control; Bacillus velezensis (BV); B. pumilus (BP); Azospirillum brasilense (AZ); B. velezensis + Bacillus pumilus (BV + BP); B. velezensis + A brasilense (BV + AZ); B. pumilus + A brasilense (BP + AZ); B. velezensis + Bacillus pumilus + A. brasilense (BV + BP + AZ), for each tomato genotype. The parameters analyzed were dry shoot and root weight, plant height, and leaf area. The results showed an increase in root dry weight with BV + BP + AZ treatment in the MT genotype, an increase in shoot dry weight in the dgt genotype with BV and no increase in any parameter in the Entire genotype for the analyzed parameters. The results also showed that coinoculation with BP + AZ decreased the shoot dry weight, dry root weight, and leaf area values of the dgt and Entire genotypes. Coinoculations with BP + AZ reduced the parameters analyzed in the dgt genotype, which is insensitive to the IAA and Entire genotypes and is sensitive. In addition, coinoculation with BV + BP + AZ, which provided the highest IAA concentration, increased the dry root weight in the MT genotype. These results suggest that the reduction in plant development caused by coinoculation is not related to IAA synthesis produced by bacteria but is probably due to negative interference in the interaction mechanisms between plants and bacteria and might also be due to negative bacteria–bacteria interactions.