Exploring the influence of the plant lateral root gravitropic set point angle on architecture in soils using X-Ray Computed Tomography.

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When we look at a growing plant what we observe are the above ground tissues comprised of leaves stems and flowers. What is not immediately obvious is that all of the above ground growth is absolutely dependent on the root system which is hidden in the soil. The root system is responsible for accessing and taking up the water and essential minerals necessary for healthy growth as well as anchoring the plant in the soil. The importance of these resources to the plant is highlighted by the need for agricultural crops to be given additional water and nutrients in the form of costly fertilizers. However, the supplies of fertilizers are not infinite and already there are worrying decreases in the stocks of rock phosphate. Added to this is the threat of climate change which is likely to lead to increased problems with both short term and long term droughts. The world's population is ever increasing leading to growing concerns about food security. Put simply this raises the question of whether enough high quality nutritious food can be produced in the future to feed the world's population. This need is placing increasing demands on the available agricultural land. In order to increase agricultural output it will be necessary to increase crop productivity. A major target to achieve this goal is to improve the efficiency of plants roots to colonise the soil and increase uptake of water and nutrients. This will provide greater drought tolerance and reduce the need for fertilizer application. A major problem in studying plant roots is the fact that they are generally hidden from view within the soil. Thus to see the root system it is necessary to remove the soil and thus lose the 3-dimensional organisation of the root system. Plant scientists have therefore used clear agar based growth media to cultivate plants in the laboratory allowing the roots to be easily visualised. However this system does not accurately mimic the natural growing environment and questions whether the results obtained and really relevant for the natural world. This project will develop a technique known as X-ray Computed Tomography (CT) as a way to visualise the root system in situ in the soil. This will allow the 3-D organisation to be observed giving a direct way to assess the volume of soil colonised by a plant root system. The necessary high-technology equipment necessary to carry out this aspect of the project is available within the newly formed CT center at the University of Southampton. The plant root system is made of a primary root which grows downwards and which forms lateral branches, elaborating the network. Initially these lateral branches grow horizontally away from the primary root before altering their direction of growth downwards. The initial angle of growth (known as the Gravitropic Setpoint Angle or GSA) of the lateral roots and the length achieved before reorienting their growth downwards are key determinants in the 3-D architecture of the root system and hence the volume of colonised soil. A few Arabidopsis mutants including rhd3 have been identified which display alterations in the GSA demonstrating that it is under genetic control. Work will be carried out to identify and characterise further Arabidopsis mutants affected in their root GSA. Gaining a better understanding of the genetic and environmental factors which influence the growth characteristics of the lateral roots and hence the overall architecture of the root system will be invaluable in attempts to improve crop productivity in the future. The techniques and results obtained in this study will be applicable and transferable to a wide range of different plants including those such as wheat and oil seed rape which form a staple part of our diets.