improving iron availability to plant roots. In general, plants acquire iron either from
bacterial siderophore-iron complex, or from the phytosiderophore-iron complex
(Ma et al. 2011 ; Rajkumar et al. 2009 ). It is known that bacterial siderophores
generally have higher affinity for iron than phytosiderophores and that siderophore
producing bacteria can help plants accumulate more iron than the plant alone under
iron-limited conditions (Ma et al. 2011 ). After the iron is complexed by side-
rophores produced by endophytic actinobacteria, plant roots are able to uptake it
directly from bacterial siderophore-iron complexes (Chen et al. 1998 ; Rungin et al.
2012 ).
Endophytic actinobacteria can provide phytohormones to hosts in order to
facilitate nutrients accumulation (Gopalakrishnan et al. 2016 ). Recently, Phetcharat
and Duangpaeng ( 2012 ) investigated the role of phytohormones produced by
endophytes in protecting plants against environmental stress. The authors found that
the success of endophytic colonization was associated with increases in plant
nutrient uptake and biomass yield. Indole-3-acetic acid (IAA) has been considered
as a major auxin, which plays a vital role in stimulating plant development (Gravel
et al. 2007 ; Shi et al. 2009 ), inducing plant self-defense or adaptation system
(Navarro et al. 2006 ), and functioning as a signaling molecule (Spaepen et al.
2007 ). The IAA synthesized by endophytic actinobacteria is considered to have
great potential to modulate the establishment and development of plant-endophyte
association (Goudjal et al. 2013 ). Endophytic actinobacteria, such asStreptomyces,
Nocardia,Nocardiopsis,Spirillospora,Microbispora,andMicromonosporawere
found to be involved in the production of this phytohormone, therefore benefiting
plants in situ (Goudjal et al. 2013 ; Shutsrirung et al. 2013 ). El-Tarabily et al. ( 2009 )
demonstrated that some endophytic actinobacterial strains greatly enhanced growth
ofCucumis sativusby synthesizing indole-3-pyruvic acid and IAA. However,
unfavorable effects of phytohormones have also been reported by Patten and Glick
( 2002 ), who found that low concentrations of bacterial IAA induced the elongation
of plant primary root, whereas high IAA concentrations caused the formation of
plant lateral and adventitious roots with negative effects on primary root growth.
Therefore, the endophytic actinobacteria that can modify the balance of phyto-
hormones might be good candidates for hastening plant development.
Ethylene, a universal phytohormone, is involved in plant growth and physio-
logical responses to both abiotic and biotic environmental stresses (Sun et al. 2006 ).
The pathway of ethylene synthesis has been extensively reviewed (Glick et al.
2007 ). It is well known that plants exposed to environmental stresses such as
extreme temperature, drought and salinity can induce the production of ethylene,
which is able to hamper elongation of roots as well as formation of root hairs.
Under such stresses, some endophytic actinobacteria might mitigate the negative
impact of stress by hydrolyzing 1-aminocyclopropane-1-carboxylic acid (ACC) and
subsequent diminishing plant ethylene production. It has been reported that the
enzyme ACC deaminase produced by some endophytic actinobacteria may
hydrolyze ACC intoa-ketobutyrate and ammonia, which then serves as a nitrogen
source for such microbes (Viterbo et al. 2010 ; Xing et al. 2012 ).
8 Endophytic Actinobacteria for Sustainable Agricultural Applications 177