Figure 4. Phloem-mobile signals during water stress and biotic stress.

Water stress (blue arrows) signals, derived in both the roots and the shoots, are shown on the left. When water limitation is sensed in the roots, CLE25 translocates in the SE to leaf tissues and is perceived by BARELY ANY MERISTEM 1 (BAM1). The CLE25-BAM1 module promotes the expression NINE-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3), which promotes synthesis of the phytohormone, ABA. ABA signaling regulates the closing of stomata (light green) in response to water stress. PLAFP and phosphatidic acid (PtdOH) are produced in response to ABA and may play a role in systemic drought response. Both ABA and PLAFP could be root- and/or shoot-derived. Biotic stress (magenta arrows; right half): AED15 and PR1 act directly as anti-herbivory or antimicrobial agents in phloem sap. Several proteins and lipophilic compounds, as well as electrical signals like Ca²⁺, are produced in wounded tissues and act in systemic signaling for resistance in distal tissues. ACBP 3 and 6 affect the fatty acid composition in the phloem, including oxylipins. The phloem-mobile, defense-response protein DIR1 may facilitate the transport of SAR signals like DHA into and through the phloem. Mobile signals such as DHA, AZA, or G3P derivatives move to distal tissues, where they can trigger defense response signals like JA signaling, involving the bioactive JA conjugate JA–Isoleucine (JA–Ile), and induce resistance systemically. Abbreviations, plants species, and references for all compounds are detailed in Table 1.