Lloret L, Martínez-Romero E

Idioma: Español
Referencias bibliográficas: 219
Paginas: 43-60
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RESUMEN

La fijación biológica del nitrógeno (diazotrofía) es un proceso muy antiguo que probablemente se originó en el Eon arqueano bajo las condiciones anoxigénicas de la atmósfera primitiva, y es exclusivo de procariontes. Sólo ocurre en Euryarchaeota y en 6 de los 54 phyla de Bacteria. Algunos de estos linajes coevolucionaron conjuntamente con las angiospermas estableciendo las bases moleculares de una relación de simbiosis mutualista que, en el caso de los rhizobios, el nitrógeno es fijado dentro de estructuras especializadas, los nódulos, que se desarrollan en la raíz o raramente en el tallo de las leguminosas. Su organogénesis se inicia con los factores de nodulación codificados en el genoma de la bacteria, en plásmidos grandes o islas de simbiosis. Los genes de la nodulación son mucho más recientes que los de la fijación de nitrógeno ya que su origen se asocia con la aparición de su hospedero, la leguminosa. La filogenia del gen 16S rRNA agrupa a los rhizobios dentro de la división Proteobacteria, mayoritariamente en α-Proteobacteria con 7 géneros: Bradyrhizobium, Mesorhizobium, Rhizobium, Sinorhizobium, Methylobacterium y Devosia, y en Β-Proteobacteria con dos géneros recientemente descritos Burkholderia y Wautersia. La topología que resulta con el gen 16S rRNA coincide a nivel de género con la de otros genes cromosomales sin embargo, es incongruente con la de los genes simbióticos (nif y nod) debido a que por estar codificados en elementos móviles, la transferencia horizontal de los genes simbióticos les ha permitido evolucionar en función de la adecuación a la planta hospedera.

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