Attenuated accumulation of jasmonates modifies stomatal responses to water deficit

Abstract

To determine whether drought-induced root jasmonate [jasmonic acid (JA) and jasmonic acid-isoleucine (JA-Ile)] accumulation affected shoot responses to drying soil, near-isogenic wild-type (WT) tomato (Solanum lycopersicum cv. Castlemart) and the def-1 mutant (which fails to accumulate jasmonates during water deficit) were self- and reciprocally grafted. Rootstock hydraulic conductance was entirely rootstock dependent and significantly lower in def-1, yet def-1 scions maintained a higher leaf water potential as the soil dried due to their lower stomatal conductance (gs). Stomatal sensitivity to drying soil (the slope of gsversus soil water content) was low in def-1 self-grafts but was normalized by grafting onto WT rootstocks. Although soil drying increased 12-oxo-phytodienoic acid (OPDA; a JA precursor and putative antitranspirant) concentrations in def-1 scions, foliar JA accumulation was negligible and foliar ABA accumulation reduced compared with WT scions. A WT rootstock increased drought-induced ABA and JA accumulation in def-1 scions, but decreased OPDA accumulation. Xylem-borne jasmonates were biologically active, since supplying exogenous JA via the transpiration stream to detached leaves decreased transpiration of WT seedlings but had the opposite effect in def-1. Thus foliar accumulation of both ABA and JA at WT levels is required for both maximum (well-watered) gs and stomatal sensitivity to drying soil.

Fig. 1.

Transpiration rate of wild-type (WT) and def-1 self- and reciprocal grafts (top scion, bottom rootstock), normalized to whole-plant leaf area. Bars represent the mean ±SE of eight replicates per graft combination; different letters denote significant (P<0.05) differences between graft combinations. The table summarizes the significance (P-values) of scion, rootstock, and their interaction (S×R) after ANOVA.

Fig. 2.

Stomatal conductance (g_s) versus soil water content (SWC) of (A) Self-grafted wild-type (WT) plants (filled circles) and WT scions grafted onto def-1 rootstocks (open circles) and (B) self-grafted def-1 plants (open triangles) and def-1 scions grafted onto WT rootstocks (filled triangles). (C) Slope of the regression line between relative g_s (normalized to maximum g_s) and SWC of the four graft combinations, with different letters showing significant differences in slope. In (A) and (B), each point is an individual plant with linear regression lines fitted for each graft combination with r² and P-values reported. The table summarizes the significance (P-values) of scion, rootstock, SWC, and their interactions after ANOVA.

Fig. 3.

(A) Leaf and (B) root water potential versus soil water content (SWC) of self-grafted wild-type (WT) plants (filled circles), def-1 self-grafts (open triangles) and reciprocal graft (scion/rootstock) combinations (WT/def-1, open circles; and def-1/WT, filled triangles). Each point is an individual plant, with lines defining significant correlations of (A) WT (dashed line) and def-1 (dotted line) scions (linear regressions) and (B) all plants (second-order regression) with r² and P-values reported. The tables summarize the significance (P-values) of scion, rootstock, SWC, and their interactions after ANOVA.

Fig. 4.

Stomatal conductance versus leaf water potential (LWP) of (A) self-grafted wild-type (WT) plants (filled circles) and WT scions grafted onto def-1 rootstocks (open circles) and (B) self-grafted def-1 plants (open triangles) and def-1 scions grafted onto WT rootstocks (filled triangles). Each point is an individual plant, with lines defining significant correlations of (A) WT self-grafts and WT/def-1 plants (linear regression) and (B) def-1 scions [exponential growth model (y=ax^b) with r² and P-values reported. The table summarizes the significance (P-values) of scion, rootstock, LWP, and their interactions after ANOVA.

Fig. 5.

(A) Root xylem sap flow rate (normalized to root area) versus applied pressure for detached roots of self-grafted wild-type (WT) plants (filled circles), def-1 self-grafts (open triangles), and reciprocal graft (scion/rootstock) combinations (WT/def-1, open circles; and def-1/WT, filled triangles). Symbols are the mean ±SE of three replicates. Lines denote a significant correlation between WT (dashed line) and def-1 (dotted line) rootstocks. (B) Root hydraulic conductance (the slope of flow rate versus applied pressure) of the different graft combinations. Letters denote a significant (P<0.05) difference between graft combinations after Duncan’s HSD.

Fig. 6.

Foliar ABA (A), JA (B), OPDA (C), and JA-Ile (D) concentrations of wild-type (WT) and def-1 self-grafts and reciprocal graft (scion/rootstock) combinations under high (0.3<SWC<0.5 cm³ cm^–3) and low (0.2<SWC<0.3 cm³ cm^–3) soil water content (SWC), represented by open and filled bars, respectively. Bars are the mean ±SE of four replicates, with different letters denoting significant (P<0.05) differences between graft combinations after Duncan’s HSD within a panel. The tables summarize the significance (P-values) of scion, rootstock, SWC, and their interactions after ANOVA.

Fig. 7.

Stomatal conductance versus foliar ABA (A), JA (B), OPDA (C), and JA-Ile (D) concentrations in wild-type (WT) self-grafts (filled circles), def-1 self-grafts (open triangles), and reciprocal graft (scion/rootstock) combinations (WT/def-1, open circles; and def-1/WT, filled triangles). Correlations between hormone concentration and stomatal conductance reported (r² and P-values) for WT scions (dashed lines), def-1 scions (dotted lines), and def-1/WT plants (plain line). The tables summarize the significance (P-values) of hormones, scion, rootstock, and their interactions after ANOVA.

Fig. 8.

Transpiration rate of detached wild-type (WT), def-1, and spr2 leaves fed with artificial xylem sap (open bars), 1000 nM ABA (light grey bars), or 1000 nM JA (dark grey bars). Symbols are the mean ±SE of four replicates, with different letters denoting significant differences between all genotype/treatment combinations. The table summarizes the significance (P-values) of genotype, treatment, and their interaction after ANOVA of pairwise treatments (artificial xylem sap versus 1000 nM ABA and artificial xylem sap versus 1000 nM JA).

Fig. 9.

ABA (A, B) OPDA (C, D), JA (E, F), and JA-Ile (G, H) concentrations of WT, def-1, and spr2 leaves after 5 h of xylem feeding with artificial xylem sap (open bars), 1000 nM ABA (light grey bars), or 1000 nM JA (dark grey bars) (left panels) or freshly detached leaves before (filled bars) and 5 h after (grey bars) starting the transpiration assay (right panels). Bars are the mean ±SE of four replicates, with different letters denoting significant differences between treatments after Duncan’s HSD, comparing across both panels. The tables summarize the significance (P-values) of genotype, treatment, and their interactions after ANOVA of pairwise treatments (artificial xylem sap versus 1000 nM JA and artificial xylem sap versus 1000 nM ABA).

Fig. 10.

Model of plant hydraulics and phytohormone impacts on stomatal conductance of grafted plants. Lines ending in arrowheads indicate a positive impact, while lines ending in a bar indicate negative impacts. Root hormone delivery is indicated by dashed lines, while relationships within a biosynthetic pathway are indicated by a yellow arrow.