Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/15/1998
Publication Date: N/A
Citation: Interpretive Summary: Corn, soybean, cotton, tomato and many other commercially significant crops in temperature North America are 'chilling sensitive'. These are botanical immigrants of tropical and subtropical origins where selection pressures to deal with low temperature do not exist. Photosynthetic metabolism is among the most chill sensitive process in these plants and the chilling sensitivity of photosynthesis plays a critical role both in limiting the geographical range where these crops are grown and accounts for annual variation in economic success of these crops grown at their northern border. An improvement of even one degree in temperature tolerance would have far reaching beneficial impact on the agronomy of these important crop species. Our earlier work demonstrated chilling interrupts the internal timing mechanism of chilling sensitive plants which is responsible for controlling when during the course of a day certain genes are expressed and specific proteins made. The interruption in circadian rhythm caused by chilling is expected to have adverse effects on photosynthetic metabolism. We show circadian regulation of two important enzymes under circadian regulation, sucrose phosphate synthase (SPS), a key enzyme controlling sucrose biosynthesis in leaves, and nitrate reductase is severely delayed by chilling exposure in tomato. However, the effect of chilling is different in each case. Nitrate reductase is affected at gene expression whereas protein phosphorylation is affected for SPS. This finding is an important clue in understanding the molecular basis for chilling sensitive of photosynthesis in crop plants and presents exciting avenues of research designed to improve chilling tolerance in crop plants.
Technical Abstract: In earlier work we have shown that one intriguing effect of low temperature on chilling sensitive plants is to stall the endogenous rhythm controlling transcription of certain nuclear-encoded genes causing the synthesis of the corresponding transcripts and proteins to be mistimed when the plant is rewarmed. We have also shown that the circadian rhythm controlling the activity of sucrose phosphate synthase (SPS), a key control point of carbon metabolism in plant cells, is delayed in tomato by chilling treatments. Here we show that the activity of nitrate reductase (NR) is under circadian regulation in tomato and, like SPS, is severely delayed by chilling exposure. However, using specific protein kinase and phosphatase inhibitors, we found that the chilling-induced delay in NR activity is at the level of gene transcription rather than the result of corresponding delays in the pattern of the phosphorylation status of the protein as was found for SPS. Whereas the sensitivity of the pattern in SPS activity to specific inhibitors of transcription and translation indicate that the chilling-induced delay in SPS phosphorylation status is caused by an effect on expression of the gene coding for SPS phosphatase, the chilling-induced delay in NR activity corresponds to changes in the NR mRNA and protein levels.