Large scale production of indole-3-acetic acid and evaluation of the inhibitory effect of indole-3-acetic acid on weed growth

Abstract

Indole-3-acetic acid (IAA) is the most common plant hormone of the auxin class and regulates various plant growth processes. The present study investigated IAA production by the basidiomycetous yeast Rhodosporidiobolus fluvialis DMKU-CP293 using the one-factor-at-a-time (OFAT) method and response surface methodology (RSM). IAA production was optimized in shake-flask culture using a cost-effective medium containing 4.5% crude glycerol, 2% CSL and 0.55% feed-grade L-tryptophan. The optimized medium resulted in a 3.3-fold improvement in IAA production and a 3.6-fold reduction in cost compared with those obtained with a non-optimized medium. Production was then scaled up to a 15-L bioreactor and to a pilot-scale (100-L) bioreactor based on the constant impeller tip speed (V_tip) strategy. By doing so, IAA was successfully produced at a concentration of 3569.32 mg/L at the pilot scale. To the best of our knowledge, this is the first report of pilot-scale IAA production by microorganisms. In addition, we evaluated the effect of crude IAA on weed growth. The results showed that weed (Cyperus rotundus L.) growth could be inhibited by 50 mg/L of crude IAA. IAA therefore has the potential to be developed as a herbicidal bioproduct to replace the chemical herbicides that have been banned in various countries, including Thailand.

Figure 1

IAA production with various carbon sources by R. fluvialis DMKU-CP293 using YPD base medium (2% carbon source, 2% peptone, 1% yeast extract and 0.1% l-tryptophan) in shake-flask culture at 30 °C and 200 rpm for 7 days.

Figure 2

IAA production and cell dry weight of R. fluvialis DMKU-CP293 cultivated in a production medium containing (a) 2% pure glycerol or crude glycerol, 2% peptone, 1% yeast extract and (b) 0.1% analytical-grade l-tryptophan or feed-grade l-tryptophan in shake-flask culture at 30 °C and 200 rpm for 7 days.

Figure 3

IAA production analyzed using the OFAT approach in media with varying parameters: (a) crude glycerol concentration, (b) nitrogen source, (c) CSL concentration, (d) growth factors, (e) technical-grade yeast extract concentration and (f) feed-grade l-tryptophan concentration at 34 °C and 200 rpm for 5 days. Different letters over the bars indicate significant differences between treatments using Duncan’s multiple range test (p < 0.05).

Figure 4

Contour plots of IAA production by the yeast R. fluvialis DMKU-CP293 using central composite design showing interactions between crude glycerol, CSL, technical-grade yeast extract, and feed-grade l-tryptophan after 5 days of incubation at 200 rpm and 34 °C.

Figure 5

The IAA concentration (mg/L), remaining l-tryptophan and cell dry weight in the batch fermentation of R. fluvialis DMKU-CP293 in (a) 2-L and (b) 15-L stirred tank bioreactors using the optimal IAA production medium at 34 °C, constant agitation speed (400 rpm), 10% inoculum size and 1 vvm aeration.

Figure 6

IAA production, l-tryptophan consumption and cell dry weight in batch fermentation by R. fluvialis DMKU-CP293 at the 100-L pilot scale at 34 °C, constant agitation speed (170 rpm), 10% inoculum size, and 1 vvm aeration.

Figure 7

Effect of (a) crude IAA produced by R. fluvialis DMKU-CP293 on weed shoot growth and (b) weed growth in plastic planting bags in control and crude IAA treatments after 9 days in a greenhouse. Different letters above the bars indicate significant differences between treatments using Duncan’s multiple range test (p < 0.05).