The regulation of plant growth in response to soil volume and neighbour density

Plants must carefully integrate environmental information and plan for their future growth according to their current and predicted resource availability. Soil volume and the presence of neighbouring plants are two such environmental factors that plants must take into consideration. Soil volume has been shown to strongly inhibit plant growth independently of nutrient availability, and plants sown at high densities show similar inhibition of growth. However, the mechanisms underpinning soil volume and neighbour density detection and response have remained elusive. In this thesis I have assessed inter- and intraspecies responses to soil volume and neighbour density in the shoot and root system, characterising the responses in agronomically important crops such as wheat, barley and pea, as well as Arabidopsis. By carrying out a large phenotyping screen of barley germplasm, I have identified genotypes with elevated and reduced sensitivity to soil volume. I have shown that although plant responses to soil volume and neighbour density are similar, they are not completely interchangeable. I have proposed a two-phase root exudate-based system that plants use to detect and respond to their available soil volume and the presence of neighbouring plants. The first phase involves the recognition of available space by dilution of an exudate and this results in changes in shoot growth. The second phase involves a different signal with much lower mobility, which increases when roots are at high density and results in shoot and root growth inhibition. I have identified that the first phase root exudate is likely exuded strigolactones. This thesis demonstrates a new role for exuded strigolactones as plant-to-plant signals that inhibit the shoot growth of neighbouring plants. This thesis provides a strong basis for integrating the soil volume and neighbour density responses into breeding programs to produce larger plants within a given space without the need for elevated fertiliser inputs.