How to grow a flat leaf?

Project Information

Flat_Leaf

Grant agreement ID: 796443

DOI

10.3030/796443

Project closed

EC signature date 3 April 2018

Start date 1 August 2019

End date 31 July 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 175 419,60

EU contribution

€ 175 419,60

Coordinated by

UNIVERSITE DE LAUSANNE
Switzerland

Periodic Reporting for period 1 - Flat_Leaf (How to grow a flat leaf?)

Reporting period: 2019-08-01 to 2021-07-31

Summary of the context and overall objectives of the project

Photosynthetic organisms, including plants, transform light from the Sun into organic materials, capturing CO2 from the atmosphere. As such, plants are key in the fight against global warming. Moreover, biomass produced by photosynthesis is the base of all trophic chains, providing us with food and multiple materials.
In most plant species photosynthesis occurs predominantly in the leaves, which capture light and exchange CO2 and oxygen with the atmosphere. One conserved feature of leaves is their flat shape. This shape allows them to efficiently capture light, minimizing weight, and optimizes carbon and water flux for photosynthesis. However, under certain environmental conditions, flat growth is not optimal. For instance, under high irradiance conditions reducing light interception is beneficial to prevent photoinhibition. Concomitantly, leaf flattening depends on the light environment. The red light receptors phytochromes (PHY) promote leaf curling when they are active, and promote leaf flattening when they are inactive, for example when light is scarce due to shading by other plants. On the contrary, the blue light receptors phototropins (PHOT) are required to grow flat leaves. The general aim of this project is to understand when, where and how the developing leaf perceives environmental signals, and how photoreceptors affect cell differentiation and tissue patterning in order to control flattening.
Our results showed that plants monitor light direction throughout their development, and in turn control their curvature, which could improve photosynthesis in natural light environments. The blue light receptors phototropins (PHOT) perceive light direction and regulate leaf shape accordingly. We identified molecular mechanisms used by PHOT to achieve this, which ultimately control plant hormones and cell growth.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To achieve our objectives, we started by performing various light treatments on the model plant Arabidopsis thaliana and measuring their impact on leaf shape. Among light receptors, the blue light receptors PHOT have a strong impact on leaf flattening, shown by the downwards curled leaves in plants lacking these receptors (phot1phot2 mutants). So, we focused on these receptors, which are well known for allowing plant stems to perceive light direction and bend towards the light. Based on this previous knowledge we asked whether PHOT can also perceive light direction in leaves and regulate their curvature. Illuminating the upper and lower parts of the leaves of wild type, mutant and transgenic plants, we found that PHOT perceive light direction in the leaf. When light reaches the leaf from the top, as it usually does in rosette plants like Arabidopsis, this signal instructs the plant to grow flat leaves. On the contrary, when light reaches the plant from below or from all directions, PHOT perceive this signal and leaves curl in a way that probably allows them to intercept light more efficiently. Using the same amount of light in our incubators, we could grow bigger plants when light reached the leaves from the top and from below, using aluminum foil to reflect light instead of being absorbed by the dark soil.
PHOT could regulate leaf shape throughout leaf development, before the leaf reached its final size, controlling cell expansion. This is similar to what happens in the stem when the plant bends towards the light. Epidermal cells on the lit side grow less than those in the non-irradiated side, causing a growth imbalance and the consequent shape of the organ. To achieve this, PHOT use similar molecular factors in the leaf and in the stem. Members of the NRL and PKS families of proteins are required to respond to light. However, their role is not exactly the same in leaves and stems. In the case of the PKS family, some members are important for stem phototropism, and others (PKS2 and PKS3) are more important for leaf flattening. Interestingly, studying the NRL family we found that the PHOT signaling network is different in each side of the leaf. This discovery will allow us to identify new proteins involved in light signaling.
Finally, we asked how PHOT and their signaling networks affected cell growth. One well studied mechanism controlling cell growth is the one involving the plant growth hormone auxin. In the stem, auxin accumulates in the shaded part, promoting growth. So, we used fluorescent auxin reporters to study auxin signaling in the upper and lower parts of the leaf, in response to light signals perceived by PHOT. We found that when PHOT are activated in the upper part of the leaf, they inhibit auxin signaling, which in turn would inhibit growth. In these conditions, auxin signaling is still high in the lower part of the leaf, allowing cell growth and leaf flattening. In conditions when PHOT are inactive (phot1phot2 mutants, or in the absence of blue light) and in conditions where PHOT are activated in the lower part of the leaf, leaf curling correlates with an increase in auxin signaling in the upper part of the leaf and a decrease in the lower part, creating again a growth imbalance which results in bending downwards. To control the spatial pattern of auxin signaling, PHOT regulate auxin transport, as shown by the requirement of members of the three main families of auxin transporters to respond to light.
To increase the impact of our work it is imperative to communicate these results to as many people as possible. Importantly, these results have been presented in scientific conferences, in a preprint (https://www.biorxiv.org/content/10.1101/2021.05.25.445665v1) and have recently been revised by other scientists, experts in the field, who endorsed their publication in a scientific journal (https://academic.oup.com/plphys/advance-article/doi/10.1093/plphys/kiab410/6364194). Additionally, we have presented this work to people outside the scientific community, such as school students, and families curious about science. We organized in person (https://ec.europa.eu/research-and-innovation/en/events/upcoming-events/research-innovation-days) and online (https://wp.unil.ch/mysteres/) interactive activities aimed at sharing experiences and bursting people’s curiosity about plants and their amazing capacity to adapt their development to their environment.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The results obtained in this project allow us and other scientists to better understand the interaction between leaf development and external light signals, which would optimize plant growth to the prevailing environmental conditions. Knowing this is the basis to design strategies to optimize plant growth, either modifying plant responses to light, or creating optimized light environments for plant development. Moreover, the actions implemented to communicate our results to a broader audience contribute to fight plant blindness, a common bias in people’s vision of the world which disregards the key role of plants in our environment and everyday life. In addition, we collaborated with school teachers suggesting experiments and activities to include in their schedules.

Schematic summary of the published results