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Genomics

Scientists uncover genetic link to heat-tolerant photosynthesis

A simple regulatory DNA sequence helps localize photosynthetic machinery to specialized cells

by Max Barnhart
November 25, 2024

 

A photo of a field of sorghum plants.
Credit: Shutterstock
Sorghum, an important cereal crop, is a model for C4 photosynthesis.

The RuBisCO enzyme, the most abundant protein on the planet, is an essential component of photosynthesis, but it isn’t terribly efficient, especially when it gets hot. Over time, some plants evolved a modified form of photosynthesis that improves the efficiency of the carbon-fixing enzyme RuBisCO at high temperatures. New research published in Nature has revealed how genetic rewiring contributed to this specialized form of photosynthesis (2024, DOI: 10.1038/s41586-024-08204-3).

The ancestral form of photosynthesis is called C3 photosynthesis, and it’s used in roughly 90% of plants. In C3 photosynthesis, RuBisCO is found in the chloroplasts of mesophyll cells, which are broadly distributed across the leaves of a plant. At temperatures between 25 °C and 30 °C, when carbon assimilation via photosynthesis is at its peak, roughly 20% of the time RuBisCO will inadvertently grab ahold of oxygen instead of carbon dioxide in a process called photorespiration. But when temperatures increase beyond 30 °C, the binding efficiency of RuBisCO to CO2 starts to drop, and photorespiration becomes more common.

A microscope image of C4 plant leaf tissue showing a ring of mesophyll cels in green surrounding a bundle sheath cell in what is called Kranz anatomy.
Credit: Shutterstock
C4 photosynthetic plants have Kranz anatomy where mesophyll cells form a ring around RuBisCO containing bundle-sheath cells in leaf tissue.

But the plants that evolved a heat-tolerant form of photosynthesis, called C4 photosynthesis, localize RuBisCO in a different cell type called the bundle sheath. “C4 photosynthesis evolved from C3 ancestors by partitioning parts of the photosynthesis pathway into the bundle sheath cell type, which is deeper in the leaf tissue,” says lead author and molecular biologist at the Salk Institute, Joseph Swift. Oxygen has a harder time getting into the bundle sheath cells and binding to RuBisCO, meaning photorespiration is rarer in C4 plants and photosynthesis can remain efficient at high temperatures.

According to Swift, how C4 photosynthesis managed to evolve has been a “big mystery.” He says that the evolution of this form from C3 photosynthesis doesn’t require the evolution of new genes; instead, “you just need to rewire or change the expression patterns of existing genes.” So to figure out what kind of rewiring took place, Swift used single-cell RNA sequencing to compare the gene expression patterns between rice, a C3 plant, and sorghum, a C4 plant, and found that many of the photosynthesis genes that were turned on in C4 bundle sheath cells had acquired a small cis-regulatory element, a series of four nucleotides reading AAAG. “If you’re a gene and you want to be turned on in the bundle sheath, you need this particular transcriptional element to regulate your expression,” Swift says. The research team then confirmed that this motif was associated with bundle-sheath specific gene expression in other plant species too.

According to Dominique Bergmann, a plant developmental biologist at Stanford and the Howard Hughes Medical Institute who was not involved in the research, the finding that the genes associated with C4 photosynthesis in sorghum acquired a cis-regulatory element is somewhat counterintuitive. “I think there used to be this idea that you would have some sort of hierarchy,” she says, explaining that in order for evolution to tweak the expression of a set of genes, usually it would just modify the transcription factor that regulates them. “What this shows is that actually the regulator stays exactly where it is, and all of these different parts all acquire the ability to be regulated by it. So it’s kind of backwards from what seems the most straightforward way of doing it.”

That counterintuitive finding could actually be quite the boon for developing more heat-tolerant crops. Some of the collaborators on this project are working on a Gates Foundation–funded initiative to engineer C4 photosynthesis into rice, and Swift says that this research could help make that happen.

“If we could achieve that, that would allow crops like rice to be more environmentally resilient at high temperatures in drier climates,” Swift says. “These sorts of discoveries, I think, are going to be fundamental for helping us transition and adapt to a warmer climate.”

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