Plants are at constant risk of being attacked by a wide
variety of insect herbivores and microbial pathogens. To
defend themselves, plants possess a powerful innate immune system by which they recognize non-self molecules
or signals from injured cells, and respond by activating
an effective defense response (Jones and Dangl
2006; Howe and Jander 2008). The importance of the
phytohormones salicylic acid (SA), jasmonic acid (JA),
ethylene, and abscisic acid as primary signals in the
regulation of the plant’s immune response is well established
(Pieterse et al. 2009; Verhage et al. 2010). Upon
pathogen or insect attack, the quantity, composition, and
timing of the phytohormonal blend produced by the plant
depends greatly on the lifestyle and infection strategy of
the invading attacker. This so-called ‘signal signature’
results in the activation of a specific set of defense-related
genes that eventually determines the nature and effectiveness
of the immune response that is triggered by the
invader (De Vos et al. 2005). In recent years, molecular,
genetic, and genomic tools have been used to uncover the
complexity of the hormone-regulated induced defense
signaling network. Besides balancing the relative abundance
of different hormones, intensive interplay between
hormone signaling pathways emerged as an important
regulatory mechanism by which the plant is able to tailor
its immune response to the type of invader encountered
(Pieterse et al. 2009). Pathogens and insects, on the other
hand, can manipulate the plant’s defense signaling network
for their own benefit by affecting phytohormone
homeostasis to antagonize the host immune response
(Pieterse and Dicke 2007; Walling 2008; Grant and Jones
2009).