Chemical elicitors are compounds, which activate defense in plants through
various biosynthetic pathways depending on the compound used; nevertheless it is
possible to elicit the plant due to physical conditions such as UV and gamma
radiation and high and low temperature, among others. These topics were discussed in Sect. 5.3.
Depending on the type of attack, the plant activates different signaling pathways
to synthesize a specific set of defensive compounds such as terpenes, phenolics,
and nitrogen and sulfur containing compounds (Mazid 2011). Some physiological
responses to elicitors are not directly defensive, but can also serve to defend plants
indirectly (Rohwer and Erwin 2008). It is commonly known that the salicylic (SA)
pathway is initiated by biotrophic pathogens, while jasmonic acid (JA) and ethylene (ET) response is initiated by necrotrophic pathogens, although the final
defense response may involve crosstalk among these biosynthetic pathways.
Commonly, chemical elicitors include salicylic acid, methyl jasmonate, ethylene,
and chitosan. Natural and synthetic compounds (carbohydrate polymers, lipids,
glycopeptides, and glycoproteins) that can be used as elicitors have emerged due to
the better understanding of plant signaling (Montesano et al. 2003), for example
oxalic acid, calcium chloride, benzo(1,2,3)thiadiazole-7-Carbothioic Acid
S-methyl Ester (BTH), potassium silicate, carrageenan, harpin, 2,6-Dichloroisonicotinic (INA), b-Aminobutyric acid (BABA) (Conrath 2009).
Elicitors could be delivered to the plant in different manners: fumigation,
spraying, pellets, paste, and irrigation. They can be applied to the whole plant or to
a selected part of it (pre or postharvest). To obtain fruit and vegetables with high
concentration of phytochemicals, postharvest elicitor treatments might be used
either singularly or in combination. Also, the concentration of the elicitor may
differ as well as the growth stage of the plants at the time of elicitation and the
contact time of elicitation (Rijhwani and Shanks 1998).
There is no consensus on these topics, since each plant family, genus, and
species produces a characteristic mix of secondary metabolites, however, they have
in common that elicitors induce plant signaling that serves as a guide to a series of
intracellular events (e.g., production of ROS) (Shilpa et al. 2010), that finally lead to
the expression of defense genes that are activated when the elicitor is recognized on
plant by a receptor (protein), causing the release of defense strategies such as the
production of secondary metabolites and plant volatiles. Jeong and Park (2005)
state that further knowledge is needed about the mode of action of elicitors; consequently the effect of elicitation on a plant cannot be easily predicted. For this
reason, it is common that elicitation methods are performed by trial and error.
It is indicated that the use of elicitors might result in a more resistant plant
through the production of secondary metabolites, but their production reduces plant
fitness (plant growth and reproduction). Nevertheless, other studies stated that it is
possible to reconcile crop yield with the production of secondary metabolites with
bioactive properties. García-Mier et al. (2013) discussed this option in a review.
Apart from inducing secondary metabolites when applied to plants, other
advantages derived from the use of elicitors are the reduced environmental hazard
due to their low toxicity compared with pesticides (Tripathi and Dubey 2004) and
1 Strategies for Sustainable Plant Food Production
35
various biosynthetic pathways depending on the compound used; nevertheless it is
possible to elicit the plant due to physical conditions such as UV and gamma
radiation and high and low temperature, among others. These topics were discussed in Sect. 5.3.
Depending on the type of attack, the plant activates different signaling pathways
to synthesize a specific set of defensive compounds such as terpenes, phenolics,
and nitrogen and sulfur containing compounds (Mazid 2011). Some physiological
responses to elicitors are not directly defensive, but can also serve to defend plants
indirectly (Rohwer and Erwin 2008). It is commonly known that the salicylic (SA)
pathway is initiated by biotrophic pathogens, while jasmonic acid (JA) and ethylene (ET) response is initiated by necrotrophic pathogens, although the final
defense response may involve crosstalk among these biosynthetic pathways.
Commonly, chemical elicitors include salicylic acid, methyl jasmonate, ethylene,
and chitosan. Natural and synthetic compounds (carbohydrate polymers, lipids,
glycopeptides, and glycoproteins) that can be used as elicitors have emerged due to
the better understanding of plant signaling (Montesano et al. 2003), for example
oxalic acid, calcium chloride, benzo(1,2,3)thiadiazole-7-Carbothioic Acid
S-methyl Ester (BTH), potassium silicate, carrageenan, harpin, 2,6-Dichloroisonicotinic (INA), b-Aminobutyric acid (BABA) (Conrath 2009).
Elicitors could be delivered to the plant in different manners: fumigation,
spraying, pellets, paste, and irrigation. They can be applied to the whole plant or to
a selected part of it (pre or postharvest). To obtain fruit and vegetables with high
concentration of phytochemicals, postharvest elicitor treatments might be used
either singularly or in combination. Also, the concentration of the elicitor may
differ as well as the growth stage of the plants at the time of elicitation and the
contact time of elicitation (Rijhwani and Shanks 1998).
There is no consensus on these topics, since each plant family, genus, and
species produces a characteristic mix of secondary metabolites, however, they have
in common that elicitors induce plant signaling that serves as a guide to a series of
intracellular events (e.g., production of ROS) (Shilpa et al. 2010), that finally lead to
the expression of defense genes that are activated when the elicitor is recognized on
plant by a receptor (protein), causing the release of defense strategies such as the
production of secondary metabolites and plant volatiles. Jeong and Park (2005)
state that further knowledge is needed about the mode of action of elicitors; consequently the effect of elicitation on a plant cannot be easily predicted. For this
reason, it is common that elicitation methods are performed by trial and error.
It is indicated that the use of elicitors might result in a more resistant plant
through the production of secondary metabolites, but their production reduces plant
fitness (plant growth and reproduction). Nevertheless, other studies stated that it is
possible to reconcile crop yield with the production of secondary metabolites with
bioactive properties. García-Mier et al. (2013) discussed this option in a review.
Apart from inducing secondary metabolites when applied to plants, other
advantages derived from the use of elicitors are the reduced environmental hazard
due to their low toxicity compared with pesticides (Tripathi and Dubey 2004) and
1 Strategies for Sustainable Plant Food Production
35