plant surface microbiology.pdf

4 Microorganisms on the Rhizosphere Microcosm 55
structure, and then the depolymerisation itself occurs. The resultant disaccharide,
cellobiose, is hydrolysed by the enzyme cellobiase to glucose (Paul
and Clark 1989).
The amylolytic group hydrolyses starch, which is a common reserve of
polysaccharide that serves as an energy storage product in plants. Starch is
called amylose when it is a linear polymer of glucose linked in the a1-4 position.
The a1-4 linkage facilitates a more rapid breakdown rate than the b1-4
linkage found in cellulose. Glucose can also be found linked in a1-6 positions
to produce a polymer known as amylopectin.
Extracellular enzymes known as amylases are produced by numerous
fungi, actinomycetes and some bacteria. a-amylases hydrolyse both amylose
and amylopectin to units consisting of several glucose molecules. b-amylase
reduces amylose to maltose (two glucose units), subsequent hydrolysis of maltose
by an a1-4 glucosidase (maltase) yields glucose, and amylopectin is broken
down to a mix of maltose and dextrins.
The proteolytic functional group can act in both the carbon and nitrogen
cycles, described later. Many microorganisms, such as fungi, actinomycetes
and bacteria may produce extra-cell enzymes called proteinases and peptidases.
The proteinases degrade proteins releasing peptides which in turn are
attacked by the peptidases releasing amino acids which are transported inside
the cells (Fig. 3).
The amino acids may be used as a source of either carbon or nitrogen. In
the carbon cycle, the amino acids are catabolised into various compounds, as
intermediate metabolites of the glucolytic path or tricarboxylic acid cycle. In
this conversion, the amino acid undergoes a de-amination process where the
amine group is removed and converted into ammonia (NH 3 + ) which may be
excreted by the cells. The carboxylic group can enter in the tricarboxylic acid
cycle or undergo a process of de-carboxylisation (removal of COOH) and dehydrogenisation,
releasing carbon dioxide and nitrogen compounds, such as
amines and di-amines.
4 Functional Groups of Microrganisms of the Nitrogen Cycle
Plants, animals, and most microorganisms require combined forms of nitrogen
for incorporation into cellular biomass, but the ability to fix atmospheric
nitrogen is restricted to a limited number of bacteria and symbiotic associations.Whereas
many habitats depend on plants for a supply of organic carbon
that can be used as a source of energy, all organisms depend on the bacterial
fixation of atmospheric nitrogen (Atlas and Bartha 1993).
Several functional groups in the nitrogen cycle can be used as bioindicators
of disturbances in the soil. Among these, the groups to be considered are the
symbiotic or free-living nitrogen fixers for legumes and non-legumes plants,
respectively, and others which participate in the mineralisation process of the