Some aspects of defense gene expression by SA have been elucidated. Genes for transcription fac-
tors, such as ethylene response binding element protein 1 (EREBP1) and NPR-1, are SA inducible
[67–69]. Despres et al. [69] have shown that interactions of NPR1 with combinations of other transcrip-
tion factors are necessary for PR-1 expression during SAR and ISR. In addition, some data suggest that
combinations of SA with JA or ethylene are necessary for the induction of some PR proteins [70,71]. This
suggests that the genes for those proteins have regulatory elements for transcription factors specific to SA,
JA, and ethylene and require combinations of specific transcription factors to enact transcription.
SYSTEMIN. Systemin is an 18-amino-acid polypeptide arising from the carboxy terminal region of a
200-amino-acid precursor, prosystemin [72,73]. Although the systemin transcript is constitutively pro-
duced at low levels, it is inducible by wounding [74]. Systemin has been shown to be released in tomato
leaves in response to pathogenic attack, as a phloem-mobile, long-range signaling molecule, and is re-
sponsible for the synthesis of numerous biotic stress defense proteins. The signal transduction pathway
by which it operates is the octadecanoid pathway, which is membrane lipid derived, generating jasmonic
acid [74] (Figure 1 and see later). Systemin protein or prosystemin complementary DNAs (cDNAs) have
664 ARTLIP AND WISNIEWSKI
Figure 1 Biotic stress defense signal transduction model. SA, salicylic acid; JA, jasmonic acid; C 2 H 4 , ethy-
lene; ROI, reactive oxygen species; AOX, alternative oxidase. Solid lines indicate strong evidence for the path-
way, (--) indicates an inhibitory pathway of unknown mechanism, (—-) indicates that some genes are ex-
pressed by combinations of JA, C 2 H 4 , and SA. (Adapted from Refs. 2, 53, 56, 61, 71.)