Indole acetic acid and phytase activity produced by rhizosphere bacilli as affected by pH and metals

The abilities to produce indole acetic acid (IAA) and mineralize organic phosphorus by phytase are desirable traits in plant-growth promotion rhizobacteria (PGPR) particularly in Chilean Andisols which are characterized by low pH and high total P. However, little is known about the influence of soil properties that are specific to Andisol (low pH and metal toxicity) on the effectiveness of PGPR. Here, we assessed the effect of pH and metal cations on IAA and phytase activity of cell-associated proteins produced by two bacilli strains isolated from the rhizosphere of pasture plants. The production in vivo of IAA by Paenibacillus sp. SPT-03 was significantly increased (7-fold) when incubated in tenfold diluted culture medium, compared to the full-strength medium. At low pH (pH<5), phytase activity of cell-associated proteins and IAA production of Bacillus sp. MQH-19 was decreased, whereas they were increased in Paenibacillus sp. SPT-03. Moreover, phytase activity in vitro of cell-associated proteins and IAA production in both bacilli strains were significantly inhibited by 30-100% and 44-70% by concentrations of 10 mM and 350 μM Fe ³⁺ and Al ³⁺, respectively. At 350 μM Mn ²⁺ IAA production was inhibited by 30-100% in both strains but there was no effect on phytase activity. This study shows that certain properties of Andisol may differentially affect some mechanisms related with PGPR efficiency.