8 Biotechnology and Bioinformatics of Endophytes …
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Bacterial siderophores are implicated directly by stimulating nutrient uptake and
indirectly by sequestering Fe
3+ in the areas around the roots, to prevent its assimilation
by pathogenic microorganisms and thus help in disease inhibition (Hayat et al. 2010).
8.6.4 Phytohormones Synthesis and Regulation
Indole-3-acetic acid (IAA): Microbial auxin is a vital regulatory phytohormone
(Kuhn et al. 2017). Synthesis of phytohormone auxin by microbes is well known
for a long time ago. Also, IAA contributes to plant cell elongation by increasing
cell osmosis, increasing cell permeability to water, increasing cell wall synthesis,
decreasing wall pressure, and inhibiting or decaying leaves abscission (Muday et al.
2012; Mohite 2013). Tryptophan is an amino acid synthesized by beneficial microbes
and acts as the major precursor of IAA (Gamalero and Glick 2015) and determines
the induced and constitutive nature of IAA in various microorganisms.
Cytokinins (CKs): Cytokinins (CKs) are compounds with a structure resembling
adenine (Sakakibara et al. 2006). Their name comes from their capacity of cytokinesis
or plants mitosis enhancement. Cytokinins are synthesized by plants, and several
soil microorganisms (Dodd et al. 2003). Cytokine mediating-beneficial microbes act
as plant growth promotors and biotic and abiotic stress inhibitors by producing or
altering CKs homeostasis (O’Brien and Benkova 2013; Ritika and Mohinder 2016;
Großkinsky et al. 2016).
Gibberellins: The phytohormone gibberellin has an important effect on host
plant development as it could regulate numerous biological processes, starting from
cell division, elongation, and differentiation to fruit development and senescence
(Bueso et al. 2016). Gibberellic acid (GA), is the main gibberellin product. GA
producing PGPRs act by regulating GA levels in plants by increasing root surface
and length (Sharma and Kaur 2018; Khan et al. 2018) and enhance plant growth and
development.
Abscisic Acid (ABA): Abscisic acid (ABA) is a critical plant stress hormone (Sah
et al. 2016). It is responsible for the regulation of various physiological processes in
stressed plants, such as limiting seed germination, inhibiting the growth of shoots
and roots, and stomatal sealing (Cohen et al. 2015). PGPM’s ability to alter ABA
levels in plants suggests their importance in influencing plant growth and abiotic
stress resistance (Dodd et al. 2003).
Jasmonic acid (JA): Being a signal molecule, jasmonic acid (JA) responds to
wound and pathogenesis attacks, by upholding secondary metabolites production in
plants (Gu et al. 2012; Du et al. 2013). It also increases the abundances of bacterial
populations, having phytopathogens, and insects suppressive abilities (Carvalhais
et al. 2013). Additionally, plants under pathogenic attack may evolve mechanisms
to recruit symbionts that synthesize JA to enhance their tolerance to both the biotic
and abiotic stresses (Liu et al. 2017; Ahmad et al. 2017).
189
Bacterial siderophores are implicated directly by stimulating nutrient uptake and
indirectly by sequestering Fe
3+ in the areas around the roots, to prevent its assimilation
by pathogenic microorganisms and thus help in disease inhibition (Hayat et al. 2010).
8.6.4 Phytohormones Synthesis and Regulation
Indole-3-acetic acid (IAA): Microbial auxin is a vital regulatory phytohormone
(Kuhn et al. 2017). Synthesis of phytohormone auxin by microbes is well known
for a long time ago. Also, IAA contributes to plant cell elongation by increasing
cell osmosis, increasing cell permeability to water, increasing cell wall synthesis,
decreasing wall pressure, and inhibiting or decaying leaves abscission (Muday et al.
2012; Mohite 2013). Tryptophan is an amino acid synthesized by beneficial microbes
and acts as the major precursor of IAA (Gamalero and Glick 2015) and determines
the induced and constitutive nature of IAA in various microorganisms.
Cytokinins (CKs): Cytokinins (CKs) are compounds with a structure resembling
adenine (Sakakibara et al. 2006). Their name comes from their capacity of cytokinesis
or plants mitosis enhancement. Cytokinins are synthesized by plants, and several
soil microorganisms (Dodd et al. 2003). Cytokine mediating-beneficial microbes act
as plant growth promotors and biotic and abiotic stress inhibitors by producing or
altering CKs homeostasis (O’Brien and Benkova 2013; Ritika and Mohinder 2016;
Großkinsky et al. 2016).
Gibberellins: The phytohormone gibberellin has an important effect on host
plant development as it could regulate numerous biological processes, starting from
cell division, elongation, and differentiation to fruit development and senescence
(Bueso et al. 2016). Gibberellic acid (GA), is the main gibberellin product. GA
producing PGPRs act by regulating GA levels in plants by increasing root surface
and length (Sharma and Kaur 2018; Khan et al. 2018) and enhance plant growth and
development.
Abscisic Acid (ABA): Abscisic acid (ABA) is a critical plant stress hormone (Sah
et al. 2016). It is responsible for the regulation of various physiological processes in
stressed plants, such as limiting seed germination, inhibiting the growth of shoots
and roots, and stomatal sealing (Cohen et al. 2015). PGPM’s ability to alter ABA
levels in plants suggests their importance in influencing plant growth and abiotic
stress resistance (Dodd et al. 2003).
Jasmonic acid (JA): Being a signal molecule, jasmonic acid (JA) responds to
wound and pathogenesis attacks, by upholding secondary metabolites production in
plants (Gu et al. 2012; Du et al. 2013). It also increases the abundances of bacterial
populations, having phytopathogens, and insects suppressive abilities (Carvalhais
et al. 2013). Additionally, plants under pathogenic attack may evolve mechanisms
to recruit symbionts that synthesize JA to enhance their tolerance to both the biotic
and abiotic stresses (Liu et al. 2017; Ahmad et al. 2017).