PARENT SESSION

Symposium S2C Carbon and nitrogen interactions
Monday August 30th, 2004 2:40 PM-4:20 PM Room 510A
Chair: Steve Huber
Co-Chair: Graham Noctor

Nitrate assimilation in plant shoots depends on photorespiration. Arnold Bloom^*,1, Shimon Rachmilevitch¹, Asaph Cousins¹, ¹ Dept of Plant Sciences, Davis, CA, USA

ABSTRACT- Carbon and nitrogen metabolism in plants intersect at several junctures, including the photorespiratory pathway. Photorespiration has been viewed as a wasteful process, a vestige of the high carbon dioxide atmospheres under which plants evolved. At best, according to current thought, photorespiration may mitigate photoinhibition under high light and drought stress or may generate amino acids such as glycine for other metabolic pathways. Genetic modification of Rubisco to minimize photorespiration in crop plants has been the goal of many investigations. We used several independent methods to demonstrate that exposure of Arabidopsis, tomato, and wheat shoots to conditions that inhibited photorespiration also strongly inhibited nitrate assimilation. Thus nitrate assimilation in both dicotyledonous and monocotyledonous species depends on photorespiration. One part of the photorespiratory pathway is the export of malate from the chloroplast, through the cytoplasm, and into the peroxisome where it generates NADH for the reduction of hydroxypyruvate. This malate "valve" or "shuttle" increases the NADH/NAD ratio in the cytoplasm and thereby may provide NADH for the reduction of nitrate to nitrite. Malate also serves as a counter-ion to prevent alkalinization when nitrate, an anion, becomes incorporated into a neutral amino acid. This novel role for photorespiration (1) explains several responses of plants to rising carbon dioxide concentrations, including carbon dioxide acclimation and carbon dioxide suppression of respiration and (2) raises concerns about genetic manipulations to diminish photorespiration in crops.

KEY WORDS: carbon dioxide acclimation, nitrate assimilation, elevated carbon dioxide, photorespiration