Fast forward and rewind plant life

Annuals sprout from their seed, grow upward, form flowers and seeds, and then die. This is done in a fixed order with an approximately fixed time scale. Leiden researchers are changing that. They discovered new genetic drivers of the life cycle. By playing with those genes, they can harvest annual crops multiple times or make plants seed faster.

Molecular remote control

“We distinguish four stages of life in the aging process of plants,” says Remko Offringa, professor of plant genetics at Leiden University and leader of the research. A plant develops as an embryo in a seed and then grows into a seedling, an adult plant and finally passes to the reproductive phase.

The transition from one phase of life to the next is regulated by two genes. The rejuvenation gene AHL15 that the Leiden researchers discovered stops the transition, while the SPL aging genes actually push the life cycle forward. That competition ensures well-balanced growth and development. “A plant must be given enough time to develop at every stage,” says Offringa.

The scientists conducted most of their research in thale cress (Arabidopsis thaliana).

In the lab, researchers hacked this mechanism. By turning on the aging or rejuvenation gene, they could fast forward or rewind the four phases of plant life. This can be compared to a film of four scenes long and a remote control with two buttons. With this, scientists can not so much play the entire film fast or rewind a few seconds, but jump to the next or previous scene. Recently, the scientists published their first results on the rejuvenation gene in the scientific journal Nature Plants. Follow-up studies will also be published in scientific journals soon.

More harvest

Jumping from one life stage to another can be useful in agriculture. Annual plants such as rice and wheat flower only once, providing a one-time harvest. After that, farmers have to sow new crops. But by switching on the rejuvenation gene, the plants return to a previous phase of life. Instead of dying, the crops continue to grow and produce flowers and fruit a second time. That means farmers can harvest the same plants several times. As a result, they have to plow less, the soil life is maintained longer and the plants develop larger and more robust root systems, making them less susceptible to drought.

Unfortunately, this trick cannot be repeated endlessly, Offringa suspects. “I think two to three harvests is optimal. Then farmers have to plant new crops. ” This is mainly due to diseases. When crops are in a field for too long, pathogens build up and spread from plant to plant. Furthermore, the question is how many flowers and fruits such a crop will form in the second round. For example, there are rice varieties that naturally bloom twice. But the second harvest yields fewer seeds. Yet the researchers do not see that difference in the laboratory. “We work with thale cress there,” says Offringa. “And fruit and seed production can sometimes differ per crop.”

Stem cells

The link between the phases of life has everything to do with stem cells: cells that can continue to divide indefinitely and thus form new stems with leaves or flowers. Groups of these cells are located in the top of the plant and in the leaf axils.

At the end of their life, annual plants convert all their stem cells into flowers and seeds. This is a finite stage, after which the plant has no stem cells left to form new stems and leaves. “In order for annual plants to flower multiple times, we force the plant to retain clusters of stem cells,” says Offringa. We do that with the rejuvenating gene. Then new stems and leaves grow from the stem cells in leaf axils. In this way, the scientists allow annual plants, such as thale cress, to flower several times.

Breeders and chrysanthemum growers

The molecular remote control works the other way too. By switching on the SPL aging genes, or by suppressing the rejuvenating gene, the plant goes through all stages of life at a rapid pace. Time-saving for plant breeders who want to quickly obtain progeny of plants in order to acquire a desired trait in a plant or crop more quickly.

A disadvantage when such a plant switches quickly from one phase of life to another is that it does not have enough time to fully develop. “In our research we saw that such plants remained smaller and produced little seed,” says Offringa. Fortunately, that does not matter for breeding. Breeders only need a handful of seeds to continue growing.

The discovery may also offer a solution for chrysanthemum growers.

The discovery could also help chrysanthemum growers out of the fire. They notice that when temperatures are too high their flowering plants again produce stems with leaves instead of colorful flowers. “That is very similar to what we see when we turn on the rejuvenation gene,” says Offringa. He suspects that this gene is the culprit of the chrysanthemum problem. “Perhaps the heat activates the rejuvenation gene, causing the chrysanthemums to return to an earlier stage of life”. Here, scientists could use their molecular remote control to keep the chrysanthemums in the reproductive phase.

Making use of environmental factors

The rejuvenation and aging genes seem to have wide applications in the plant world. But the key question is how accurately the molecular remote control works in the outside world. “There are plenty of ways to turn genes on and off in the laboratory,” Offringa explains. “The problem is that we have to make adjustments to the DNA of the plant.”

Because such genetically modified plants are not allowed to grow in nature in Europe, Offringa is looking for other ways to switch the genes on and off. This could be done, for example, by using environmental factors. “Due to the chrysanthemum problem, I suspect that heat turns on the rejuvenating gene,” says Offringa. “We are now investigating whether, for example, heat and light can press the buttons of the molecular remote control.”

In a follow-up study, Offringa delves further into the rejuvenation protein. “I would like to find out how this substance works and what exactly changes in a cell when the rejuvenation gene is more or less active,” says Offringa. “That sounds very fundamental, but we need that information if we want to switch the gene on and off in the fields and forests without genetically modifying crops and trees.”

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