Halogenated indoles selectively modulate plant growth and root development.

Indole is a central scaffold in microbiology and also functions as an auxin precursor and signaling molecule in plants. However, the growth-modulating potential of substituted indoles remains largely unexplored. Here, we screened 87 indole and indole-derived compounds for effects on early seedling growth in two Brassicaceae species, radish and Chinese cabbage. Seed germination was largely unaffected, whereas marked variability was observed in total growth and particularly in root development. The parent compound indole strongly promoted seedling growth to approximately 140% relative to the control, while classical auxin-related derivatives, including indole-3-acetic acid and indole-3-acetonitrile, suppressed growth to approximately 31% of the untreated control at 10 μg/mL. Several substituted indoles exhibited enhanced growth, with di-halogenated derivatives such as 7-chloro-5-fluoroindole producing the strongest stimulation, with total seedling length and root growth reaching approximately 155% and 220% of the untreated control, respectively. Structure-activity relationship analysis revealed clear position- and halogen-dependent trends, with substitutions at C6-C7 and specific di-halogenation patterns associated with root-biased promotion, whereas auxin-like side chains and small halogens at C4-C5 were generally unfavorable. These findings identify halogenated indoles as a tunable chemotype capable of modulating plant growth and root development.