Bilateral NSF/BIO-BBSRC - Translational landscape to link cell growth with proliferation in the root meristem

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Our life depends on growing plants. Projected population increases together with anticipated disruptions to agricultural production by climate change create a pressing need to achieve step-change improvements in agricultural production to guarantee security of global food supplies. Increases in the application of nitrogen fertilizers underpinned the "green revolution" but are unsustainable. Work described in this proposal will contribute to an alternative route to increased agricultural production, which could be described as a "second green revolution". According to this strategy, agricultural productivity is increased through use of crops in which growth responses are optimized to sustain the increase in biomass in what would otherwise be limiting environments. Plant growth fundamentally depends on maintaining growth and proliferation of cells, which occurs in the meristems. The rate of cell production must be aligned with developmental cues, available energy, nutrient supplies and environmental conditions. Cytoplasmic growth in meristematic cells is largely constrained by protein synthesis and is coupled to cell division to maintain cell size homeostasis. There are evolutionarily conserved sensing and intracellular signalling mechanisms that inform cells on the available nutrient supply. Central to this is the so called TARGET OF RAPAMYCIN (TOR) protein, so named after an antifungal compound produced by a bacterium that was discovered in the Easter Island, Rapa Nui. TOR is central for cell growth mainly through the regulation of protein synthesis and connecting protein synthesis and cell proliferation, but these regulatory mechanisms are not well understood in plant cells. TOR is a master regulator and also functions through other output pathways. One main route of TOR function is through stimulating ribosomes to increase the translational capacity of cells for protein synthesis. Recent findings unexpectedly show that a canonical ribosomal protein target also functions as a transcriptional regulator (repressor). We found that this is in association with a key controller of the cell cycle, the RETINOBLASTOMA RELATED (RBR) protein, named after the cancer in the eye when mutated in humans. RBR and its partner proteins are thought to constitute a switch that controls cell proliferation and cell growth and can be flicked by environmental conditions. In this project we shall use root meristematic cells to systematically uncover transcriptionally and translationally regulated genes that function to connect cell growth and proliferation. We will then design experiments through which we can precisely observe the molecular behaviour of the components of the switch in time, as we alter the growth conditions, while at the same time following changes in growth through microscopic movies. These types of experiments will produce a wealth of data that allow building a comprehensive knowledge of the regulatory network. With additional help from carefully optimized computer models, we can learn the functioning of this cellular decision making circuitry and make predictions at different environmental and nutrient conditions what is the extent of cell proliferation and therefore root growth. Having achieved to construct such a predictive model we will test its performance in different real life situations, such as what happens to root growth in dark, or under limited nitrate or sucrose. We might also find that we missed some components, and this will prompt us for further experimentation. Having perfected the model we can start adapting it to other growth-altering conditions, such as stress, or to other parts of the plant important for crop yield, such as fruits or seeds.