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K. Lasudee et al.
actinobacteria produced gluconic acid as the major organic acid for phosphate solubilization (Mohandas et al. 2013; Poovarasan et al. 2015). Similarly, S. thermocarboxydus isolates S3 from F. mosseae CMU-RYA08 produced gluconic acid, malonic
acid, oxalic acid, and propionic acid to solubilize tricalcium phosphate (Lasudee et al.
2018). Also, Streptomyces spp. and Arthrobacter phenanthrenivorans isolated from
Rhizophagus intraradices were able to solubilize both insoluble organic (inositol
phosphate) and insoluble inorganic phosphates (tri-calcium phosphate). Phosphate
solubilizing ability was reported as solubilization efficiency (SE) (Battini et al. 2016).
Also, zinc solubilization was reported in some Streptomyces species isolated from
G. mosseae (Poovarasan et al. 2015).
Iron is an important micronutrient for the growth of both plants and bacteria.
Although iron is abundant in soils, it exists in the form of ferric iron which is
insoluble. Thus, the amount of iron available to plants and bacteria is low (Glick
2012; Sathya et al. 2017). Siderophores are ferric iron (Fe
3+ ) specific chelators that
can promote plant growth both by direct and indirect mechanisms. Under limiting
iron environment, actinobacteria secrete siderophores directly to bind with ferric
iron before uptake. Siderophores also act as a biocontrol agent for indirect mechanisms. With a higher affinity for iron than fungal pathogens, siderophores from
biocontrol actinobacteria outcompete pathogens causing insufficient iron necessary
for phytopathogen proliferation (Glick 2012). Siderophores have been detected from
Streptomyces species isolated from mycorrhiza by observing yellow-orange halo
around the colony. Unfortunately, the determination of siderophore types was not
investigated in the paper published (Mohandas et al. 2013; Battini et al. 2016). The
only report by Lasudee et al. (2018) determined the type of siderophore produced
by actinobacteria isolated from F. mosseae spores and it comes out as hydroxamate
and catecholate type siderophores.
10.3.2 Biocontrol Activities of Endophytic Actinobacteria
Actinobacteria can also promote plant growth via indirect mechanisms as biocontrol agents as summarized in Table 10.3. Several actinobacteria associated with
mycorrhizal spores have been reported to suppress the growth of plant pathogens
(Bharadwaj et al. 2008b; Mohandas et al. 2013; Poovarasan et al. 2013). For example,
Streptomyces and Leifsonia species isolated from G. mosseae inhibited Fusarium
oxysporum (guava wilt) and Alternaria solani (early blight of tomato) up to 96.4–
98.8% that is possible by the production of chitinase to degrade fungal cell wall.
(Mohandas et al. 2013). Other hydrolytic enzymes such as amylase, cellulase,
lipase, and protease were also reported from Streptomyces spp. and L. poae from
G. mosseae (Poovarasan et al. 2015). Later, the similar set of actinobacteria was
found to inhibit Xanthomonas axonopodis pv punicae (bacterial blight disease) as
studied by (Poovarasan et al. 2013). Actinobacteria isolated from G. moseae or G.
intraradices such as Arthrobacter spp., Micrococcus spp., and Cellulomonas flavigena exhibited antagonistic activity against Rhizoctonia solani (damping-off, cutting
K. Lasudee et al.
actinobacteria produced gluconic acid as the major organic acid for phosphate solubilization (Mohandas et al. 2013; Poovarasan et al. 2015). Similarly, S. thermocarboxydus isolates S3 from F. mosseae CMU-RYA08 produced gluconic acid, malonic
acid, oxalic acid, and propionic acid to solubilize tricalcium phosphate (Lasudee et al.
2018). Also, Streptomyces spp. and Arthrobacter phenanthrenivorans isolated from
Rhizophagus intraradices were able to solubilize both insoluble organic (inositol
phosphate) and insoluble inorganic phosphates (tri-calcium phosphate). Phosphate
solubilizing ability was reported as solubilization efficiency (SE) (Battini et al. 2016).
Also, zinc solubilization was reported in some Streptomyces species isolated from
G. mosseae (Poovarasan et al. 2015).
Iron is an important micronutrient for the growth of both plants and bacteria.
Although iron is abundant in soils, it exists in the form of ferric iron which is
insoluble. Thus, the amount of iron available to plants and bacteria is low (Glick
2012; Sathya et al. 2017). Siderophores are ferric iron (Fe
3+ ) specific chelators that
can promote plant growth both by direct and indirect mechanisms. Under limiting
iron environment, actinobacteria secrete siderophores directly to bind with ferric
iron before uptake. Siderophores also act as a biocontrol agent for indirect mechanisms. With a higher affinity for iron than fungal pathogens, siderophores from
biocontrol actinobacteria outcompete pathogens causing insufficient iron necessary
for phytopathogen proliferation (Glick 2012). Siderophores have been detected from
Streptomyces species isolated from mycorrhiza by observing yellow-orange halo
around the colony. Unfortunately, the determination of siderophore types was not
investigated in the paper published (Mohandas et al. 2013; Battini et al. 2016). The
only report by Lasudee et al. (2018) determined the type of siderophore produced
by actinobacteria isolated from F. mosseae spores and it comes out as hydroxamate
and catecholate type siderophores.
10.3.2 Biocontrol Activities of Endophytic Actinobacteria
Actinobacteria can also promote plant growth via indirect mechanisms as biocontrol agents as summarized in Table 10.3. Several actinobacteria associated with
mycorrhizal spores have been reported to suppress the growth of plant pathogens
(Bharadwaj et al. 2008b; Mohandas et al. 2013; Poovarasan et al. 2013). For example,
Streptomyces and Leifsonia species isolated from G. mosseae inhibited Fusarium
oxysporum (guava wilt) and Alternaria solani (early blight of tomato) up to 96.4–
98.8% that is possible by the production of chitinase to degrade fungal cell wall.
(Mohandas et al. 2013). Other hydrolytic enzymes such as amylase, cellulase,
lipase, and protease were also reported from Streptomyces spp. and L. poae from
G. mosseae (Poovarasan et al. 2015). Later, the similar set of actinobacteria was
found to inhibit Xanthomonas axonopodis pv punicae (bacterial blight disease) as
studied by (Poovarasan et al. 2013). Actinobacteria isolated from G. moseae or G.
intraradices such as Arthrobacter spp., Micrococcus spp., and Cellulomonas flavigena exhibited antagonistic activity against Rhizoctonia solani (damping-off, cutting