Move it: Mechanisms of light-driven leaf positioning dynamics

This thesis reports how Arabidopsis thaliana plants adapt to light conditions associated with competition for sunlight. Plants rely on photosynthesis to convert carbon dioxide into sugar, and when growing densely, they compete for light. This leads to “shade avoidance responses,” where plants adjust their leaf positioning to optimize light absorption. For rosette plants like Arabidopsis, the upward movement of leaves, called hyponasty, is a key adjustment to avoid shading. The study delves into the mechanisms behind these leaf movements, focussing on how the plant hormone auxin regulates growth responses to light signals. When leaves detect reduced red:far-red (R:FR) light ratios due to shading by neighboring plants, photoreceptors in the leaf tip trigger differential growth, with cells on the underside of the petiole growing more, causing the leaf to bend upward. The thesis explores how these directional growth responses emerge. Auxin plays a crucial role in regulating this growth and this is first thoroughly reviewed. To track leaf movement, the researchers developed an automated camera system that revealed that the most significant leaf movements occur during the night when no light is present. They also studied the sensitivity of the leaf blade and petiole to the R:FR ratio and found interactions between light intensity and the R:FR ratio in both leaf blade and petiole responses. Further experiments with light treatments and plant hormones, including gibberellin (GA), which can regulate auxin biosynthesis, revealed complex hormonal interactions. Interestingly, plants with mutations in the abscisic acid (ABA) pathway could still respond to far-red light, suggesting ABA might regulate leaf angle under white light conditions. A central aspect of the study is the flow of auxin from the leaf tip to the base of the petiole, where it forms a gradient. This gradient is important for hyponastic responses, with the research confirming roles for auxin transport-associated PIN proteins. Through confocal microscopy, the researchers observed that PINs are more abundant on the abaxial side of the petiole, and this asymmetry increases under far-red light or auxin treatments. These findings help map out how auxin flows and accumulates in different parts of the petiole, influencing leaf movement. The petiole’s radial asymmetry is another key focus, and a computational model was designed to predict how the petiole's structure influences auxin gradients. Mutants with disrupted vascular structures or cell polarity revealed that anatomical features significantly affect hyponasty. The study also compares the transcriptional response of the petiole’s abaxial and adaxial sides to far-red light treatments, showing that cell division regulation might play a role in the response. In addition, data suggest that chloroplast signals might inhibit the hyponastic response, providing further insight into the complex regulation of leaf movements. In conclusion, the study provides new insights into leaf movment kinetics in response to fa-red light and highlights the complex molecular regulation and anatomical features that underpin this process. The findings offer a deeper understanding of how plants adapt to light competition and open new avenues for future research into the mechanisms involved in light perception and growth regulation.