Lettuce growth improvement by Azospirillum argentinense and fluorescent Pseudomonas co-inoculation depends on strain compatibility

Azospirillum and fluorescent Pseudomonas are plant-associated bacteria used for inoculant formulation worldwide, both alone and in combination. Dual inoculation of these two bacterial groups is a common practice that is not usually accompanied by an assessment of strain compatibility. Here we sought to investigate to what extent Azospirillum and Pseudomonas strain compatibility influences their plant-growth promoting activity. Several strain combinations were analyzed for in vitro compatibility when growing in dual-species biofilms. The combinations A. argentinense Az39 + P. fluorescens A506 and A. argentinense Az39 + P. protegens Pf-5 were selected to investigate their plant growth-promotion capacity in lettuce, in comparison to single strains. When co-inoculated, Az39 negatively affected the early root colonization of P. fluorescens A506 but extended its survival in the roots for a longer period. Plant analysis at 35 days after inoculation showed that Az39 and A506 act synergistically to promote lettuce growth, increasing the measured parameters when compared to uninoculated plants and outperforming the rest of the bacterial treatments. In contrast, the Az39 + Pf-5 combination did not modify plant growth, indicating that these strains interfere in vivo to mutually impair their individual plant growth-promoting effects. Similarly, synergism or interference between these strain combinations was observed when plant yield was analysed at harvest time. We conclude that strain compatibility is an absolute requirement to achieve increased plant growth promotion by co-inoculation of beneficial strains. Dual inoculation of different bacterial species without making a compatibility study may impede the plant-stimulatory potential of individual strains, in turn leading to poor performance of the inoculant.