The effect of dark chilling on circadian-regulated enzyme activities in tomato

Overnight low temperature treatment inhibits next day photosynthesis in chilling-sensitive plant species such as tomato and cucumber. In tomato, net photosynthesis is reduced 60% by an overnight chill at 4$\sp\circ$C. The mechanism for the inhibition of photosynthesis is unknown. However, one effect of the low temperature treatment is to delay the circadian clock regulating the transcription of several genes. The focus of this dissertation was to determine whether effects of low temperature on the circadian timing of gene expression contribute to the inhibition of photosynthesis following dark chilling.It was of interest to determine whether low temperature induced delays in rhythms of gene transcription might cause corresponding changes in enzyme activity rhythms, since mistimed activity patterns of key enzymes might have direct effects on photosynthesis. Thus, I identified two central metabolic enzymes that exhibit circadian activity in tomato, sucrose phosphate synthase (SPS) and nitrate reductase (NR), and investigated the effect of low temperature on these activity rhythms.SPS and NR are both negatively regulated by protein phosphorylation, and I have shown that the circadian rhythm in both SPS and NR activity is the result of circadian oscillations in protein phosphorylation state. In both cases, the basis for the rhythm is circadian regulation of the activity of the phospho-protein phosphatase that dephosphorylates and activates the enzyme. My data show that SPS-phosphatase gene transcription is circadian, however, the mechanism for the circadian oscillation in NR-phosphatase activity appears not to be transcription and is at this time unknown.Low night temperature delays the circadian rhythm in SPS and NR activity, resulting in dramatically altered patterns of enzyme activity the following day. Chilling suspends the circadian oscillation in SPS and NR phosphorylation state by disrupting the oscillation in phosphatase activity. The chilling sensitivity of photosynthesis may be the result of temporally inappropriate SPS and NR activity levels, as well as other, presently unidentified, circadian regulated enzyme activities that are mistimed by a chilling night.U of I OnlyETDs are only available to UIUC Users without author permissio