Systemic Regulation of Plant Development
My lab is focussed on understanding how long distance signalling within plants allows their growth and development to be precisely regulated in both space and time. Plant shoot and root systems are shaped in response to range of environmental stimuli, including soil nutrient status and light availability. For instance, the degree of shoot branching in plants is strongly sensitive to both nitrate and phosphate levels in the soil – indicating long distance communication between the root and shoot system. A small number of hormonal signals, including auxin, cytokinin and strigolactone are implicated in the systemic coordination of plant development, both within root and shoot systems, and between them. We use a wide range of approaches, anchored by molecular genetics, to try and dissect these signalling mechanisms, and to understand how such a simple set of signals can allow such precise control of development at a whole plant level.
Strigolactone signalling and root development
The hormone strigolactone (SL) coordinates multiple aspects of shoot development by triggering ubiquitin-mediated degradation of a small family of ‘SMAX1-LIKE’ (SMXL) proteins (Soundappan et al, 2015). SL acts through the DWARF14 (D14) receptor protein, and the E3 ubiquitin ligase complex SCFMAX2. Intriguingly, a second pathway also acts via SCFMAX2, and triggers degradation of SMAX1 itself in response to signalling through the KAI2 receptor, which is a close homologue of D14. Loss of MAX2 activity alters root system architecture, including increasing lateral root density, but it is currently unclear whether these effects are related to SL or KAI2 signalling, or to both. We are characterising the relative contribution of these signalling pathways to root development, and aim to define their downstream targets in the root system. SLs are secreted into the rhizosphere by many species of plant, and we are also investigating their possible role in plant-plant signalling.
Evolution and diversity in the strigolactone receptor family
The parallel nature of the SL and KAI2 signalling pathways points to an intriguing evolutionary history. It is currently thought that KAI2 signalling evolved in the algal ancestors of land plants, while D14-mediated SL signalling evolved much later. Paradoxically, SLs are synthesised and regulate development in early-diverging land plants that apparently lack the means for SL perception. We are interested in understanding how SLs are perceived in these early-diverging land plants, and how and when ‘modern’ SL receptors have evolved. Phylogenetic and structural analysis of the D14/KAI2 family suggests that, across land plants, a wide variety of small molecules may be perceived through this receptor family. We are interested in characterising this diversity of receptors through structural and chemical biological approaches, and in exploring the potential to use these signalling circuits to modulate development in crop plants.
Control of seed set
The production of fruits and seeds in shoot systems is self-limiting, such that existing fruits tend to inhibit the formation or development of new ones. This is not caused by nutrient limitation, but seems to be a process whereby seed set is actively restricted, for reasons that are currently unclear. This ‘carpic dominance’ is similar to the better-characterised phenomenon of ‘apical dominance’, in which actively shoot branches actively inhibit the formation of new branches. In the crop species oilseed rape, this process results in up to 50% of fruits being shed post-fertilisation; understanding this phenomenon thus has very obvious ramifications for improving crop yields. Almost nothing is known of the mechanisms that regulate carpic dominance, or the signals that allow fruits to communicate with each other. We aim to make rapid progress in characterising this phenomenon, and isolating novel genetic loci that control seed set.