Session Y14: Plant Physics

Sap flow and sugar transport in a pine needle: how to deal with an adverse environement

The survival of green plants depends on efficient use of photosynthesis in the leaves, where sunlight, water, and CO₂ are transformed to sugar – the raw material, which builds up even the largest trees. The dissolved sugars are transported by osmosis through the sieve tubes of the phloem, a vascular system, which runs through the veins of the leaves and on through the stem, all the way down into the roots. This basic mechanism is at work in all trees - somewhat surprisingly since the older class of gymnosperms (conifers) predate the “modern” angiosperms (broad leaf trees) by more than 100 mio years. The sugar production sites (mesophyll cells) are distributed along the entire leaf, and it is important for the functionality of the leaf that they are all able to export their sugars. For conifer leaves (needles) the linear venation architecture makes this challenging, since the osmotic sugar transport in a long sugar filled pipe will tend to stagnate at the tip. To overcome this, the phloem tubes (sieve tubes) of conifer needles are divided in groups starting at different points along the needle. In addition the needles contain an extra “transfusion tissue”, taking up a considerable volume, which bridges between production and transport and directs the sugars to the correct (outermost) sieve tube group. Micro X-ray tomography on intact needles reveals the complex network of interdigitated water -and sugar carrying cells (both much larger than the xylem (tracheid) cells and the phloem (sieve) cells) making up the transfusion tissue and allows us to understand the pathways for water and for sugars (running in opposite directions) with huge pressure differences (say 3 MPa) across tiny length scales (say 5 microns). Presumably this structure is also important for understanding why conifers are among the largest and most drought/cold resistant organisms on earth.    

Marchantia polymorpha gemmae interact as capillary multipoles on fluid interfaces

Marchantia polymorpha, or common liverworts, reproduce asexually by growing cup-like structures on their leafy thalli which contain multicellular propagules called gemmae. When rain fills the gemmae cups, mature gemmae are released, adsorb to the air-water interface, and subsequently are splashed out to grow into new plants. Gemmae have long been assumed to be passive agents, but their capillary interactions while attached to the air-water interface suggest they play a much more active role in liverwort reproduction. To investigate this, we directly measure capillary interactions of gemmae on both flat and curved fluid interfaces using brightfield and phase contrast microscopy as well as optical profilometry, and find that gemmae adsorbed to the water surface interact as capillary multipoles with a dominant quadrupolar character. Intriguingly, we observe different self-assembly behavior of multiple gemmae on flat fluid interfaces versus in their natural environment. This suggests that the gemmae cups may have evolved to shape the water surface to arrange the gemmae for better splash dispersal.   

Biogirders: how the leaves of the giant Amazonian waterlily occupy a large surface area at an economical material cost

The giant Amazonian waterlily (genus Victoria) produces the largest floating leaves of all plants. We studied how the structural form of the vasculature system underpins gigantism in these extraordinary leaves. and infer how this unique form of leaf gigantism evolved. Specifically, by means of mechanical testing and geometrical modelling, we found that the bending resistance of the Amazonian waterlily is considerably higher than of an elastic floating sheet of the same amount of material. Our analysis suggests that the unique pattern of branching veins on the underside of the Victoria leaf provides structural support at an economical material cost and, as such, enables gigantism. Considering the evolutionary history of the waterlily family, and the life history of Victoria in particular, we inferred that this multi-purpose system may have evolved to maximise photosynthesis and enable rapid growth in fast-drying pools, thereby conferring a selective advantage in an unstable environment.   

Dehydration-induced corrugated folding in shape-morphing leaves

Plant leaf morphogenesis, giving rise to vast varieties of flower and leaf shapes, has been of interest to researchers in both biology and engineering. Leaves also demonstrate intriguing shape-morphing behaviors upon changes in the environment. For example, the leaves of Rhapis excelsa fold into a corrugated structure with the loss of water. To understand this interesting phenomenon, we explore the mechanical basis and the cellular mechanism of the corrugated leaf folding. We experimentally examine the cellular structures in the leaf that induce folding - the ‘hinge’ cells that undergo higher volume shrinkage than the others, which introduce the strain-mismatch within the leaf - and investigate the mechanism by numerical simulations. Additionally, we develop a mathematical model for describing the dynamics of the water loss, as well as predicting the folding angle. These findings provide insights of how plant leaves in nature change their shapes by mechanical principles in response to environment change, which may have biological functions for plant survival. To further explore its potential applications, the mechanism of leaf folding we observed is implemented into the design of biomimetic soft machines that made of hydrogels and show self-folding ability with water loss.   

