Delivering Force and Amplifying Signals in Plant Mechanosensing

A Molecular Pinball Machine of the Plasma Membrane Regulates Plant Growth-A New Paradigm

Novel molecular pinball machines of the plasma membrane control cytosolic Ca²⁺ levels that regulate plant metabolism. The essential components involve: 1. an auxin-activated proton pump; 2. arabinogalactan glycoproteins (AGPs); 3. Ca²⁺ channels; 4. auxin-efflux "PIN" proteins. Typical pinball machines release pinballs that trigger various sound and visual effects. However, in plants, "proton pinballs" eject Ca²⁺ bound by paired glucuronic acid residues of numerous glycomodules in periplasmic AGP-Ca²⁺. Freed Ca²⁺ ions flow down the electrostatic gradient through open Ca²⁺ channels into the cytosol, thus activating numerous Ca²⁺-dependent activities. Clearly, cytosolic Ca²⁺ levels depend on the activity of the proton pump, the state of Ca²⁺ channels and the size of the periplasmic AGP-Ca²⁺ capacitor; proton pump activation is a major regulatory focal point tightly controlled by the supply of auxin. Auxin efflux carriers conveniently known as "PIN" proteins (null mutants are pin-shaped) pump auxin from cell to cell. Mechanosensitive Ca²⁺ channels and their activation by reactive oxygen species (ROS) are yet another factor regulating cytosolic Ca²⁺. Cell expansion also triggers proton pump/pinball activity by the mechanotransduction of wall stress via Hechtian adhesion, thus forming a Hechtian oscillator that underlies cycles of wall plasticity and oscillatory growth. Finally, the Ca²⁺ homeostasis of plants depends on cell surface external storage as a source of dynamic Ca²⁺, unlike the internal ER storage source of animals, where the added regulatory complexities ranging from vitamin D to parathormone contrast with the elegant simplicity of plant life. This paper summarizes a sixty-year Odyssey.

Barbara G. Pickard - Queen of Plant Electrophysiology

Barbara Gillespie Pickard (1936-2019) studied plant electrophysiology and mechanosensory biology for more than 50 y. Her first papers on the roles of auxin in plant tropisms were coauthored with Kenneth V. Thimann. Later, she studied plant electrophysiology. She made it clear that plant action potentials are not a peculiar feature of so-called sensitive plants, but that all plants exhibit these fast electric signals. Barbara Gillespie Pickard proposed a neuronal model for the spreading of electric signals induced by mechanical stimuli across plant tissues. In later years, she studied the stretch-activated plasma membrane channels of plants and formulated the plasma-membrane control center model. Barbara Pickard summarized all her findings in a new model of phyllotaxis involving waves of auxin fluxes and mechano-sensory signaling.

Static Modulation Wave of Arrays of Halogen Interactions Transduced to a Hierarchy of Nanoscale Change Stimuli of Crystalline Rotors Dynamics

Here we present a study where what can be seen as a static modulation wave encompassing four successive arrays of interacting iodine atoms in crystalline 1,4-Bis((4'-(iodoethynyl)phenyl) ethynyl)bicyclo[2,2,2]octane rotors changes the structure from one-half molecule to three-and-a-half molecules in the asymmetric unit below a phase transition at 105 K. The remarkable finding is that the total ¹H spin-lattice relaxation rate, T₁^-1, of unprecedented complexity to date in molecular rotors, is the weighted sum of the relaxation rates of the four contributing rotors relaxation rates, each with distinguishable exchange frequencies reflecting Arrhenius parameters with different activation barriers ( E_a) and attempt frequencies (τ_o^-1). This allows us to show in tandem with rotor-environment interaction energy calculations how the dynamics of molecular rotors are able to decode structural information from their surroundings with remarkable nanoscale precision.

Back to the future with the AGP-Ca2+ flux capacitor

BACKGROUND

Arabinogalactan proteins (AGPs) are ubiquitous in green plants. AGPs comprise a widely varied group of hydroxyproline (Hyp)-rich cell surface glycoproteins (HRGPs). However, the more narrowly defined classical AGPs massively predominate and cover the plasma membrane. Extensive glycosylation by pendant polysaccharides O-linked to numerous Hyp residues like beads of a necklace creates a unique ionic compartment essential to a wide range of physiological processes including germination, cell extension and fertilization. The vital clue to a precise molecular function remained elusive until the recent isolation of small Hyp-arabinogalactan polysaccharide subunits; their structural elucidation by nuclear magentic resonance imaging, molecular simulations and direct experiment identified a 15-residue consensus subunit as a β-1,3-linked galactose trisaccharide with two short branched sidechains each with a single glucuronic acid residue that binds Ca(2+) when paired with its adjacent sidechain.

