Thidiazuron delays postharvest kiwifruit softening with temporal shifts in ethylene, starch and cell-wall programmes

Fruit ripening is a biological countdown that begins the moment a fruit is picked. For the kiwifruit, this countdown is notoriously fast. Once harvested, the fruit undergoes a rapid transformation where its hard flesh turns soft, its starch turns to sugar, and its tart acidity fades. This process is driven by a natural plant hormone called ethylene, which acts as a chemical signal telling the fruit to mature. While this softening makes the fruit sweet and ready to eat, it also makes it fragile and short-lived, creating a major challenge for farmers and grovers who need to transport and store the fruit without it turning to mush. Scientists have long known that if they can slow down this chemical signaling or the structural changes in the fruit's cell walls, they can extend the fruit's shelf life, but the precise timing of how different treatments affect these internal changes has remained a mystery.

Researchers at Nanchang Normal University and the Jiangxi Academy of Sciences set out to solve this puzzle by testing a synthetic plant regulator called thidiazuron, or TDZ, on fresh kiwifruit. They treated the fruit with a specific concentration of this chemical and then watched how it changed over time compared to untreated fruit. The goal was not just to see if the fruit stayed firm, but to understand the exact moment when the chemical treatment began to alter the fruit's internal instructions. By measuring the fruit's firmness, acidity, and chemical composition alongside a detailed reading of its genetic activity, the team discovered that the treatment did not simply stop the ripening process; instead, it pushed the entire timeline of ripening events further into the future, effectively buying the fruit more time before it became soft.

The experiment began with a simple physical test. The researchers dipped harvested kiwifruit into a solution containing 600 milligrams of TDZ per liter of water, while a control group was dipped in plain water. They stored both groups at a constant room temperature and checked them at regular intervals over twenty days. The results were striking. After twenty days, the untreated fruit had become so soft that it was barely firm, measuring only 3.78 newtons of force, which is the point where the fruit is considered overripe and ready to eat immediately. In contrast, the treated fruit remained firm at 27.72 newtons, a level of hardness that would allow it to be stored and shipped for much longer. Beyond just feeling hard to the touch, the treated fruit also held onto its tart flavor, its starch content, and the structural components of its cell walls, which are the microscopic building blocks that give the fruit its shape.

To understand why this happened, the scientists looked inside the fruit at the level of its genes. They collected samples from the fruit at different times and sequenced the RNA, which is the molecule that carries instructions from the DNA to the rest of the cell. This allowed them to see which genes were turned on or off at each stage of the ripening process. They found that the treated fruit and the untreated fruit started to diverge in their genetic instructions as early as four to eight days after harvest, but the differences became very clear between twelve and twenty days. The untreated fruit followed a predictable path where genes responsible for breaking down starch and softening the cell walls were switched on aggressively. The treated fruit, however, kept these genes switched off for much longer.

The study identified twelve specific genes that played a central role in this delay. These genes fell into three main categories: those that control the production of ethylene, those that manage the breakdown of starch and sugar, and those that remodel the cell wall. In the untreated fruit, the genes that produce ethylene and break down starch were highly active during the critical window of twelve to twenty days. In the treated fruit, these same genes were significantly less active. For example, the genes responsible for making the enzyme that breaks down pectin—a substance that acts like glue holding the cell walls together—were kept in a low state. This meant the cell walls remained intact, and the fruit stayed firm. Interestingly, the researchers also found that some genes related to the fruit's internal structure actually increased in activity later in the storage period, suggesting that the treatment did not freeze the fruit in time but rather shifted the entire schedule of events.

The researchers confirmed that their findings were not unique to this specific chemical by comparing their results with data from other studies involving ethylene and a substance that blocks ethylene. They found that the same twelve genes responded in a similar way when the fruit was exposed to these different treatments, reinforcing the idea that these genes are the key switches for the ripening process. The study suggests that thidiazuron works by delaying the activation of these switches, creating a "temporal displacement" where the fruit's ripening program is pushed back. This delay allows the fruit to retain its firmness, acidity, and starch for a longer period, effectively extending its commercial life.

This research provides a clear map of how a chemical treatment can alter the biological clock of a fruit. By pinpointing the exact genes and the specific times when they change, the scientists have moved beyond simply observing that the fruit stays firm. They have shown that the treatment creates a coordinated delay across the fruit's entire system, from the production of ripening hormones to the breakdown of its internal structure. This understanding offers a new way to think about postharvest management, suggesting that the key to preserving fruit quality lies in managing the timing of these genetic instructions rather than just trying to stop them entirely. The findings point to a future where fruit can be stored for longer periods without losing the texture and flavor that consumers expect, simply by shifting the timing of its natural ripening cycle.