Here comes the sun: How optimization of photosynthetic light reactions can boost crop yields

Abstract

Photosynthesis started to evolve some 3.5 billion years ago CO₂ is the substrate for photosynthesis and in the past 200-250 years, atmospheric levels have approximately doubled due to human industrial activities. However, this time span is not sufficient for adaptation mechanisms of photosynthesis to be evolutionarily manifested. Steep increases in human population, shortage of arable land and food, and climate change call for actions, now. Thanks to substantial research efforts and advances in the last century, basic knowledge of photosynthetic and primary metabolic processes can now be translated into strategies to optimize photosynthesis to its full potential in order to improve crop yields and food supply for the future. Many different approaches have been proposed in recent years, some of which have already proven successful in different crop species. Here, we summarize recent advances on modifications of the complex network of photosynthetic light reactions. These are the starting point of all biomass production and supply the energy equivalents necessary for downstream processes as well as the oxygen we breathe.

Keywords: bioengineering; crop improvement; electron transfer; light reactions; photosynthesis; photosystem; stress tolerance.

Figure 1

Global developments calling for solutions for improved crop yields (A) Annual world population (in billion people) recorded from 1950 to 2020 with future predictions from 2021 to 2100. (B) Agricultural land use development and (C) forest land development per continent over the past 30 years. The shares of annual agricultural land use and forested land (in %) per total land area was determined for each area. All data were obtained from the Food and Agriculture Organization (FAO) in October–December 2021. Agricultural land use includes both crop and pasture land. Timeseries data for Asia and Europe were started at 1993 to avoid the discontinuity in 1992 due to the end of the USSR. Groupings by continent or sub‐continent follow the FAO country groupings. The data for group “South America” also includes the Caribbean countries. The data for group “Asia” combines FAO country groups for central, eastern, southern, and western Asia.

Figure 2

Schemes of terrestrial and marine photosynthesis Carbon dioxide (CO₂) from the air and water (H₂O) from the soil are taken up by land plants and converted into sugars and biomass using the light energy of the sun (according to the equation 6 CO₂+6 H₂O↔C₆H₁₂O₆+6 O₂). As a photosynthetic by‐product, molecular oxygen (O₂) is released into the air. Percentages indicate the proportions of total sequestered CO₂ and released O₂ by terrestrial and marine photosynthesis (source: Food and Agriculture Organization https://www.fao.org/3/y0900e/y0900e06.htm, World Ocean Review).

Figure 3

Overview of light reactions in the thylakoid membrane In the thylakoid membrane system inside the chloroplast, two pigment–protein photosystems (PSII and PSI) operate in series in order to generate the energy equivalents nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) and adenosine triphosphate (ATP). Absorbed excitation energy is channeled by light‐harvesting complexes LHCII and LHCI toward the reaction centers of both photosystems (P680 in PSII and P700 in PSI), where an electron is liberated and passed along several electron acceptors down the linear electron transfer (LET) chain (blue arrows). Downstream of PSII, the plastoquinone/plastoquinol (PQ) pool transfers electrons to the cytochrome b6f complex (cyt b6f) and further to PSI via plastocyanin (PC). In PSI, several iron‐sulfur clusters (F_x, F_A, F_B) are the primary electron acceptors which then reduce ferredoxin (Fd). Lastly in LET, the Fd‐NADP⁺ reductase (FNR) is released from its possible anchor thylakoid rhodanase‐like (TROL) protein, which contains a rhodanese‐like motif (RHO) in the lumen and a membrane recruiting motif (MRM) in the stroma, and regenerates NADPH by oxidation of Fd. Simultaneously in PSII, oxidized P680⁺ is reduced by an electron deriving from the splitting of water (H₂O) at the oxygen‐evolving complex, which also releases molecular oxygen (O₂) and protons (H⁺). Protons are also transported across the thylakoid membrane by cyt b6f and the NADH dehydrogenase‐like 1 (NDH‐1) complex (orange arrows) in order to fuel ATP production at the ATP synthase complex. Both NADPH and ATP are then metabolized in the Calvin–Benson–Bassham (CBB) cycle for carbon fixation. In case of overexcitation of the LET chain, AET routes are activated downstream of PSI (magenta dashed arrows), including cyclic electron transfer via the Proton Gradient Regulation 5 (PGR5)/PGR5‐like photosynthetic phenotype 1 (PGRL1) or the NDH‐1 complexes. NDH‐1 also diverts excess electrons to the Plastid Terminal Oxidase PTOX, which reduces O₂ to H₂O. Photoprotection via qE‐type nonphotochemical quenching (NPQ) involves the PSII subunit S protein PsbS, which senses the acidification of the lumen upon high light exposure via protonatable residues and initiates the rearrangement of the LHCII complexes, thus inducing the dissipation of excess excitation energy or NPQ. In parallel, the xanthophyll cycle is also activated by the lumen pH, inducing the reversible conversion of violaxanthin (yellow pigment) into antheraxanthin and zeaxanthin (red pigment) in light via the violaxanthin de‐epoxidase (VDE). In dark, the xanthophyll cycle is then reversed by zeaxanthin epoxidase (ZEP).

