CHILLING DELAYS CIRCADIAN PATTERN OF SUCROSE-PHOSPHATE SYNTHASE AND NITRATE REDUCTASE-ACTIVITY IN TOMATO

Overnight low-temperature exposure inhibits photosynthesis in chilling -sensitive species such as tomato (Lycopersicon esculentum) and cucumb er by as much as 60%. In an earlier study we showed that one intriguin g effect of low temperature on chilling-sensitive plants is to stall t he endogenous rhythm controlling transcription of certain nuclear-enco ded genes, causing the synthesis of the corresponding transcripts and proteins to be mistimed when the plant is rewarmed. Here we show that the circadian rhythm controlling the activity of sucrose phosphate syn thase (SPS) and nitrate reductase (NR), key control points of carbon a nd nitrogen metabolism in plant cells, is delayed in tomato by chillin g treatments. Using specific protein kinase and phosphatase inhibitors , we further demonstrate that the chilling-induced delay in the circad ian control of SPS and NR activity is associated with the activity of critical protein phosphatases. The sensitivity of the pattern of SPS a ctivity to specific inhibitors of transcription and translation indica tes that there is a chilling-induced delay in SPS phosphorylation stat us that is caused by an effect of low temperature on the expression of a gene coding for a phosphoprotein phosphatase, perhaps the SPS phosp hatase. In contrast, the chilling-induced delay in NR activity does no t appear to arise from effects on NR phosphorylation status, but rathe r from direct effects on NR expression. It is likely that the mistimin g in the regulation of SPS and NR, and perhaps other key metabolic enz ymes under circadian regulation, underlies the chilling sensitivity of photosynthesis in these plant species.