8.5.2 Endophytic Actinobacteria as Biocontrol Tools
Endophytes are becoming very interesting biocontrol candidates, because of their
crucial role in host–plant association, such as competition with phytopathogens for
colonization sites and mineral nutrients (Ma et al. 2016 ). Bacterial endophytes have
great potential to inhibit the growth of phytopathogens, and to stimulate the growth
and development of host plants (Ma et al. 2011 ). The understanding of the endo-
phytic actinobacteria–host plant interaction might accelerate the application of these
microbes in sustainable agriculture. Currently, endophytic actinobacteria have been
isolated from various plant species, such asBrassica rapa(Lee et al. 2008 ),
Brassica oleracea(Kanchanadevi et al. 2013 ),Oryza sativa(Mingma et al. 2015 ),
Lycopersicon esculentum(Cao et al. 2004 ; Kanchanadevi et al. 2013 ),Jatropha
curcas(Xing et al. 2012 ),Glycine max(Mingma et al. 2014 ),Triticum aestivum
(Jog et al. 2014 ), andZea mays(Costa et al. 2013 ); however, only few crop species
have been investigated in terms of their endophytic actinobacterial diversity and
their effect as biocontrol agents. Additionally, the mechanisms involved in endo-
phytic actinobacteria–host plant interaction are still very poorly understood given
the limited data currently available.
Endophytic actinobacteria have been attracting interest because of their capa-
bility to produce bioactive chemicals and/or allelochemicals, such as siderophores,
antibiotics, biocidal volatiles, lytic enzymes, chitinases, and detoxification enzymes
(Bérdy 2005 ; Clardy et al. 2006 ; El-Tarabily et al. 2010 ; Quecine et al. 2008 )
(Fig.8.1). Siderophores chelate or complex soluble iron from the soil; antibiotics
hinder pathogenic colonization; biocidal volatiles (e.g., hydrogen cyanide) inhibit
the growth of pathogenic fungi; lytic enzymes (e.g., chitinases) degrade some
organic compounds (e.g., chitin) conferring plant resistance/tolerance to pathogens;
detoxifying enzymes protect against pathogen and toxins. Moreover, endophytic
actinobacteria are capable of successfully competing with pathogens for specific
niches and mineral nutrients in plant tissues, and of inducing systemic resistance
(Doumbou et al. 2001 ). For instance, antibiotics produced byStreptomycesspp. are
able to hinder the growth of a wide range of pathogenic microbes (Gopalakrishnan
et al. 2016 ). Moreover, these compounds are considered as important agents to
control soil-borne diseases with low toxic impacts on the environment and human
health (Cao et al. 2004 ).
8.5.3 Endophytic Actinobacteria as Helpers of Agricultural
Crops
The mechanisms of plant growth promotion and biological control involved in host
plant–microbe association have been discussed above. In this section, we have
summarized some recent publications on the beneficial functions of endophytic
actinobacteria in enhancing sustainable agriculture via acting as helpers of
178 M.F. Carvalho et al.