Agriculture Reference
In-Depth Information
of non-legumes by rhizobia, the production of gibberellins by the bacterium is
reported to facilitate this process (Lievens et al.
2005
).
2.3.2 Negative Plant Growth Regulator
Abscisic acid is one of the strong inhibitor of growth and germination and promotes
seed dormancy (Miransari and Smith
2009
; Yang et al.
2009
). Apart from these,
ABA also helps plants to tolerate abiotic stresses. When plants are exposed to
drought stress, the hormonal balance of plants changes and ABA content in the
leaves increases, which reduce the level of cytokinin. This in turn elicits stomata
closure (Yang et al.
2009
). Cohen et al. (
2009
) in a similar study suggested that
ABA produced along with gibberellins by PGPR strain significantly contributed to
water stress alleviation of maize plants. Some rhizobial strains such as
B. japonicum
USDA110 also produce ABA (Boiero et al.
2007
) and function in the same way as
do the other PGPR (Zhang et al.
2012
).
2.3.3 Growth Modulation Enzyme ACC Deaminase
Ethylene is a plant hormone which under normal conditions regulates many physio-
logical processes, such as (1) seed germination, (2) root hair development and root
elongation, (3) leaf and organ senescence, (4) leaf and petal abscission, (5) epinasty
and (6) fruit ripening (Abeles et al.
1992
; Frankenberger and Arshad
1995
; Arshad
and Frankenberger
2002
; Siddikee et al.
2011
). Also, ethylene regulates nod factor
signalling and nodule formation and has primary functions in plant defence sys-
tems. Besides its physiological role in different developmental processes of plants,
ethylene is also considered as a stress hormone, whose synthesis in plants is
increased substantially by a number of biotic and abiotic stresses. At higher
concentrations, ethylene, however, inhibits growth and development of plants
(Grichko and Glick
2001
). The enzyme 1-aminocyclopropane-1-carboxylate
(ACC) deaminase (E.C. 4.1.99.4) which however mitigates the ethylene stress
was first purified to homogeneity from
Pseudomonas
sp. strain ACP (Honma and
Shimomura
1978
), later on partially purified from
P. chlororaphis
6G5 (Klee
et al.
1991
) and
P. putida
GR12-2 (Jacobson et al.
1994
) and then purified to
homogeneity from
P. putida
UW4 (Hontzeas et al.
2004
). Enzyme ACC deaminase
(a multimeric enzyme) has been found thereafter to be synthesized by a variety of
PGPR (Belimov et al.
2005
; Rajkumar et al.
2006
; Madhaiyan et al.
2007
; Mellado
et al.
2007
).
