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Biological Chemistry

Engineered Bacteria Ripen Fruit By Belching Ethylene

Bioengineering: Researchers program E. coli to produce ethylene, a gas commonly used by the food industry to ripen produce

by Erika Gebel Berg
November 25, 2014

BIG RED
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Credit: Laboratory of Cristina Del Bianco
Both pairs of cherry tomatoes sat in jars attached to flasks with cultures of engineered Escherichia coli. The fruits on the right (Eth 2) were connected to cultures producing ethylene, allowing the tomatoes to ripen. Those on the left (Ctrl 2.1) had cultures that didn’t produce the gas, so the fruits remained green.
Photographs of fruit ripened in a lab.
Credit: Laboratory of Cristina Del Bianco
Both pairs of cherry tomatoes sat in jars attached to flasks with cultures of engineered Escherichia coli. The fruits on the right (Eth 2) were connected to cultures producing ethylene, allowing the tomatoes to ripen. Those on the left (Ctrl 2.1) had cultures that didn’t produce the gas, so the fruits remained green.

The ruby red rows of tomatoes at the local grocery store don’t come off the vine in such a pretty state. Food producers pick fruits while unripe and later douse them with ethylene, a gas that plants naturally produce to trigger ripening. The ethylene used by food producers comes from cracking fossil fuels. As a green alternative, Cristina Del Bianco of the University of Trento, in Italy, and her team engineered Escherichia coli to produce ethylene to accelerate fruit ripening (ACS Synth. Biol. 2014, DOI: 10.1021/sb5000077).

JUICY FRUITS
[+]Enlarge
Credit: Laboratory of Cristina Del Bianco
Researchers grew cultures of ethylene-producing bacteria in flasks connected to jars containing unripe fruits such as cherry tomatoes, pears, apples, and bananas. The ethylene helped ripen the fruit.
Photograph of fruit ripening in a lab.
Credit: Laboratory of Cristina Del Bianco
Researchers grew cultures of ethylene-producing bacteria in flasks connected to jars containing unripe fruits such as cherry tomatoes, pears, apples, and bananas. The ethylene helped ripen the fruit.

To program E. coli to make the gas, the scientists turned to another microbe called Pseudomonas syringae. This plant pathogen has an enzyme that converts 2-oxoglutarate, a citric acid cycle intermediate, to ethylene in a single step. The researchers inserted the gene into E. coli so that they could turn it on in the presence of the sugar arabinose. When they added arabinose to liquid cultures of the bacteria, ethylene levels in the flasks reached 100 ppm.

Next, the researchers grew the bacteria in flasks connected to jars filled with unripe cherry tomatoes, kiwis, or apples. After eight days, the fruits connected to flasks that received a dose of arabinose were significantly riper than those connected to bacterial cultures that didn’t get the sugar. The tomatoes were redder, the kiwis were softer, and the apples had less starch—a signature of ripening.

To make ethylene production easier for commercial applications, the team reengineered the bacteria so that they expressed the ethylene gene when exposed to blue light. The researchers detected 92 ppm of ethylene in a culture of these bacteria grown in blue light, whereas no ethylene was detected from bacteria grown in the dark.

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