Seeds are the final product of plant reproduction. Whether directly as food, or indirectly as animal feed, they provide about 80 percent of human caloric consumption. In the centuries since humans first settled, we have bred countless varieties of plants with beneficial traits, such as increased yield, better quality, resistance to pests or hardiness. Where possible, farmers use hybrid varieties, which are created by crossing two inbred lines and are more resistant and higher yielding than common varieties. The problem is that these desirable traits are lost during propagation and so hybrid seeds have to be regenerated every year.

A signal from the sperm stimulates cell division of the female gamete.

If we could find a way to propagate crop plants through asexual reproduction through seeds — known as apomixis — it would revolutionize agriculture. If it were possible to bypass the reductive division and fertilization of the female gametes, the resulting seeds would be genetically identical to the mother plant. In this way plant varieties with desired characteristics can be propagated more easily — as seed clones. Now, Ueli Grossniklaus and his team at the Department of Plant and Microbial Biology at the University of Zurich (UZH) are one step closer to achieving this goal. “In the model plant Thallus cress, we have discovered the signal that triggers the female gamete to form a new seed,” says Grossenklaus.

The process of fertilization in plants consists of two events. Two sperm cells fuse with each female gamete – one sperm cell fertilizes the egg, which forms the embryo and eventually the next generation, while the other fuses with the central cell, which forms the placenta-like tissue. Produces what supplies. Fetus with nutrients. Together, they develop into mature seeds. For fertilization to be successful, the sperm cell and the female gametes must be in the same phase of the cell cycle — in other words, they must be “in sync” with each other.

Synchronization precedes gamete division.

Scientists already knew that the sperm cells in the thallus (Arabidopsis thaliana) are in the preparation stage for cell division. Grossniklaus’ team has now shown that the quiescent egg cell is also in this stage. On the other hand, the central cell is stuck in the middle of the previous phase, in which the genetic material is copied. While sperm and egg cells are in the same cell cycle phase, the central cell must complete DNA synthesis after fertilization before dividing first.

This interruption in the cell cycle is caused by a protein in the central cell that has not been completely degraded and thus is still present. When the sperm fertilizes this gamete, it introduces the protein cyclin, which then triggers the breakdown of the inhibited protein. Only then can the central cell complete DNA synthesis and proceed to the next phase of the cell cycle. “For the first time, we have been able to detect the molecular mechanism of how the signal from the sperm to the female gamete is transmitted out of its quiescent state. This signals the central cell that fertilization has been successful and That cell division can now happen,” says first author Sarah Simonini.

Asexual reproduction for crops

If watercress is genetically modified so that the central cells produce the protein cyclin themselves, they will begin to divide even without fertilization. “We can now deliberately trigger this activation in the absence of fertilization. This allows us to introduce apomixis into crop plants, especially hybrid varieties that are more resilient and yield more than conventional varieties. Opportunities open up,” says Grossenklaus. If Apoximes can be used in crop plants, millions of small-scale farmers in the Global South will be able for the first time to grow hybrid varieties whose seeds can be saved for the next planting.