Agriculture Reference
In-Depth Information
reduced microbial population (Wani and Khan
2013
) thereby affecting the soil
fertility and making it unsuitable for sustainable agriculture (Cheng
2003
). Germi-
nation rate and root vitality of the plant are also affected by the metal stress (Shu
et al.
1997
). Heavy metals were also known to affect the cell division by causing
inhibition of DNase and RNase activity; damaging nucleolus and disrupting DNA
synthesis; and causing chromosomal aberration, coagulation, and fragmentation
(Yang and He
1995
; Musarrat et al.
2011
). Reduced cell division and elongation
along with decreased cell membrane integrity are some other effects of membrane
toxicity. Some of the visible symptoms include interfoliar chlorosis, wilting, necro-
sis, crinkling of leaf, reddening, and purpling (Reichman
2002
). Lessened chloro-
phyll content, reduced photosynthetic rate, and augmented carotenoid breakdown
are also some of the results of metal toxicity. Accumulated metals are believed to
replace Mg ion of the chlorophyll molecule thus affecting photosynthesis (Kupper
et al.
1996
). Heavy metals are also known to disrupt the photosystems ensuing
decreased proton availability, consequently affecting photosynthesis. Reduced ATP
synthesis and disrupted activity of chloroplast are some other effects reported for
metal toxicity by disruption of enzymatic systems (Teige et al.
1990
). Like any
other stress, free radical production is increased in plant as a response to metal
stress. The concentration of metal plays an important role here as at low concen-
tration the protective antioxidant enzymes balance the effect, but at higher metal
toxic condition these accumulated free radicals damage membranes by lipid peroxi-
dation (Yadav
2010
) followed by injury to surrounding cells. Free radicals also
damage macromolecules like nucleic acids and proteins, thus disrupting normal
metabolism and leading to cell death. Leaf senescence is another effect of oxidative
damage due to ROS accumulation (Luna et al.
1994
). Since growth, yields, and
many other physiological functions of plants are affected negatively by toxic metals
(Yadav
2010
; Selvakumar et al.
2012
), remedial measures are urgently required for
its cleanup from the contaminated sites (Khan et al.
2011
; Zaidi et al.
2012
). In this
context, scientists around the world have attempted to use molecular tools and
breeding programs for exploiting physiological traits of plants, developing new
stress-tolerant crop varieties, altering crop calendars, and managing agronomic
resources to circumvent stress-related impact on plants. Another well-considered
option in this direction is the use of microorganisms for combating stress (Khan
et al.
2009
). In this regard, reports on the individual/combined use of metal-tolerant/
normal microorganisms in growth promotion and other positive effects on plants
are available (Selvakumar et al.
2012
; Ahmad et al.
2013
; Oves et al.
2013
).
11.3 Plant Growth-Promoting Rhizobacteria
Soil is inhabited by numerous microorganisms, which can be categorized as
beneficial or detrimental based on their effect on the soil, plants, and ultimately
plant's yield (Singh et al.
2011a
). The diverse microbial population of soil plays a
pivotal role in processes determining soil fertility and plant's productivity (Tilak
