The efficiency of light harvesting in higher plant photosynthesis is regulated in response to external environmental conditions. Under conditions of excess light, the normally highly efficient light‐harvesting system of photosystem II is switched into a state in which unwanted, potentially harmful, energy is dissipated as heat. This process, known as nonphotochemical quenching, occurs by the creation of energy quenchers following conformational change in the light‐harvesting complexes, which is initiated by the build up of the thylakoid pH gradient and controlled by the xanthophyll cycle. In the present study, the evidence to support the notion that this regulatory mechanism is dependent upon the organization of the different antenna subunits in the stacked grana membranes is reviewed. We postulate that nonphotochemical quenching occurs within a structural locus comprising the PsbS subunit and components of the light‐harvesting antenna, CP26, CP24, CP29 and LHCIIb (the major trimeric light‐harvesting complex), formed in response to protonation and controlled by the xanthophyll cycle carotenoids.

The main trimeric light-harvesting complex of higher plants (LHCII) consists of three different Lhcb proteins (Lhcb1-3). We show that Arabidopsis thaliana T-DNA knockout plants lacking Lhcb3 (koLhcb3) compensate for the lack of Lhcb3 by producing increased amounts of Lhcb1 and Lhcb2. As in wild-type plants, LHCII-photosystem II (PSII) supercomplexes were present in Lhcb3 knockout plants (koLhcb3), and preservation of the LHCII trimers (M trimers) indicates that the Lhcb3 in M trimers has been replaced by Lhcb1 and/or Lhcb2. However, the rotational position of the M LHCII trimer was altered, suggesting that the Lhcb3 subunit affects the macrostructural arrangement of the LHCII antenna. The absence of Lhcb3 did not result in any significant alteration in PSII efficiency or qE type of nonphotochemical quenching, but the rate of transition from State 1 to State 2 was increased in koLhcb3, although the final extent of state transition was unchanged. The level of phosphorylation of LHCII was increased in the koLhcb3 plants compared with wild-type plants in both State 1 and State 2. The relative increase in phosphorylation upon transition from State 1 to State 2 was also significantly higher in koLhcb3. It is suggested that the main function of Lhcb3 is to modulate the rate of state transitions.

The PsbS subunit of photosystem II (PSII) plays a key role in nonphotochemical quenching (NPQ), the major photoprotective regulatory mechanism in higher plant thylakoid membranes, but its mechanism of action is unknown. Here we describe direct evidence that PsbS controls the organization of PSII and its light harvesting system (LHCII). The changes in chlorophyll fluorescence amplitude associated with the Mg 2؉ -dependent restacking of thylakoid membranes were measured in thylakoids prepared from wild-type plants, a PsbS-deficient mutant and a PsbS overexpresser. The Mg 2؉ requirement and sigmoidicity of the titration curves for the fluorescence rise were negatively correlated with the level of PsbS. Using a range of PsbS mutants, this effect of PsbS was shown not to depend upon its efficacy in controlling NPQ, but to be related only to protein concentration. Electron microscopy and fluorescence spectroscopy showed that this effect was because of enhancement of the Mg 2؉ -dependent re-association of PSII and LHCII by PsbS, rather than an effect on stacking per se. In the presence of PsbS the LHCII⅐PSII complex was also more readily removed from thylakoid membranes by detergent, and the level of PsbS protein correlated with the amplitude of the psi-type CD signal originating from features of LHCII⅐PSII organization. It is proposed that PsbS regulates the interaction between LHCII and PSII in the grana membranes, explaining how it acts as a pH-dependent trigger of the conformational changes within the PSII light harvesting system that result in NPQ.Light harvesting antennae in plant photosynthesis increase the rate at which absorbed photons excite the photosynthetic reaction centers. In the case of photosystem II (PSII) 3 in higher plants, light harvesting is carried out by a complex assembly of chlorophyll protein antenna complexes (LHCII) located in the grana membranes of the chloroplast (1). The main components are trimeric LHCIIb and monomeric CP24, CP26, and CP29. These are associated together with the dimeric PSII reaction center core complexes to form LHCII⅐PSII supercomplexes (2). The minimal unit of the supercomplex comprises two cores (C 2 ) and two strongly bound LHCIIb trimers (S 2 ), together with two copies of CP26 and CP29, referred to as the C 2 S 2 LHCII⅐PSII supercomplex (3). Two further trimers bound with moderate strength (M 2 ) together with CP24 make up the most predominant supercomplex, C 2 S 2 M 2 . LHCII⅐PSII supercomplexes are segregated from photosystem I, associating together in the lateral plane of the thylakoid membrane, sometimes forming highly ordered semi-crystalline domains. Transverse associations between the outer stromal surfaces of these proteins result in membrane appression and the characteristic grana stack. Although this highly conserved macro-organization of the PSII antenna has been rationalized in terms of efficient capture and utilization of light (4), it can be argued that it is the enabled structural and functional flexibility that is probably more important. Changes ...

BackgroundPlants exhibit phenotypic plasticity and respond to differences in environmental conditions by acclimation. We have systematically compared leaves of Arabidopsis thaliana plants grown in the field and under controlled low, normal and high light conditions in the laboratory to determine their most prominent phenotypic differences.ResultsCompared to plants grown under field conditions, the "indoor plants" had larger leaves, modified leaf shapes and longer petioles. Their pigment composition also significantly differed; indoor plants had reduced levels of xanthophyll pigments. In addition, Lhcb1 and Lhcb2 levels were up to three times higher in the indoor plants, but differences in the PSI antenna were much smaller, with only the low-abundance Lhca5 protein showing altered levels. Both isoforms of early-light-induced protein (ELIP) were absent in the indoor plants, and they had less non-photochemical quenching (NPQ). The field-grown plants had a high capacity to perform state transitions. Plants lacking ELIPs did not have reduced growth or seed set rates, but their mortality rates were sometimes higher. NPQ levels between natural accessions grown under different conditions were not correlated.ConclusionOur results indicate that comparative analysis of field-grown plants with those grown under artificial conditions is important for a full understanding of plant plasticity and adaptation.