Recent Advances and Future Prospects on Practical Use of Salicylic Acid - Salicylic Acid: Plant Growth and Development - page 357

activity suppresses the jasmonic acid pathway. The activation of SAR has been
shown to suppress JA signaling in plants, thereby prioritizing SA-dependent
resistance to microbial pathogens over JA-dependent defense against insect her-
bivory (Pieterse et al. 2001 ). Pharmacological and genetic experiments have
shown that SA is a potent suppressor of JA-inducible gene expression (Peña-
Cortes et al. 1993 ).
It appears that different branches of these two pathways combine in different
situations, making for a complex interplay. Complicating this is the induction of
jasmonic-SAR, or really a priming or strengthening of SAR, by rhizosphere
bacteria, composts, and certain bacterial isolates. Additionally, some insects
induce SAR channels that are normally induced only by microbes. Common to
both the SA and JA pathways is the shikimic acid pathway, a branch of Krebs
cycle phospoenolpyruvate, and source of the majority of phenolics in the plant.
One of the three aromatic (ring-structured) amino acids, phenylalanine, is an
important intermediate here. In SAR, phenylalanine is diverted from protein
synthesis by an enzyme, phenylalanine ammonialyase (PAL), which de-ammo-
niates the molecule and converts it to phenol precursor cinnamic acid.
JA and MeJA mediate plants defense against insects, and appears to mobilize
antimicrobial defense response, while SA-mediated defense responses are effective
against biothropic fungi, bacteria and viruses (Thomma et al. 2001 ).
Besides SA/JA cross talk, interactions between SA and ET, JA and ABA, and JA
and ET have been shown to function in the adaptive response of plants to herbivores
and pathogens with different lifestyles. Cross talk between defense-signaling
pathways is thought to help the plant decide which defensive strategy to follow,
depending on the type of attacker it is encountering. It seems that attackers have
also evolved to manipulate plants for their own benefit by suppressing induced
defenses or modulating the defense-signaling network (Pieterse and Dicke 2007 ).
SA-, JA-, and ET-dependent pathways regulate defense responses that are differ-
entially effective against specific types of attackers. In general it can be stated that
pathogens with a biotrophic lifestyle are more sensitive to SA-dependent responses,
whereas necrotrophic pathogens and herbivorous insects are resisted by JA/ET-
dependent defenses (Thomma et al. 2001 ). However, there are many exceptions to
this basic framework, and recent work suggests that interactions between the JA and
SA pathways may play important roles in fine-tuning the defense responses (Thaler
et al. 2004 ). The few studies that have addressed plant responses to parasitism by
other plants suggest that both salicylates and jasmonates can mediate effective
defenses. A question remains as to the relevance of synergistic signal interactions to
resistance responses. Often simultaneous activation of signaling pathways has no
additive effects with resistance patterns to discrete pathogens and pests being
maintained (van Wees et al. 2000 ), although there are instances where both SA-and
JA-signaling pathways are required (Ellis et al. 2002 ). As discussed by Mur et al.
( 2006 ), it seems likely that variably employed synergistic/antagonistic mecha-
nisms, which need not involve only SA and JA but also, for example, ET may
represent more flexible plant response to a particular stress.