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This article comments on: Turc B, Sahay S, Haupt J, de Oliveira Santos T, Bai G, Glowacka K. 2024. Up-regulation of non-photochemical quenching improves water use efficiency and reduces whole-plant water consumption under drought in Nicotiana tabacum. Journal of Experimental Botany 75, 3959–3972. [ABSTRACT FROM AUTHOR]

Non-photochemical chlorophyll fluorescence quenching (NPQ) remains one of the most studied topics of the 21st century in photosynthesis research. Over the past 30 years, profound knowledge has been obtained on the molecular mechanism of NPQ in higher plants. First, the largely overlooked significance of NPQ in protecting the reaction center of photosystem II (RCII) against damage, and the ways to assess its effectiveness are highlighted. Then, the key in vivo signals that can monitor the life of the major NPQ component, qE, are presented. Finally, recent knowledge on the site of qE and the possible molecular events that transmit ΔpH into the conformational change in the major LHCII [the major trimeric light harvesting complex of photosystem II (PSII)] antenna complex are discussed. Recently, number of reports on Arabidopsis mutants lacking various antenna components of PSII confirmed that the in vivo site of qE rests within the major trimeric LHCII complex. Experiments on biochemistry, spectroscopy, microscopy and molecular modeling suggest an interplay between thylakoid membrane geometry and the dynamics of LHCII, the PsbS (PSII subunit S) protein and thylakoid lipids. The molecular basis for the qE-related conformational change in the thylakoid membrane, including the possible onset of a hydrophobic mismatch between LHCII and lipids, potentiated by PsbS protein, begins to unfold. [ABSTRACT FROM AUTHOR]

High light intensities raise photosynthetic and plant growth rates but can cause damage to the photosynthetic machinery. The likelihood and severity of deleterious effects are minimised by a set of photoprotective mechanisms, one key process being the controlled dissipation of energy from chlorophyll within PSII known as non-photochemical quenching (NPQ). Although ubiquitous, the role of NPQ in plant productivity is important because it momentarily reduces the quantum efficiency of photosynthesis. Rice plants overexpressing and deficient in the gene encoding a central regulator of NPQ, the protein PsbS, were used to assess the effect of protective effectiveness of NPQ (pNPQ) at the canopy scale. Using a combination of three-dimensional reconstruction, modelling, chlorophyll fluorescence, and gas exchange, the influence of altered NPQ capacity on the distribution of pNPQ was explored. A higher phototolerance in the lower layers of a canopy was found, regardless of genotype, suggesting a mechanism for increased protection for leaves that experience relatively low light intensities interspersed with brief periods of high light. Relative to wild-type plants, psbS overexpressors have a reduced risk of photoinactivation and early growth advantage, demonstrating that manipulating photoprotective mechanisms can impact both subcellular mechanisms and whole-canopy function. [ABSTRACT FROM AUTHOR]

During winter evergreens maintain a sustained form of thermal energy dissipation that results in reduced photochemical efficiency measured using the chlorophyll fluorescence parameter Fv/Fm. Eastern white pine (Pinus strobus L.) and white spruce [Picea glauca (Moench) Voss] have been shown to differ in their rate of recovery of Fv/Fm from winter stress. The goal of this study was to monitor changes in photosynthetic protein abundance and phosphorylation status during winter recovery that accompany these functional changes. An additional goal was to determine whether light-dependent changes in light harvesting complex II (LHCII) phosphorylation occur during winter conditions. We used a combination of field measurements and recovery experiments to monitor chlorophyll fluorescence and photosynthetic protein content and phosphorylation status. We found that pine recovered three times more slowly than spruce, and that the kinetics of recovery in spruce included a rapid and slow component, while in pine there was only a rapid component to recovery. Both species retained relatively high amounts of the light harvesting protein Lhcb5 (CP26) and the PsbS protein during winter, suggesting a role for these proteins in sustained thermal dissipation. Both species maintained high phosphorylation of LHCII and the D1 protein in darkness during winter. Pine and spruce differed in the kinetics of the dephosphorylation of LHCII and D1 upon warming, suggesting the rate of dephosphorylation of LHCII and D1 may be important in the rapid component of recovery from winter stress. Finally, we demonstrated that light-dependent changes in LHII phosphorylation do not continue to occur on subzero winter days and that needles are maintained in a phosphorylation pattern consistent with the high light conditions to which those needles are exposed. Our results suggest a role for retained phosphorylation of both LHCII and D1 in maintenance of the photosynthetic machinery in a winter conformation that maximizes thermal energy dissipation. [ABSTRACT FROM AUTHOR]

