9.2.1.4 Fe Nutrient
In the dry soil, iron (Fe
2 +
) is oxidized to Fe
3 + to form insoluble compounds, which
cause Fe deficiency to microbes or plants (Ma 2005). Therefore, there is intense
competition between bacteria, fungi, and plants in the rhizosphere. However, some
bacterial strains synthesize low molecular weight proteins known as siderophores
(Sharma et al. 2003; Vansuyt et al. 2007). This molecule is similar to a chelate, and
this chelate-like compound has a high affinity with iron ions, then Fe-chelate
compounds are formed. Plants and microbes can absorb Fe from Fe-chelate compounds (Machuca et al. 2007; Miethke and Marahiel 2007). The cell membrane
secretes siderophore ions into the cell by connecting the inner and outer membranes
(Boukhalfa and Crumbliss 2002). In addition, bacteria produce extracellular
siderophore that is useful for controlling pathogenic organisms, which are produced
under limited conditions of Fe, and extracellular siderophore forms complexes with
other heavy metals (Zn, Pb, In, Cu, Ga, Cd, and Al) (Schalk et al. 2011). Under the
presence of heavy metals, some bacteria produce siderophores, which can increase
the availability of Fe in the rhizosphere (Wang et al. 1993).
However, as the Fe content in peat is very limited, when plants are planting in
peatland, the Fe application method is important, by applying (1) mineral materials
such as volcanic ash, zeolite, etc. and (2) Fe-chelate compounds. Not only Fe, but
also Mg, Cu, Mn, and Mo are essential elements, especially in electron transport
processes such as photosynthesis, respiration, and nitrogen fixation. Because these
essential nutrients are very limited in peat, it is important to apply them when sowing
seeds or planting seedlings.
9.2.1.5 Plant Growth Factors of PGPR
Rhizosphere space is a relatively nutrient-rich environment, which includes amino
acids, sugars, fatty acids, and other organic compounds, which attract microbes that
utilize various nutrients released by plant roots (Vorholt 2012). Conversely,
microbes biologically synthesize active compounds, including phytohormones
(e.g., auxins, cytokinin, gibberellins, and ABA), antifungal compounds, enzymes,
and valuable compatible compounds, which in some cases will give benefits to
plants or stimulate plant growth promotion (PGP) (Pereira et al. 2016; Etesami
et al. 2014; Sgroy et al. 2009; Khan and Lee 2013; Kudoyarova et al. 2015).
Endophytes that live in plant tissue or on the root surface (Rhizosphere and Rhizoplane) work together with each other by producing various metabolically active
substances (Egamberdieva 2012; Berg et al. 2013; Asaf et al. 2017). Plant-like
hormones produced by microbes have a similarity to exogenous phytohormones
(Egamberdieva 2009; Shahzad et al. 2016).
306
S. Antonius et al.
In the dry soil, iron (Fe
2 +
) is oxidized to Fe
3 + to form insoluble compounds, which
cause Fe deficiency to microbes or plants (Ma 2005). Therefore, there is intense
competition between bacteria, fungi, and plants in the rhizosphere. However, some
bacterial strains synthesize low molecular weight proteins known as siderophores
(Sharma et al. 2003; Vansuyt et al. 2007). This molecule is similar to a chelate, and
this chelate-like compound has a high affinity with iron ions, then Fe-chelate
compounds are formed. Plants and microbes can absorb Fe from Fe-chelate compounds (Machuca et al. 2007; Miethke and Marahiel 2007). The cell membrane
secretes siderophore ions into the cell by connecting the inner and outer membranes
(Boukhalfa and Crumbliss 2002). In addition, bacteria produce extracellular
siderophore that is useful for controlling pathogenic organisms, which are produced
under limited conditions of Fe, and extracellular siderophore forms complexes with
other heavy metals (Zn, Pb, In, Cu, Ga, Cd, and Al) (Schalk et al. 2011). Under the
presence of heavy metals, some bacteria produce siderophores, which can increase
the availability of Fe in the rhizosphere (Wang et al. 1993).
However, as the Fe content in peat is very limited, when plants are planting in
peatland, the Fe application method is important, by applying (1) mineral materials
such as volcanic ash, zeolite, etc. and (2) Fe-chelate compounds. Not only Fe, but
also Mg, Cu, Mn, and Mo are essential elements, especially in electron transport
processes such as photosynthesis, respiration, and nitrogen fixation. Because these
essential nutrients are very limited in peat, it is important to apply them when sowing
seeds or planting seedlings.
9.2.1.5 Plant Growth Factors of PGPR
Rhizosphere space is a relatively nutrient-rich environment, which includes amino
acids, sugars, fatty acids, and other organic compounds, which attract microbes that
utilize various nutrients released by plant roots (Vorholt 2012). Conversely,
microbes biologically synthesize active compounds, including phytohormones
(e.g., auxins, cytokinin, gibberellins, and ABA), antifungal compounds, enzymes,
and valuable compatible compounds, which in some cases will give benefits to
plants or stimulate plant growth promotion (PGP) (Pereira et al. 2016; Etesami
et al. 2014; Sgroy et al. 2009; Khan and Lee 2013; Kudoyarova et al. 2015).
Endophytes that live in plant tissue or on the root surface (Rhizosphere and Rhizoplane) work together with each other by producing various metabolically active
substances (Egamberdieva 2012; Berg et al. 2013; Asaf et al. 2017). Plant-like
hormones produced by microbes have a similarity to exogenous phytohormones
(Egamberdieva 2009; Shahzad et al. 2016).
306
S. Antonius et al.