April 16, 2013

Scarecrow Gene Helps Plants in Dry Climates

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Worldwide hunger may become a problem as the population continues to grow, yet a solution might lie within plants. In the lab of Prof. Robert Turgeon, plant biology,  postdoctoral research associate Thomas Slewinski found a gene that is currently repressed in most plant leaves that has the potential to grow crops 50 percent more efficiently in dry climates. This research was part of a collaborative effort led by Prof. Timothy Nelson, Yale University.

Previously, this gene, dubbed the “Scarecrow gene,” has been studied in the stems and roots of plants. Slewinski, however, discovered that the gene is also found in the leaves of certain plants.

Plants use photosynthesis to obtain energy through light, but, according to Slewinski, not all plants use the same pathway to do this. The C3 pathway is the most common photosynthetic pathway in plants and is found in important crops such as rice. The C4 photosynthetic pathway is used in corn, sorghum and millet, which are grown in arid climates.

Plants that use the C4 synthetic pathway use water, nitrogen, and sunlight 50 percent more effectively in areas where water is more scarce than those that use the C3 pathway.

According to Nelson, the C4 pathway is more effective in dry climates because it separates the processes of photosynthesis into two locations within the plant: bundle sheath cells and mesophyll cells.

Bundle sheath cells, not found in C3 plants, are cells that surround a leaf’s vascular core where nutrients are transported. Mesophyll cells form around the bundle sheath. The Scarecrow gene plays a role in this type of separate anatomy used by C4 plants called Kranz anatomy.

Because the C4 pathway splits a step of the C3 pathway into two locations, it is less energy efficient in areas where water is abundant. In dry climates, however, C4 is better than C3 because the enzyme RuBisCO is relocated to the bundle sheath cells, separating itself from the other half of the pathway in the mesophyll cells.

According to Slewinski, RuBisCO is a central enzyme to both C3 and C4 photosynthesis pathways because it is involved in the uptake of carbon dioxide. However, RuBisCo can also cause energy loss when exposed to oxygen. Unlike in mesophyll cells, oxygen levels are kept low to prevent this type of energy loss in bundle sheath cells, Slewinski said.

“C4 has evolved independently 66 times,” Slewinski said. This fact led Slewinski to the hypothesis that the gene existed in every plant, but was being repressed.

By breeding plants with a mutated Scarecrow gene with plants with no mutations in the Scarecrow gene, Alyssa Anderson ’13, an undergraduate researcher in the Turgeon lab, determined the effect of the scarecrow gene.

While both C3 and C4 plants with a mutated Scarecrow gene had impairment in their stems and roots, as was expected, the Kranz anatomy of the C4 leaf samples was also damaged.

According to Slewinski, the leaves of C3 plants with the mutated gene looked normal. This supports the hypothesis that the Scarecrow gene is repressed in C3 leaves but has an effect on Kranz anatomy in plants with C4 photosynthesis.

“Evolution works in a very simple way: by using things it already has in new ways. In this case, C4 leaves took basic housekeeping genes that are used in lower parts of the plant,”  Slewinski said.

The energy loss wasted in C3 photosynthesis is a difficult biological problem, but, according to Turgeon, Slewinski’s work gives scientists a clearer direction toward understanding the development of Kranz anatomy.

The lab’s future studies will include identifying what activates or deactivates the Scarecrow gene.

Original Author: Samantha Klasfeld