Systemic acquired resistance: the elusive signal(s)

Systemic acquired resistance (SAR) is a form of inducible resistance that is triggered in systemic healthy tissues of locally infected plants. The nature of the mobile signal that travels through the phloem from the site of infection to establish systemic immunity has been sought after for decades. Several candidate signaling molecules have emerged in the past two years, including the methylated derivative of a well-known defense hormone (methyl salicylate), the defense hormone jasmonic acid, a yet undefined glycerolipid-derived factor, and a group of peptides that is involved in cell-to-cell basal defense signaling. Systemic SAR signal amplification increasingly appears to parallel salicylic acid-dependent defense responses, and is concomitantly fine-tuned by auxin.