Effectiveness of Light-Quality and Dark-White Growth Light Shifts in Short-Term Light Acclimation of Photosynthesis in Arabidopsis.

Elisabeth Hommel,Monique Liebers,Sascha Offermann,Thomas Pfannschmidt

doi:10.3389/fpls.2021.615253

Abstract

Photosynthesis needs to run efficiently under permanently changing illumination. To achieve this, highly dynamic acclimation processes optimize photosynthetic performance under a variety of rapidly changing light conditions. Such acclimation responses are acting by a complex interplay of reversible molecular changes in the photosynthetic antenna or photosystem assemblies which dissipate excess energy and balance uneven excitation between the two photosystems. This includes a number of non-photochemical quenching processes including state transitions and photosystem II remodeling. In the laboratory such processes are typically studied by selective illumination set-ups. Two set-ups known to be effective in a highly similar manner are (i) light quality shifts (inducing a preferential excitation of one photosystem over the other) or (ii) dark-light shifts (inducing a general off-on switch of the light harvesting machinery). Both set-ups result in similar effects on the plastoquinone redox state, but their equivalence in induction of photosynthetic acclimation responses remained still open. Here, we present a comparative study in which dark-light and light-quality shifts were applied to samples of the same growth batches of plants. Both illumination set-ups caused comparable effects on the phosphorylation of LHCII complexes and, hence, on the performance of state transitions, but generated different effects on the degree of state transitions and the formation of PSII super-complexes. The two light set-ups, thus, are not fully equivalent in their physiological effectiveness potentially leading to different conclusions in mechanistic models of photosynthetic acclimation. Studies on the regulation of photosynthetic light acclimation, therefore, requires to regard the respective illumination test set-up as a critical parameter that needs to be considered in the discussion of mechanistic and regulatory aspects in this subject.

Highlights

In oxygenic photosynthesis of plant and algae chloroplasts photosystem II (PSII) and photosystem I (PSI) work electrochemically in series
Recent studies reported 77K fluorescence emission spectra indicating that both, light-quality and dark-white light shifts, induce a significant lateral movement of the mobile light harvesting complex of PSII (LHCII) antenna between PSII and PSI resulting in considerable changes of the respective antenna cross sections
The state transition deficient Arabidopsis mutant stn7 was devoid of such changes indicating that the differences in the observed 77K spectra of wild-type control plants (WT) and mutant were caused by short-term antenna movements, i.e., state transitions (Dietzel et al, 2011; Koskela et al, 2018)

Summary

Introduction

In oxygenic photosynthesis of plant and algae chloroplasts photosystem II (PSII) and photosystem I (PSI) work electrochemically in series. Efficient electron transport from the donor at PSII, water, to the final acceptor at PSI, NADP+, requires a balanced action of both photosystems. Enrichment of either wavelength in the incident light, can cause imbalances in photosystems excitation which in turn reduces the efficiency in photosynthetic energy conversion (Allen and Pfannschmidt, 2000). Many abiotic and biotic influences can lead to variations in the illumination of plants. A number of highly sophisticated regulation mechanisms evolved that acclimate the process of photosynthetic light harvesting to variations in both light intensity and light-quality that can occur at time scales ranging from seconds to minutes as well as from daily to seasonal variations (Kanervo et al, 2005; Walters, 2005; Eberhard et al, 2008)

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