PsbS genotype in relation to coordinated function of PS II and PS I in Arabidopsis leaves.

Application of multiple probes to systems that carry specific mutations provides a powerful means for studying how known regulators of light utilization interact in vivo. Two lines of Arabidopsis thaliana were studied, each carrying a unique lesion in the nuclear psbS gene encoding a 22-kDa pigment-binding protein (PS II-S) essential for full expression of photoprotective, rapid-phase, nonphotochemical quenching of chlorophyll fluorescence (NPQ). The PS II-S protein is absent in line npq4-1 due to deletion of psbS. Line npq4-9 expresses normal levels of PS II-S but carries a single amino acid substitution that lowers NPQ capacity by about 50%. A prior report [Peterson RB and Havir EA (2001) Planta 214: 142-152] described an altered pattern of redox states of the acceptor side of Photosystem II (PS II) and donor side of Photosystem I (PS I) for npq4-9 suggesting that interphotosystem electron transport may be restricted by a higher transthylakoid DeltapH in this line. In vivo steady state fluorescence and absorbance measurements (820 nm) confirmed these earlier observations for line npq4-9 but not for npq4-1. Thus, the prior results cannot be correlated simply to a loss of NPQ capacity. Likewise, the kinetics of the 820-nm absorbance change did not indicate a substantial effect of psbS genotype on electron flow from plastoquinol to PS I. A simple model is proposed to relate linear electron transport rate (measured gasometrically) to a parameter (based on fluorescence) that provides a relative measure of the density of excitation available for photochemistry in PS II. Surprisingly, analyses using this model suggested that the in vivo midpoint potential of the primary quinone acceptor in PS II (Q(A)) is lowered in both psbS mutant lines. This heretofore-unsuspected role for PS II-S is discussed with regard to: (1) numerous prior reports indicating plasticity of the redox potential of Q(A) and (2) the basis for the contrasting regulation of quantum yields of PS I and II in npq4-1 and npq4-9.