Direct and individual analysis of stress-related phytohormone dispersion in the vascular system of Cucurbita maxima after flagellin 22 treatment

The stress-related phytohormones, salicylic acid (SA) and abscisic acid (ABA), and the three jasmonates, jasmonic acid (JA), cis-12-oxo-phytodienoic acid (cis-OPDA), and (+)-7-isojasmonoyl-l-isoleucine (JA-Ile), were investigated in phloem and xylem exudates of Cucurbita maxima. Phloem and xylem exudates were separately collected and analysed via liquid chromatography– mass spectrometry. We show direct evidence for all three jasmonates, ABA, and SA in both phloem and xylem exudates of C. maxima. JAand JA-Ile concentrations are higher in xylem (JA: c xylem ≈ 199.5 nM, c phloem ≈ 43.9 nM; JA-Ile: c xylem ≈ 7.9 nM, c phloem ≈ 1.6 nM), whereas ABA and SA concentrations are higher in phloem exudates (ABA: c xylem ≈ 37.1 nM, c phloem ≈ 142.6 nM; SA: c xylem ≈ 61.6 nM, c phloem ≈ 1319 nM). During bacteria-derived flagellin 22 (flg22)-triggered remote root-to-shoot signalling, phytohormone concentration changed rapidly both in phloem and xylem. The unequal distribution of phytohormones suggests that phloem and xylem have distinct roles in defence responses. Our data shed light on systemic phytohormone signalling and help explain how plants cope with environmental challenges by lateral exchange between phloem and xylem. Our analysis is a starting point for further investigations of how phytohormones contribute to phloem- and xylem-based defence signalling.