Indole-3-Butyric Acid (IBA) Derivatives for Root Architecture Modulation: Potent Promoters via <i>In Vivo</i> Conversion to IBA and Selective Inhibitors for Lateral Root Formation

Indole-3-butyric acid (IBA)-derived compounds, methyl 2-hydroxy-5-(1<i>H</i>-indole-3-yl)-pentanoate (JAX-44) and its acid derivative (JAX-77), emerge as potent modulators of root architecture, exhibiting significant growth-promoting activity for lateral and adventitious roots. Crucially, unlike native IBA, which often inhibits primary root elongation at higher concentrations, JAX-44 and JAX-77 maintain primary root length while substantially increasing lateral root numbers, thereby optimizing the plant's overall root system. Mass spectrometry analyses in <i>Arabidopsis thaliana</i> elucidated their mechanism: JAX-44 undergoes <i>in vivo</i> conversion to IBA via enzymatic hydrolysis to JAX-77, followed by oxidation to a keto-acid intermediate and finally YUCCA-mediated oxidative decarbonylation. This conversion process specifically requires an <i>N</i>-unsubstituted indole structure. Conversely, <i>N</i>- or <i>O</i>-methylation of JAX-44, yielding compounds JAX-86 and JAX-83, switches their activity from promotion to selective inhibition of lateral root formation. This inhibitory effect is attributed to the methylation preventing the necessary oxidative decarbonylation pathway. These results suggest that JAX-44 and JAX-77 are not only potentially superior plant growth regulators with enhanced flexibility and optimized root development for practical use but also valuable chemical biology tools that provide insight into auxin biology and the specific structural requirements for IBA bioactivity.