Alleviating the barrier of adventitious roots formation in recalcitrant mature tissue by slow release of a synthetic auxin

Clonal propagation of plants by induction of adventitious roots (ARs) from stem cuttings is a requisite step in breeding programs. Nevertheless, a major barrier exists for propagating valuable plants that naturally have low capacity to form ARs. Due to the central role of auxin in organogenesis, indole-3-butyric acid (IBA) is often utilized, yet many recalcitrant plants do not form ARs in response to such treatment. We describe the synthesis and screening of a focused library of synthetic auxin conjugates in Eucalyptus grandis cuttings, highlighting 4-chlorophenoxyacetic acid-L-tryptophan-OMe as a competent enhancer of adventitious rooting in a number of recalcitrant woody plants. Comprehensive metabolic and functional analyses revealed that this activity is engendered by prolonged auxin signaling due to initial fast uptake and slow release and clearance of the free auxin 4-chlorophenoxyacetic acid. This work highlights the utility of a slow-release strategy for bioactive compounds and provides an exemplar for further rational development of more effective plant-growth regulators for agriculture. ### Competing Interest Statement The authors have declared no competing interest.