Figure 8.

Physiological Relevance of Cys-Induced Stomatal Closure.

(A) Stomatal aperture in detached leaves of wild type and mutants affected in provision of sulfide for Cys synthesis (sir1-1), synthesis of GSH from Cys (cad2-1), and the sir1-1 cad2-1 double mutant (s1c2). Detached leaves were fed with ABA (gray, 50 μM) or sulfate (white, 15 mM MgSO4) dissolved in water (black, Control) for 180 min before analysis. Data represent mean ± sd (n = 50, derived from five individual plants). Letters indicate statistically significant differences between groups determined with one-way ANOVA (P < 0.05). Please note that feeding of ABA via the petiole can close the stomata in sir1-1 and s1c2.

(B) Correlation analysis between endogenous foliar Cys steady-state levels and stomatal aperture in wild type (black), sir1-1 (orange), cad2-1 (red), and s1c2 (blue). The data were dynamically fitted with a linear equation (y = m x +b). The negative slope demonstrates that higher endogenous Cys levels correlate with stomatal closure in mutants with functional ABA biosynthesis and ABA response. The regression coefficient was 0.997 and the coefficient of determination (r²) was 0.995, demonstrating the significant correlation between endogenous Cys steady-state levels and stomatal aperture.

(C) and (D) Cys-synthesis–depleted mutants (serat2;1 and oastl-b) are sensitive to soil drying. Five-week-old soil-grown wild type and Cys-synthesis–depleted mutants were subjected to drought stress for 25 d. The serat2;1 and oastl-b mutants suffered from only mild depletion of Cys synthesis capacity and thus grow like the wild type under nonstressed conditions (Heeg et al., 2008; Watanabe et al., 2008). Application of drought resulted in a more pronounced wilting of both Cys-synthesis–depleted mutants when compared with the wild type (C) caused by a significantly greater water loss of both mutants upon soil drying (D). Scale bar = 4 cm. The relative water content of the leaves was determined according to the following equation: (fresh weight – dry weight) / (turgid weight – dry weight). Data represent mean ± se (n = 4 individual plants).

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