Recent Advances and Future Prospects on Practical Use of Salicylic Acid - Salicylic Acid: Plant Growth and Development - page 353

treatment with a pathogen elicitor (Kauss et al. 1992 , 1993 ). Latter, Park et al.
( 2007) suggested that MeSA is a critical, phloem-mobile SAR long-distance signal
in tobacco. In addition, MeSA has been suggested to act as a volatile intraplant
signal that is capable of activating SAR in distant leaves of the same plant
(Shulaev et al. 1997 ). The authors conclude that MeSA may function as an air-
borne signal which activates disease resistance and the expression of defense-
related genes in neighbouring plants and in the healthy tissues of the infected plant.
Another recent study extended this putative signaling function of MeSA to SAR in
Arabidopsis (Vlot et al. 2008 ).
Among the compounds that are thought to be involved in interplant commu-
nication are two jasmonates (cis-jasmone and MeJA), MeSA, terpenes, and some
C6-C10 alkenals and alkanals (Preston et al. 2001 ).
Since the pioneer work reported by Farmer and Ryan ( 1990 ), JAs have been the
most studied compounds as well as the stronger candidates for aerial signals in
interplant communication (Shonle and Bergelson 1995 ; Farmer 2001 ; Preston et al.
2001 , 2004 ). Different approaches have been used for understanding the mecha-
nisms of interplant communications. Most of them are based on the integration of
molecular biology, biochemistry, physiology, and ecology. Manipulation of the
volatile emission of a plant using a chemical elicitor allows for the investigation of
the possible effects of plant volatiles on community ecology. The use of an elicitor
has the advantage of being able to induce (part of) the volatile blend without
removal of plant tissue, and offers the possibility to apply a controlled dose,
whereas it is difficult to control the amount of damage inflicted by herbivore
feeding (Bruinsma et al. 2009 ).
Plant defenses against pathogens and insects are regulated differentially by
cross-communicating signal transduction pathways in which SA and JA play key
roles. SA and JA accumulate in response to pathogen infection or herbivore
damage, resulting in the activation of distinct sets of defense-related genes. Mutant
and transgenic plants that accumulate SA are often more susceptible to pathogen
infection than wild-type plants (Delaney et al. 1994 ; Wildermuth et al. 2001 ).
Blocking the response to JA generally renders plants more susceptible to herbivore
damage (Howe et al. 1996 ; McConn et al. 1997 ), although enhanced susceptibility
toward necrotrophic pathogens has been reported as well (Thomma et al. 2001 ).
SA- and JA-dependent defense pathways have been shown to cross-communicate
(Felton and Korth 2000 ; Feys and Parker 2000 ), providing the plant with a reg-
ulatory potential to fine-tune the defense reactions depending on the type of
attacker encountered.
Recently nitric oxide (NO) has emerged as a key signalling molecule in plants.
It is a small, water and lipid soluble gas that in recent years has been pointed as a
major signalling molecule of ancient origin and ubiquitous importance However,
research on NO and plant signalling was mainly restricted to a few 'pioneers' such
as Leshem (Leshem and Haramaty 1996 ) and Laxalt (Laxalt et al. 1997 ) until the
landmark publication in 1998 describing NO as a plant defense signal (Delledonne
et al. 1998 ). Since then, studies on NO and its role on plant biology have increased
dramatically and intensive reviewed (Durner and Klessig 1999 ; Beligni and