Final Report Summary - MEXTIM (Measurement of temperature exposure and integration over time)
The MEXTIM project undertook the ambitious goal of elucidating the molecular mechanisms plants use to register the natural fluctuating temperatures they experience over weeks and months of winter. This information is integrated to influence the timing of the transition to flowering. Genetic, mutational and computational analyses were used to study the thermosensory mechanisms that govern the quantitative expression and epigenetic silencing of the gene encoding the plant developmental repressor FLOWERING LOCUS C (FLC).
Three interconnected objectives were successfully completed to give new viewpoints to the understanding of the thermosensory mechanisms in plants.
In the first objective we defined the molecular mechanisms behind multiple thermo-sensors regulating FLC, the Arabidopsis developmental timer. This included showing how a particular structure in the chromatin – the DNA and protein that makes up chromosomes- is affected by temperature. We also studied the temperature regulation of the nucleation event that triggers cold-induced silencing at FLC. Cold leads to establishment of a metastably silenced state, and only when plants return to warmer conditions does the silencing complex spread over the gene to give long-term epigenetic silencing (Yang et al 2017). We modelled how trans factor activity interfaces with local chromatin environment to vary transcriptional output – and this has been the platform to then build in temperature inputs (Ietswaart et al 2017, Berry et al Cell Systems 2017). We also defined long-term, short-term, circadian-regulated and current temperature modules, and connected the long-term sensor with NTL8 function ((Berry et al 2017, Zhao, Kourounioti et al in preparation). In the cold we have found that a particular protein (NTL8) accumulates, with these increasing levels of NTL8 caused primarily by slowed growth at low temperatures which impedes growth-dependent dilution. In this way, the temperature dependence of growth itself is used as the long-term thermosensor in an entirely novel mechanism.
In our second objective we dissected the contributions of the different thermosensory pathways in the monitoring of natural environments. Our combination of experiments and modelling was very successful and showed a key factor in plants sensing seasonal progression in the absence of warm temperature in autumn, in addition to requiring near freezing temperature at night. This was unexpected and has important implications in forecasting plant phenology in changing climates (Hepworth et al 2018). We have also integrated the fluctuating field temperature data with parameterization data from chamber experiments and generated a mathematical model to predict vernalization response in field conditions (Kourounioti et al 2018). This analysis clearly shows temperature affects every aspect of the regulatory network modulating VIN3 levels and affecting FLC expression and silencing. This considerably changes the understanding in the field, as to date one or two major thermosensors were considered to be how plants monitored ambient temperature.
Our third objective analysed how the thermosensory mechanism has changed during the adaptation of Arabidopsis accessions to different climates. We progressed the molecular mechanism by which the single changes in the DNA sequence at FLC affect its regulation (Questa, Kourounioti et al, in preparation). In addition, we analysed the major differences contributed by FLC to field behaviour of different types of Arabidopsis. Our major finding was that differences in the starting levels of FLC expression were the key feature in determining adaptation of different Arabidopsis accessions to their natural habitats.
Our work and the models we have produced change the view of how plants register winter temperatures and provide the community with the tools and knowledge to understand temperature registration in other plants, a particularly important goal to produce climate-change ready crops.
Three interconnected objectives were successfully completed to give new viewpoints to the understanding of the thermosensory mechanisms in plants.
In the first objective we defined the molecular mechanisms behind multiple thermo-sensors regulating FLC, the Arabidopsis developmental timer. This included showing how a particular structure in the chromatin – the DNA and protein that makes up chromosomes- is affected by temperature. We also studied the temperature regulation of the nucleation event that triggers cold-induced silencing at FLC. Cold leads to establishment of a metastably silenced state, and only when plants return to warmer conditions does the silencing complex spread over the gene to give long-term epigenetic silencing (Yang et al 2017). We modelled how trans factor activity interfaces with local chromatin environment to vary transcriptional output – and this has been the platform to then build in temperature inputs (Ietswaart et al 2017, Berry et al Cell Systems 2017). We also defined long-term, short-term, circadian-regulated and current temperature modules, and connected the long-term sensor with NTL8 function ((Berry et al 2017, Zhao, Kourounioti et al in preparation). In the cold we have found that a particular protein (NTL8) accumulates, with these increasing levels of NTL8 caused primarily by slowed growth at low temperatures which impedes growth-dependent dilution. In this way, the temperature dependence of growth itself is used as the long-term thermosensor in an entirely novel mechanism.
In our second objective we dissected the contributions of the different thermosensory pathways in the monitoring of natural environments. Our combination of experiments and modelling was very successful and showed a key factor in plants sensing seasonal progression in the absence of warm temperature in autumn, in addition to requiring near freezing temperature at night. This was unexpected and has important implications in forecasting plant phenology in changing climates (Hepworth et al 2018). We have also integrated the fluctuating field temperature data with parameterization data from chamber experiments and generated a mathematical model to predict vernalization response in field conditions (Kourounioti et al 2018). This analysis clearly shows temperature affects every aspect of the regulatory network modulating VIN3 levels and affecting FLC expression and silencing. This considerably changes the understanding in the field, as to date one or two major thermosensors were considered to be how plants monitored ambient temperature.
Our third objective analysed how the thermosensory mechanism has changed during the adaptation of Arabidopsis accessions to different climates. We progressed the molecular mechanism by which the single changes in the DNA sequence at FLC affect its regulation (Questa, Kourounioti et al, in preparation). In addition, we analysed the major differences contributed by FLC to field behaviour of different types of Arabidopsis. Our major finding was that differences in the starting levels of FLC expression were the key feature in determining adaptation of different Arabidopsis accessions to their natural habitats.
Our work and the models we have produced change the view of how plants register winter temperatures and provide the community with the tools and knowledge to understand temperature registration in other plants, a particularly important goal to produce climate-change ready crops.