by Alice Ren
|Fruit Basket by Giuseppe Arcimboldo, 1590|
Fruits play a fundamental role in diets of all cultures, and are renowned “superfoods” that improve general health due to the vast amount of vitamins, fibre and antioxidants contained. With all the benefits that can be reaped by regularly consuming fruits, it is surprising that few people ask the question of how exactly they are made and how they are formed.
Fruit development can be divided into 3 main stages. The first stage is ovarian development and the decision to abort the seed or to proceed with further cell division, the second stage is primarily the cell division phase of fruit growth, and the third final stage is further fruit development via both cell division and cell expansion, until the fruit reaches its terminal size. These stages are only a rough outline in practise, as different fruits show different variations of the sequence.
Let’s take a look at the first phase. Although the mechanism of ovary development is not known on a molecular level, it is now well established that the synthesis and action of auxin, cytokinin and gibberellins are required (though their specific functions are unknown). The choice between abortion or continuation of the seed is decided on the basis of successful pollination and fertilisation. After pollination, fertilisation requires the pollen to be germinated as well as the penetration and growth of the pollen tube in the stylar tissue into the embryo sac to fuse with the egg cell. The presence of these fertilised ovules is what triggers the development of the ovary into a fruit. Without successful pollination, the fruit could be undersized, or the ovary could be aborted altogether; most fruit primordia that are not pollinated and fertilised cease cell division, and the flower abscises (falls off the plant).
In the second stage, cell division is activated in the ovary and continues for around 7-10 days. It is known that the rate of cell division is determined by the developing embryo/seed. That is, the number of fertilised ovules has been observed to determine the mitosis rate; if ovules do not develop seeds in only a section of the fruit, the fruit will grow to become lopsided. After the cell division phase ends, each individual cell in the fruit enlarges which also increases the size of the fruit itself. Prior to the cell enlargement, the cells in the fruit are small, tightly compressed and contain mostly cytoplasmic elements with smaller vacuoles, but after enlargement, the cell wall and cytoplasm become significantly thinner, and the vacuoles become the largest organelles.
In the final stage, the cell volumes increase more than tenfold. Auxins are responsible for this cell expansion by causing an increase in the extensibility of the cell walls and inducing both the uptake and retention of water and solutes. However, there are alternative arguments to explain this process. It is equally possible that signal molecules other than auxins could be used to regulate expansion, or a certain amount of seed-produced auxin could be used to sustain the cell wall extension in the fruit tissues, but this cannot be investigated as the auxin would be used up too quickly to detect. The cell wall extension and uptake of solution also cannot account for the huge increase in volume alone; it has been concluded that there must be synthesis of new cell wall material involved too. This idea is consistent with proof that the gene pTPRPF1 is expressed, which codes for a proline-rich cell wall protein used in cell wall synthesis.