Drought-induced curvature changes in leaf

Water stress directly affects crop growth and is used as a key indicator in evaluating the crop yield. There were several non-destructive approaches suggested to utilize the relationship between water stress and leaf vibration frequency, but the tendency of the frequency due to the dryness was controversial. Here, we proposed a new perspective of leaf surface curvature effect to vibration frequency in order to understand this discrepancy. The change of leaf curvature due to dryness was measured through a 3D scan of individual leaves each day it underwent drought conditions. The leaves were observed to undergo two types of drying patterns. In some leaves, drought conditions led to an increase in upward transversal curvature, while in others, they wilted without significant change in their surface curvature. We show that topological changes due to dryness influence the bending stiffness of the leaf and contribute to increasing the frequency when vibrating. These results indicate that the morphological feature of leaves is key to the vibration frequency.   

Rearrangement of primary cell wall polymers during mechanical stretch

Plant cells grow by the loosening of their primary cell walls and expansion from turgor. Therefore, characterization of how cell wall polymers respond to mechanical strain will aid our understanding of plant growth. This is also the foundation for the development of plant biomass-derived materials. Primary cell wall consists of semi-crystalline cellulose microfibrils embedded in a matrix of pectin and hemicellulose and is organized in a cross-lamellate structure. The interactions between these components play important roles in the mechanical properties of the wall. Onion outer epidermal peel, a model system for primary cell wall, was excised into strips amenable for mechanical testing, microscopy, and X-ray scattering. We have developed a method to stretch onion epidermal wall to various levels of longitudinal strain to examine both reversible and irreversible structural change of wall polymer architecture. Atomic force microscopy revealed reversible reorientation and straightening of cellulose microfibrils at high strain in the innermost lamellae of onion epidermal wall. Small-angle X-ray scattering revealed overall reorientation of cellulose microfibrils, a part of which appeared irreversible at high strains. A prominent scattering feature at high strains indicated that cellulose microfibrils packed together with a center-to-center spacing of 7.4 nm. We hypothesize that matrix polymers, most notably pectin, fill up this space and prevent the microfibrils from fully collapsing onto each other. Overall, these observations will shed light on the structure-mechanical property relationships of cell wall polymers and refine our models of cell wall microstructure.   

Frequency response analysis of gravitropism in rice roots

Active sensory response behaviors in plants, known as tropisms, are key to a root’s ability to navigate soil. These behaviors, which can alter root growth trajectory in response to light (phototropism), touch (thigmotropism), or gravity (gravitropism) have been widely studied for single stimulus events. However, how these responses are influenced by dynamic stimuli to facilitate root negotiation of complex, changing substrate conditions is not well understood. In the past, dynamic sensory response in animals such as cockroaches, knifefish, and hawk moths have been characterized using concepts from control theory [Cowan et al, ICB, 2014]. Here we extend this analysis framework to plants, using rice (O. sativa) roots. We develop an apparatus that provides programmable, time-dependent gravitational stimuli to roots grown in a transparent gel, and image root growth with two orthogonal cameras. We use this apparatus to track the three-dimensional growth trajectory of rice roots that are rotated between 0° and 90° with respect to gravity at frequencies between 1.5 and 144 cycles/day with square and sine wave stimulus profiles. We present a control model to characterize root sensory response and describe how plant growth trajectory is impacted by periodic stimuli using frequency response techniques such as Bode analysis.

    

Plant root inspired protrusions enhance intruder anchoring force in dry granular media

Diverse systems that interact with soft substrates can take advantage of complex structures to modify local stresses and flows to improve intrusion resistance [Agarwal et al., Soft Matter 2021]. Less is known about how extrusion resistance is modified by complex structures. This is particularly relevant in plants as studies have shown that networks of hairs lining roots can increase anchoring force and enhance soil stability [Bengough et al.,J Exp Bot 2016]. To discover the principles for biological and engineering anchoring resistance, we study forces experienced by rigid cylinders (8.3mm diameter) incorporating rigid protrusions (3mm by 5mm) as it is pulled from a model noncohesive soft substrate, dry granular media (1mm diameter particles). In laboratory experiments and validated Discrete Element Method simulations we measured peak pullout force as a function of distance between hairs. We observed that roots exhibited limited anchoring force for both too sparse and too dense hairs and peaked by approximately 3x in force at intermediate spacing of 1-2cm between hairs. Our results are in accord with tests of a novel burrowing soft robot, generating hair-like structures during intrusion. Investigating the plant-inspired complex interactions with GM could inform future robot designs capable of effective robotic burrowing, and anchoring.

