Phytochrome photoreceptors and the environmental regulation of plant water use and development.

Predicted increases in global temperatures due to climate change mean that plants must find ways to adapt. These increased temperatures may be coupled with a reduction in the availability of fresh water and so studies need to be undertaken to determine how plants can use their water more efficiently. Stomata, the microscopic pores on the surface of leaves that regulate gas exchange, are the main ways in which plants lose water and are therefore primary candidates to study in the context of water use efficiency (WUE). A key regulator of WUE, through the control of stomatal development and photosynthetic traits, is the red-light photoreceptor phytochrome B (phyB). Increased temperatures cause faster thermal reversion of phyB from its active Pfr form to the inactive Pr form. Therefore, this study aims to determine the impact of temperature on phytochrome activity and how this could impact upon plant development and WUE. I hypothesise that under increased temperatures, faster thermal reversion rates reduce the pool of active Pfr, impacting stomatal development, photosynthesis and morphology. The trade-off between these traits is decisive in determining the outcome on WUE. It was demonstrated that previously documented improvements in WUE of phytochrome mutants is not robust and that impacts on photosynthesis or canopy structures negate any gains in WUE resulting from a reduction in transpiration due to fewer stomata. The inclusion of end of day (EoD) far-red light (FR) demonstrated that night time temperature has an impact on the ability of plants to be more water use efficient. These warmer nights in turn impact upon epidermal patterning, with phyB being a buffer that regulates progression of cell divisions through the stomatal lineage and this could be acting through cell cycle cyclins and SMR4. This work also examined the interaction between phyB and fertiliser treatments in rice. Previous studies had shown that wild-type (WT) rice plants do not flower without the addition of fertiliser whereas OsphyB-1 do. The removal of OsphyB caused improvements in WUE but at the expense of yield independent of fertiliser regimes, however, intermediate fertiliser treatments had similar impacts as full fertiliser treatments under most measurements and so there is the potential that lower levels of fertiliser could be used to grow rice in the future.