Single-strain and consortium inoculations with plant-beneficial <i>Pseudomonas</i> spp. promote lettuce growth under field conditions

Plant-beneficial <i>Pseudomonas</i> spp. can promote plant growth by a wealth of mechanisms. Developing microbial inoculants encompassing multiple <i>Pseudomonas</i> strains (consortia) could represent an attractive solution to some of the problems encountered by single-strain inoculants, such as inconsistent rhizosphere colonization under field conditions. In this study, we evaluated the potential of <i>Pseudomonas protegens</i> B21-024 and <i>Pseudomonas putida</i> B21-029, inoculated alone or in combination, to promote lettuce growth under greenhouse and representative commercial field conditions. Plant growth metrics, including shoot fresh and dry weights, were used to evaluate plant growth promotion. In addition, unmanned aerial vehicle imaging was used to monitor plant growth in the field. Strain-specific primer-probe sets were also developed to study, using the quantitative polymerase chain reaction, the population dynamics of the two strains in rhizosphere soil. <i>P. putida</i> B21-029 and the <i>Pseudomonas</i> consortium significantly increased lettuce shoot fresh/dry weight under greenhouse conditions. In the field, only inoculation with <i>P. protegens</i> B21-024 and the <i>Pseudomonas</i> consortium promoted lettuce growth by significantly increasing shoot dry weight. Interestingly, the nitrogen-rich bacterial growth medium used to prepare the inocula played a role in the plant growth promotion achieved. Overall, the rhizosphere population of both <i>Pseudomonas</i> strains remained relatively stable during the growing season. However, the population of <i>P. protegens</i> B21-024 was significantly higher when it was co-inoculated with <i>P. putida</i> B21-029 than when inoculated alone, suggesting the existence of a beneficial relationship. These findings support the use of <i>Pseudomonas</i> consortia to promote the growth of leafy greens under field conditions.IMPORTANCESome bacteria belonging to the genus <i>Pseudomonas</i> can establish mutually beneficial relationships with many crop species, resulting in disease suppression and/or plant growth promotion. These microorganisms show potential to replace, at least in part, synthetic fertilizers and pesticides in agricultural field settings. While some <i>Pseudomonas</i> strains have been shown to promote lettuce growth, successfully developing <i>Pseudomonas</i> consortia would have many advantages over single-strain inoculations, including better effectiveness and rhizosphere colonization. In this study, a consortium encompassing two <i>Pseudomonas</i> strains successfully promoted lettuce growth under greenhouse and field conditions. The nitrogen-rich bacterial growth medium used for the preparation of inocula contributed to the plant growth promotion achieved. Strain-specific molecular markers were also developed to monitor the abundance of each strain in rhizosphere soil throughout the growing season. The results obtained in this study indicate that the two <i>Pseudomonas</i> strains under study successfully colonized the lettuce rhizosphere.