The influence of the circadian clock and environmental conditions on the calcium-dependent response to pathogens

An increase in the cytosolic concentration of Ca2+ ([Ca2+]cyt) is a ubiquitous response to a variety of plant stresses and stimuli. The circadian clock, which allows plants to coordinate their internal mechanisms to their environment, can influence the magnitude of these calcium-dependent responses. Data in this thesis demonstrate that pathogen-induced second messenger signalling and downstream gene expression is gated by the circadian clock and heightened in the subjective morning. Using core circadian clock mutants, LHY and CCA1 were found to be important core components involved in controlling this gated response. While searching for intermediates acting between the clock and the [Ca2+]cyt response to flg22, the circadian influence on the FLS2 receptor was investigated. Although no circadian control of FLS2 protein or localisation was detected in this work, artificially increasing FLS2 levels was found to restore the [Ca2+]cyt response to flg22 in the evening. This clearly suggests that there may be a circadian clock-dependent inhibition of FLS2, potentially post-translationally, at this time. Another component investigated was the photosynthetic electron transport chain (ETC). Specific blockers were used to inhibit the ETC causing a reduced [Ca2+]cyt response to flg22. ETC activity, and output, was found to be higher during the subjective day, suggesting that the circadian clock may control ETC efficiency, which may in turn regulate the magnitude of the [Ca2+]cyt response to flg22. As well as the circadian clock, this work established that the calcium-dependent response to PAMPs was regulated by a change in the environmental temperature. A pre-treatment at an increased temperature, or pharmacologically increasing membrane fluidity, caused a reduction in flg22-dependent second messenger signalling and ICS1 and EDS1 gene expression. This was found to be, at least in part, because an increase in membrane fluidity reduces FLS2 protein levels.