Effect of Exogenously Applied L-Tryptophan on Allelochemical Activity of Plant-Growth-Promoting Rhizobacteria (PGPR)

The effect of L-tryptophan on root elongation of lettuce seedlings (Lactuca sativa), as well as on allelochemical activity of three species of root growth promoting rhizobacteria (PGPR), was studied under controlled axenic conditions. L-Tryptophan exhibited an hormonelike effect, promoting root elongation of lettuce seedlings at 10−5 and 10−3 mM by ca. 20%, but at 10 mM, root elongation was inhibited by ca. 52% compared to the control (L-tryptophan-free). Comamonas acidovorans 26, Agrobacterium sp., and Alcaligenes piechaudii promoted root elongation of lettuce seedlings by 15, 30, and 44%, respectively. When 10 mM L-tryptophan was applied, Agrobacterium sp. and A. piechaudii inhibited root elongation by 57.6 and 63.5%, respectively. However, at the same concentration of L-tryptophan, C. acidovorans 26 promoted root growth. It is suggested that L-tryptophan reverted root growth promotion through enhancing the secretion of indole-3-acetic acid by the rhizobacteria. L-Tryptophan may have both a direct effect on root growth and an indirect effect through affecting activity of root growth-mediating bacteria. Since the growth of C. acidovorans 26 was exclusively inhibited by L-tryptophan (10 mM), it is conjectured that reduced population of this bacterium could not generate IAA at a level sufficient to inhibit root elongation. Despite the mild root growth promoting effect of C. acidovorans 26, its consistent root growth promotion, perhaps justifies further experimentation.