dehydrogenase, phosphatase, protease, urease and others, derived from plant, animal
or microbial origins (Gupta et al. 2016). These enzymes can be accumulated,
stabilized and or decomposed in the soil.
Lignin degradation is principally an oxidative process catalysed by enzymes
broadly categorized as phenol oxidases, peroxidases and dehydrogenases. Phenol
oxidases are enzymes that oxidize phenolic compounds using oxygen as an electron
acceptor. Peroxidases have heme prosthetic groups that use H 2 O 2 as an electron
acceptor. With redox potentials up to 1490 mV, they can oxidize lignin linkages
either directly or through redox intermediates such as Mn
3+ . The third group of
ligninases, the dehydrogenases, are primarily intracellular oxidative enzymes that
transfer hydride groups from a substrate to an acceptor such as NAD
+
. They are
generally considered substrate-specific but play a key role in the decomposition
process, particularly for bacteria. However, at least a few bacteria,
e.g. Sphingomonas, depolymerize lignin extracellularly using dehydrogenases.
The extracellular oxidative enzymes associated with the degradation of recalcitrant
plant and microbial components include saccharide-oxidizing enzymes such as
glyoxal oxidase, galactose oxidase and glucose oxidase that reduce oxygen to
H 2 O 2 in support of peroxidase activity; and cellobiose dehydrogenase, which
reduces phenoxy radicals, quinones and metal cations, contributing to the supply
of redox mediators.
1.5 Role of Enzymes and Plant Growth Regulators
Plant growth–promoting substances, regulators such as auxins and gibberellins, are
present in root exudates and thus enter the rhizosphere. Auxin is the generic term for
growth substances that typically stimulate cell elongation, while IAA (indoleacetic
acid) is recognized as the principal auxin in plants. The level of auxin is usually
higher in the rhizosphere than in the free bulk soil, a consequence of an increased
microbial population or of accelerated metabolism owing to the presence of root
exudates. A large number of gibberellins have been isolated from bacteria, fungi and
ferns and identified as GA-like substances. The best known GA response is the
stimulation of internode growth.
Microorganisms present in the rhizosphere of various crops appear to have a
greater potential to synthesize and release plant growth substances as secondary
metabolites because of the rich supply of substances, and it is an important factor in
soil fertility. According to several reports, 86% of the bacterial isolates from the
rhizosphere of various plants produced phytohormones such auxins, gibberelins and
kinetin-like substances, but also different hydrolytic enzymes such protease, lipase,
pectinase and amylase.
Acid and alkaline phosphatase activities in wheat rhizosphere were strongly
correlated with the depletion of organic P. Protease activity is involved in the
hydrolysis of N compounds to NH 4 , using low-molecular-weight protein substrates,
and microorganisms are responsible for breaking down urea into ammonium. Urease
8
M. Gomathy et al.
or microbial origins (Gupta et al. 2016). These enzymes can be accumulated,
stabilized and or decomposed in the soil.
Lignin degradation is principally an oxidative process catalysed by enzymes
broadly categorized as phenol oxidases, peroxidases and dehydrogenases. Phenol
oxidases are enzymes that oxidize phenolic compounds using oxygen as an electron
acceptor. Peroxidases have heme prosthetic groups that use H 2 O 2 as an electron
acceptor. With redox potentials up to 1490 mV, they can oxidize lignin linkages
either directly or through redox intermediates such as Mn
3+ . The third group of
ligninases, the dehydrogenases, are primarily intracellular oxidative enzymes that
transfer hydride groups from a substrate to an acceptor such as NAD
+
. They are
generally considered substrate-specific but play a key role in the decomposition
process, particularly for bacteria. However, at least a few bacteria,
e.g. Sphingomonas, depolymerize lignin extracellularly using dehydrogenases.
The extracellular oxidative enzymes associated with the degradation of recalcitrant
plant and microbial components include saccharide-oxidizing enzymes such as
glyoxal oxidase, galactose oxidase and glucose oxidase that reduce oxygen to
H 2 O 2 in support of peroxidase activity; and cellobiose dehydrogenase, which
reduces phenoxy radicals, quinones and metal cations, contributing to the supply
of redox mediators.
1.5 Role of Enzymes and Plant Growth Regulators
Plant growth–promoting substances, regulators such as auxins and gibberellins, are
present in root exudates and thus enter the rhizosphere. Auxin is the generic term for
growth substances that typically stimulate cell elongation, while IAA (indoleacetic
acid) is recognized as the principal auxin in plants. The level of auxin is usually
higher in the rhizosphere than in the free bulk soil, a consequence of an increased
microbial population or of accelerated metabolism owing to the presence of root
exudates. A large number of gibberellins have been isolated from bacteria, fungi and
ferns and identified as GA-like substances. The best known GA response is the
stimulation of internode growth.
Microorganisms present in the rhizosphere of various crops appear to have a
greater potential to synthesize and release plant growth substances as secondary
metabolites because of the rich supply of substances, and it is an important factor in
soil fertility. According to several reports, 86% of the bacterial isolates from the
rhizosphere of various plants produced phytohormones such auxins, gibberelins and
kinetin-like substances, but also different hydrolytic enzymes such protease, lipase,
pectinase and amylase.
Acid and alkaline phosphatase activities in wheat rhizosphere were strongly
correlated with the depletion of organic P. Protease activity is involved in the
hydrolysis of N compounds to NH 4 , using low-molecular-weight protein substrates,
and microorganisms are responsible for breaking down urea into ammonium. Urease
8
M. Gomathy et al.