In order to ripen, fruit needs to start breaking down the rigidity of cell walls. It has not been clear how this happens. Explanations include the action of wall enzymes or hydroxyl radicals. Thurayya Z.S. Al Hinai and colleagues developed a method for examining what enzymes are active in ripening fruits. They were able to demonstrate for the first time that an enzyme, pectate lyase, is active in the walls of soft fruits, and this could contribute to fruit softening.
For many plants, the conditions to grow seeds are very different from the conditions to spread seeds. A firm fruit protects its cargo with rigid cell walls. If a plant wishes to attract something to eat the fruit and carry the seed, these cells need to change to become more digestible. This change could happen through reactions with reactive oxygen species but also with enzymes. But pulling apart how the enzymes work is difficult. People have examined them in the lab, but Al Hinai and colleagues write: “Rhe link between enzyme activities (measured in vitro after extraction of the enzymes) and fruit softening was often contradictory.”
Tomatoes are one of the most extensively studied fruits. In tomatoes, the most significant cell-wall change during ripening is a modification of pectin. The team decided to examine dates (Phoenix dactylifera), another fruit rich in pectin. The scientists developed a new method to examine what is left when the pectin is broken down. If pectate lyase was involved, it should leave a distinctive signal in the broken polymer chains of pectin.
Sure enough, when Al Hinai and colleagues looked for the signal, they found it. The pectate lyase had broken pectin in a way that radical oxygen species or other enzymes could not do.
After finding this signal in dates, the team examined other fruits, apples, pears and rowan berries. They found the same signal in these fruits, showing that the machinery to break down pectin must be fairly widespread in plants. Understanding how these fruits ripen could help develop fruits with longer shelf lives without chemical interventions.