Researchers discover new shadow avoidance mechanism

Plants close together do everything they can to intercept light. This “shadow avoidance” response has been extensively researched. It is therefore even more remarkable that researchers from the Molecular Biology Laboratory at Wageningen University have discovered another entirely new mechanism: the important role of the hormone cytokinin.

Their research was published in Natural communications.

Plants in the wild, in the field or in a greenhouse compete with each other for light, moisture and nutrients. The more densely they are planted, the tougher the competition. But how do they know there are a few people there?

“In densely planted crops, red light is absorbed more quickly than far-red light, which is reflected instead. The red/far-red ratio therefore decreases with greater density. Plants ‘see’ this through the pigment phytochrome sensitive to light,” says Ronald Pierik, professor of molecular biology at Wageningen University and Research.

Pigment is like a switch: it can be on or off. The red/far-red ratio flips the switch, so to speak. This triggers a whole series of responses.

Pierik explains: “With relatively high levels of far-red light, as is the case in densely planted crops, the stems elongate, as do the petioles. The leaves themselves move from a horizontal position to a more vertical position. their neighbors and intercept more light.

Overhead and underground competition

We already know a lot about this response and the mechanisms that drive it.

“How plants process light information is important for our crops. Because we always plant them close to each other. The question is how far we can go,” explains Pierik.

However, plants not only compete for light, but also for nutrients, for example.

“So you have to consider shadow avoidance in conjunction with other responses to competition. You would then get much closer to the situation on the ground. This line of thinking led our postdoctoral researcher Pierre Gautrat, who started this work in our former group at Utrecht University, with the idea of ​​jointly examining above-ground and below-ground competition. One of the research questions was whether the plant even if it does not receive. lots of nutrition in the form of nitrogen, can still respond well to far-red light,” he says.

Cytokinin serves as a messenger

To do this, growing tissues must know the amount of nitrogen available in the soil. They know this because a message travels from the roots to the growing points. In this case, the messenger is the plant hormone cytokinin. This hormone is formed in the roots and passes through the veins to the aerial part of the plant. If there is a lot of nitrogen, there will also be a lot of cytokinin.

“In fact, the shade avoidance response appears to be inhibited when nitrogen is low. However, we have demonstrated that it is possible to trick the plant. If you give it a cytokinin supplement, when the nitrogen is low, you still get substantial length growth with supplemental far red light. This is the first time anyone has shown that cytokinin plays a role in shade avoidance. a new mechanism, because these processes have already been studied very intensively,” explains Pierik.

And it gets even more remarkable: until now, cytokinin was known to be the very hormone that inhibits growth in length. “In hindsight, all of the trials on which this conclusion is based involved plants grown in the dark. You only get this answer when you grow them in the light. And not with ordinary white light, but only with excess far-red light. “, he explains.

The inhibitor inhibited

Researchers have also studied how this mechanism works at the genetic level.

“There are specific proteins that inhibit the sensitivity of plants to cytokinin. The genes encoding these proteins are themselves inhibited when exposed to far-red light. In other words, the inhibitor is inhibited. And this is precisely what stimulates sensitivity. These are also very new proteins,” says Pierik.

The architecture of crops can be very important.

“We learned this from the Green Revolution. This resulted in much higher yields because agronomists began growing varieties of rice and wheat that put less energy into growing in length and more into growing the grains “Such new knowledge can help agronomists and producers achieve better production of crops like barley, wheat, corn and rice,” he concludes.