A Biologist Considers Apples and Oranges, Rice and Rubber

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There is a scientific basis to the old adage “one rotten apple spoils the whole bunch,” says Dartmouth plant biologist Eric Schaller.

“As an apple ripens and rots, it releases the gaseous plant hormone ethylene,” says Schaller, a professor in the Department of Biological Sciences. “We know that ethylene stimulates the ripening process and it can accelerate the ripening and eventual decay of the others around it.”

Eric Schaller

Dartmouth plant biologist Eric Schaller examines experimental rice plants growing in the greenhouse atop the Class of 1978 Life Sciences Center. (Photo by Eli Burakian ’00)

Schaller works with plant hormones, naturally produced chemicals that regulate plant growth. He studies the molecular mechanisms by which a plant recognizes a hormone and then responds to it. Ethylene is an important natural plant hormone used in agriculture to force the ripening of fruit, but it can also be a problem for growers who are storing fruits or vegetables.

“If you don’t want them to over-ripen and decay, you store them in silos under special atmospheric conditions to reduce the rate of ripening,” Schaller says. “Ethylene scrubbers remove the gas that is being made by the fruits or vegetables.”

He also points to the plastic “green bags,” sold to retard spoilage at home. “They have a compound built into them that binds ethylene so that its content doesn’t increase in the bag and your vegetables stay crisp longer.”

On the other hand, ethylene can be used to induce ripening when that is the goal. “This goes back to biblical times, with people doing work unknowingly to affect the ethylene levels in plants,” says Schaller, referring to the practice of “fig-gashing.” When you wound a plant, he explains, it releases ethylene that stimulates ripening.

In another application, all the oranges you buy have been treated with ethylene, notes Schaller. “Ethylene is used not because it induces ripening, because it doesn’t affect ripening of citrus fruit, but it does affect the skin color of the citrus fruit. It produces a more uniform ‘orange-colored’ skin for the fruit,” he says, using cosmetics to improve agricultural marketing.

Schaller says that in the collection of sap (latex) from rubber trees, the sap will flow longer in the presence of ethylene. An ethylene-releasing compound may be applied where the tree is tapped. Schaller muses half-seriously that this might possibly work in the sap harvesting of New England’s maple trees.

Schaller conducts research under the hallmark of basic science, but enjoys it more when there are direct applications. “Personally, I need to see where things fit into a bigger picture,” he says. ”Otherwise I would get bored. I like to see that it does have some impact down the line.”

In Schaller’s case, his work is directed at the improvement of agricultural products upon which we depend: fruits, vegetables, and most recently grains.

Schaller is growing rice in the greenhouse atop the Class of 1978 Life Sciences Center. This is where the other hormone he studies—cytokinin—comes into play. He regards cytokinin as having a role opposite to that of ethylene.

“Whereas ethylene can be considered an aging hormone, cytokinin is more of an anti-aging hormone,” he says. “If you have higher cytokinin levels, leaves will stay greener longer and fruit and vegetable growth will be stimulated.”

In the case of rice, cytokinin seems to play a role in regulating the expression of genes that affect grain yield. Higher levels of the hormone result in increased yield—more grains per plant. “We are looking carefully at cytokinin and how it is regulating grain development and yield in rice. We expect that a better understanding of cytokinin activity could lead to improved productivity.”

Schaller is drawn to the study of rice not only because is it a crop plant, but also because its relatively simple genome makes it easy to work with.

While rice does not constitute a primary staple for many Americans, what we learn from rice can be applied to other crops, such as wheat, which is a more prominent component of our diet.