8.5.1 Endophytic Actinobacteria as Plant Growth
Promoting Agents
Although plants are able to adjust their activities and metabolism in the presence of
stresses during their life cycle, for instance, they can synthesize various specific
defensive proteins to overcome stress (Hossain et al. 2012 ), the exposure of plants
to abiotic and biotic stresses still causes major losses in yield of agricultural crops.
Certain actinobacteria have been found to be able to help their host plants by either
completely or partially avoiding abiotic and biotic stresses (Gopalakrishnan et al.
2016 ). Especially, beneficial endophytic actinobacteria that promote plant growth
under favorable and unfavorable conditions have recently received attention
(Hasegawa et al. 2006 ). Like rhizosphere actinobacteria, beneficial endophytic
actinobacteria are capable of improving plant growth via one or more plant growth
promoting mechanisms, includingfixation of atmospheric nitrogen, solubilization
of mineral nutrients, secretion of phytohormones, and siderophores (Dudeja et al.
2012 ) (Fig.8.1).
Endophytic actinobacteria are able to express nitrogenase and occupy an
essential ecological niche in the living plant tissue by providingfixed nitrogen to
their hosts (Soe et al. 2012 ). It is well known that endophytic actinobacteria pos-
sessing strong nitrogen-fixing property may confer plants the capacity to tolerate
nitrogen-poor soil environment. For instance, the endophytic genera Frankia
(Callaham et al. 1978 ),Micromonospora(Trujillo et al. 2015 ) andStreptomyces
(Soe et al. 2012 ) were capable of enhancing plant growth under nitrogen-limited
environment byfixing nitrogen. Recently, nitrogen-fixing endophytic actinobacteria
have been reported to be able to increase the number of nodules, nitrogenfixation
rate, as well as nitrogen uptake by plants in low nitrogen ecosystems (Le et al.
2016 ;Rafik et al. 2014 ; Trujillo et al. 2015 ).
Phosphorus is involved in various enzymatic reactions in living organisms, such
as transport of glucose, stimulation of cell proliferation and promotion of organ
development (Ahemad 2015 ). Although most of soil phosphorus is immobile and
thus unavailable for plant uptake (Ezawa et al. 2002 ), some endophytic bacteria are
able to solubilize precipitated phosphates through acidification, chelation, redox
changes (Nautiyal et al. 2000 ), or to mineralize organic P through production of
phosphatase (van der Hiejden et al. 2008 ) under environmental stress conditions,
thus enhancing P bioavailability. Jog et al. ( 2014 ) found that two root endophytic
actinobacteria (Streptomycesspp.) isolated fromTriticum aestivumsignificantly
improved plant growth through phosphate solubilization and secretion of phytases
as well as some other plant growth promoting traits. This is probably because the
phytase-P complex process facilitates plant uptake of P.
Iron is a vital element for life and is needed by almost all organisms; since it
plays a significant role in physiological processes (e.g., transpiration) and enzy-
matic activities (Bothwell 1995 ). In general, most iron in soil exists in highly
insoluble ferric (Fe
3+
) form and is unavailable for plants. Siderophores produced by
soil bacteria are able to solubilize iron under iron-limited conditions, therefore
176 M.F. Carvalho et al.