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M. R. DE FELIPE ANTÓN Y COLS. AN. R. ACAD. NAC. FARM.

La segunda parte se refiere a los mecanismos fisiológicos, bioquímicos y mole-
culares que regulan la simbiosis, especialmente los relacionados con la regulación
de oxígeno en el nódulo, de gran importancia para la fijación de N2, ya que la ni-
trogenasa necesita una atmósfera microaeróbica para fijar nitrógeno. Se descri-
ben algunos mecanismos de regulación, como la leghemoglobina y la barrera de
resistencia a la difusión de O2. Por último también se incluye el estudio genético
que realizamos para conocer los genes que regulan la difusión de O2 en el nódulo:
lenod 2, leghemoglobin y nifH.

Palabras clave: Aldehído oxidasa.—Barrera de resistencia a la difusión de oxí-
geno.—Fijación biológica de nitrógeno.—Leghemoglobina.—Lupino.—MAPKina-
sas.—Proceso de infección.—Regulación de oxígeno.

SUMMARY

Structural, biochemical and molecular factors of the simbiosis
Bradyrhizobium sp. (Lupinus)-Lupinus

The present revision summarises some of the original works carried out in our
Department to study different aspects of the peculiar symbiosis Bradyrhyzobium sp.
Lupinus, by integrating structure and function. The first part of the revision is
focused to a structural study on the infection mechanisms, since differ of other
legumes. This study was realized by applying immunofluorescent techniques with
the green fluorescent protein, immunocytochemistry and different microscopies. It
could be observed that the infection by the Bradyrhizobium follows the intercellular
way. The infection study includes the presence of enzymatic components, as MAPKs
(Mitogen-Activated Protein Kinase) and aldehyde oxidase in the first steps of the
symbiosis. Both enzymes are very much related with auxins and cell multiplication.

The second part takes up the physiological, biochemical and genetic mechanisms
that regulates nitrogen fixation, speccially those related to oxygen diffusion, since
nitrogenase activity is very sensible to oxygen and needs a microaerobic atmosphere
for its function. Between these mechanisms, the leghemoglobine and the oxygen-
diffusion resistance barrier have been considered the more important in oxygen
regulation. Finally it is included the results of a genetic study carried out to know
the genes implicated in nodule oxygen diffusion as: lenod 2, leghemoglobin and nifH.

Key words: Aldehyde oxidase.—Biological nitrogen fixation.—Infection
process.—Leghemoglobin.—Lupin plants.—MAPKinases.—Oxygen regulation.—
Oxygen diffusion barrier.

1. INTRODUCCIÓN: FIJACIÓN BIOLÓGICA DE NITRÓGENO

La Fijación Biológica de Nitrógeno (FBN) aporta la mayor parte
del nitrógeno fijado a los ecosistemas terrestres. Se estima que apro-

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