Submitted to: Photosynthesis Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/4/2002
Publication Date: 12/20/2002
Citation: TUCKER, D.E., ORT, D.R. LOW TEMPERATURE INDUCES EXPRESSION OF NITRATE REDUCTASE THAT TEMPORARILY OVERRIDES CIRCADIAN REGULATION OF NITRATE REDUCTASE ACTIVITY IN TOMATO. PHOTOSYNTHESIS RESEARCH. 2002. v 74. p. 1-9. Interpretive Summary: Many of the commercially most significant crops in temperature North America are referred to as 'chilling sensitive. 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 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 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 photosynthetic metabolism. Our current work reveals that there is a temporary stimulation of NR production caused by chilling which results in toxic byproducts of nitrogen assimilation to accumulate in chilling sensitive plant species. This finding is an important clue in understanding the molecular basis for the chilling sensitive of photosynthesis in crop plants and of interest to agricultural researchers working 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. The circadian rhythm controlling the activity of sucrose phosphate synthase (SPS) is delayed in tomato by chilling treatments due an effect of low temperature on the expression of a phosphoprotein phosphatase, perhaps the SPS phosphatase. The activity of nitrate reductase (NR), the first and rate limiting step in the assimilation of nitrate into amino acids in photosynthetic organisms, is subjected to a myriad of regulatory influences including a robust circadian rhythm in activity. The similarity in regulation to SPS suggested that NR might also be subject to low temperature disruptions in its endogenous rhythm in chilling-sensitive species such as tomato. We show here that NR regulation is in fact disrupted by low temperatures, however the change is transient and does not alter the phase of the NR endogenous rhythm following the chill. There is a temporary induction of NR transcription, regardless of the time in the circadian cycle that the chilling episode is initiated, thereby causing an increase in both NR protein and activity. Untimely depletion of reductant and TCA carbon skeletons as well as accumulations of toxic byproducts of nitrogen assimilation that this would cause is likely to be an important contributor to the low temperature induced disruption of metabolism in photosynthetic cells that takes place in chilling sensitive plant species.