Session Z36: Plant and Fungal Physics II

Chloroplasts' Choreography: On Mechanisms of Light Adaptation in Plant Cells

Plant cells have to adapt to the ever-changing conditions of their environment, leading to the evolution of intriguing strategies across scales, from cells to the organism. Using experiments and mathematical models, we will discuss how plants re-arrange the internal structure of cells by the active motion of chloroplasts, to remain efficient during photosynthesis. We will show that the chloroplasts can behave like densely packed light-sensitive active particles, whose non-gaussian athermal fluctuations can lead to various self-organization scenarios, including active glassy dynamics under dim lights and highly packed active clusters under intense light. In the end, I will discuss a series of open problems in the physics and biology of chloroplast motion.   

Tickling plant veins with electroshock to manipulate vascular traffic lights

Sugar-rich whole crops are used for biofuel production. To avoid harvesting, we could extract sweet sap from plant veins. However, the defense mechanisms of plants that lead the sap flow to stop, make extraction difficult. In principle, it is possible to fatigue the plant's defense responses [Pickard, et al., J. Exp. Bot., (1990)]. Hence, we propose to overstimulate the plant using continuous electroshocks. For probe positioning of the electroshock (or laser, or continuous touch) probe, we will use a home-built, cost-effective, high-resolution robot. The effect of the probes is explored on the plant Arabidopsis Thaliana. To quantify the effect on vascular transport, we use Fluorescence Recovery after Photobleaching (FRAP) measurements to visualize the sap translocation. Optimising the electroshock frequency and magnitude is considered. Finally, challenges such as scaling across species are discussed.   

Biomechanics of fast actuation in Mimosa pudica

The "touch-me-not" plant, Mimosa pudica, folds its leaves in less than a second in response to mechanical or electrical stimuli. This movement is reversible, and the leaf snaps back into place within a few tens of minutes.

The rapid transport of water by osmosis within the pulvinus - a flexible, hinge-like bulge at the base of the leaf, acting as the motor organ - is the classic explanation for these movements. However, this scenario lacks experimental evidence. We first characterize the kinematics of the movement and the mechanics of the pulvinus using macroscopic measurements. We then propose a simple model based on an osmotic motor to capture the macroscopic dynamic. Finally, we propose an alternative mechanism based on local redistribution of water from cells to neighboring air cavities, rather than organ-wide water transport.   

Cracking the code of foliar desalination: the structural basis of reverse osmosis in chambered salt glands

Excess salt inhibits plant growth and development and, if severe, results in mortality. In addition to metabolic complications, high salinity also poses a physical problem for plants, as large osmotic gradients can impair water uptake from the soil. Unlike animals, plants do not have dedicated excretory systems; however, some salt-tolerant plants have evolved glands that secrete salt onto the leaf surface in the form of a persistent brine. Here we investigate the structure-function relationships involved in desalination via chambered salt glands using both theoretical and experimental approaches. Fractures in the waxy cuticle allow the brine to escape but provide physical continuity between the concentrated brine and the living cells, putting the leaf at risk of desiccation. We have developed a steady-state, multicompartment, mathematical model to explore how the size and distribution of cuticle fractures affect the balance of pressure- and osmotically-driven fluxes through the gland. Whereas most efforts to understand and engineer salt tolerance in plants have focused on biochemical and genetic pathways, we find that the chambered salt gland leverages its structure and material properties to achieve reverse osmosis across the leaf surface.   

How the Softest Part of the Plant Cell Wall Makes It Strong: Pectin-Cellulose Entanglement Prevents Embolism Spread in the Plant Vascular System

Water transport in plants is blocked by the entry and spread of air embolisms in the vascular system. Pit membranes are cell walls found in the vasculature that help prevent embolism spread, but the relationship between their structure and ability to block air remains controversial. The current paradigm is that the spread of air across pit membranes is determined by defects in a rigid cellulose network, with the other main component of the wall -- pectin hydrogel -- being too soft to have a measurable impact. In contrast, we hypothesize that the pectin network plays a critical role in preventing embolism spread by locally stabilizing the dynamic cellulose network via polymer entanglements and calcium crosslinks. Using red maple (Acer rubrum) as our model system, we found that 1) removing pectin network increases the permeability to air, supporting the idea that the pectin network locally restrict the deformation of the cellulose network; and 2) calcium removal in the pectin network has a small effect on embolism spread, which suggests that pectin-cellulose entanglements may be more important than calcium crosslinks. Our study is relevant to predicting plant responses to drought in a warming climate.   

