Ca2+ pulsation in BY-2 cells and evidence for control of mechanosensory Ca2+-selective channels by the plasmalemmal reticulum

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.

Arabinogalactan proteins are involved in root hair development in barley

The arabinogalactan proteins (AGPs) are involved in a range of plant processes, including cell differentiation and expansion. Here, barley root hair mutants and their wild-type parent cultivars were used, as a model system, to reveal the role of AGPs in root hair development. The treatment of roots with different concentrations of βGlcY (a reagent which binds to all classes of AGPs) inhibited or totally suppressed the development of root hairs in all of the cultivars. Three groups of AGP (recognized by the monoclonal antibodies LM2, LM14, and MAC207) were diversely localized in trichoblasts and atrichoblasts of root hair-producing plants. The relevant epitopes were present in wild-type trichoblast cell walls and cytoplasm, whereas in wild-type atrichoblasts and in all epidermal cells of a root hairless mutant, they were only present in the cytoplasm. In all of cultivars the higher expression of LM2, LM14, and MAC207 was observed in trichoblasts at an early stage of development. Additionally, the LM2 epitope was detected on the surface of primordia and root hair tubes in plants able to generate root hairs. The major conclusion was that the AGPs recognized by LM2, LM14, and MAC207 are involved in the differentiation of barley root epidermal cells, thereby implying a requirement for these AGPs for root hair development in barley.

Structures formed by a cell membrane-associated arabinogalactan-protein on graphite or mica alone and with Yariv phenylglycosides

BACKGROUND

Certain membrane-associated arabinogalactan-proteins (AGPs) with lysine-rich sub-domains participate in plant growth, development and resistance to stress. To complement fluorescence imaging of such molecules when tagged and introduced transgenically to the cell periphery and to extend the groundwork for assessing molecular structure, some behaviours of surface-spread AGPs were visualized at the nanometre scale in a simplified electrostatic environment.

METHODS

Enhanced green fluorescent protein (EGFP)-labelled LeAGP1 was isolated from Arabidopsis thaliana leaves using antibody-coated magnetic beads, deposited on graphite or mica, and examined with atomic force microscopy (AFM).

KEY RESULTS

When deposited at low concentration on graphite, LeAGP can form independent clusters and rings a few nanometres in diameter, often defining deep pits; the aperture of the rings depends on plating parameters. On mica, intermediate and high concentrations, respectively, yielded lacy meshes and solid sheets that could dynamically evolve arcs, rings, 'pores' and 'co-pores', and pits. Glucosyl Yariv reagent combined with the AGP to make very large and distinctive rings.

CONCLUSIONS

Diverse cell-specific nano-patterns of native lysine-rich AGPs are expected at the wall-membrane interface and, while there will not be an identical patterning in different environmental settings, AFM imaging suggests protein tendencies for surficial organization and thus opens new avenues for experimentation. Nanopore formation with Yariv reagents suggests how the reagent might bind with AGP to admit Ca(2+) to cells and hints at ways in which AGP might be structured at some cell surfaces.

A β-glucuronosyltransferase from Arabidopsis thaliana involved in biosynthesis of type II arabinogalactan has a role in cell elongation during seedling growth

We have characterized a β-glucuronosyltransferase (AtGlcAT14A) from Arabidopsis thaliana that is involved in the biosynthesis of type II arabinogalactan (AG). This enzyme belongs to the Carbohydrate Active Enzyme database glycosyltransferase family 14 (GT14). The protein was localized to the Golgi apparatus when transiently expressed in Nicotiana benthamiana. The soluble catalytic domain expressed in Pichia pastoris transferred glucuronic acid (GlcA) to β-1,6-galactooligosaccharides with degrees of polymerization (DP) ranging from 3-11, and to β-1,3-galactooligosaccharides of DP5 and 7, indicating that the enzyme is a glucuronosyltransferase that modifies both the β-1,6- and β-1,3-galactan present in type II AG. Two allelic T-DNA insertion mutant lines showed 20-35% enhanced cell elongation during seedling growth compared to wild-type. Analyses of AG isolated from the mutants revealed a reduction of GlcA substitution on Gal-β-1,6-Gal and β-1,3-Gal, indicating an in vivo role of AtGlcAT14A in synthesis of those structures in type II AG. Moreover, a relative increase in the levels of 3-, 6- and 3,6-linked galactose (Gal) and reduced levels of 3-, 2- and 2,5-linked arabinose (Ara) were seen, suggesting that the mutation in AtGlcAT14A results in a relative increase of the longer and branched β-1,3- and β-1,6-galactans. This increase of galactosylation in the mutants is most likely caused by increased availability of the O6 position of Gal, which is a shared acceptor site for AtGlcAT14A and galactosyltransferases in synthesis of type II AG, and thus addition of GlcA may terminate Gal chain extension. We discuss a role for the glucuronosyltransferase in the biosynthesis of type II AG, with a biological role during seedling growth.

