Plant growth promoting activities of endophytic bacteria from Melia azedarach (Meliaceae) and their influence on plant growth under gnotobiotic conditions

Bacteria that live asymptomatically within plant tissues are known as endophytes. Because of the close relation with the plant host, they have been a matter of interest for application as plant growth promoters. <i>Melia azedarach</i> is a widely distributed medicinal tree with proven insecticidal, antimicrobial, and antiviral activity. The aim of this study was to isolate and characterize endophytic bacteria from <i>M. azedarach</i> and analyze their plant growth promoting activities for the potential application as biological products. Bacteria were isolated from roots and leaves of trees growing in two locations of Northeastern Argentina. The isolates were characterized by repetitive extragenic palindromic sequence PCR and 16S rDNA sequence analysis. The plant growth-promoting activities were assayed <i>in vitro</i>, improvement of plant growth of selected isolates was tested on <i>M. azedarach</i> plantlets, and the effect of selected ACC deaminase producing isolates was tested on tomato seedlings under salt-stress conditions. The highest endophytic bacterial abundance and diversity were obtained from the roots. All isolates had at least one of the assayed plant growth-promoting activities and 80 % of them had antagonistic activity. The most efficient bacteria were <i>Pseudomonas monteilii</i>, <i>Pseudomonas farsensis, Burkholderia</i> sp. and <i>Cupriavidus</i> sp. for phosphate solubilization (2064 μg P ml^-1), IAA production (94.7 μg ml^-1), siderophore production index (5.5) and ACC deaminase activity (1294 nmol α-ketobutyrate mg^-1 h^-1). <i>M. azedarach</i> inoculation assays revealed the bacterial growth promotion potential, with <i>Pseudomonas monteilii</i>, <i>Pseudomonas farsensis</i> and <i>Cupriavidus</i> sp. standing out for their effect on leaf area, leaf dry weight, specific leaf area, and total Chl, Mg and N content, with increases of up to 149 %, 58 %, 65 %, 178 %, 76 % and 97.7 %, respectively, compared to NI plants. Efficient ACC deaminase-producing isolates increased stress tolerance of tomato plants under saline condition. Overall, these findings indicate the potential of the endophytic isolates as biostimulant and biocontrol agents.