Eye-plant coordination determined by the eye

Publication in PNAS

Plants are sun worshippers and shade-avoidant. As soon as a leaf ‘sees’ shade, it either points its tip up, has the petiole grow further towards the light, or does a bit of both. When a team of researchers from Utrecht and Wageningen wanted to develop computer simulations of plant growth, they were surprised to learn that the coordination between the plant’s ‘eye’ and its response to shade was not well understood. Since then, plant scientists at Utrecht University have deciphered the shade reaction step by step. Their joint results were published in the scientific journal PNAS on 26 June.

“With the dramatic growth of the world’s population, we will have to work hard to make agriculture more efficient”, explains research leader Prof. Ronald Pierik from Utrecht University. “We therefore want to know how we can help plants achieve maximum growth at high densities; conditions under which plants compete with one another for light.  Thanks to the combination of our molecular plant research and the simulation models at Wageningen, we can predict what will work, and what won’t.”

Eye-plant coordination determined by the eye

Fundamental knowledge lacking

When the researchers wanted to expand their simulation model to include the effects of shade, they discovered that they lacked fundamental knowledge about how plants observe light. Scientists knew that plants respond to shade by observing the ratio of red light to far-red light. Red light is essential for photosynthesis, but more far-red light means that the plant is in the shade. "What we didn’t know, however, was where exactly the plant observes and processes the light colours", Pierik says. "Our research shows that the plant observes the colours everywhere, but that its response can differ significantly."

The ‘eye’ has it

More far-red light at the tip of the leaf makes the leaf move up, while at the petiole it results in faster elongation growth for the petiole itself. It is also possible for both reactions to occur: the petiole can grow a bit, while the leaf moves slightly upwards. This means that the ‘eye’ determines how the plant responds, which in turn leads to new questions. Why are there different responses depending on where the change in colour is observed? And how does a plant ensure that the change in colour provokes a response elsewhere in the plant?

Explanatory model

In order to explain the differences in the responses, the researchers developed a model that they tested using simulations as well as real plants. The far-red alarm signal in the petiole appeared to cause unnecessary leaf movement at low plant densities, which resulted in the leaf capturing less light, while at high densities the response came too late to avoid the shade. Far-red information at the leaf tip, on the other hand, appeared to predict the vicinity of nearby plants at all plant densities. This means that the leaf tip is the optimal location for ‘eye-leaf coordination’ if the leaf needs to move elsewhere.

Crucial role for auxin

Next, the researchers asked how the observation of colour changes leads to the leaf’s upward movement or to the growth of the petiole. Their research confirms the suspicion that the hormone auxin plays a crucial role in this process. For example, excess far-red light on the tip of the leaf leads to higher production of the hormone in the leaf tip. The auxin then travels through the plant to initiate the necessary responses.  

As effective as possible

"PhD candidate Franca Bongers incorporated all of these insights into her simulation models, showing that this is indeed the best way for plants to respond as effectively as possible to neighbouring plants at high plant densities", Pierik explains. Bongers will defend her dissertation on Tuesday, 4 July in Wageningen.

Photo and simulation

Figure of the auxin reaction in plants in different circumstances: top left: no changes in the light; top right, white arrow: far-red light on the tip of the leaf; bottom left: far-red light on the whole plant; bottom right, white arrow: the effect of external administration of auxin to the tip of the leaf.

Video: Simulation of two extreme responses to shade by a plant. The green plants use the far-red signal on the tip of their leaves to orient the leaves. The red plants use the far-red signal at the leaf petiole to make the leaf move up.

 

This research was funded in part by the Netherlands Organisation for Scientific Research (NWO): Open Competition Grant to Prof. Niels Anten (Wageningen University), Graduate School Horticulture and Starting Materials Grant to Prof. Ronald Pierik & Jesse Küpers MSc and VIDI Grant to Prof. Ronald Pierik (both Utrecht University).​

Publication

‘Neighbor detection at the leaf tip adaptively regulates upward leaf movement through spatial auxin dynamics’
Chrysoula Pantazopoulou1, Franca Bongers1,2, Jesse Küpers1, Emilie Reinen1, Debatosh Das1, Jochem Evers2, Niels Anten2, Ronald Pierik1
1 Affiliated with Utrecht University, 2 Affiliated with Wageningen University
PNAS, 26 June 2017

Background information

Future Food Utrecht

This research is part of the Future Food Utrecht programme, a multi-disciplinary programme in which 25 research groups collaborate to solve social challenges in the field of food safety and security.

Contact

Monica van der Garde, Public Information Officer, Faculty of Science, m.vandergarde@uu.nl, +31 (0)6 13 66 14 38.

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