Eye-plant coordination determined by the eye

Plants are sun worshippers and shade-avoidant. As soon as a leaf 'sees' shade, it 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 reaction 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."

Fundamental knowledge lacking

When the researchers wanted to expand their simulation model with the effects of shade, they discovered that they lacked fundamental knowledge about how plants observe light. Scientists knew that plants react 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 the plant observes and processes the light colours, exactly", according to Pierik. "Our research shows that the plant observes the colours everywhere, but that the reaction 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. Both reactions are also possible: the petiole can grow a bit, while the leaf moves a bit upwards. This means that the 'eye' determines how the plant reacts, which in turn leads to new questions. Why are there different reactions depending on where the change in colour is observed? And how does a plant ensure that the change in colour provokes a reaction elsewhere in the plant?

In order to explain the differences in the reactions, the researchers developed a model that they can test using simulations or real plants. The far-red alarm signal in the petiole appeared to cause unnecessary leaf movement at low plant densities, which result in the leaf capturing less light, while at high densities the reaction 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 the '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 moving upwards 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 reactions.

"PhD Candidate Franca Bongers processed all of these insights into her simulation models. That showed that this is indeed the best way for plants to react as effectively as possible to neighbouring plants at high plant densities", according to Pierik. Bongers will defend her dissertation on Tuesday, 4 July in Wageningen.

More information:
Chrysoula K. Pantazopoulou el al., "Neighbor detection at the leaf tip adaptively regulates upward leaf movement through spatial auxin dynamics," PNAS (2017). www.pnas.org/cgi/doi/10.1073/pnas.1702275114

Provided by Utrecht University Faculty of Science