Enhanced isoprene-related tolerance of heat- and light-stressed photosynthesis at low, but not high, CO2 concentrations

The principal function of isoprene biosynthesis in plants remains unclear, but emission rates are positively correlated with temperature and light, supporting a role for isoprene in maintaining photosynthesis under transient heat and light stress from sunflecks. Isoprene production is also inversely correlated with CO2 concentrations, implying that rising CO2 may reduce the functional importance of isoprene. To understand the importance of isoprene in maintaining photosynthesis during sunflecks, we used RNAi technology to suppress isoprene production in poplar seedlings and compared the responses of these transgenic plants to wild-type and empty-vector control plants. We grew isoprene-emitting and non-emitting trees at low (190 ppm) and high (590 ppm) CO2 concentrations and compared their photosynthetic responses to short, transient periods of high light and temperature, as well as their photosynthetic thermal response at constant light. While there was little difference between emitting and non-emitting plants in their photosynthetic responses to simulated sunflecks at high CO2, isoprene-emitting trees grown at low CO2 had significantly greater photosynthetic sunfleck tolerance than non-emitting plants. Net photosynthesis at 42A degrees C was 50% lower in non-emitters than in isoprene-emitting trees at low CO2, but only 22% lower at high CO2. Dark respiration rates were significantly higher in non-emitting poplar from low CO2, but there was no difference between isoprene-emitting and non-emitting lines at high CO2. We propose that isoprene biosynthesis may have evolved at low CO2 concentrations, where its physiological effect is greatest, and that rising CO2 will reduce the functional benefit of isoprene in the near future.