Editing Leghemoglobin (section)

== Debate on principal function ==

Results of a 1995 study suggested that the low free oxygen concentration in root nodule cells is actually due to the low oxygen permeability of root nodule cells.<ref>{{cite book |vauthors=Ludwig RA, de Vries GE |veditors=Broughton WJ, Pühler S |title=Nitrogen Fixation, Vol. 4: Molecular Biology |chapter=Biochemical physiology of Rhizobium dinitrogen fixation |publisher=Clarendon University Press |year=1986 |location=Oxford, UK |pages=[https://archive.org/details/nitrogenfixation0000unse_h1p7/page/50 50–69] |isbn=978-0-19-854575-0 |chapter-url=https://archive.org/details/nitrogenfixation0000unse_h1p7}}</ref> It follows that the main purpose of leghemoglobin is to scavenge the limited free oxygen in the cell and deliver it to [[mitochondria]] for respiration. But, scientists of a later 2005 article suggest that leghemoglobin is responsible both for buffering oxygen concentration, and for delivery of oxygen to mitochondria.<ref name=pubmed.15797021>{{Cite journal |last1=Ott |first1=Thomas |last2=van Dongen |first2=Joost T. |last3=Günther |first3=Catrin |last4=Krusell |first4=Lene |last5=Desbrosses |first5=Guilhem |last6=Vigeolas |first6=Helene |last7=Bock |first7=Vivien |last8=Czechowski |first8=Tomasz |last9=Geigenberger |first9=Peter |last10=Udvardi |first10=Michael K. |date=2005-03-29 |title=Symbiotic leghemoglobins are crucial for nitrogen fixation in legume root nodules but not for general plant growth and development |journal=Current Biology |volume=15 |issue=6 |pages=531–535 |doi=10.1016/j.cub.2005.01.042 |issn=0960-9822 |pmid=15797021|doi-access=free |bibcode=2005CBio...15..531O }}</ref> Their leghemoglobin [[Gene knockout|knockout]] studies showed that leghemoglobin actually does significantly decrease the free oxygen concentration in root nodule cells, and that nitrogenase expression was eliminated in leghemoglobin knockout mutants, assumably due to the degradation of nitrogenase with high free oxygen concentration. Their study also showed a higher [[Adenosine triphosphate|ATP]]/[[Adenosine diphosphate|ADP]] ratio in wild-type root nodule cells with active leghemoglobin, suggesting that leghemoglobin also assists with delivery of oxygen for respiration. 

Plants contain both symbiotic and nonsymbiotic hemoglobins. Symbiotic hemoglobins are thought to be important for symbiotic nitrogen fixation (SNF). In legume, SNF takes place in specialized organs called nodules which contain bacteroids, or nitrogen fixing rhizobia. The induction of nodule-specific plant genes, which include those that encode for symbiotic leghemoglobins (Lb), accompany nodule development. Leghemoglobins accumulate to millimolar concentrations in the cytoplasm of infected plant cells prior to nitrogen fixation to buffer free oxygen in the nanomolar range, which can avoid inactivation of oxygen-labile nitrogenase while keeping a high enough oxygen flux for respiration in the cell. The leghemoglobins are required for SNF but are not required for plant growth and development in the presence of an external source of fixed nitrogen. Leghemoglobins make the essential contribution of establishing low free-oxygen concentrations while keep a high energy status in cells. These are the conditions necessary for effective SNF.<ref name=pubmed.15797021/>