title: The haemoglobin/nitric oxide cycle: involvement in flooding stress and effects on hormone signalling.
abstract: BACKGROUND
  Class 1 haemoglobins (Hbs) are induced in plant cells under hypoxic conditions. They have a high affinity for oxygen, which is two orders of magnitude lower than that of cytochrome oxidase, permitting the utilization of oxygen by the molecule at extremely low oxygen concentrations. Their presence reduces the levels of nitric oxide (NO) that is produced from nitrate ion during hypoxia and improves the redox and energy status of the hypoxic cell.
  
  
  METHODS
  The mechanism by which Hb interacts with NO under hypoxic conditions in plants is examined, and the effects of Hb expression on metabolism and signal transduction are discussed.
  
  
  CONCLUSIONS
  The accumulated evidence suggests that a metabolic pathway involving NO and Hb provides an alternative type of respiration to mitochondrial electron transport under limited oxygen. Hb in hypoxic plants acts as part of a soluble, terminal, NO dioxygenase system, yielding nitrate ion from the reaction of oxyHb with NO. NO is mainly formed due to anaerobic accumulation of nitrite. The overall reaction sequence, referred to as the Hb/NO cycle, consumes NADH and maintains ATP levels via an as yet unknown mechanism. Hb gene expression appears to influence signal transduction pathways, possibly through its effect on NO, as evidenced by phenotypic changes in normoxic Hb-varying transgenic plants. Ethylene levels are elevated when Hb gene expression is suppressed, which could be a factor leading to root aerenchyma formation during hypoxic stress.
authors: Department of Plant Science, University of Manitoba, Winnipeg, MB, R3T 2N2 Canada.
authors: Abir U
authors: AU
authors: Igamberdiev
authors: Kevin
authors: K
authors: Baron
authors: Nathalie
authors: N
authors: Manac'h-Little
authors: Maria
authors: M
authors: Stoimenova
authors: Robert D
authors: RD
authors: Hill
mesh_terms: D004190:Disasters; D006454:Hemoglobins / Q000502:physiology; D008928:Mitochondria / Q000502:physiology; D018384:Oxidative Stress; D010101:Oxygen Consumption; D010937:Plant Growth Regulators / Q000502:physiology*; D018521:Plant Physiological Phenomena; D015398:Signal Transduction; D014872:Water Movements
chemical_list: D006454:Hemoglobins; D010937:Plant Growth Regulators