SCOPE

AGPs bind Ca(2+) (Kd ∼ 6 μm) at the plasma membrane (PM) at pH ∼5·5 but release it when auxin-dependent PM H(+)-ATPase generates a low periplasmic pH that dissociates AGP-Ca(2+) carboxylates (pka ∼3); the consequential large increase in free Ca(2+) drives entry into the cytosol via Ca(2+) channels that may be voltage gated. AGPs are thus arguably the primary source of cytosolic oscillatory Ca(2+) waves. This differs markedly from animals, in which cytosolic Ca(2+) originates mostly from internal stores such as the sarcoplasmic reticulum. In contrast, we propose that external dynamic Ca(2+) storage by a periplasmic AGP capacitor co-ordinates plant growth, typically involving exocytosis of AGPs and recycled Ca(2+), hence an AGP-Ca(2+) oscillator.

CONCLUSIONS

The novel concept of dynamic Ca(2+) recycling by an AGP-Ca(2+) oscillator solves the long-standing problem of a molecular-level function for classical AGPs and thus integrates three fields: AGPs, Ca(2+) signalling and auxin. This accounts for the involvement of AGPs in plant morphogenesis, including tropic and nastic movements.

Dynamic actin controls polarity induction de novo in protoplasts

Cell polarity and axes are central for plant morphogenesis. To study how polarity and axes are induced de novo, we investigated protoplasts of tobacco Nicotiana tabacum cv. BY-2 expressing fluorescently-tagged cytoskeletal markers. We standardized the system to such a degree that we were able to generate quantitative data on the temporal patterns of regeneration stages. The synthesis of a new cell wall marks the transition to the first stage of regeneration, and proceeds after a long preparatory phase within a few minutes. During this preparatory phase, the nucleus migrates actively, and cytoplasmic strands remodel vigorously. We probed this system for the effect of anti-cytoskeletal compounds, inducible bundling of actin, RGD-peptides, and temperature. Suppression of actin dynamics at an early stage leads to aberrant tripolar cells, whereas suppression of microtubule dynamics produces aberrant sausage-like cells with asymmetric cell walls. We integrated these data into a model, where the microtubular cytoskeleton conveys positional information between the nucleus and the membrane controlling the release or activation of components required for cell wall synthesis. Cell wall formation is followed by the induction of a new cell pole requiring dynamic actin filaments, and the new cell axis is manifested as elongation growth perpendicular to the orientation of the aligned cortical microtubules.

Gravitropism and mechanical signaling in plants

Mechanical stress is a critical signal affecting morphogenesis and growth and is caused by a large variety of environmental stimuli such as touch, wind, and gravity in addition to endogenous forces generated by growth. On the basis of studies dating from the early 19th century, the plant mechanical sensors and response components related to gravity can be divided into two types in terms of their temporal character: sensors of the transient stress of reorientation (phasic signaling) and sensors capable of monitoring and responding to the extended, continuous gravitropic signal for the duration of the tropic growth response (tonic signaling). In the case of transient stress, changes in the concentrations of ions in the cytoplasm play a central role in mechanosensing and are likely a key component of initial gravisensing. Potential candidates for mechanosensitive channels have been identified in Arabidopsis thaliana and may provide clues to these rapid, ionic gravisensing mechanisms. Continuous mechanical stress, on the other hand, may be sensed by other mechanisms in addition to the rapidly adapting mechnaosensitive channels of the phasic system. Sustaining such long-term responses may be through a network of biochemical signaling cascades that would therefore need to be maintained for the many hours of the growth response once they are triggered. However, classical physiological analyses and recent simulation studies also suggest involvement of the cytoskeleton in sensing/responding to long-term mechanoresponse independently of the biochemical signaling cascades triggered by initial graviperception events.

Mechano-perception in Chara cells: the influence of salinity and calcium on touch-activated receptor potentials, action potentials and ion transport

This paper investigates the impact of increased salinity on touch-induced receptor and action potentials of Chara internodal cells. We resolved underlying changes in ion transport by current/voltage analysis. In a saline medium with a low Ca(2+) ion concentration [(Ca(2+))(ext)], the cell background conductance significantly increased and proton pump currents declined to negligible levels, depolarizing the membrane potential difference (PD) to the excitation threshold [action potential (AP)(threshold)]. The onset of spontaneous repetitive action potentials further depolarized the PD, activating K(+) outward rectifying (KOR) channels. K(+) efflux was then sustained and irrevocable, and cells were desensitized to touch. However, when [Ca(2+)](ext) was high, the background conductance increased to a lesser extent and proton pump currents were stimulated, establishing a PD narrowly negative to AP(threshold). Cells did not spontaneously fire, but became hypersensitive to touch. Even slight touch stimulus induced an action potential and further repetitive firing. The duration of each excitation was extended when [Ca(2+)](ext) was low. Cell viability was prolonged in the absence of touch stimulus. Chara cells eventually depolarize and die in the saline media, but touch-stimulated and spontaneous excitation accelerates the process in a Ca(2+)-dependent manner. Our results have broad implications for understanding the interactions between mechano-perception and salinity stress in plants.