Natural capacity has evolved in higher plants to absorb and harness excessive light energy. In basic models, the majority of absorbed photon energy is radiated back as fluorescence and heat. For years the proton sensor protein PsbS was considered to play a critical role in non-photochemical quenching (NPQ) of light absorbed by PSII antennae and in its dissipation as heat. However, the significance of PsbS in regulating heat emission from a whole leaf has never been verified before by direct measurement of foliar temperature under changing light intensity. To test its validity, we here investigated the foliar temperature changes on increasing and decreasing light intensity conditions (foliar temperature dynamics) using a high resolution thermal camera and a powerful adjustable light-emitting diode (LED) light source. First, we showed that light-dependent foliar temperature dynamics is correlated with Chl content in leaves of various plant species. Secondly, we compared the foliar temperature dynamics in Arabidopsis thaliana wild type, the PsbS null mutant npq4-1 and a PsbS-overexpressing transgenic line under different transpiration conditions with or without a photosynthesis inhibitor. We found no direct correlations between the NPQ level and the foliar temperature dynamics. Rather, differences in foliar temperature dynamics are primarily affected by stomatal aperture, and rapid foliar temperature increase during irradiation depends on the water status of the leaf. We conclude that PsbS is not directly involved in regulation of foliar temperature dynamics during excessive light energy episodes. [ABSTRACT FROM AUTHOR]

The PsbS protein plays an important role in dissipating excess light energy as heat in photosystem II (PSII). However, the physiological importance of PsbS under naturally fluctuating light has not been quantitatively estimated. Here we investigated energy allocation in PSII in PsbS-suppressed rice transformants (ΔpsbS) under both naturally fluctuating and constant light conditions. Under constant light, PsbS was essential for inducing the rapid formation of light-inducible thermal dissipation (ΦNPQ), which consequently suppressed the rapid formation of basal intrinsic decay (Φf,D), while the quantum yield of electron transport (ΦII) did not change. In the steady state phase, the difference between the wild type (WT) and ΔpsbS was minimized. Under regularly fluctuating light, the reduced PsbS resulted in higher ΦII upon the transition from high light to low light and in lower ΦII upon the transition from low light to high light, indicating that ΦII was, to some extent, controlled by PsbS. Under naturally fluctuating light in a greenhouse, rapid changes in ΦII were compensated by ΦNPQ in the WT, but by Φf,D in ΔpsbS. As a consequence, a significantly lower ΣNPQ integrated ΦNPQ over a whole day) and higher Σf,D were found in ΔpsbS. Furthermore, thermal dissipation associated with photoinhibtion was enhanced in ΔpsbS. These results suggest that PsbS plays an important role in photoprotective process at the induction phase of photosynthesis as well as under field conditions. The physiological relevance of PsbS as a photoprotection mechanism and the identities of ΦNPQ and Φf,D are discussed. [ABSTRACT FROM PUBLISHER]

The thermal dissipation (TD) of absorbed light energy in PSII is considered to be an important photoprotection process in photosynthesis. A major portion of TD has been visualized through the analysis of Chl fluorescence as energy quenching (qE) which depends on the presence of the PsbS subunit. Although the physiological importance of qE-associated TD (qE-TD) has been widely accepted, it is not yet clear how much of the absorbed light energy is dissipated through a qE-associated mechanism. In this study, the fates of absorbed light energy in PSII with regard to different TD processes, including qE-TD, were quantitatively estimated by the typical energy allocation models using transgenic rice in which psbS genes were silenced by RNA interference (RNAi). The silencing of psbS genes resulted in a decrease in the light-inducible portion of TD, whereas the allocation of energy to electron transport did not change over a wide range of light intensities. The allocation models indicate that the energy allocated to qE-TD under saturating light is 30–50%. We also showed that a large portion of absorbed light energy is thermally dissipated in manners that are independent of qE. The nature of such dissipations is discussed. [ABSTRACT FROM AUTHOR]

Homeodomain-leucine zipper proteins constitute a family of transcription factors found only in plants. Expression patterns of the sunflower homeobox-leucine zipper gene Hahb-10 (Helianthus annuus homeobox-10), that belongs to the HD-Zip II subfamily, were analysed. Northern blots showed that Hahb-10 is expressed primarily in mature leaves, although expression is clearly detectable in younger leaves and also in stems. Considerably higher expression levels were detected in etiolated seedlings compared with light-grown seedlings. Induction of Hahb-10 expression was observed when seedlings were subjected to treatment with gibberellins. Transgenic Arabidopsis thaliana plants that express Hahb-10 under the 35S cauliflower mosaic virus promoter show special phenotypic characteristics such as darker cotyledons and planar leaves. A reduction in the life cycle of about 25% allowing earlier seed collection was also observed, and this phenomenon is clearly related to a shortened flowering time. When the number of plants per pot increased, the difference in developmental rate between transgenic and non-transformed individuals became larger. After gibberellin treatment, the relative difference in life cycle duration was considerably reduced. Several light-regulated genes have been tested as possible target genes of Hahb-10. One of them, PsbS, shows a different response to illumination conditions in transgenic plants compared with the response in wild-type plants while the other genes behave similarly in both genotypes. We propose that Hahb-10 functions in a signalling cascade(s) that control(s) plant responses to light quality and quantity, and may also be involved in gibberellin transduction pathways. [ABSTRACT FROM AUTHOR]

The PsbS protein of photosystem II functions in the regulation of photosynthetic light harvesting. Along with a low thylakoid lumen pH and the presence of de-epoxidized xanthophylls, PsbS is necessary for photoprotective thermal dissipation (qE) of excess absorbed light energy in plants, measured as non-photochemical quenching of chlorophyll fluorescence. What is known about PsbS in relation to the hypothesis that this protein is the site of qE is reviewed here. [ABSTRACT FROM PUBLISHER]