Submitted to: Photosynthesis International Congress Symposium Proceedings and Abstracts
Publication Type: Proceedings
Publication Acceptance Date: 8/24/1998
Publication Date: N/A
Citation: N/A Interpretive Summary: Many of the commercially most significant crops in temperature North America (e.g., corn, soybean, cotton and others) are referred to as "chilling sensitive." Plants that fall into this category are botanical immigrants from tropical and subtropical origins where selection pressures to deal with low temperature do not exist. The chilling sensitivity of these crops plays a critical role both in limiting the geographical range where these species are grown as well as accounting for the annual variation in the economic success of these crops grown at the northern border of their cultivation. An improvement of even one degree in the low temperature tolerance would have a far reaching beneficial impact on the agronomy of these important crop species. Our earlier work demonstrated that chilling interrupts the internal timing mechanism (i.e., circadian rhythm) of chilling sensitive plants. Since this internal clock is responsible for controlling when during the course of a day that certain genes are expressed and specific proteins made, the interruption caused by chilling would be expected to have adverse effects on cellular metabolism. Our current work reveals that the natural rhythm in nitrate reductase activity is severely delayed by chilling. This finding is a very important clue in understanding the molecular basis for the chilling sensitive in crop plants and presents exciting avenues for research designed to improve chilling tolerance.
Technical Abstract: Overnight low-temperature exposure inhibits photosynthesis in chilling-sensitive species, such as tomato and cucumber, by as much as 60 percent. In earlier work we showed 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. Here we show that the circadian rhythm controlling the activity nitrate reductase (NR), a key control point of nitrogen metabolism in plant cells, is delayed in tomato by chilling treatments. Using specific protein kinase and phosphatase inhibitors, we further demonstrate that the chilling-induced delay in the circadian control NR activity is associated with the activity of a critical protein phosphatase. Although protein dephosphorylation and the subsequent debinding of an inhibitor protein play a central role in NR activation, the chilling-induced delay in NR activity does not appear to arise from effects on NR phosphorylation status but rather from effects directly on NR expression. It is likely that the mistiming in the regulation of NR, and other key enzymes of nitrogen and carbohydrate metabolism that are under circadian regulation, underlies the chilling sensitivity of photosynthesis in these plant species.