Indoleacetic acid producing rhizobia promote growth of Tanzania grass (Panicum maximum) and Pensacola grass (Paspalum saurae)

Production of phytohormones, such as the indoleacetic acid (IAA), is possibly the main mechanism of plant growth promotion by rhizobia. The greenhouse present study aimed to quantify the production of IAA by rhizobia and evaluate the effect of inoculation with rhizobia on four forage grasses i.e., Tanzania grass (Panicum maximum), Pensacola grass (Paspalum saurae), brachiaria grass (Brachiaria decumbens) and ryegrass (Lolium multiflorum). The work also aimed to characterize genotypically the rhizobia strains studied in this work. All rhizobia were able to produce IAA and promote plant growth in at least one of the grasses studied. Tanzania grass plants inoculated with rhizobia SEMIA816, Lc134, Lc323, Lc348, Lc510 and Lc524 showed increased shoot and root dry mass as compared to control or non-inoculated plants. The inoculation with Lc336 only increased the root dry mass in Tanzania grass. In Pensacola grass increases in shoot and root dry mass were observed with inoculation of SEMIA816, Lc134, Lc336 and Lc394. The inoculation with Lc323 and Lc348 only increased the root dry mass in Pensacola grass. On the other hand, no increases on shoot and root dry mass was observed in plants of brachiaria grass and ryegrass by inoculation with the studied rhizobia. The rhizobia SEMIA 816 and Lc134 promoted the plant growth in both Tanzania grass and Pensacola grass. The rhizobia SEMIA 816, Lc336, Lc348 and Lc524 also increased the number of panicles in brachiaria grass. A higher accumulation of total nitrogen was noted in the shoots of Tanzania grass inoculated with the UFRGS Lc323, UFRGS Lc336 and SEMIA 816. After characterization of rhizobia genomic DNA by BOX PCR and 16S rRNA sequencing the unidentified rhizobia were grouped together in a dendrogram of similarity and found belonging to the genus Mesorhizobium or Bradyrhizobium.