Induction of Poplar Leaf Nitrate Reductase: A Test of Extrachloroplastic Control of Isoprene Emission Rate

Several recent studies have suggested that control of isoprene emission rate is in part exerted by supply of extrachloroplastic phosphoenolpyruvate to the chloroplast. To test this hypothesis, we altered PEP supply by differential induction of cytosolic nitrate reductase (NR) and PEP carboxylase (PEPC) in plants of Populus deltoides grown with NO₃^- or NH ₄⁺ as the sole nitrogen source. Growth with 8 mM NH ₄⁺ produced a high leaf nitrogen concentration, compared with 8 mM NO₃^-, as well as slightly elevated rates of photosynthesis and significantly enhanced rates of isoprene emission and content of dimethylallyl diphosphate (DMAPP, a precursor to isoprene biosynthesis), chlorophyll (a+b) and carotenoids. Growth with 8 mM NO ₃^- resulted in parallel reductions in both leaf isoprene emission rate and DMAPP. The differential effects of growth with NH ₄⁺ or NO₃^- were not observed when plants were grown with 4 mM nitrogen. The effects of reduced DMAPP availability were specific to isoprene emission and were not propagated to higher isoprenoids, as the correlations between nitrogen content and either leaf chlorophyll (a+b) or total carotenoids were unaffected by nitrogen source. Biochemical analysis revealed significantly higher levels of NR and PEPC activity in leaves of 8 mM NO₃^--grown plants, consistent with their fundamental roles in nitrate assimilation. Taken together, these results support the hypothesis that foliar assimilation of NO₃ ^- reduces isoprene emission rate by competing for carbon skeletons (mediated by PEPC) within the cytosol and possibly reductant within the chloroplast. Cytosolic competition for PEP is a major regulator of chloroplast DMAPP supply, and we offer a new "safety valve" hypothesis to explain why plants emit isoprene.