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?
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