Growth promotion of Arabidopsis thaliana using oxygen-radical-treated l-tryptophan solution

Recently, plant-growth-promoting effects of oxygen-radical-treated amino-acid solutions are attracting interests. In this study, we evaluated the growth-promoting effect of oxygen-radical-treated l-tryptophan (l-Trp) solutions, reported to have strong bactericidal effects as well, on Arabidopsis thaliana (A. thaliana).  An atmospheric-pressure radical source, which can selectively irradiate only electrically-neutral species in the plasma, was used to treat a 50 mM l-Trp solution. The radical source was driven with a gas mixture of Ar and O2, their flow rates were set at 4.96 and 0.03 slm, respectively. The treated solution was diluted to 1, 5, 10, 20, 50 μM, and then mixed with MS medium (supplemented with 1% sucrose and 1% agar). Seeds were germinated on control MS or with the diluted solution. Plants were grown vertically in a growth chamber at 22 °C under 16-h light/8-h dark conditions. We assessed whole root length when treated for 7days. On oxygen-radical-treated l-Trp medium, the root length was longer than that of MS medium. The maximum promoting effect was observed on the 5 µM oxygen-radial-treated l-Trp plate. These results suggest that oxygen-radical-treated l-Trp solutions with strong bactericidal effects have a potential to promote the growth of plants as well.