Slow release of a synthetic auxin induces formation of adventitious roots in recalcitrant woody plants

Content snapshot: I will present the identification and comprehensive mechanistic study of a new family of compounds that leverage slow-release of synthetic auxins to enhance the rooting capacity of woody plants and discuss the underlying mechanism that support their function.

Background: Adventitious roots (ARs) are defined as roots that regenerate from non-root tissues. Clonal propagation of plants by induction of ARs from stem cuttings is a requisite step in selection and breeding programs as well as in routine agricultural practices and has tremendous economic importance. Due to the central role of auxin in organogenesis, indole-3-butyric acid (IBA) is often used as part of commercial rooting mixtures, yet many recalcitrant plants do not form ARs in response to this treatment.

Knowledge gap: A major barrier exists for propagating valuable plants that naturally have none or low capacity to form ARs.

Methods and results: we synthesized and screened of a focused library of synthetic auxin conjugates in Eucalyptus grandis cuttings, identifying 4-chlorophenoxyacetic acid-L-tryptophan-OMe as a competent enhancer of adventitious rooting. Comprehensive metabolic and functional analyses reveal that this activity is engendered by prolonged auxin signaling due to initial efficient uptake and slow-release and clearance of a free synthetic auxin 4-chlorophenoxyacetic acid. Amidohydrolases from the ILR/ILLs family were found to facilitate the slow-release. Phylogenetic analysis revealed that these enzymes are highly conserved across the plant kingdom. Based on this analysis, successfully utilized the new compounds for rooting of multiple difficult-to-root woody plants.

Discussion: This work highlights the utility of a slow-release strategy for bioactive compounds for more effective plant growth regulation. In addition, it supports a previous observation that callus creation and AR development are temporally distinct processes in AR formation.