Asexual propagation of cultivated plants gets closer thanks to new study

When the female gametes of plants are fertilized, a signal from the sperm activates cell division, leading to the formation of new plant seeds. This activation can also be triggered deliberately without fertilization, as the UZH researchers have shown. Their findings, published in Scienceopen new avenues for the asexual propagation of cultivated plants.

Seeds are the end product of plant reproduction. Whether directly in food form or indirectly in the form of animal feed, they provide approximately 80% of human caloric consumption. Over the millennia since human settlement, we have bred countless varieties of plants with advantageous traits, such as increased yield, improved quality, pest resistance, or hardiness.

Where possible, farmers use hybrid varieties, created by crossing two inbred lines, which are more resistant and more productive than normal varieties. The problem is that these desired properties are lost during propagation and therefore the hybrid seeds must be recreated every year.

Sperm-derived signal activates female gamete cell division

If we could find a way to propagate crop plants through asexual reproduction using seeds – known as apomixis – it would revolutionize agriculture. If it were possible to bypass the reductional division and fertilization of female gametes, the seeds produced would be genetically identical to the parent plant. Plant varieties with the desired characteristics could thus be multiplied much more easily, in the form of seed clones.

Today, Ueli Grossniklaus and his team at the Department of Plant and Microbial Biology at the University of Zurich (UZH) have taken another step towards this goal. “In the model plant Thale Cress, we discovered the signal that activates the female gamete to form a new seed,” explains Grossniklaus.

The plant fertilization process involves two events. Two sperm fuse with a female gamete each: one sperm fertilizes the egg, from which the embryo and ultimately the next generation is formed, while the other fuses with the central cell, which develops into a tissue similar to a placenta which supplies the embryo with nutrients. Together they develop into mature seeds. For fertilization to be successful, the female sperm and 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 of Thale watercress (Arabidopsis thaliana) are in the preparatory phase for cell division. Grossniklaus’ team has now shown that the resting egg is also in this phase. The central cell, on the other hand, gets stuck in the middle of the previous phase, during which the genetic material duplicates itself. While sperm and eggs are in the same phase of the cell cycle, the central cell must first complete DNA synthesis after fertilization before the first division can begin.

This interruption of the cell cycle is caused by a protein in the central cell that is not completely degraded and is therefore still present. When the sperm fertilizes this gamete, it introduces the protein cyclin, which then activates the breakdown of the inhibitor protein. Only then can the central cell complete DNA synthesis and move on to the next phase of the cell cycle.

“For the first time, we have managed to understand the molecular mechanism by which the signal is transmitted from the sperm to the female gamete in order to bring it out of its resting state. It signals to the central cell that fertilization has been successful and this division cellular can now take place,” says first author Sara Simonini.

Asexual reproduction for crops

If the watercress were to be genetically modified so that the central cells produced the protein cyclin themselves, they would begin to divide even without fertilization. “We can now deliberately trigger this activation in the absence of fertilization. This opens the possibility of introducing apomixis into crop plants, particularly in hybrid varieties which are more resistant and produce higher yields than normal varieties “, explains Grossniklaus.

If apomixis could be exploited in cultivated plants, millions of small farmers in the South could for the first time cultivate hybrid varieties whose seeds could be saved for future sowing.