Agriculture Reference
In-Depth Information
6.3.4 Actinomycetes as a Potential Candidate for Increased
Plant Growth and Yield
The agronomic use of PGPR at field scale in general has been hampered by poor
understanding of mechanisms that facilitate plant growth, inability of bacterial
strains to establish in various soils, rhizosphere incompetence, and changing envi-
ronmental conditions. On the contrary, the actinobacteria with their ability to
(1) sporulate, (2) transform various complex soil nutrients into simple and acces-
sible forms, (3) extensively and efficiently colonize plant roots, (4) manage phyto-
pathogens, and (5) secrete other plant-growth-promoting substances make these
organisms as preferred choices for developing potential field bio-inoculants. Acti-
nomycetes are metabolically flexible soil/rhizosphere-colonizing microorganisms
(Miller et al.
1990
; Euanorasetr et al.
2010
; Lei et al.
2013
) capable of producing a
range of compounds of interest, including (1) antifungal compounds which have
been found useful in controlling fungal root diseases (Rothrock and Gottlieb
1984
;
Zucchi et al.
2010
; Bungonsiri et al.
2011
; Sreevidya and Gopalakrishnan
2012
;
Francisco et al.
2013
) (2) siderophores (Lee et al.
2012
; Nakouti et al.
2012
; Najwa
et al.
2013
), ACC deaminase (El-Tarabily
2008
), and (3) plant-growth-promoting
hormones (Hamdali et al.
2008a
; Khamna et al.
2010
). Generally, plant root
exudates stimulate growth, proliferation, and rhizosphere colonization of actino-
mycetes that may act as a strong antagonist to fungal pathogens. The root exudates
are utilized as a source of carbon and energy by the actinomycetes and, concom-
itantly, enhance the synthesis of antimicrobial substances (Crawford et al.
1993
;
Yuan and Crawford
1995
). In addition, actinomycetes synthesize an array of
biodegradative enzymes which includes chitinases (Blaak et al.
1993
; Gupta
et al.
1995
; Pattanapipitpaisal and Kamlandharn
2012
; Sowmya et al.
2012
),
glucanases (Hopwood
1990
; Damude et al.
1993
; Mahadevan and Crawford
1996
; Harchand and Singh
1997
; Thomas and Crawford
1998
; Trejo-Estrada
et al.
1998
; Fayad et al.
2001
; Huiling et al.
2014
), peroxidases (Ramachandra
et al.
1988
; Djamila et al.
2011
), and other enzymes possibly involved in
mycoparasitic activity. Considering the potential role of actinomycetes in the
management of plant diseases and plant growth promotion by certain other mech-
anisms, actinomycetes in recent times are considered as one of the important
aspects in sustainable plant production (Palaniyandi et al.
2013
) as presented in
Table
6.2
.
The root-colonizing soil actinomycetes
S. lydicus
WYEC108, for instance, have
been reported to influence pea root nodulation and increase the nodulation fre-
quency possibly at the level of infection by
Rhizobium
spp. Following colonization,
S. lydicus
sporulate inside the surface cell layers of the nodules which in turn led to
a massive increase in the nodules size. Subsequently, the forms and vigor of
bacteroids were greatly improved due to enhanced assimilation of iron and possibly
other soil nutrients within nodules. Moreover, bacteroid accumulation of the C
storage polymer, poly-
-hydroxybutyrate (PHB), was reduced in colonized nodules
(Solans
2007
). The co-inoculation of rhizoactinomycetes
Streptomyces
MM40,
β
