Molecule traps carbon dioxide in leaf tissue

Florence Danila of Australian National University’s ARC Centre of Excellence for Translational Photosynthesis has discovered how plants improve photosynthesis by trapping carbon dioxide inside cell layers. | ANU photo

Researchers think adding suberin to plants that don’t have this ability could increase their yields by 30 to 50 percent

Scientists at the Australian National University in Canberra along with international partners have discovered that the molecule suberin forms a tight layer that keeps carbon dioxide inside a layer of cells called the bundle sheath, and that can improve photosynthesis.

Florence Danila with the ARC Centre of Excellence for Translational Photosynthesis at ANU said C4-type plants have evolved to overcome the limitations of the enzyme Rubisco.

She said CO2 is fixed into a four-carbon acid and once in the bundle sheath cell, it is regenerated for the Rubisco to convert it to energy-rich sugars. But the Rubisco is not entirely efficient.

In C4 plants, CO2 is concentrated near the site of the Rubisco by dividing the work between the mesophyll cells and bundle sheath cells during photosynthesis. This increases the efficiency of photosynthesis.

“Our research provides several pieces of evidence about the responsibility of suberin on making the leaf cells of C4 plants gas tight,” said Danila in a news release. “We have grown mutant plants that don’t develop this layer and we have seen the deleterious effect this mutation has in their growth and in their capacity to photosynthesize.”

Danila said that the mutant plants were created to identify mutants with impaired photosynthetic capacity and, ultimately, the genes involved.

The research started in 2012 at the International Rice Research Institute in the Philippines as part of the C4 Rice Project, a global initiative aiming to increase rice yields by changing its photosynthetic mechanism from C3 to C4.

Danila’s team has experimentally shown the importance of suberin in C4 photosynthesis.

She said that the difference between a C3 plant like rice and a C4 plant like the foxtail millet is that the latter has the ability to concentrate CO2 near the site of Rubisco resulting in a more efficient carbon capture.

“Because of this, C4 plants can tolerate and survive at a very low CO2 concentration environment whereas C3 plants cannot. Therefore, subjecting foxtail millet mutant plants to low CO2 concentration allowed the identification of mutant plants with impaired C4 photosynthetic capacity, manifesting as foxtail millet plants struggling to grow at low CO2 concentration.”

The plants were grown in trays inside 10 airtight custom-made chambers, which could house 900 plants per chamber capable of drawing down the carbon concentration. By combining low CO2 treatment with chlorophyll fluorescence measurement (the ability of plants to use light energy for photosynthesis), the degree of impairment of C4 photosynthetic efficiency could be measured. For the first time, the researchers were able to clearly see the anatomical differences between plants with and without suberin.

“It is predicted that successful installation of a C4 pathway into rice could result in a 30 percent to 50 percent increase in rice yield given the same amount of water, fertilizer, and sunlight,” she said. “Plants performing C4 photosynthesis are also more resilient to drought because they can tolerate low CO2 concentration, therefore they can afford to have their stomata closed to prevent water loss.”

She said that the process of adapting rice will require genetic engineering. While the experimental stage may require genetic modification, the final product may rely on gene editing, depending on technological advances. Gene editing does not require the introduction of foreign material into a plant that GM requires. The end game is a crop plant with higher yield and future food security.

“Once C4 rice is successfully made, seeds will be made available and accessible at an affordable price to smallholder farmers in developing countries,” she said.

The research paper was recently published in Communications Biology.

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