Modelling and Inferring Protein Dynamics in Fission Yeast Mechanosensing

Mechanical forces play an important role in determining the growth and the shape of a cell, yet they can also be a potential cause of damage. Indeed, cells are endowed with mechanosensors, i.e. receptors at the subcellular scale able to detect mechanical stimuli. In fission yeast, the transmembrane protein Wsc1 is such a mechanosensory. It stimulates glucan synthesis to reinforce the cell wall, the protective thin layer that surrounds the cell. Interestingly, Wsc1 clusters in the region of the cell wall where stress is applied. 

This work aims at describing clustering by building a mathematical model based on reaction-diffusion equations that take into account the interactions between cell wall polysaccharides and Wsc1. To quantify the dynamical parameters of this model, we developed a new inference method for Fluorescence Recovery after Photobleaching (FRAP) experiments which only requires minimal hypotheses. Thanks to the flexibility of this method, it is possible to carry out the experiment with the Wsc1 protein as the cell wall is under compression and obtain accurate protein mobility estimations as a function of mechanical stress.

This study offers fresh methodologies for quantifying and comprehending intricate protein dynamics within cells and tissues.

    

Pattering and growth are coordinated early in the cell cycle

During development, cells in multicellular organisms undergo a series of heavily regulated steps that lead to their differentiation into specific cell types. When this process goes wrong – it can lead to diseases like cancer and developmental malformations. Unlike animals, where differentiation stops primarily after embryogenesis, plants grow continuously and have differentiating cells throughout their lifespan. One major differentiation process occurs when a specific stem cell divides formatively into the endodermis and cortex. In the model system - Arabidopsis thaliana, SHORTROOT (SHR) and SCARECROW (SCR) are transcription factors (TFs) that regulate the decision-making process of whether to divide formatively or proliferatively. Using an inducible SHR system, we followed the dynamics of these TFs and observed the cell outcome in long timelapses with high temporal resolution of more than 1000 cells. We linked the trajectories of SCR to SHR and showed that monostable models explain better the SCR dynamics than the previously suggested bistable switch [1]. Furthermore, we propose and validate an alternative model, where a low threshold of SHR and SCR levels in the early stages of the cell cycle can lead to asymmetric divisions. This model suggests that the decision to divide formatively must occur early in the cell cycle.

[1] Cruz-Ramírez, et al. https://doi.org/10.1016/j.cell.2012.07.017.   

Stochastic simulations of cortical microtubule dynamics using Cytosim to derive biophysical principles of morphogenesis in Arabidopsis leaf trichomes.

Morphogenesis, a fundamental process in life, orchestrates development of functional anatomical features through intricate cell growth and movement. In plant cells, cortical microtubules (CMT) guide cellulose synthase complex (CSC) along the plasma membrane, thereby influencing cellulose microfibril (CMF) orientation in cell walls. This preferred orientation of CMFs induces anisotropy, leading to asymmetric cell expansion. While current research has comprehensively described fundamental cellular mechanisms involved in morphogenesis, a cohesive modeling framework to accurately simulate CMT dynamics in real 3D cell shapes and assess the resulting alterations in wall architecture is currently absent. Such a framework is crucial to investigate cytoskeleton’s regulatory role in cell growth.

This study establishes a biophysical link between CMT-CSC dynamics and wall structure in Arabidopsis trichomes - an ideal system because they are single-celled, easily accessible, and reproducible. The organization of CMT, predicted in silico using Cytosim - a stochastic simulation suite, is meticulously compared with confocal image data obtained before and after trichome branching. Subsequently, simulation of CMT-CSC interaction enables the calculation of local CMF orientation. This comprehensive framework can be readily applied to other biological systems, and serve as an initial foundation to faithfully reproduce various functional anatomical features experimentally.   

Peristaltic flow in nonlinear networks

Many biological systems, such as Physarum polycephalum, utilize peristaltic pumping to direct solutes through flow networks. When the fluid satisfies a linear pressure-flow relationship, pumping is optimized by coordinated peristalsis with a wavelength comparable to the system size. However, when a fluid interacts with flexible structures, such as the valves in the venous and lymphatic systems, pressure-flow relationships can be highly nonlinear. Interestingly, synchronous contractions can be more effective than peristaltic waves in driving flow through nonlinear edges. Another possibility in nonlinear edges is that flow may be induced opposite the peristalsis direction due to the rectification of oscillating flows by valves. These effects can persist in networks, but only if the valves are carefully placed throughout the network. A theory of peristaltic fluid flow through vessels with a nonlinear pressure-flow relationship is developed, and it is argued that coordination in valve placement is vital for the system to operate, while coordination between contractions is less important than in the linear network.