Microtiter method for quantitative assay of IAA from fungal isolates, demonstrated with Trichoderma

A quantitative and accurate method is needed to estimate indole-3-acetic acid (IAA) production by Trichoderma to compare strains and allow the repeatability of experiments in plant-growth promotion studies. Suspensions of 1.0 × 106 conidia. mL-1 of 98 Brazilian strains were grown in potato + dextrose broth in the absence or presence of L-tryptophan (100 mg L-1) in 1 mL microtubes. After 12,000 rpm spinning for 15 min, 150 μL of the supernatant were added to 100 μL of Salkowski reagent with 30 min incubation at room temperature in the dark, with IAA yield spectrophotometrically estimated at 540 nm. Isolates BRM-29474 and BRM-29574 that outperformed in the IAA production increased tomato root and shoot mass and number of root tips in a greenhouse test, endorsing their potential for plant growth promotion. This adjustment allows a quantitative method to estimate an IAA production by Trichoderma spp., which is more informative than the standard qualitative method. •There are limitations to measure indol acetic acid (IAA) production by Trichoderma spp.•We describe a microtiter-based method to quantify IAA produced by Trichoderma spp.•Reactions use the Salkowsky reagent and L-Triptophan in 2.0 mL microtubes.•Adjusted concentrations of conidia allow isolate screening and treatment comparisons.•Growth promotion was confirmed in tomato plants with increased biomass and root tips.