Evolving Photosynthesis

Evolving Photosynthesis: Mutations in Key Enzyme Show Promise for Enhancing Plant Growth

New research suggests that subtle changes to a fundamental enzyme in plants could improve their ability to fix carbon dioxide, potentially leading to faster-growing crops. In a study published in Nature Plants, scientists used laboratory evolution techniques to identify mutations in Rubisco—the enzyme central to photosynthesis—that boost its efficiency or production. Results indicated encouraging improvements in plant growth rates, particularly when applied to hybrid enzyme forms, though further testing is needed to confirm broader applications.

The Persistent Challenge of Boosting Plant Productivity

Photosynthesis, the process by which plants convert sunlight, water, and carbon dioxide into energy, is remarkably inefficient in many crops. Rubisco, often called the most abundant enzyme on Earth, plays a starring role but has long been a bottleneck. It not only fixes CO2 to fuel plant growth but also mistakenly reacts with oxygen, leading to a wasteful cycle known as photorespiration that can reduce productivity by up to half in some conditions.

The study notes that improving the carboxylation properties of plant Rubisco has proven challenging. Traditional approaches, such as introducing CO2-concentrating mechanisms from other organisms, are complex and genetically demanding. This has prompted scientists to explore direct tweaks to Rubisco itself, aiming for enhancements that could support global food needs without heavy reliance on fertilizers or irrigation.

A Tailored Laboratory Evolution Screen

To overcome hurdles in engineering plant Rubisco, the team developed a specialized system in Escherichia coli bacteria, which they adapted to mimic the chloroplast environment where the enzyme assembles. Think of it as a simplified factory: The bacteria were engineered to depend on Rubisco activity for survival, allowing researchers to screen for beneficial mutations efficiently.

They created a library of mutated tobacco Rubisco genes using error-prone PCR—a method that introduces random variations, much like shuffling a deck of cards to find winning combinations. This library was then tested in the bacterial system under controlled conditions, selecting for colonies that grew faster, indicating improved enzyme function. The paper describes the development and use of a new chloroplast-compatible E. coli-RDE screen, highlighting how this tool condenses complex plant biology into a manageable lab setup.

The road block to plant Rubisco engineering in E. coli was overcome by co-expressing seven auxiliary chloroplast proteins, which help fold and assemble the enzyme's large and small subunits

Catalytic Speed and Solubility Gains

From the screen, two standout mutations emerged. The first, a change from methionine to leucine at position 116 (M116L), acts as a catalytic switch, increasing Rubisco's CO2-fixation rate—the speed at which it processes carbon—without severely compromising its specificity for CO2 over oxygen. This mutation boosted the rate in various plant Rubiscos, including those from tobacco, Arabidopsis, carrot, and strawberry.

The second mutation, from alanine to valine at position 242 (A242V), enhances the enzyme's solubility, meaning more functional Rubisco can be produced in the cell. The study reports that incorporating A242V enhanced Nt Rubisco biogenesis up to 3-fold, with even greater effects in hybrid forms.

Structural analysis revealed why these changes work: The M116L site is near the enzyme's active center, subtly influencing its reactivity, while A242V stabilizes assembly across subunits. Simulations suggested these mutations could elevate photosynthetic rates in certain plants, prompting real-world tests in tobacco.

Translating Lab Results to Living Plants

To evaluate practical impacts, the mutations were introduced into tobacco plants via plastome transformation—a technique targeting the chloroplast genome. In native tobacco Rubisco, neither mutation significantly altered leaf enzyme levels, photosynthesis, or growth. However, when applied to a hybrid Rubisco (combining Arabidopsis large subunits with tobacco small subunits), the results were more promising.

Plants with the M116L hybrid enzyme showed an exponential growth rate increase relative to the unmodified hybrid, while the A242V version boosted both enzyme production and overall plant growth. The abstract states that tobacco transformed with low-abundance hybrid Arabidopsis Rubisco coding M116L improved plant exponential growth rate by ~75% relative to unmutated hybrid enzyme, with the A242V substitution increasing both hybrid Rubisco production and plant growth by ~50%.

These gains align with the study's modeling, which predicted benefits under CO2-limiting conditions common in C3 plants like wheat and rice.

Looking Ahead: Cautious Optimism

The findings open doors to exploring more Rubisco variations, potentially yielding substantive productivity gains for staple crops. The abstract indicates that the identification of mutations with the potential to enhance plant growth bodes well for broadening the survey of Rubisco sequence space.

Yet, the authors stress that these are early steps. The mutations' effects varied by plant type, and long-term field trials are essential to assess stability, yield impacts, and environmental interactions. As global efforts intensify to meet food demands sustainably, such innovations offer grounded hope, but they must be validated through rigorous, multi-stage testing.