Distinct roles for jasmonate synthesis and action in the systemic wound response of tomato

Abstract

Plant defense responses to wounding and herbivore attack are regulated by signal transduction pathways that operate both at the site of wounding and in undamaged distal leaves. Genetic analysis in tomato indicates that systemin and its precursor protein, prosystemin, are upstream components of a wound-induced, intercellular signaling pathway that involves both the biosynthesis and action of jasmonic acid (JA). To examine the role of JA in systemic signaling, reciprocal grafting experiments were used to analyze wound-induced expression of the proteinase inhibitor II gene in a JA biosynthetic mutant (spr-2) and a JA response mutant (jai-1). The results showed that spr-2 plants are defective in the production, but not recognition, of a graft-transmissible wound signal. Conversely, jai-1 plants are compromised in the recognition of this signal but not its production. It was also determined that a graft-transmissible signal produced in response to ectopic expression of prosystemin in rootstocks was recognized by spr-2 but not by jai-1 scions. Taken together, the results show that activation of the jasmonate biosynthetic pathway in response to wounding or (pro)systemin is required for the production of a long-distance signal whose recognition in distal leaves depends on jasmonate signaling. These findings suggest that JA, or a related compound derived from the octadecanoid pathway, may act as a transmissible wound signal.

Figure 1

Induction of the PI-II gene in response to wounding and MeJA. Two-leaf stage WT and mutant (spr-2 and jai-1²) tomato plants were wounded once with a hemostat across the midvein of the lower leaf. Leaf tissue was harvested separately (8 hr later) from the wounded leaf (L, local response) and the upper undamaged leaf (S, systemic response) for RNA isolation. RNA was also isolated from a set of plants treated for 12 h with MeJA (J) and a set of untreated control plants (C). Aliquots (5 μg) of total RNA were analyzed by RNA blot analysis for PI-II mRNA levels. As a loading control, a duplicate blot was probed with a cDNA encoding eIF4A.

Figure 2

Photograph of a typical grafted tomato plant. The arrow indicates the position of the graft junction between the stock and scion. Systemic PI-II expression was measured in undamaged scion leaves 11 h after wounding of the stock leaves. The distance between wounded leaflets on the stock and undamaged leaflets on the scion is ≈20 cm.

Figure 3

Wound-inducible PI-II expression in grafts between WT plants and mutants defective in jasmonate signaling (jai-1) or jasmonate biosynthesis (spr-2). WT and jai-1 plants (A) or WT and spr-2 plants (B) were grafted in the four combinations indicated. The genotypes listed above and below the horizontal line correspond to the scion and stock, respectively. For each graft combination, plants were divided into a control (C) and experimental (W) group consisting of three grafted plants per group. For the experimental group, each leaflet on the stock was wounded as described in Materials and Methods. Eleven hours after wounding, leaf tissue was harvested separately from wounded stock leaves (stock) and undamaged scion leaves (scion) for RNA extraction. The control set of plants received no wounding, other than that inflicted by the grafting procedure itself. Levels of PI-II mRNA were analyzed by RNA blot analysis, using an eIF4A cDNA probe as a loading control.

Figure 4

Wound-induced systemic PI-II expression in grafts between jai-1 and spr-2 plants. Wound-induced systemic expression of PI-II was assessed in the various graft combinations indicated, as described in Fig. 3. PI-II and eIF4 (loading control) mRNA levels in the undamaged scion leaves of unwounded control (C) plants, and plants wounded on the stock leaves (W), are shown.

Figure 5

Genetic model for the role of jasmonate synthesis and signaling in the systemic activation of wound-responsive PI genes in tomato plants. Wounding or expression of 35S∷prosys leads to the production of a long-distance signal that activates PI gene expression in distal leaves (i.e., scion). A functional octadecanoid pathway for jasmonate biosynthesis (hatched line), which is disrupted by def-1 and spr-2, is required for the production of the long-distance signal (X). Jasmonate signaling, which is blocked by jai-1, is required for the recognition of the long-distance signal.