Many highly contagious plant viruses can drastically reduce crop yield and
quality depending on the cultivar involved. Viruses affect different parts of the
plant and cause local lesions and systemic damage due to stunting, leaf curl, mosaic,
chlorosis, and malformations. Furthermore, parasitic nematodes feed on all parts of
the plant including roots and cause wilting or stunting. The root-knot nematodes,
Meloidogyne spp., are amongst the important plant fauna that limit the productivity
of many susceptible crops (Mostafa et al. 2014). In addition, insects and mites also
damage plants by their feeding and egg laying activities, and as vectors of various
pathogens (Schumann and D’Arcy 2006).
Various biotic threats especially diseases caused by pathogens are managed with
the help of protectant and systemic pesticides, which are applied routinely to
suppress the spread of the causative agents of these stresses. However, majority of
pesticides in vogue are non-biodegradable and have unwanted side effects such as
pathogen resistance towards pesticides, environmental pollution, ground and surface
water contamination, and other non-target deleterious effects on beneficial soil
microorganisms, humans, insects, birds, and fishes (Savci 2012; Muñoz-Leoz et al.
2013). Therefore, alternative approaches to handle this problem of agri-sector are
utmost.
10.3 How Do Plants Manage Biotic Stress?
Plants have evolved an array of constitutive or inducible morphological, genetic,
biochemical, and molecular mechanisms of resistance to various biotic stresses
caused by pathogens and insect pests as a part of their immune system (Howe and
Jander 2008; Nurnberger and Kemmerling 2009). The passive defense that prevents
pathogens or insect herbivores from getting access into plant-body has physical
barriers such as waxes, thick cuticles, specialized trichomes, and production of
antimicrobial compounds (Nejat and Mantri 2017; Singh 2017). Additionally, as a
second line of defense, plants also possess an inducible defense mechanism against
pathogens that break primary constitutive defense system. This includes the steps of
recognition of invaders and actual defense reaction.
10.3.1 Recognition of Biotic Stress
During invasion, pathogens release a plethora of chemicals in the plant-body, called
as elicitors, which pertain to various classes of biomolecules such as proteins, lipids,
oligosaccharides, and nucleotides. Having conserved structures and essential roles in
their producers, elicitors are capable of provoking the defense response of the host
on their recognition. Further, elicitors may be general elicitors, also termed as
pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs), if produced
by a number of microorganisms belonging to a group, and specific elicitors, also
termed as effectors, if produced by a particular microorganism after successful
invasion (Yu et al. 2017). Further, insects also release herbivore-associated elicitors
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quality depending on the cultivar involved. Viruses affect different parts of the
plant and cause local lesions and systemic damage due to stunting, leaf curl, mosaic,
chlorosis, and malformations. Furthermore, parasitic nematodes feed on all parts of
the plant including roots and cause wilting or stunting. The root-knot nematodes,
Meloidogyne spp., are amongst the important plant fauna that limit the productivity
of many susceptible crops (Mostafa et al. 2014). In addition, insects and mites also
damage plants by their feeding and egg laying activities, and as vectors of various
pathogens (Schumann and D’Arcy 2006).
Various biotic threats especially diseases caused by pathogens are managed with
the help of protectant and systemic pesticides, which are applied routinely to
suppress the spread of the causative agents of these stresses. However, majority of
pesticides in vogue are non-biodegradable and have unwanted side effects such as
pathogen resistance towards pesticides, environmental pollution, ground and surface
water contamination, and other non-target deleterious effects on beneficial soil
microorganisms, humans, insects, birds, and fishes (Savci 2012; Muñoz-Leoz et al.
2013). Therefore, alternative approaches to handle this problem of agri-sector are
utmost.
10.3 How Do Plants Manage Biotic Stress?
Plants have evolved an array of constitutive or inducible morphological, genetic,
biochemical, and molecular mechanisms of resistance to various biotic stresses
caused by pathogens and insect pests as a part of their immune system (Howe and
Jander 2008; Nurnberger and Kemmerling 2009). The passive defense that prevents
pathogens or insect herbivores from getting access into plant-body has physical
barriers such as waxes, thick cuticles, specialized trichomes, and production of
antimicrobial compounds (Nejat and Mantri 2017; Singh 2017). Additionally, as a
second line of defense, plants also possess an inducible defense mechanism against
pathogens that break primary constitutive defense system. This includes the steps of
recognition of invaders and actual defense reaction.
10.3.1 Recognition of Biotic Stress
During invasion, pathogens release a plethora of chemicals in the plant-body, called
as elicitors, which pertain to various classes of biomolecules such as proteins, lipids,
oligosaccharides, and nucleotides. Having conserved structures and essential roles in
their producers, elicitors are capable of provoking the defense response of the host
on their recognition. Further, elicitors may be general elicitors, also termed as
pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs), if produced
by a number of microorganisms belonging to a group, and specific elicitors, also
termed as effectors, if produced by a particular microorganism after successful
invasion (Yu et al. 2017). Further, insects also release herbivore-associated elicitors
300
L. Thomas and I. Singh