8.2 Actinobacterial Endophytes
Many actinobacteria have the capacity to establish intimate associations with plants
and colonize their inner tissues without causing apparent disease symptoms, being
defined as endophytic actinobacteria (Qin et al. 2009 ; Schulz and Boyle 2006 ;
Stone et al. 2000 ; Strobel and Daisy 2003 ). Different parts of the plant can be
colonized, including roots, stems, leaves, seeds,flowers, and fruits.Frankiawas the
first isolated actinobacterial endophyte, and is known for inducing the formation of
nitrogen-fixing nodules in non-leguminous plants (Benson and Silvester 1993 ;
Callaham et al. 1978 ; Coombs and Franco 2003 ). Endophytic actinobacteria play an
important role in the protection of plants by producing bioactive compounds that
can act as plant growth promoters, biological control agents or alleviate stress
effects in plants, while in return these microorganisms can obtain nutrition and
protection from the host plant (Cao et al. 2005 ; Conn et al. 2008 ; Goudjal et al.
2013 ; Igarashi et al. 2002 ; Yandigeri et al. 2012 ). Many studies also indicate that
endophytic actinobacteria are capable of producing a wide range of pharmaceuti-
cally relevant bioactive compounds such as antimicrobial, antitumor,
anti-inflammatory, antiviral agents, etc., including the production of metabolites
bioactive against drug resistant pathogens (Golinska et al. 2015 ; Savi et al. 2015 ;
Singh and Dubey 2015 ; Zhang et al. 2012 ). In addition, PKS and NRPS gene
clusters, which are behind the synthesis of a wide variety of secondary metabolites,
have also been shown to be present in many endophytic actinobacteria (Luo et al.
2013 ).
It is thought that almost every plant on earth hosts one or more endophytic
microorganisms, where actinobacteria are included (Golinska et al. 2015 ; Kekuda
2016 ; Strobel and Daisy 2003 ). Endophytic microorganisms may originate both
from the rhizosphere or phyllosphere and may enter plants through naturally
occurring wounds or epidermal root hairs, or through the production of hydrolytic
enzymes, such as cellulase and pectinase (Dudeja et al. 2012 ; Suman et al. 2016 ).
Once inside the plant they can become installed at the entry location or spread
through the different parts of the plant, where they may colonize the interior of the
cells, intercellular spaces, or vascular systems (Suman et al. 2016 ). Due to their
several beneficial effects in plants, endophytic actinobacteria are very promising
biological resource that can be applied in environmentally friendly and sustainable
agricultural approaches to control plant diseases and promote plant health and
growth (Kunoh 2002 ). The capacity of endophytic actinobacteria to colonize seeds
is particularly relevant due to the possibility of transmission of the endophytic
community to the next generation (Tchinda et al. 2016 ).
Culture-dependent and culture-independent methods have revealed an increasing
number of plants, including crops, hosting endophytic actinobacteria. Studies
indicate that these microorganisms are among the predominant phyla inside the
plants (Manter et al. 2010 ; Sessitsch et al. 2012 ). The potential of these microor-
ganisms for agricultural applications is enormous, where they can be used as
microbial inoculants for increasing crop yields and controlling pathogenic agents
8 Endophytic Actinobacteria for Sustainable Agricultural Applications 165