Unlocking the adventitious-root-formation barrier in mature trees by new rooting enhancers

The loss of rooting capability in mature woody plants is a serious barrier for selection and breeding programs. Instead of roots, woody plants often form callus which was found to consist parenchymal cells with patches of differentiated unorganized tracheary elements. Such organization suggests aberrations in auxin polar transport and canalization. In aim to unlock this barrier we have designed, synthesized and tested new rooting enhancers made from four synthetic auxins conjugated to various residues, enabling a slow release of the active compound in plant cells. The examination of 82 different conjugates resulted in the isolation of two; 4-chloropenoxy acetic acid (4-CPA) conjugated to L-Trp or L-Val which enhanced adventitious roots (AR) from cuttings of mature Eucalyptus grandis significantly better than control (50% and 15% respectively). Interestingly, conjugates of 4-CPA to D-Trp and D-Val exhibited a lower auxin and rooting activity. Application of the conjugates to the leaves was necessary to enhanced AR formation. Accordingly, basipetal transport of 4-CPA was more efficient than acropetal one, and hydrolysis of the conjugate in leaves was better than in the cutting bases. Levels of endogenous auxins changed such that the gradient from the leaves to the cutting base became stronger. Expression profiling in cambium enriched cell fraction revealed candidate transcripts performing differently in response to the new enhancer. Crossing these with lists of transcripts functioning in lateral root formation or in AR formation in poplar, an easy to root tree, suggested that 4-CPA contributed to cell wall remodeling, and auxin canalization.