Submitted to: Journal of Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/1999
Publication Date: N/A
Citation: 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. Photosynthetic metabolism is among the most chill sensitive process in these plants and the chilling sensitivity of photosynthesis plays a critical role in limiting the geographical range where these crops are grown. 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 inhibited the activation of two key photosynthetic enzymes that normally occurs when plants are exposed to light. A puzzling aspect of the inhibition of these enzymes was the fact that several other chloroplast enzymes that are activated by the same mechanism involving the transfer of electrons from a small protein called thioredoxin to the photosynthetic enzymes, were not impaired by the low temperature treatment. We have now discovered that more energy is required to transfer electrons to the inhibited enzymes than is needed for the other enzymes not inhibited by low temperature. In addition, we found that chilling lowers the energy that thioredoxin has to transfer its electrons explaining why only the two enzymes requiring the most energy were inactivated. This finding is an important clue in understanding the molecular basis for the chilling sensitive of photosynthesis in crop plants.
Technical Abstract: Photosynthesis in plant species that are evolutionarily adapted for growth in warm climates is highly sensitive to illumination under cool conditions. Although it is well documented that illumination of these sensitive species under cool conditions results in the photosynthetic production of reactive oxygen molecules, the underlying mechanism for the inhibition of photosynthesis remains uncertain. Determinations of chloroplast fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase activity showed that the light-dependent, reductive activation of these key carbon reduction cycle enzymes was substantially inhibited in tomato (Lycopersicon esculentum) following illumination at 4oC. However, other chloroplast enzymes also dependent on thioredoxin mediated reductive activation were largely unaffected. We performed equilibrium redox titrations to investigate the thermodynamics of the thiol/disulfide exchange between thioredoxin f and the regulatory sulfhydryl groups of fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, NADP-glyceraldehyde phosphate dehydrogenase, and the chloroplast ATPsynthase. We determined that the redox midpoint potentials for the regulatory sulfhydryl groups of the various enzymes, while not affected by illumination under cool conditions, spanned a broad range (>50 mV at pH 7.9). The electron sharing equilibria among thioredoxin f and its target enzymes largely explained the differential effects of low temperature induced oxidation on thioredoxin mediated activation of chloroplast enzymes in tomato.