Figure 4

Arrangements of photosystems with light‐harvesting complexes (LHC) in higher plants and cyanobacteria in the thylakoid membranes Top views of the pigment–protein photosystem II (PSII)‐LHCII (A) and PSI‐LHCI (B) supercomplex core components in higher plants (recreated from Protein Data Bank (PDB) entries 5MDX and 2WSC). The dimeric PSII core consists of the reaction center (RC) proteins D1 and D2 and the core proteins CP43 and CP47. The minor antennae CP29/CP24 (heterodimer) and CP26 connect moderately and strongly bound M‐ and S‐LHCII trimers to the PSII RC via CP47 and CP43, respectively. Loosely bound L‐LHCII trimers are often detached from the supercomplex. In monomeric PSI, the RC is surrounded by LhcA1‐4 in a fan‐like fashion, with LhcA1 and A4 and LhcA2 and A3 forming heterodimers. In contrast, cyanobacterial light‐harvesting antennae are not embedded within the thylakoid membrane but are attached to the core proteins on the stromal surface. These large pigment‐protein complexes are called phycobilisomes (PBS) and come in different shapes. Pentacylindrical PBS are predominantly present in filamentous cyanobacteria and consist of five allophycocyanin (APC) core cylinders (red) from which eight phycoerythrocyanin (PEC; blue)/phycocyanin (PHC; green) rods radiate (C). The colors represent the wavelengths the different PBS discs absorb upon binding phycocyanobilin pigments, inducing energy transfer from PEC→PHC→APC→PSII RC. Unicellular cyanobacteria mostly contain tricylindrical PBS with three APC core cylinders and up to six rods (D). The structures were generated from PDB entries 7EYD (Anabaena sp. PCC 7120) and 7EXT (Synechococcus sp. PCC 7002), respectively. (E) Phylogenetic tree of Arabidopsis thaliana LHC family proteins generated in MEGAX64. Protein sequences were aligned by MUSCLE with the UPGMA cluster method. The tree was built using Maximum Likelihood as statistical method, in conjunction with the bootstrap method (500 replications), the rtREV with Freqs. (+F) model, Gamma Distributed rates with Invariant Sites (G + I; five discrete gamma categories), partial deletion of gaps (95% cutoff) and the Nearest‐Neighbor‐Interchange heuristic model.

Figure 5

Light distribution across the plant canopy between wild type (A) and mutants with decreased light‐harvesting complex (LHC) antenna sizes (B) (A) In a dense crop canopy, light distribution across the leaves of the plant is very uneven because of shading effects from other leaves. Leaves of the upper canopy closest to the light source absorb most of the photosynthetic active radiation (PAR) and deplete the light in the lower canopy of its PAR, resulting in a strong decrease of photosynthesis with increasing canopy depth (see insert). (B) Introduction of mutants with a decreased cross‐section of the LHC antennae allows PAR to travel deeper into the crop canopy, as the fractional PAR absorption of leaves closest to the light source has decreased. This approach may marginally decrease photosynthesis of the light‐exposed layers but improves light distribution across the entire canopy, thus improving overall photosynthesis in the otherwise shaded layers and the entire plant (see insert).

Figure 6

Activity of nonphotochemical quenching (NPQ) mechanisms and xanthophylls under changing light conditions and their potential for improving photosynthetic efficiency Plants often encounter sudden changes in light intensities (sunflecks or shading), which they quickly need to respond to and adjust their metabolic setup in order to avoid photodamage or maintain their photosynthetic capacity. Upon exposure to high light intensities, NPQ (black lines) components are swiftly initiated to dissipate excess excitation energy as heat and prevent photoinhibition of the photosynthetic machinery. Acidification of the thylakoid lumen initiates the fastest NPQ component qE (energy‐dependent quenching) within seconds to minutes, which is subsequently further enhanced through activation of the xanthophyll cycle, that is, the conversion of violaxanthin (V) into photoprotective zeaxanthin (Z) via antheraxanthin (A). Simultaneously, NPQ inhibits photosynthetic efficiency (red lines), which drops to very low levels under high light conditions. Upon the shift to low light or dark conditions, NPQ relaxes and pigment–protein photosystem II (PSII) efficiency recovers, observable through reconversion of zeaxanthin to violaxanthin. However, full relaxation of NPQ after high light stress is a rather slow process (30–60 min or longer), during which photosynthetic capacity is still inhibited to some extent under otherwise optimal conditions, thereby possibly losing time for biomass production. By overexpressing the lumenal pH sensor protein PsbS and the xanthophyll‐converting enzymes in tobacco (dashed lines), transgenic VPZ plants displayed faster NPQ relaxation under changing light conditions and thus faster recovery of photosynthesis, which resulted in higher biomass accumulation compared to control plants (Kromdijk et al., 2016). Graphs displayed here are schematic representations.

Figure 7

Schematic overview of light reaction components that have been targeted for bioengineering improved crops Strategies for boosting plant photosynthesis through manipulations of light reaction components are highlighted in either red or blue in this depiction of Figure 3. Components that have not been investigated further yet are depicted in gray. Proteins and protein complexes that are highlighted in red have been directly overexpressed or their expression has been indirectly induced and resulted in a measurably improved phenotype compared to control plants. Blue highlighting of protein complexes indicates downregulation of these components. Yellow–orange proteins symbolize the introduction of alternative pathways deriving from lower plants, microalgae and cyanobacteria, including an algal zeaxanthin epoxidase (ZEP), cytochrome c6 (cyt c6), flavodoxin (FLD), and flavodiiron proteins (FDPs). Phenotypes were either associated with higher biomass accumulation (labeled with yellow asterisks) or enhanced abiotic stress tolerance (see also Table 1). For further explanation of the light reactions see the original Figure 3.