Celebrating photosynthesis

Editorial https://doi.org/10.1038/s41477-026-02250-7 Celebrating photosynthesis Check for updates Photosynthesis is both an elegantly simple and dazzlingly complex process. The year 2026 marks anniversaries of the discoveries of both of these layers. T his year marks two significant anniversaries in plant biology, both concerning photosynthesis. The earliest of these is the 65th anniversary of the awarding of the Nobel Prize in Chemistry to Melvin Calvin for the elucidation of the biochemical pathways of what we now call C3 photosynthesis. The second is the 60th anniversary of the discovery of C4 photosynthesis by Marshall Hatch and Roger Slack. Why this second achievement did not also garner a Nobel Prize is well worth pondering. During the Nobel Prize award ceremony in 1961, Calvin was introduced by the Swedish pharmacologist Göran Liljestrand with these words: “No chemical process has a greater importance than the incorporation of atmospheric carbon dioxide into the starch molecule of the green plants under the influence of light from the sun. This reaction is the foundation of life, not only for the green plants themselves but also for all higher animals. This complicated process — the object of intense studies for more than a century — has now been unravelled, Professor Calvin, by your establishing the intermediate steps in the reaction. We express our deep admiration of your achievements.” There is little that needs to be added to such sentiments. The cycle of biochemical reactions elucidated by Calvin and his colleagues — initially called the Calvin cycle but now more often referred to as the Calvin–Benson nature plants or Calvin–Benson–Bassham (CBB) cycle in recognition of the contributions of James Bassham and Andrew Benson to its discovery — remains central to plant biochemistry. In a series of papers in the late 1940s and early 1950s, these researchers used carbon isotope labelling experiments in the green alga Chlorella pyrenoidosa (reclassified as Auxenochlorella pyrenoidosa in 2015) to follow the incorporation of a carbon atom from carbon dioxide into ribulose-1,5-bisphosphate, as catalysed by RuBisCO, to form two molecules of 3-phosphoglyceric acid, and the subsequent rearrangement of these 3-carbon compounds to regenerate ribulose-1,5-bisphosphate, while providing triose phosphate molecules to be used as ‘raw materials’ for the synthesis of sugars and other organic compounds by multiple metabolic pathways. For about ten years it was assumed that the CBB cycle was the system used by all plants, indeed all photosynthetic organisms, to capture sunlight and fix carbon. But studies on crops such as sugar cane and maize began to suggest that things were not quite so clear cut. When carbon isotope experiments similar to Calvin’s on Chlorella were performed on these plants, the initial labelled product was not a 3-carbon glyceride but the 4-carbon dicarboxylic acids malate and aspartate. The first person to observe this was probably a Russian scientist, called Yuri Karpilov, working with maize, who published the results in the 1960 Annual Report of the Kazan Agricultural Institute. However, it was work on sugar cane at the Hawaiian Sugar Planters’ Association research laboratory in Honolulu a few years later that came to the attention of the Australians Hatch and Slack, who were working in Brisbane. Using further ‘pulse–chase’ experiments, they worked out that the 4-carbon compounds were being used to effectively transport carbon dioxide from mesophyll cells, where it was absorbed from the atmosphere, to bundle sheath cells surrounding a leaf’s veins, where the biochemistry of the CBB cycle was taking place. By so doing, they both explained the anomalous biochemical results and provided a function for the Kranz anatomy of mesophyll and bundle sheaths that had been enigmatic for at least 100 years. The ultimate result of the biochemical shunt at the heart of C4 photosynthesis (so named because the initial products of carbon dioxide assimilation have four carbon atoms) was to increase the effective concentration of carbon dioxide around the enzyme RuBisCO, making it more efficient. In the 60 years since Hatch and Slack’s discovery, we have found that far from the relative simplicity of Calvin’s C3 photosynthesis, multiple different ‘flavours’ of C4 photosynthesis have evolved multiple times during evolution. There are plants that use both C3 and C4 approaches depending on their environment, and others that have systems intermediate between the two (often now called C2 photosynthesis). We have also found other methods of concentrating carbon dioxide around the ubiquitous RuBisCO enzyme such as carboxysomes in cyanobacteria and pyrenoids in algae and hornworts. None of this fascinating complexity changes the veracity of Liljestrand’s assertion that “no chemical process has a greater importance”, and we look forward to publishing many more studies on all the myriad varieties of photosynthesis in this anniversary year and beyond. Published online: 23 February 2026 Volume 12 | February 2026 | 261 | 261  (...truncated)