BIOFERTILIZERS AS COMPLEMENTS TO SYNTHETIC AND ORGANIC FERTILIZATION - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 161

and IAA can be found in up to 80% of the bacteria isolated from the rhizosphere of some
plants (Loper & Schroth, 1986).
Synthesis of Vitamins
The production of certain vitamins contributes significantly to the growth promoting
activity of certain microorganisms. For example, it has been shown that Pseudomonas
fluorescens strain 267 produces water soluble vitamins of the B group, which stimulate the
growth of red clover, Trifolium pratense (Marek-Kozaczuk & Skorupska, 2001). Also, some
strains of Azotobacter and Azospirillum produce B vitamins that increase the rooting capacity
of plants and affect soil microbial populations (Rodelas et al., 1993; Revillas et al., 2000).
Regulation of the Ethylene Levels
Ethylene is a plant hormone that can inhibit the development of the root and, therefore,
limit the ability of plants to absorb nutrients and water from soil. In higher plants, the enzyme
S-adenosyl-L-methionine (SAM) synthase catalyzes the conversion of methionine to SAM
(Giovanelli et al. , 1980). In response to various types of stress, including mechanical injury,
water stress (drought and flooding), salinity, herbicides, among others, the enzyme ACC
synthase catalyzes the conversion of SAM to 1-aminocyclopropane-1-carboxylic acid (ACC),
which is the immediate precursor of ethylene. Subsequently, ACC oxidase enzyme catalyzes
the conversion of ACC to ethylene, carbon dioxide and hydrogen cyanide (John, 1991). This
increase in the levels of ethylene in the root causes a delay in root growth. Some
microorganisms, e.g . different species of the genera Pseudomonas and Bacillus , possess an
enzyme called ACC deaminase, which hydrolyzes 1-aminocyclopropane-1-carboxylic acid,
the immediate precursor of ethylene, to form ammonia and -ketobutyrate (Mayak et al. ,
1999; Shaharoona et al. , 2006; Glick et al. , 2007) and, thereby, prevent the formation of
ethylene. Consequently, when the ACC deaminase activity increases, the levels of ethylene in
the plant decreases and root development is visibly increased (Muhammad et al. , 2007).
Siderophore Production
Siderophores are molecules (mainly non-ribosomal peptides) with a high affinity for iron
that are produced by various microorganisms and grasses (Neilands, 1952; 1995) to increase
the bioavailability of this element. At neutral pH, the iron availability in the soil is limited to
plants due to the low solubility of the mineral phases (such as iron oxides) that associate to
this element. Siderophores dissolve these mineral phases forming soluble complexes of iron
can be then introduced into plant cells by active transport mechanisms. Under anoxic
conditions (low oxygen), iron is usually found in the oxidation state of Fe +2 (ferrous ion)
which is a soluble form. However, under oxic conditions ( i.e . high concentrations of oxygen),
iron is found as ferric ion (Fe +3 ) that is capable of forming different insoluble minerals. To
obtain the iron from these minerals, cells produce siderophores that binds to iron with high