Photosynthetic properties of an Arabidopsis thaliana mutant possessing a defective PsbS gene

We describe the properties of npq4-9, a new mutant of Arabidopsis thaliana (L.) Heynh. with reduced nonphotochemical quenching (NPQ) capacity that pos sesses a single amino acid substitution in the PsbS gene encoding PSII-S. a ubiquitous pigment-binding protein associated with photosystem II (PSII) o f higher plants. Growth. photosynthetic pigment contents. and levels of the major PS11 antenna proteins were not affected by npq4-9. Although the exte nt of de-epoxidation of violaxanthin to antheraxanthin plus zeaxanthin for leaves displaying the mutant phenotype equaled or exceeded that observed fo r the wild typed the relative effectiveness of de-epoxidized xanthophylls i n promoting NPQ was consistently lower for the mutant. Energy partitioning in PSII was analyzed in terms of the competition for singlet chlorophyll a among the processes of fluorescence. thermal dissipation. and photochemistr y. The key processes of NPQ and photochemistry in open PSII centers are rep resented by the relative in vivo rate constants k(N) and k(P0), respectivel y. The magnitude of k(P0) in normal leaves declined only slightly with incr easing k(N), consistent with localization of NPQ primarily in the antenna c omplex. Conversely. a highly significant linear decline in k(P0) with incre asing k(N) was observed for the mutant, consistent with a role for the PSII reaction center in the NPQ mechanism. Although the PSII absorption cross-s ection for white light was not significantly different relative to that of the wild type. PSII quantum yield was significantly lower in the mutant. Th e resulting lower capacity for linear electron transport in the mutant prim arily affected reduction of terminal acceptors other than CO,. Parallel mea surements of fluorescence and in vivo absorbance at 820 nm indicated a cons istently higher steady-state level of reduction of PSII acceptors and accum ulation of P700(+) for the mutant. This suggests that inter-photosystem ele ctron transport in the mutant is restricted either by a higher transthylako id Delta pH or by diminished accessibility to reduced plastoquinone.