Figure 3.

Sulfate Triggers ABA Production in Guard Cells in a Concentration-Dependent Manner.

(A) The upper panel shows the ABAleon2.1 emission ratio. Signals from guard cells treated with water alone (n = 308), or water supplemented with 2 mM MgSO4 (n = 125), 15 mM MgSO4 (n = 67), or 15 mM MgCl2 (n = 110), respectively. Average ABAleon2.1 emission ratio is calculated per stomatal area. Statistical tests are performed with respect to the water control. The lower panel shows a representative stoma in the given treatment. Letters indicate statistically significant differences between groups determined with one-way ANOVA (P < 0.05).

(B) Transcript levels of ABA-responsive genes in sulfate-treated epidermal peels. Epidermal peels were collected from 5-week-old wild-type plants and incubated on water supplemented without (black) or with 2 mM MgSO₄ (white, sulfate) for 3 h. RNA was extracted and the steady-state transcript levels of ABA-marker genes (LEA7, HAl1, RD20, and RD29B) were quantified by RT-qPCR. The transcript levels of respective genes from water-treated samples were set to 1. Data represent mean ± sd (n = 3). Asterisks indicate statistical significant differences as determined with the Student’s t test (*P < 0.05).

(C) Impact of petiole-fed ABA or sulfate on the expression of the ABA signaling marker ProRAB18:GFP in detached leaves. Leaves of 25-day-old soil grown ProRAB18:GFP plants were detached and fed via the petiole with ABA (gray, 50 μM) or sulfate (white, 15 mM MgSO₄) dissolved in water (black, Control) for 180 min before quantification of the GFP signal. The upper panel displays a representative image of the epidermis containing guard and pavement cells. Bright field image of the same area is shown for orientation. The lower panel depicts the quantification of GFP-signal intensities in guard- or pavement cells after the treatment. Data represent mean ± se (guard cells: n = 666 for water, n = 534 for sulfate, n = 894 for ABA, from five individual leaves, pavement cells n = 20 regions of interest containing multiple pavement cells for each treatment, from five individual leaves).

Letters indicate statistically significant differences between groups determined with one-way ANOVA (P < 0.05).