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6.5 Plant Growth Stimulating Endophytic Bacteria
Even, Dharni et al. (2014) and Ma et al. (2016) noted that it is known that PGPB
participates in plant growth-promotion and heavy-metal phytoremediation. Various
workers resumed that there is little knowledge about plant endophytic bacteria interactions and their potential role in phytoremediation (He et al. 2013; Chen et al. 2014;
Babu et al. 2015; Ma et al. 2015). Phetcharat and Duangpaeng (2012) named these
bacteria as plant growth-promoting endophytic (PGPE) bacteria and suggested the
role of internal colonization for plant-propagation, enhancement of soil fertility, and
stimulation of host plant growth by plant growth regulators production. It also has
been demonstrated by Ma et al. (2011) that endophytic bacteria may help host plants
to adapt under unfavorable environmental conditions and increase the phytoremediation efficiency, promoting the plant growth, alleviating the metal stress, reducing
metal phytotoxicity, and finally altering the metal bioavailability and translocation
inside the plants. Plant growth-promoting bacteria are a broad group of bacteria that
are present in the soil rhizosphere and enter into root-cells and tissues under complex
signaling mechanisms. A group of these bacteria called ‘endophyte–s’ which are able
to reach toward plant host using chemotaxis, and enter into plant tissues through the
lenticels, wounds caused by trichome breaks, stomata, exit zone of lateral roots and
the area of root radical (Hallmann et al. 1997; 1998). These microorganisms grow in
the apoplast or simplest space of plant root tissues without causing obvious damage
(Gimenez et al. 2007). They often proliferate in the intercellular space of the root or
may enter the peripheral circular cells. These develop systemic-infection and enter
into parenchymal cells (Hallmann et al. 1997; 1998). Specific genetic systems are
then activated between the bacterium and plant (Hardoim et al. 2008). The above
symbiotic bacteria provide various benefits, such as increased level of resistance to
stress and improvement of plant growth conditions (Dheeman et al. 2017). This relationship provides a balance between the plant and endophyte, but if environmental
conditions are manipulated in favor of endophyte, the endophyte becomes pathogenic
to disturb homeostasis drastically (Aly et al. 2010).
There are various ways by which bacterial endophytes can confer resistance or
tolerance to the host plant from different biotic and abiotic stresses (Santoyo et al.
2016). Endophytic bacteria utilize the nutrients, assimilated in plants and stabilizing
nitrogen in-turn. These are also involved in production of 2-, 3-butanol, and acetone
(Sturz et al. 2000), as well as secreting hormones such as ethylene, auxin, cytokinin,
gibberellin, etc. By the production of terpenoids, flavonoids, isoflavonoids they help
plant in developing resistance against pathogens and counter-acting environmental
stresses. In addition to these, endophytic bacteria produce antifungal compounds,
i.e., siderophore, which enhance iron-absorption, competition for food, limiting
ecological-niche, and favoring plant-resistance mechanisms. Hence, it is worthy
to understand that, PGPR is an important player of biological control (Hardoim
et al. 2008; Jha et al. 2013). Research on Chinese medicinal plant Ferula songorica
revealed that this plant is a rich reservoir of endophytic bacteria with an ability to
solubilize phosphate and producing enzymes such as protease and cellulose (Yadav
A. Ghasemnezhad et al.
6.5 Plant Growth Stimulating Endophytic Bacteria
Even, Dharni et al. (2014) and Ma et al. (2016) noted that it is known that PGPB
participates in plant growth-promotion and heavy-metal phytoremediation. Various
workers resumed that there is little knowledge about plant endophytic bacteria interactions and their potential role in phytoremediation (He et al. 2013; Chen et al. 2014;
Babu et al. 2015; Ma et al. 2015). Phetcharat and Duangpaeng (2012) named these
bacteria as plant growth-promoting endophytic (PGPE) bacteria and suggested the
role of internal colonization for plant-propagation, enhancement of soil fertility, and
stimulation of host plant growth by plant growth regulators production. It also has
been demonstrated by Ma et al. (2011) that endophytic bacteria may help host plants
to adapt under unfavorable environmental conditions and increase the phytoremediation efficiency, promoting the plant growth, alleviating the metal stress, reducing
metal phytotoxicity, and finally altering the metal bioavailability and translocation
inside the plants. Plant growth-promoting bacteria are a broad group of bacteria that
are present in the soil rhizosphere and enter into root-cells and tissues under complex
signaling mechanisms. A group of these bacteria called ‘endophyte–s’ which are able
to reach toward plant host using chemotaxis, and enter into plant tissues through the
lenticels, wounds caused by trichome breaks, stomata, exit zone of lateral roots and
the area of root radical (Hallmann et al. 1997; 1998). These microorganisms grow in
the apoplast or simplest space of plant root tissues without causing obvious damage
(Gimenez et al. 2007). They often proliferate in the intercellular space of the root or
may enter the peripheral circular cells. These develop systemic-infection and enter
into parenchymal cells (Hallmann et al. 1997; 1998). Specific genetic systems are
then activated between the bacterium and plant (Hardoim et al. 2008). The above
symbiotic bacteria provide various benefits, such as increased level of resistance to
stress and improvement of plant growth conditions (Dheeman et al. 2017). This relationship provides a balance between the plant and endophyte, but if environmental
conditions are manipulated in favor of endophyte, the endophyte becomes pathogenic
to disturb homeostasis drastically (Aly et al. 2010).
There are various ways by which bacterial endophytes can confer resistance or
tolerance to the host plant from different biotic and abiotic stresses (Santoyo et al.
2016). Endophytic bacteria utilize the nutrients, assimilated in plants and stabilizing
nitrogen in-turn. These are also involved in production of 2-, 3-butanol, and acetone
(Sturz et al. 2000), as well as secreting hormones such as ethylene, auxin, cytokinin,
gibberellin, etc. By the production of terpenoids, flavonoids, isoflavonoids they help
plant in developing resistance against pathogens and counter-acting environmental
stresses. In addition to these, endophytic bacteria produce antifungal compounds,
i.e., siderophore, which enhance iron-absorption, competition for food, limiting
ecological-niche, and favoring plant-resistance mechanisms. Hence, it is worthy
to understand that, PGPR is an important player of biological control (Hardoim
et al. 2008; Jha et al. 2013). Research on Chinese medicinal plant Ferula songorica
revealed that this plant is a rich reservoir of endophytic bacteria with an ability to
solubilize phosphate and producing enzymes such as protease and cellulose (Yadav