"Second extrinsic organizational mechanism" for orienting cellulose: modeling a role for the plasmalemmal reticulum

Oriented deposition of cellulose fibers by cellulose-synthesizing complexes typically occurs across the plasma membrane from microtubule bundles and is guided by them. However, aligned movement of the complexes can be shown even after applied oryzalin has depolymerized microtubules. Further, there is a claim that when (1) microtubules are depolymerized with oryzalin, (2) a microtubule-orienting stimulus is applied temporarily, and (3) oryzalin is washed out, the newly forming cellulose fibers are oriented with respect to the stimulus. With this in mind, the present paper gathers evidence from a diverse literature to suggest that the plasmalemmal reticulum, a major and structurally important form of cytoskeleton which connects cortical cytoplasm with wall, is a candidate to both independently and cooperatively participate in orienting microtubules and routing movements of cellulose-synthesizing complexes. Critical to this proposed function, the adhesion sites of the plasmalemmal reticulum have some morphological and molecular similarities to animal cell adhesion sites, known to play numerous integrative roles. The reticulum itself may be the morphological manifestation of the so-called lipid raft, previously known only on the basis of biochemical properties. According to the working model, the trusses interconnecting the adhesion sites shape the reticulum into apparently situation-dependent geometries. For example, in nongrowing or nonpolarized cells in which cellulose is deposited in brushy meshes, they form a nonpolar or weakly polar net; however, in elongating cells with oblique or otherwise polarized microtubules and newly forming cellulose fibers, there is suggestive evidence that net formation is dominated by trusses organized with correspondingly biased orientation. Consideration of such geometries and roles of the reticulum suggests several tests that could affirm, deny, or replace key aspects of this proposal to expand the theory of the peripheral cytoskeleton.

Roles of amyloplasts and water deficit in root tropisms

Directed growth of roots in relation to a moisture gradient is called hydrotropism. The no hydrotropic response (nhr1) mutant of Arabidopsis lacks a hydrotropic response, and shows a stronger gravitropic response than that of wild type (wt) in a medium with an osmotic gradient. Local application of abscisic acid (ABA) to seeds or root tips of nhr1 increased root downward growth, indicating the critical role of ABA in tropisms. Wt roots germinated and treated with ABA in this system were strongly gravitropic, even though they had almost no starch amyloplasts in the root-cap columella cells. Hydrotropically stimulated nhr1 roots, with or without ABA, maintained starch in the amyloplasts, as opposed to those of wt. Hence, the near-absence (wt) or abundant presence (nhr1) of starch granules does not influence the extent of downward gravitropism of the roots in an osmotic gradient medium. Starch degradation in the wt might help the root sustain osmotic stress and carry out hydrotropism, instead of reducing gravity responsiveness. nhr1 roots might be hydrotropically inactive because they maintain this starch reserve in the columella cells, sustaining both their turgor and growth, and in effect minimizing the need for hydrotropism and at least partially disabling its mechanism. We conclude that ABA and water stress are critical regulators of root tropic responses.

Disruption of arabinogalactan proteins disorganizes cortical microtubules in the root of Arabidopsis thaliana

The cortical array of microtubules inside the cell and arabinogalactan proteins on the external surface of the cell are each implicated in plant morphogenesis. To determine whether the cortical array is influenced by arabinogalactan proteins, we first treated Arabidopsis roots with a Yariv reagent that binds arabinogalactan proteins. Cortical microtubules were markedly disorganized by 1 microM beta-D-glucosyl (active) Yariv but not by up to 10 microM beta-D-mannosyl (inactive) Yariv. This was observed for 24-h treatments in wild-type roots, fixed and stained with anti-tubulin antibodies, as well as in living roots expressing a green fluorescent protein (GFP) reporter for microtubules. Using the reporter line, microtubule disorganization was evident within 10 min of treatment with 5 microM active Yariv and extensive by 30 min. Active Yariv (5 microM) disorganized cortical microtubules after gadolinium pre-treatment, suggesting that this effect is independent of calcium influx across the plasma membrane. Similar effects on cortical microtubules, over a similar time scale, were induced by two anti-arabinogalactan-protein antibodies (JIM13 and JIM14) but not by antibodies recognizing pectin or xyloglucan epitopes. Active Yariv, JIM13, and JIM14 caused arabinogalactan proteins to aggregate rapidly, as assessed either in fixed wild-type roots or in the living cells of a line expressing a plasma membrane-anchored arabinogalactan protein from tomato fused to GFP. Finally, electron microscopy of roots prepared by high-pressure freezing showed that treatment with 5 microM active Yariv for 2 h significantly increased the distance between cortical microtubules and the plasma membrane. These findings demonstrate that cell surface arabinogalactan proteins influence the organization of cortical microtubules.