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VOL. 70 (3), 743-776, 2004 INTERACCIONES MICROORGANISMOS-SUELO-PLANTA...

There is an increasing interest for the use of microorganisms as biofertilizers,
to achieve the dream of a sustantaible agriculture and silviculture to exclude the
use of phitosanitary products that are expensive and toxic for human health. These
microorganisms can be clasified as beneficious or harmful and infective and non
infective. The beneficious infective microorganisms include the N2 fixing bacteria,
Gram-negatives, from the Rhizobiaceae family, including the genera Rhizobium,
Bradyrhizobium Azorhizobium, Sinorhizobium and Mesorhizobium, all of them li-
ving in symbiosis with legume roots. The actinomicete Frankia fix N2 in some trees
and bushs such us Alnus, Myrica, Eleagnus, Casuarina, etc. The ectomycorrhiza
fungi stablish simbiosis with a great number of trees, while the arbuscular myco-
rrhiza (AM) stablish simbiosis with most of the higher plants.

Between the non-symbiotic fixing bacteria are the eubacteria Rhodobacter and
some cyanobacteria and green-bleu algae as Azolla, Anabaena and Nostoc, specially
important in flooding soils cultures as rice, by permiting a notable saving of nitro-
gen fertilizers and avoiding soil contamination.

Some beneficious non-infective microorganisms have developed the ability to
colonize the plant root surface and they are known as promoting growth plant
rhizobacteria (PGPRs). They stimulate plant growth throught different ways: by
hormon secretion, by inhibiting phitopatogenes or by activating nutrients assimi-
lation, mainly nitrogen and phosphorous. The genera Pseudomonas and Bacillus
(B. thuringensis) are good examples of rhizobacteria.

The dissertation also focuses on the great interest of legume plants in agricul-
ture and environment for being able to fix N2 in arid and semiarid zones under
symbiosis with the Rhizobiaceae family increasing nitrogen content in these areas.
The legumes are divided in three subfamilies: Cesalpinoideae, Mimosoideae and
Papilionoideae, there are about 750 genera and 18000 to 19000 species. The 88%
of the examined species from the three families, form N2-fixing nodules in their
roots infected by Rhizobium. The process of nitrogen fixation is controlled by the
plant from the initiation of the infection by the bacteria, and succesful nodulation
depends on the availability of the combined nitrogen and the adequated photosyn-
thetic energy. As the whole process is governed by genes, the genetic programmes
that order the symbiotic compartments, infection thread and symbiosomes, are
discussed in the text, as also the programmes controlling the nodule histogenesis,
nodule autoregulation and differentation of bacteria in bacteroids. It is also men-
tioned some biotecnology strategies to obtain more efficient and competitive stra-
ins as good nitrogen fixers.

Besides the symbiosis bacteria-plant, it is described briefly the symbiosis fun-
gi-plant which is carried out by the mycorrhizal fungi of the soil. Most of the
plants present mycorrhized roots, helping to them to obtain mineral nutrients
and water from the soil, as complementary roots and inducing resistence against
pathogens. Two types of mycorrhiza fungi are described: the ectomycorrhiza, of
great value in forestry since they help to the growth and development of the trees
and are of great economical interest for industry by the commercialization of the
fungi carpophores (musrhoms). The other type, the arbuscular mycorrhiza stablish
symbiosis with agricultural plants acting as biofertilizers of crops under stress
conditions.

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