Scarecrows and Wreaths: Genetic Secrets of Efficient Food Crops
• Ancient plants, like rice, wheat and barley, originating in the Mesozoic and Paleozoic eras, still form 95% of the Earth’s plant biomass. They use an enzyme known as RuBisCo (the most abundant protein on the planet!) to fix atmospheric carbon dioxide on to a 5-carbon sugar (ribulose bis-phosphate) to make 2 molecules of a 3-carbon sugar that eventually becomes sucrose. This is the C3 pathway, but it’s not too efficient: the enzyme RuBisCo also catalyzes a competing reaction called “photorespiration” that adds oxygen to the 5-carbon sugar making a byproduct that takes many tedious and expensive steps to convert back to the useful sugar. These plants can also lose 97% of the water absorbed by the roots through stomata or pores on the underside of the leaves. If they close their stomata, they limit the diffusion of CO2 into leaves, so they have limited growth in hot, dry areas.
• Fortunately, in the last 6-7 million years, another group of plants (sugarcane, maize, grasses) began to flourish that bypassed this problem. They evolved from the C3 plants independently, more than 60 times- a spectacular example of convergent evolution. In these plants, a different enzyme is used to fix CO2 to make a 4-carbon sugar in the leaf cells, that is then shuttled into special wreath-like layer around the veins, known as Kranz sheath (German for wreath). Kranz cells release CO2 from this intermediate, insulating and concentrating it around the Rubisco enzyme so that the wasteful side reaction does not occur. This highly effective C4 pathway boosts productivity by 50%. Even though C4 plants make up only 3% of plant species, they account for 30% of all carbon fixation on land.
• How does one coax C3 plants to follow C4 pathways and boost food production in hot, dry areas, while removing more CO2 from the atmosphere? C3 plants have all the enzymes needed, but lack the specialized anatomy of the wreaths and the tight spacing between veins. It was assumed that engineering Kranz anatomy would be exceptionally difficult. In a breakthrough study, scientists noted common features of the Kranz sheath with root and stem bundles, suggesting a common developmental pathway. Working on a hunch, they showed that a gene called Scarecrow, regulates the special anatomy in both roots and leaves. “Recapitulating the evolution of C4 structure in C3 plants is likely to be a much more manageable goal if the underlying regulatory components are already in place in roots and stems”.
Image: Kranz anatomy in French Millet, a C4 plant. Note the bundle sheath, packed with green chloroplasts, around the central vein, and the tight spacing of less than 4 cells between the bundles. http://goo.gl/J004P
Paper: Scarecrow plays a role in establishing Kranz anatomy in maize leaves. Slewinsky, T.L., et al. Plant Cell Physiol. 2012 Dec;53:2030-7. doi: 10.1093/pcp/pcs147.
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