Photorespiration and nitrate assimilation: a major intersection between plant carbon and nitrogen

C-3 carbon fixation has a bad reputation, primarily because it is associated with photorespiration, a biochemical pathway thought to waste a substantial amount of the carbohydrate produced in a plant. This review presents evidence collected over nearly a century that (1) Rubisco when associated with Mn2+ generates additional reductant during photorespiration, (2) this reductant participates in the assimilation of nitrate into protein, and (3) this nitrate assimilation facilitates the use of a nitrogen source that other organisms tend to avoid. This phenomenon explains the continued dominance of C-3 plants during the past 23 million years of low CO2 atmospheres as well as the decline in plant protein concentrations as atmospheric CO2 rises.