Fig. 1.

Anaerobic operation of plant mitochondria. Externally facing dehydrogenases oxidize NADH and NADPH and transfer electrons to ubiquinone (Q). At levels of oxygen below saturation of cytochrome c oxidase (COX), nitrite produced in the cytoplasm serves as an alternative electron acceptor at sites of complexes III (bc1) and IV (COX). NO formed in this reaction is converted in the cytosol by hypoxically induced haemoglobin (Hb) to nitrate NO₃⁻. The latter is reduced to nitrite (NO₂⁻) by nitrate reductase. ATP is synthesized due to proton pumping at the COX site. Modified from Stoimenova et al. (2007).