Recent Advances and Future Prospects on Practical Use of Salicylic Acid - Salicylic Acid: Plant Growth and Development - page 352

attacking insects/herbivores. These warning volatiles also induce the emission of
similar defensive volatiles by neighboring plants. Volatiles can also enhance a
plants reproductive by attracting pollinators and/or seed dispersers. Volatiles
emitted from roots can contribute to below ground defense in similar ways: acting
as antimicrobial and antiherbivore substances, or attracting enemies of root
feeding herbivores. The presence of volatiles, and abilities for plants to produce
them, greatly increases plants resiliency to a variety of threats (reviewed by
Dudareva et al. 2006 ).
Plant volatiles provide herbivorous arthropods with information that allow them
to discriminate between host and non-host plants. Volatiles may also indicate plant
stress status, and natural enemies can use herbivore-induced plant volatiles as
cuses for prey location. Neighbouring plants may also make use of volatiles cuses
to prepare for herbivore attack. Since both constitutive and inducible plant volatile
emissions can be modified by plant breeding or metabolic engineering, the pos-
sibility exists to improve plant resistance against important pests both directly and
indirectly via improved biological control.
Plants are always producing volatiles, the most common of which are the
typical green-leaf volatiles (GLVs), which include several saturated and unsatu-
rated six-carbon alcohols, aldehydes, and esters. These GLVs are typically
released in higher amounts after mechanical damage (Pare and Tumlinson 1999 ).
Plants also have the ability to produce volatiles after being induced by insect
feeding. The most obvious result of induction is a marked increase in the amount
of terpenes produced by the plant, especially 1,3,6-octatriene, 3,7-dimethyl (b-
ocimene), and 1,6-octadien-3-ol, 3,7-dimethyl (linalool) (Pare and Tumlinson
1999 ). Induced volatile production can lead to attraction of natural insect enemies
of the herbivorous insect (Turlings et al. 1993 ). There is typically a delay between
the onset of insect feeding and emission of induced volatile release, therefore, a
series of biochemical reactions must be involved (Pare and Tumlinson 1999 ).
In recent years, major advances have been made in identifying metabolites that
are candidate for systemic signals in plant defense against different attacks. Many
chemicals are critical for plant growth and development and play an important role
in integrating various stress signals and controlling downstream stress responses
by modulating gene expression machinery and regulating various transporters/
pumps and biochemical reactions. These chemicals include calcium (Ca 2+ ), cyclic
nucleotides, polyphosphoinositides, nitric oxide (NO), sugars. Plant hormones:
abscisic acid (ABA), jasmonates (JAs), salicylic acid (SA) and polyamines, which
occupy a central role in regulating these highly dynamic and adaptive responses
(Satner and Estelle 2009 ).
MeSA, JAs, azelaic acid and a diterpenoid have been implicated as mobile
signals associated with the activation of systemic acquired resistance (SAR),
which confers enhanced resistance against a broad spectrum of pathogens. By
contrast, auxins probably contribute to negative regulation of systemic defenses.
The first systematic studies into the priming phenomenon were carried out by
Kauss and Conrath laboratories in 1990s. They discovered that exogenous appli-
cation

of SA parsley cells can enhanced cellular defenses upon secondary