Plant cells exposed to anaerobic stress generate copious amounts of the gaseous free radical nitric oxide (NO). At this time, the concomitant expression of the ubiquitous class 1 plant hemoglobins establishes one component of a soluble terminal NO dioxygenase system, which yields nitrate ions via reaction of oxyhemoglobin with NO. Class 1 hemoglobin expression also enhances the cellular energy status, redox status, and NO metabolism of plant cells exposed to hypoxic stress. The ability of class

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... 1 hemoglobins to ligate oxygen at concentrations two orders of magnitude lower than cytochrome c oxidase suggests that hemoglobin and NO may serve as components of an alternative type of respiration that is operative during conditions that impair the operation of mitochondrial terminal oxidases. We suggest that, under hypoxic conditions, NO can be formed by anaerobic reduction of nitrite by a portion of the mitochondrial electron transport chain. NADH and NADPH, accumulated due to glycolytic fermentation and lipid breakdown, contribute electrons to the process, generating a chemiosmotic potential capable of generating ATP. The overall anaerobic reaction sequence is referred to as the Hb/NO cycle.