Guard cell K+ channels of Kalanchoë follow the diel cycle of crassulacean acid metabolism

Abstract

The activity of outward-rectifying but not inward-rectifying K + channels of Kalanchoë stomata follows the diel cycle of crassulacean acid metabolism.

Figure 1.

Outward-rectifying K⁺ currents of K. fedtschenkoi vary with the diel cycle. A) Mean steady-state current-voltage (I/V) curves (left) and representative clamp current traces (right) recorded at times between 6 and 22 h over the diel cycle. Plants were grown and measurements were carried out as described previously (Lefoulon et al. 2020) by 2-electrode voltage clamp of guard cells in epidermal peels superfused with 5 mm Ca²⁺-MES (2-(N-morpholino)propanesulfonic acid), pH 6.1 (1 mm Ca²⁺) and 10 mm KCl. Microelectrodes were filled with 200 mm K⁺ acetate, pH 7.5 (Blatt 1987). Currents were analyzed using Henry IV EP suite and corrected for guard cell surface area. Data points are color-coded, each point the mean ± SE of n > 7 for each data point, each from guard cells on different plants. Scale (traces): 100 µA vertical, 1 s horizontal. Curves are fittings of the mean currents to the Boltzmann function$$\mathrm{I}={\mathrm{G}}_{\max }(\mathrm{V}\text{-}{\mathrm{E}}_{\mathrm{K}})/(1+\exp (\delta F(\mathrm{V}\text{-}{\mathrm{V}}_{1/2})/RT))[1]$$ where G_max is the maximum conductance, V_1/2 is the midpoint voltage where G = 0.5 G_max, δ is the apparent gating charge and defines voltage sensitivity around V_1/2, F is the Faraday constant, R is the gas constant, and T is the temperature in K. The value for δ was 2.4 ± 0.4 and was held in common for fitting. B) Means ± SE of values for V_1/2 from the fittings in (A) plotted as a function of diel time. Shading marks the dark period of the diel cycle. Lettering (above) indicates significant differences at P < 0.001 by post hoc Tukey analysis.

Figure 2.

The diel cycle in outward-rectifying K⁺ current of K. laxiflora is suppressed or reversed in rPEPC mutants that affect mesophyll Mal metabolism. Mean steady-state current-voltage (I/V) curves recorded as in Fig. 1. Scale (traces): 100 µA vertical, 1 s horizontal. Current means ± SE (n > 8) shown were recorded at 6 h (open symbols) and 20 h (filled symbols) of the diel cycle, marking the light–dark maxima–minima only shown for clarity. Data are for the wild-type (A), and the mutants rPEPC-A (B) and rPEPC-B (C). Data are means ± SE of n > 8 each from guard cells on different plants for each genotype. Curves are fittings to Eqn. [1] with δ held in common (δ = 2.4 ± 0.2) and V_1/2 values between night and day for each genotype. Values for V_1/2 are −21 ± 0.9 mV (wt), −24 ± 1.7 mV (rPEPC-A), and −14 ± 1.4 mV (rPEPC-B).