one of the key mechanisms of plant growth promotion by bacteria and fungi. Thus,
the work described here is central to effectively using plant growth-promoting
bacteria and fungi as biological tools in agricultural practice.
19.1 Introduction
In the absence of beneficial soil microorganisms, i.e., plant growth-promoting
bacteria (PGPB) and mycorrhizal fungi, the growth of plants would be severely
limited, especially during periods of environmental stress. However, by various
estimates, these plant-helper soil microorganisms have been interacting with plants
and facilitating their growth for somewhere between fifty and several hundred
million years. The environmental stresses that these soil microorganisms help the
plants to overcome include, but are not limited to, abiotic stresses such as drought,
flooding, and high salt concentrations; extremes of temperature and sunlight; the
presence of inhibitory metals and organic compounds; and the paucity of certain
nutrients in the soil such as fixed nitrogen, iron, and phosphorus. These soil
microorganisms help plants to overcome biotic as well as abiotic stresses; these
include fungal and bacterial plant pathogens, and nematode and insect predation.
Given the effectiveness of certain soil microorganisms in promoting plant growth, it
is not surprising that the scientific literature is filled with countless examples
describing PGPB and mycorrhizae, both separately and together, facilitating plant
growth under laboratory, greenhouse, and field conditions (Glick 2012; Reed and
Glick 2013).
While PGPB utilizes a range of different mechanisms to promote plant growth,
arguably, one of the key mechanisms utilized by PGPB to facilitate plant growth is
the use of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase.
Importantly, ACC is the immediate precursor, in all higher plants, of the plant
hormone ethylene. Ethylene is involved in seed germination, tissue differentiation,
root development, lateral bud development, flowering, anthocyanin synthesis, fruit
ripening, aroma production, leaf senescence, leaf and fruit abscission, and maintenance of plant–microbe interaction in Rhizobia nodule formation and mycorrhizae–
plant interactions (Ali et al. 2017; Glick 2015). Moreover, ethylene is also a plant
stress hormone, with its level rising as a consequence of various environmental
stresses. While this hormone is typically required in very low concentrations for
normal plant growth and development, the much higher levels of ethylene that plants
produce when they are subjected to either abiotic or biotic stresses are generally
deleterious to plant growth and development. Thus, by lowering the amount of ACC,
and hence the level of ethylene, in plant tissues, ACC deaminase can ameliorate
many of the harmful inhibitory effects of stress ethylene.
It has been widely demonstrated in laboratory experiments that many different
plants exhibit a much higher level of resistance to a wide range of environmental
stresses, especially abiotic stresses, when those plants have first been inoculated with
ACC deaminase-containing PGPB. These laboratory observations are consistent
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S. Ali and B. R. Glick
the work described here is central to effectively using plant growth-promoting
bacteria and fungi as biological tools in agricultural practice.
19.1 Introduction
In the absence of beneficial soil microorganisms, i.e., plant growth-promoting
bacteria (PGPB) and mycorrhizal fungi, the growth of plants would be severely
limited, especially during periods of environmental stress. However, by various
estimates, these plant-helper soil microorganisms have been interacting with plants
and facilitating their growth for somewhere between fifty and several hundred
million years. The environmental stresses that these soil microorganisms help the
plants to overcome include, but are not limited to, abiotic stresses such as drought,
flooding, and high salt concentrations; extremes of temperature and sunlight; the
presence of inhibitory metals and organic compounds; and the paucity of certain
nutrients in the soil such as fixed nitrogen, iron, and phosphorus. These soil
microorganisms help plants to overcome biotic as well as abiotic stresses; these
include fungal and bacterial plant pathogens, and nematode and insect predation.
Given the effectiveness of certain soil microorganisms in promoting plant growth, it
is not surprising that the scientific literature is filled with countless examples
describing PGPB and mycorrhizae, both separately and together, facilitating plant
growth under laboratory, greenhouse, and field conditions (Glick 2012; Reed and
Glick 2013).
While PGPB utilizes a range of different mechanisms to promote plant growth,
arguably, one of the key mechanisms utilized by PGPB to facilitate plant growth is
the use of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase.
Importantly, ACC is the immediate precursor, in all higher plants, of the plant
hormone ethylene. Ethylene is involved in seed germination, tissue differentiation,
root development, lateral bud development, flowering, anthocyanin synthesis, fruit
ripening, aroma production, leaf senescence, leaf and fruit abscission, and maintenance of plant–microbe interaction in Rhizobia nodule formation and mycorrhizae–
plant interactions (Ali et al. 2017; Glick 2015). Moreover, ethylene is also a plant
stress hormone, with its level rising as a consequence of various environmental
stresses. While this hormone is typically required in very low concentrations for
normal plant growth and development, the much higher levels of ethylene that plants
produce when they are subjected to either abiotic or biotic stresses are generally
deleterious to plant growth and development. Thus, by lowering the amount of ACC,
and hence the level of ethylene, in plant tissues, ACC deaminase can ameliorate
many of the harmful inhibitory effects of stress ethylene.
It has been widely demonstrated in laboratory experiments that many different
plants exhibit a much higher level of resistance to a wide range of environmental
stresses, especially abiotic stresses, when those plants have first been inoculated with
ACC deaminase-containing PGPB. These laboratory observations are consistent
366
S. Ali and B. R. Glick