Right place, right time: Targeted auxin delivery to improve tree cutting propagation

The Government has set a target to plant 10,000 hectares of trees per year in England by 2025. To address the shortfall in tree propagation knowledge new funding has been made available. We applied and have successfully obtained 6-month proof-of-concept funding, which is expected to become a further 3 years funding to upscale to meet government targets. However, there are species that require more detailed research to understand the underlying biology before large scale production will be possible which is what this PhD proposal addresses.

Tree cutting propagation depends on root formation from stem bases, however there is significant variation in rooting success between species. This variation is dependent on signalling networks including plant hormones such as auxin which is the core ingredient of many rooting powders. However auxin treatments do not always improve rooting on difficult-to-root species, partly due to increases in auxin degradation. In this project the student will identify whether auxin responses and degradation previously discovered by the primary supervisor can be applied to industry-relevant cuttings of Cornus species (edible and biodiversity value) while developing a novel method for targeted delivery of auxin. By adding auxin to the right cells at the right time for root initiation we can overcome low-auxin-related blocks to rooting.

Objective 1: Identify if auxin responses previously discovered in model systems also apply to Cornus cuttings.

Building on Rasmussen et al 2015, auxin levels and responsiveness will be measured in harder to propagate Cornus cuttings collected from Whetman Plants International. Grafting will be used to confirm if the signal is shoot or root-derived. Auxin, cytokinins, strigolactones and jasmonic acid levels will be measured in the rooting zone to compare to our pea model. Laser Ablation Tomography will be used to track root initiation and emergence processes in 3D.

Objective 2: Develop pharmaceutical carriers for novel auxin delivery to manipulate local auxin at key times post-cutting

The student will synthesise nanocarriers to deliver auxin and auxin metabolites in a spatially and temporally targeted manner. This will involve producing (bio)polymer-based capsules (e.g., PLGA, PCL, chitin, chitosan) trapping Auxin inside. These will be fed to the cut end of DR5:GUS lines of Pea which will provide a marker to demonstrate auxin release. They will then be fed to commercial Cornus sp. cuttings to test the impact on root formation compared to alternative protocols including cotton feeding, microinjection, microdialysis delivery and mixed in lanolin.

Objective 3: Interference with auxin movement and metabolism to improve rooting

The auxins IBA, 2-4-D and NAA are more stable with different mobility to IAA and will be applied with and without NPA (auxin transport inhibitor) to cuttings along with the three most effective polyphenols for stabilising auxin (ferulic acid, caffeic acid and vanillin). For each of these compounds PsDR5:GUS will be used to observe changes in auxin response parallel to rooting response in Cornus sp.

Deliverables:
Protocols for effective tree propagation to profit the industry partner while meeting government tree-planting targets.
New technology for targeted release of molecules (no genetic modification required)
Novel insights into how traditional physiology methods can be used to manipulate internal auxin levels and the cause behind the adventitious root development decline with age

Milestones:
Year 1: Review paper (aligned with first year review requirements).
Year 2: Research paper 1: Auxin regulation of adventitious rooting: a comparison between a model and woody species (New Phytologist/Journal of Experimental Botany for example).
Year 3: Research paper 2: Targeted release of auxin and auxin degradation blockers in plant development (Nature/Science)