Interactions of bacterial-feeding nematodes and indole-3-acetic acid (IAA)-producing bacteria promotes growth of Arabidopsis thaliana by regulating soil auxin status

To clarify the ‘auxin-driven’ effect of bacterial-feeding nematodes on plant growth, the influence of an indole-3-acetic acid (IAA)-producing bacteria, Bacillus amyloliquefaciens, non-IAA-producing bacteria Variovorax sp., and two bacterial-feeding nematodes (Cephalobus sp. and Mesorhabditis sp.) on the growth of Arabidopsis thaliana after various exposure durations was investigated using a gnotobiotic microcosm incubation system. The nematodes, responses of soil chemistry (IAA and mineral nitrogen), plant growth (root architecture, shoot dry weight, and plant nitrogen), and the expression of two IAA-responsive genes, IAA3 and IAA13, were measured. Bacterial-feeding nematodes stimulated the activity of IAA-producing bacteria by grazing, and promoted the mineralization of nitrogen in the soil, thus increasing the soil IAA and nitrogen content and promoting shoot dry weight and plant nitrogen content. The A. thaliana root architecture consisted of a highly branched root system, with bigger surface area. The IAA3 and IAA13 genes in A. thaliana roots significantly increased and there were significant correlations with the soil IAA content. This study further confirms the existence of an auxin-driven effect that promotes A. thaliana growth by bacterial-feeding nematodes through changes in the expression of root genes.