Abstract
The growth of plants is determined by complex interactions between microorganisms within a complex physical environment. Soil segregates water, air and solid particles into a spatially heterogeneous medium, through which live observation or precise manipulation has proved difficult to date. The plant is also changing the environment chemically so that interactions occurring are also highly dynamic.Non-invasive optical manipulations have been widely used to address similar issues in mammalian biological studies, in particular, to address problems with high temporal and spatial resolution. It is surprising that little of these techniques have benefited the plant sciences.
This thesis reports the development of non-invasive optical approaches to study plant interactions with their environment. The overall objective is to develop optical manipulation techniques for the precise control of interactions between plants and their direct environment, including with microorganisms. The study is broken down into three main parts to achieve this goal. The first part is the study focused on the in situ manipulation of roots within transparent soils. A key challenge is to realise minimal damage to the root epidermis through the complex structured medium. The second part of the thesis develops optical trapping and guiding method in transparent soils as a way to perform controlled studies of root interactions with pathogenic bacteria. The last part is focused on the development of techniques for the levitation and coagulation of aerosols as a way to enable controlled interactions between small biological particles in the phyllosphere (pollen or spores from pathogens).
Soil microorganisms play a very important role in crop growth, with beneficial microbes assisting with the acquisition of mineral elements and protecting against pathogens. However, it is not clear how root growth properties affect associations between plant roots and bacterial populations. Therefore, we developed laser ablation techniques to induce small lesions and modify the elongation rate of roots within transparent soil. We characterise the effect of the optical heterogeneity of transparent soil using an established technique for determining beam quality, that of hole burning through a controlled substrate. This work opens new opportunities for investigating the factors which contribute to the colonisation of the root surface and how it heals following a physical injury.
Studying the attachment and subsequent interactions between a bacterium and plant roots is difficult because the processes involved are stochastic, occur at the microscopic scale, but across a plant that is macroscopic in scale. Controlling the location and time during which bacteria are in contact with the plant would greatly facilitate the acquisition of quantitative data. In the second part of this thesis, we develop optical tweezers using low NA aspherical lenses to manipulate bacteria in transparent soil. We demonstrate that trapping and guiding bacteria to the precise location of the surface of the root and through transparent soil is possible. We characterise factors that are controlling the guiding speeds and number of bacteria that can be attached to plant cells. Using the technique, we also characterise factors that affect the attachment and detachment of bacteria to the surface of the root.
Pathogens in the atmosphere can be transported as aerosols and contribute to the spread of disease in the field. Moreover, many bioaerosols are wrapped in water droplets, involving the fusion of different aerosols. The characterisation of aerosol behaviour is achieved using an optical levitation method. By using this technique, we showed that two aerosols could coagulate. Precise control of aerosols in laboratory conditions using optical levitation opens avenues for further research in plant sciences, including controlled inoculation of pathogens or heterogeneous freezing.
| Date of Award | 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Michael MacDonald (Supervisor) & Tom Vettenburg (Supervisor) |