Principles in plant roots enable robotic self-anchoring

Plants require no above-ground reaction force to burrow deep underground, using several key principles that could help enable robotic burrowing. 1) Tip extension allows skin friction to counter resistive forces. 2) Root hairs increase anchoring through friction and local interactions. 3) Angled branches increase anchoring force and reduce reaction force. And 4), radial expansion increases soil loading and anchoring force. In our study, we have developed a soft, pneumatic device capable of all four of these plant-inspired behaviors. Tip extension is achieved by a thin-walled tube, inverted back inside itself. When pressurized, it everts and extends from its tip. Flexible root-hair-like protrusions are attached along its skin that passively deploy as the device grows. Branching is done with an array of separate anchors, and radial growth is achieved with nested devices. Our preliminary results on a 1 cm diameter device with 5 mm long protrusions show that combining these principles enables self-anchoring, where the device needs little to no external reaction force to burrow 15 cm or more into granular media. This could be useful in reduced gravity environments where reaction forces are difficult to produce.   

Emergent behaviors of heterogeneous cell growth patterns in plant root growth

Circumnutation, the endogenous helical motion of a plant root tip, has been recently shown to facilitate exploration, anchoring and penetration in heterogeneous subterranean environments [Taylor et al, PNAS, 2021]. However, the regulatory mechanism that controls root circumnutation at the cellular level is not well understood. To investigate the effects of cell growth on circumnutation, we developed a multiparticle Discrete Element Method (DEM) simulation in which each "particle" represents a single root cell. Previously, we had developed a small scale simulation that could mimic circumnutation. To scale this model , we integrated LAMMPS, a molecular dynamics software, to run efficient parallelized simulations. Our model implements a perimeter of 90 particles to match the azimuthal cell count in rice roots (O. sativa, 0.2 mm diameter) as determined from confocal imaging, and can model up to 15,000 total cells during root growth. Based on our confocal imaging data, the heterogeneity in cell growth is crucial to the emergence of circumnutation. We emulated root meristematic (cell division) and elongation (cell growth) zones in our model, connecting cell level signals and behaviors to the trajectory of the simulated root tip to test the effects of heterogeneous growth. We validated the simulation results through comparison with experimental data from rice root imaging to understand how circumnutation is regulated.   

Experiments and simulations of Turing patterns in vegetation

Many pattern formations exist in ecosystems. From labyrinth patterns of bushy vegetation and spotted patterns of isolated tree patches in Niger to regular maze patterns of shrubs in West Siberia, and striped pattern of tree lines in tape forests in the USA, among various others. Several of these systems can, and have been described by reaction diffusion equations, however because in general ecosystems have large spatiotemporal scales there is little direct experimentation to quantify and validate the models which in addition have limited prediction powers.  We present controlled experiments of Chia seeds growing in half-meter square domains growing in four different substrates with different diffusion coefficients, amounts of daily irrigation and evaporation that display evolving transitions that include Turing patters ranging from spots, labyrinth, and mazes to continuum. We then model the experiments with a reaction diffusion system with parameters fitted to the experiments and used it to predict the patterns obtained in two different diffusive substrates as a function of daily irrigation and evaporation. We believe this is the first time a vegetation model has been validated directly with experiments and used for predictions.

    

The network architecture of plant-pathogen interactions

Plants must contend with diverse pathogens to grow and propagate, with major implications for biodiversity, agriculture, and food security. Many pathogens use virulence molecules called effectors to modify or disable important host proteins. Plants lack an organism-wide adaptive immune system, so they are unable to maintain a repertoire of antibodies recognizing individual effectors. Instead, each plant cell maintains a smaller set of resistance genes (“R genes”) which guard the host proteins. Recent experiments suggest that each effector and each R protein interacts with many different host proteins in a complex 3-layer network. How does the structure of this network govern immune activation? We propose a simple model to study how networks of interacting molecules can be used to build an immune sensor. We show that sharp immune activation emerges from even the simplest models and that redundant R genes can act to suppress the response rather than enhancing it. We then use our model to analyze the empirical interactions between A. thaliana and P. syringae, providing an opportunity to connect molecular networks with evolutionary outcomes.   

Improving the Water Solubility of Hydrophobic Carotenoids Through Association with Phytoglycogen

Phytoglycogen (PG) is a soft, compact, highly branched polysaccharide that is produced as nanoparticles in the kernels of sweet corn. Because it is non-toxic and digestible, it is ideal for applications in human health and nutrition. We have evaluated the effectiveness of PG in improving the solubility of hydrophobic carotenoids, such as astaxanthin (ASX). We developed a method to produce lyophilized powders of ASX associated with PG (ASX-PG) that can be easily redispersed in water, increasing the solubility of ASX by a factor of 10⁹. We used Ultra-Violet visible spectroscopy (UV-vis), Dynamic Light Scattering (DLS), Surface Plasmon Resonance imaging (SPRi) and Nuclear Magnetic Resonance (NMR) to explore the nature of the interaction between ASX and native PG, and ASX and PG that was hydrophobically modified with octenyl succinic anhydride (OSA).

Sorting Category: 02.01.47 Biomaterials and Nanotechnology