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(si apre in una nuova finestra)) 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(si apre in una nuova finestra)). 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(si apre in una nuova finestra)) and online (
https://wp.unil.ch/mysteres/(si apre in una nuova finestra)) interactive activities aimed at sharing experiences and bursting people’s curiosity about plants and their amazing capacity to adapt their development to their environment.