Microtubules, signalling and abiotic stress

Plant microtubules, in addition to their role in cell division and axial cell expansion, convey a sensory function that is relevant for the perception of mechanical membrane stress and its derivatives, such as osmotic or cold stress. During development, sensory microtubules participate in the mechanical integration of plant architecture, including the patterning of incipient organogenesis and the alignment with gravity-dependent load. The sensory function of microtubules depends on dynamic instability, and often involves a transient elimination of cortical microtubules followed by adaptive events accompanied by subsequent formation of stable microtubule bundles. It is proposed that microtubules, because of their relative rigidity in combination with their innate nonlinear dynamics, are pre-adapted for a function as mechanosensors and, in concert with the flexible actin filaments and the anisotropic cell wall, comprise a tensegral system that allows plant cells to sense geometry and to respond to fields of mechanical strains such that the load is minimized. Microtubules are proposed as elements of a sensory hub that decodes stress-related signal signatures, with phospholipase D as an important player.

Periplasmic arabinogalactan glycoproteins act as a calcium capacitor that regulates plant growth and development

Arabinogalactan glycoproteins (AGPs) are implicated in virtually all aspects of plant growth and development, yet their precise role remains unknown. Classical AGPs cover the plasma membrane and are highly glycosylated by numerous acidic arabinogalactan polysaccharides O-linked to hydroxyproline. Their heterogeneity and complexity hindered a structural approach until the recent determination of a highly conserved repetitive consensus structure for a 15-sugar residue arabinogalactan subunit with paired glucuronic carboxyls. Based on NMR data and molecular dynamics simulations, we identify these carboxyls as potential intramolecular Ca(2+)-binding sites. Using rapid ultrafiltration assays and mass spectrometry, we verified that AGPs bind Ca(2+) tightly (K(d) ~ 6.5 μM) and stoichiometrically at pH 5. Ca(2+) binding is reversible in a pH-dependent manner. As typical AGPs contain c. 30 Ca(2+)-binding subunits and are bulk components of the periplasm, they represent a Ca(2+) capacitor discharged at low pH by stretch-activated plasma membrane H(+)-ATPases, hence a substantial source of cytosolic Ca(2+). We propose that these Ca(2+) waves prime the 'calcium oscillator', a signal generator essential to the global Ca(2+) signalling pathway of green plants.

A fasciclin-like arabinogalactan-protein (FLA) mutant of Arabidopsis thaliana, fla1, shows defects in shoot regeneration

BACKGROUND

The fasciclin-like arabinogalactan-proteins (FLAs) are an enigmatic class of 21 members within the larger family of arabinogalactan-proteins (AGPs) in Arabidopsis thaliana. Located at the cell surface, in the cell wall/plasma membrane, they are implicated in many developmental roles yet their function remains largely undefined. Fasciclin (FAS) domains are putative cell-adhesion domains found in extracellular matrix proteins of organisms from all kingdoms, but the juxtaposition of FAS domains with highly glycosylated AGP domains is unique to plants. Recent studies have started to elucidate the role of FLAs in Arabidopsis development. FLAs containing a single FAS domain are important for the integrity and elasticity of the plant cell wall matrix (FLA11 and FLA12) and FLA3 is involved in microspore development. FLA4/SOS5 with two FAS domains and two AGP domains has a role in maintaining proper cell expansion under salt stressed conditions. The role of other FLAs remains to be uncovered.

METHOD/PRINCIPAL FINDINGS

Here we describe the characterisation of a T-DNA insertion mutant in the FLA1 gene (At5g55730). Under standard growth conditions fla1-1 mutants have no obvious phenotype. Based on gene expression studies, a putative role for FLA1 in callus induction was investigated and revealed that fla1-1 has a reduced ability to regenerate shoots in an in vitro shoot-induction assay. Analysis of FLA1p:GUS reporter lines show that FLA1 is expressed in several tissues including stomata, trichomes, the vasculature of leaves, the primary root tip and in lateral roots near the junction of the primary root.

CONCLUSION

The results of the developmental expression of FLA1 and characterisation of the fla1 mutant support a role for FLA1 in the early events of lateral root development and shoot development in tissue culture, prior to cell-type specification.

