9.2.2 Indirect Influence of PGPR
Soil fertility problems and the presence of pests and diseases in agricultural cultivation are the most significant threats. Therefore, the use of inorganic chemical
fertilizers and pesticides is almost inevitable. However, recently demands for
environment-friendly agricultural production have become more intense. In fact, a
higher dosage of inorganic synthetic chemical fertilizer and chemical eradication of
pests and diseases would have negative impacts on the soil ecosystems by reducing
soil biochemical properties, polluting environment, and contaminating food. The use
of PGPR in agriculture not only reduces the use of agrochemicals but also is more
environmentally friendly (Antonius et al. 2012; Lugtenberg and Kamilova 2009).
PGPR has a very important indirect role in inducing resistance to plants against
biotic and abiotic environmental stress. Because of root colonizing bacteria, both
rhizoplane and root endophytic bacteria have a very fundamental role to support
plant adaptation to their unfavorable environment (Hallmann 1997).
Thus, the root colonizing bacteria can induce tolerance to abiotic stress, calling as
the induced systemic tolerance (IST). The IST trigger is osmotic pressure changing,
for example (Yang et al. 2009; Grover et al. 2011). According to Hardoim et al.
(2008), ethylene biosynthesis is a key IST trigger, following the bacteria biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase (Glick 2005; Zahir
et al. 2009; Niu et al. 2012; Naz et al. 2013).
In the ethylene biosynthesis pathway, S-adenosylmethionine (S-AdoMet) is
converted by 1-aminocyclopropane-1-carboxylate synthase (ACS) into
1-aminocyclopropane-1-carboxylate (ACC), which is a precursor of ethylene
(Glick et al. 1998). Under stress conditions, ethylene indigenously regulates plant
homeostasis with decreasing shoot and root growth. The ACC produced by plants is
degraded by the ACC deaminase enzyme produced by bacteria into ammonia and
α-ketobutyrate. In conclusion, through the production and breakdown of ACC by
bacterial cells, root and shoot growth are controlled to stimulate and not inhibit.
Conversely, the induced systemic tolerance (ISR) elicited by microbes affects
plant hormone signal networks composed of such salicylic acid (SA), jasmonic acid
(JA), and ethylene (ET) (Glazebrook 2005). Host plants respond to defenses through
cross-communication between SA, JA, and ET, depending on the pathogen attack
encountered (Koornneef and Pieterse 2008). Timmusk and Wagner (1999) showed a
correlation between biotic and abiotic stress responses in Arabidopsis thaliana
plants induced by PGPR Paenibacillus polymyxa. Moreover, some PGPR-produced
strains protect Arabidopsis thaliana leaves from pathogens of P. syringae and also
protect plants from salinity stress (Barriuso et al. 2008). ISR is identified by the
expression of the PR1 gene, associated with the SA-dependent pathway. This
evidence shows that ISR in plants also induces IST, responding to ET, JA, and
abscisic acid (ABA) pathways under salinity stress (Cheong et al. 2002; Timmusk
and Wagner 1999).
In addition, ISR and IST elicited by some PGPR can produce siderophore, and
these siderophores can support (1) the nutrient uptake under poor nutrient
9 Plant Growth-Promoting Rhizobacteria (PGPR) and Compost Materials for. . .
307
Soil fertility problems and the presence of pests and diseases in agricultural cultivation are the most significant threats. Therefore, the use of inorganic chemical
fertilizers and pesticides is almost inevitable. However, recently demands for
environment-friendly agricultural production have become more intense. In fact, a
higher dosage of inorganic synthetic chemical fertilizer and chemical eradication of
pests and diseases would have negative impacts on the soil ecosystems by reducing
soil biochemical properties, polluting environment, and contaminating food. The use
of PGPR in agriculture not only reduces the use of agrochemicals but also is more
environmentally friendly (Antonius et al. 2012; Lugtenberg and Kamilova 2009).
PGPR has a very important indirect role in inducing resistance to plants against
biotic and abiotic environmental stress. Because of root colonizing bacteria, both
rhizoplane and root endophytic bacteria have a very fundamental role to support
plant adaptation to their unfavorable environment (Hallmann 1997).
Thus, the root colonizing bacteria can induce tolerance to abiotic stress, calling as
the induced systemic tolerance (IST). The IST trigger is osmotic pressure changing,
for example (Yang et al. 2009; Grover et al. 2011). According to Hardoim et al.
(2008), ethylene biosynthesis is a key IST trigger, following the bacteria biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase (Glick 2005; Zahir
et al. 2009; Niu et al. 2012; Naz et al. 2013).
In the ethylene biosynthesis pathway, S-adenosylmethionine (S-AdoMet) is
converted by 1-aminocyclopropane-1-carboxylate synthase (ACS) into
1-aminocyclopropane-1-carboxylate (ACC), which is a precursor of ethylene
(Glick et al. 1998). Under stress conditions, ethylene indigenously regulates plant
homeostasis with decreasing shoot and root growth. The ACC produced by plants is
degraded by the ACC deaminase enzyme produced by bacteria into ammonia and
α-ketobutyrate. In conclusion, through the production and breakdown of ACC by
bacterial cells, root and shoot growth are controlled to stimulate and not inhibit.
Conversely, the induced systemic tolerance (ISR) elicited by microbes affects
plant hormone signal networks composed of such salicylic acid (SA), jasmonic acid
(JA), and ethylene (ET) (Glazebrook 2005). Host plants respond to defenses through
cross-communication between SA, JA, and ET, depending on the pathogen attack
encountered (Koornneef and Pieterse 2008). Timmusk and Wagner (1999) showed a
correlation between biotic and abiotic stress responses in Arabidopsis thaliana
plants induced by PGPR Paenibacillus polymyxa. Moreover, some PGPR-produced
strains protect Arabidopsis thaliana leaves from pathogens of P. syringae and also
protect plants from salinity stress (Barriuso et al. 2008). ISR is identified by the
expression of the PR1 gene, associated with the SA-dependent pathway. This
evidence shows that ISR in plants also induces IST, responding to ET, JA, and
abscisic acid (ABA) pathways under salinity stress (Cheong et al. 2002; Timmusk
and Wagner 1999).
In addition, ISR and IST elicited by some PGPR can produce siderophore, and
these siderophores can support (1) the nutrient uptake under poor nutrient
9 Plant Growth-Promoting Rhizobacteria (PGPR) and Compost Materials for. . .
307