Agriculture Reference
In-Depth Information
Volatile Production
PGPR are known to produce volatile organic and inorganic compounds which can
affect the plant growth and resistance/tolerance against biotic and abiotic stresses
via different mechanisms. Some of the volatile compounds produced by different
PGPR include 3-hydroxy-2-butanone (acetoin) and 2,3-butanediol by
B. subtilis
GB03 and
B. amyloliquefaciens
IN937a (Ryu et al.
2003
) and C-13 hydrocarbon
tridecane by
Paenibacillus polymyxa
E681 (Lee et al.
2012
). These volatile com-
pounds were found to affect the tissue-specific regulation of high-affinity K
+
transporter 1 (HKT1), which is further involved in the regulation of Na
+
homeo-
stasis in salt stress. The volatiles downregulate hkt1 in roots but upregulate them in
the shoot, lowering Na
+
levels and recirculation of Na
+
levels in plant (Yang
et al.
2009
). Other mechanisms include enhanced iron uptake by upregulating
FIT1 (Fe-deficiency-induced transcription factor) during metal stress and produc-
tion of compatible solutes like betaine under oxidative stress (Farag et al.
2013
).
These volatiles are also found to have negative effects on the plant growth under
certain circumstances (Bailly and Weisskopf
2012
). Some of the volatiles involved
in stress resistance against biotic factors like pathogens have been already discussed
in Sect.
11.3.2.1
.
Synthesis of Auxins and Similar Compounds
Microbial auxins can affect the plant's auxins governed developmental processes
such as root development including root length, surface area, and number of root
tips. This root development further enables nutrient uptake by plants, thereby
improving plant health in the presence of inhibitory compounds or under stress
conditions (Egamberdieva and Kucharova
2009
). Plant exudates contain trypto-
phan, which is when acquired by rhizobacteria converted to IAA. The microbial
IAA along with plant's pooled auxins stimulates plant growth and proliferation
(Glick
1995
).
Protective Compounds
Microorganisms are known to produce osmo-protectants such as proline, betaine,
trehalose, and glutamate which modulate their cytoplasmic osmolarity and hence
protect plants from stress conditions (Blanco
1994
). Plant also produces protective
compounds or compatible osmolytes in response to stress conditions, mainly salt
stress. Some of these compounds include amino acids, imino acids, amides, pro-
teins, quaternary ammonium compounds, and polyamines (Carmen and Roberto
2011
). Increased production of proline in response to stressors has been reported
(Lalelou et al.
2010
; Marin et al.
2010
) which plays a role in osmo-adaptation in salt
stress (Meloni et al.
2001
), and as a molecular chaperone it protects and stabilizes
