A green banana changes in several ways before it becomes yellow and sweet. Its starch breaks down into sugar, and the green material in its skin slowly disappears. A gas called ethylene controls much of this process. In plants, this gas also works as a hormone.
Ethylene is the reason a paper bag can help fruit ripen faster. When a banana is placed inside, the gas becomes trapped in the bag. It then encourages the banana and any other fruit nearby to ripen.
Once the process begins, ethylene tells the banana to make even more ethylene. Scientists call this a positive feedback loop. A recent study looked at two proteins that help manage the loop. These proteins can turn certain genes on or off.
One protein, called MaNAC029, was already known to help the banana produce ethylene. Another, MaNAC19, helps produce sucrose, the sugar that gives a ripe banana much of its sweetness.
Researchers found that the two proteins do more than separate jobs. Both become most active around the time when ethylene production rises sharply. Each protein can also affect genes linked to both ethylene and sugar production.
The proteins can even join together. When they do, their effect is stronger than when either one works alone. As more ethylene is produced, the two proteins become more active. They then help the banana make still more ethylene and sugar.
Why does the banana need two proteins that can perform similar jobs? Ripening can be disturbed by extreme temperatures and other conditions. The researchers think this shared system may make the process steadier and more efficient.
So the change from a firm, green banana to a soft, yellow one is not a simple reaction. It is a carefully connected process involving gas, sugar, genes, and two proteins working as a team.