Geoscience Reference
In-Depth Information
aspects, primarily because exudates of roots offer nutrients rich in carbon and sug-
ars. Some of the bacteria even enter into the roots and reside within the plant's many
vascular bundles without causing any harm to them. Outside in the rhizosphere,
bacteria enable prolongation of fi ne roots. Their production of antibiotics helps the
plants in their fi ght against the harmful microorganisms causing various plant dis-
eases. As the symbiotic bacteria are able to enter into all organs of the plant, they
produce special compounds in vascular bundles warning which specifi c part of the
plant is ill. These biochemical messages, equivalent to the recently developed SMS
(short message service) available today using cell phones and iPads, have been a
part of a plant's life for millions of years. One of the most benefi cial of all processes
performed by rhizobacteria is nitrogen fi xation since nitrogen in gaseous form N
2
is
not usable to plants. Nitrogen gas consists of individual molecules of pairs of N
atoms bound together by three bonds with suffi cient collective strength that plants
do not have enough energy to break any of the triple bonds. Fortunately, rhizobac-
teria are able to convert gaseous N
2
into ammonia NH
3
that is soluble in water and
easily metabolized by plants. The enzyme enabling this transformation requires
special conditions which are provided by membranes within root nodules. The host
plant supplies the bacteria with amino acids so that they have no need to consume
ammonia. Nevertheless, the symbiosis between rhizobacteria and plants is not
free - it costs each plant about 10-25 % of its total photosynthetic output. Legumes,
plants that have root nodules containing rhizobacteria, are recognized as the most
effi cient of all plants to transform nitrogen from insoluble to soluble forms. The
accessibility of other important plant nutrients like iron or phosphates is also fre-
quently enabled by rhizobacteria.
Other microorganisms and zooedaphon inhabiting soil pores were discussed in
detail in Chap.
4
.
