Agriculture Reference
In-Depth Information
light conditions. Biotic stresses are those that occurs as a result of damage done to
plants by other living organisms, such as bacteria, viruses, fungi, oomycetes, para-
sites, and herbivores These stresses can occur at multiple stages of plant develop-
ment and often more than one stress simultaneously affects the plant, potentially
restricting plant growth, plant development, or even determining plant species dis-
tribution across different types of environments.
The adaption to various stresses of all living organisms is the outcome of inter-
action between their genome and the ever-changing environment. In order to bet-
ter adapt to adverse conditions and finally complete their life cycles, plants have
evolved plenty of complex signaling systems to respond to external and internal
cues to regulate growth and development under stressful environments. These
responses are mediated by plant growth regulators (phytohormones), compounds
derived from plant biosynthetic pathways that can act both locally and systemi-
cally. Numerous studies have demonstrated that plant hormones play critical regu-
latory roles in both plant growth and development and the responses to adverse
stresses. In general, it is accepted that the plant hormones consist of five major
classes of plant hormones, namely abscisic acid (ABA), ethylene (ET), cytokinin
(CK), auxin (IAA), gibberellin (GA), as well as jasmonate (JA), brassinosteroids
(BR), salicylic acid (SA), nitric oxide (NO), strigolactone (SL), karrikins, poly-
amines, and plant peptide hormones, and it is imaginable that additional growth
regulators would be discovered in the future. So far, thanks to so many excellent
researchers' efforts in these fields, we have been made significant research pro-
gresses associated with the biosynthesis of plant hormones, their metabolism, as
well as their roles in signaling. In this chapter, we will mainly discuss the role of
ABA in different plant-attacker interactions.
20.2 Hormonal Modulation of Plant Immunity
During the long-term and constant interactions with various microbial pathogens or
herbivores, resistant plants have successfully evolved sophisticated defense mecha-
nisms to protect themselves. Normally, plants use both constitutive and inducible
defense responses to fend off various attackers. Constitutive defenses were based
on many preformed barriers, such as cell walls, waxy epidermal cuticles, and bark;
while inducible defenses include the production of toxic chemicals, pathogen-
degrading enzymes, and deliberate cell suicide, which were activated upon the rec-
ognition of pathogen attack. The constitutive and inducible defenses constitute a
multilayered defense which is initiated sequentially during plant-attacker interac-
tions (Chisholm et al. 2006 ; Jones and Dangl 2006 ; Nishimura and Dangl 2010 ).
The defensive response of the plant upon pathogen infection is the outcome of
highly coordinated sequential changes at the cellular level, during which the plant
hormones play important roles (Ton et al. 2009 ). SA, jasmonic acid (JA), and eth-
ylene (ET) are the primary defense hormones, and their importance in plant innate
immunity is well documented, particularly in the model plant Arabidopsis thaliana
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