In vivo modulation of nonphotochemical exciton quenching (NPQ) by regulation of the chloroplast ATP synthase

Abstract

Nonphotochemical quenching (NPQ) of excitation energy, which protects higher plant photosynthetic machinery from photodamage, is triggered by acidification of the thylakoid lumen as a result of light-induced proton pumping, which also drives the synthesis of ATP. It is clear that the sensitivity of NPQ is modulated in response to changing physiological conditions, but the mechanism for this modulation has remained unclear. Evidence is presented that, in intact tobacco or Arabidopsis leaves, NPQ modulation in response to changing CO(2) levels occurs predominantly by alterations in the conductivity of the CF(O)-CF(1) ATP synthase to protons (g(H)(+)). At a given proton flux, decreasing g(H)(+) will increase transthylakoid proton motive force (pmf), thus lowering lumen pH and contributing to the activation of NPQ. It was found that an approximately 5-fold decrease in g(H)(+) could account for the majority of NPQ modulation as atmospheric CO(2) was decreased from 2,000 ppm to 0 ppm. Data are presented that g(H)(+) is kinetically controlled, rather than imposed thermodynamically by buildup of DeltaG(ATP). Further results suggest that the redox state of the ATP synthase gamma-subunit thiols is not responsible for altering g(H)(+). A working model is proposed wherein g(H)(+) is modulated by stromal metabolite levels, possibly by inorganic phosphate.

Figure 1

Demonstration of a variable relationship between LEF and NPQ of excitation energy. Experiments were performed on intact wild-type tobacco leaves, under steady-state photosynthetic conditions with light intensities from 45 to 2,000 μmol of photons m⁻²⋅s⁻¹ and CO₂ levels of 2,000 ppm CO₂ (open triangles), 350 ppm CO₂ (ambient, filled circles), 50 ppm CO₂ (open squares), and 0 CO₂ (filled squares). The O₂ level was held constant at 20%. The dashed lines represent a global fit of the data points by using the equation y = A(10^(x/t) − 1). The value of A was held constant at 0.152 and that of t was fit at 35, 66, 140, and 180 at 0, 50, 350 and 2,000 ppm CO₂, respectively. The diameters of the open circles surrounding each data point were set proportional to relative values for g, as described in the text. The largest and smallest diameter symbols represented ≈15- and 80-ms decay times.

Figure 2

The relationship between light-induced pmf and NPQ. The y axis data were taken from Fig. 1, and plotted against the steady-state pmf, estimated by the decay of the ECS, as described in the text. The symbols and conditions are as in Fig. 1.

Figure 3

Relationships among light intensity, CO₂ levels, and the conductivity of the ATP synthase to protons (g). Values of g were estimated as described in the text and plotted as a function of light intensity. Conditions and symbols were as in Fig. 1. (Inset) Decay kinetics of the ECS on abrupt light-dark transitions from steady-state photosynthetic conditions. The light intensity was 320 μmol of photons m⁻²⋅s⁻¹, and the CO₂ levels were 350 ppm (top curve) and 0 ppm (bottom curve). The curves were fit to single first order decays as described in the text.

Figure 4

The relationship between pmf, estimated by LEF/g, and NPQ. The data from Figs. 1 and 4 were used to estimate pmf based on model 3 (see text). Conditions and symbols were as in Fig. 1. (Inset) The relationship between the sensitivity of NPQ to LEF, S_NPQ, and the average value of g at each CO₂ level. The dashed line represents the least-squares linear fit to the data, with slope of −0.068, y intercept of 5.9 and r value of −0.96.