AtAGP18, a lysine-rich arabinogalactan protein in Arabidopsis thaliana, functions in plant growth and development as a putative co-receptor for signal transduction

Arabinogalactan-proteins (AGPs) are a class of hyperglycosylated, hydroxyproline-rich glycoproteins that are widely distributed in the plant kingdom. AtAGP17, 18 and 19 are homologous genes encoding three classical lysine-rich AGPs in Arabidopsis. We observed subcellular localization of AtAGP18 at the plasma membrane by expressing a translational fusion gene construction of AtAGP18 attached to a green fluorescent protein (GFP) tag in Arabidopsis plants. We also overexpressed AtAGP18 without the GFP tag in Arabidopsis plants, and the resulting transgenic plants had a short, bushy phenotype. Here we discuss putative roles of AtAGP18 as a glycosylphosphatidylinositol (GPI)-anchored protein involved in a signal transduction pathway regulating plant growth and development.

Changes in gravity rapidly alter the magnitude and direction of a cellular calcium current

In single-celled spores of the fern Ceratopteris richardii, gravity directs polarity of development and induces a directional, trans-cellular calcium (Ca(2+)) current. To clarify how gravity polarizes this electrophysiological process, we measured the kinetics of the cellular response to changes in the gravity vector, which we initially estimated using the self-referencing calcium microsensor. In order to generate more precise and detailed data, we developed a silicon microfabricated sensor array which facilitated a lab-on-a-chip approach to simultaneously measure calcium currents from multiple cells in real time. These experiments revealed that the direction of the gravity-dependent polar calcium current is reversed in less than 25 s when the cells are inverted, and that changes in the magnitude of the calcium current parallel rapidly changing g-forces during parabolic flight on the NASA C-9 aircraft. The data also revealed a hysteresis in the response of cells in the transition from 2g to micro-g in comparison to cells in the micro-g to 2-g transition, a result consistent with a role for mechanosensitive ion channels in the gravity response. The calcium current is suppressed by either nifedipine (calcium-channel blocker) or eosin yellow (plasma membrane calcium pump inhibitor). Nifedipine disrupts gravity-directed cell polarity, but not spore germination. These results indicate that gravity perception in single plant cells may be mediated by mechanosensitive calcium channels, an idea consistent with some previously proposed models of plant gravity perception.

Intercourse between cell wall and cytoplasm exemplified by arabinogalactan proteins and cortical microtubules

How does a plant cell sense and respond to the status of its cell wall? Intercourse between cell wall and cytoplasm has long been supposed to involve arabinogalactan proteins, in part because many of them are anchored to the plasma membrane. Disrupting arabinogalactan proteins has recently been shown to disrupt the array of cortical microtubules present just inside the plasma membrane, implying that microtubules and arabinogalactan proteins interact. In this article, we assess possibilities for how this interaction might be mediated. First, we consider microdomains in the plasma membrane (lipid rafts), which have been alleged to link internal and external regions of the plasma membrane; however, the characteristics and even the existence of these domains remains controversial. Next, we point out that disrupting the synthesis of cellulose also can disrupt microtubules and consider whether arabinogalactan proteins are part of a network linking microtubules and nascent microfibrils. Finally, we outline several signaling cascades that could transmit information from arabinogalactan proteins to microtubules through channels of cellular communication. These diverse possibilities highlight the work that remains to be done before we can understand how plant cells communicate across their membranes.

"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.

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.

The biology of arabinogalactan proteins

Arabinogalactan proteins is an umbrella term applied to a highly diverse class of cell surface glycoproteins, many of which contain glycosylphosphatidylinositol lipid anchors. The structures of protein and glycan moieties of arabinogalactan proteins are overwhelmingly diverse while the "hydroxproline contiguity hypothesis" predicts arabinogalactan modification of members of many families of extracellular proteins. Descriptive studies using monoclonal antibodies reacting with carbohydrate epitopes on arabinogalactan proteins and experimental work using beta-Yariv reagent implicate arabinogalactan proteins in many biological processes of cell proliferation and survival, pattern formation and growth, and in plant microbe interaction. Advanced structural understanding of arabinogalactan proteins and an emerging molecular genetic definition of biological roles of individual arabinogalactan protein species, in conjunction with potentially analogous extracellular matrix components of animals, stimulate hypotheses about their mode of action. Arabinogalactan proteins might be soluble signals, or might act as modulators and coreceptors of apoplastic morphogens; their amphiphilic molecular nature makes them prime candidates of mediators between the cell wall, the plasma membrane, and the cytoplasm.