Agriculture Reference
In-Depth Information
adaptation of plants on disturbed sites, this chapter will be covered the molecular
and physiological mechanisms of the association MA, responsible for this adapta-
tion and greater tolerance of plants to biotic and abiotic stresses.
Introduction
Plants are constantly exposed to various abiotic and biotic stresses, such as radia-
tion, temperature, water, minerals, plants, animals, and microorganisms, which alter
the biosynthesis and development of plants as a consequence of oxidative burst
(Gill and Tuteja
2010
). During aerobic metabolism, molecular oxygen is partially
reduced, generating transient intermediates that are highly reactive and damaging
to the cell. This partial reduction of O
2
produces reactive oxygen species (ROS),
including the superoxide anion (O
2
), the hydroxyl radical (OH
-
), and hydrogen per-
oxide (H
2
O
2
). Further conversions can occur among these molecules, transforming
them into even more reactive species (Hajiboland and Joudmand
2009
; Gill and
Tuteja
2010
; Abdel Latef and Chaoxing
2011
). These ROS cause morphological,
physiological and molecular alterations, resulting in plant damage and eventual
death. However, plants have enzymatic and non-enzymatic defence systems to mi-
nimise the effects of ROS (Arfaoui et al.
2007
; Gill and Tuteja
2010
).
The microhabitat of the rhizosphere is a specialised ecosystem, where microbial
populations are highly favoured, allowing the growth, development and multiplica-
tion of these microorganisms. Plants can form beneficial associations with these
microorganisms, acquiring tolerance and/or resistance to biotic and abiotic stress
factors (Dimkpa et al.
2009
; Singh et al.
2011
)
Bacteria and fungi are the most numerous inhabitants of the rhizosphere. Plant
growth-promoting rhizobacteria (PGPR) are beneficial bacteria that colonise the
root system of plants and promote growth through various mechanisms including
producing plant growth regulators, increasing the cycling and availability of soil
nutrients, functioning as pathogen biocontrol agents and conferring tolerance and/
or resistance to biotic and abiotic stresses (del Mar Alguacil et al.
2009
). Arbus-
cular mycorrhizal fungi (AMF) are common inhabitants of soil that form mutu-
alistic associations within the root systems of a large number of agricultural plant
species, conferring benefits to host plants distributed throughout various habitats
(Smith et al.
2010
). Arbuscular mycorrhizae improve the nutritional status of plants,
facilitate plant adaptation to different ecosystems, and increase plant tolerance to
biotic and abiotic stress factors, and they are also considered to be biocontrol agents
(Singh et al.
2012
)
Plants that associate with microorganisms become more tolerant to stress condi-
tions. Several mechanisms have been described to explain this greater tolerance
(Pozo et al.
2010
). The physiological changes induced in the plant through relation-
ships with symbionts prevent pathogenic attacks and activate defence mechanisms.
Phytoalexins, pathogenesis-related proteins, and agents for the lignification of the
cell wall have been reported in mycorrhizal plants in areas far from infection sites,
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