Plant Biology
Plasmodesmata
50 nm tubular cytoplasmic channels through plant cell walls — symplastic transport of small molecules and viral RNA
Plasmodesmata are membrane-lined channels approximately 50 nanometers in diameter that traverse plant cell walls, joining the cytoplasm and endoplasmic reticulum of adjacent cells into a continuous network called the symplast. Each pore contains a central desmotubule (a tightly compressed strand of endoplasmic reticulum) surrounded by an annular cytoplasmic sleeve that allows passive diffusion of molecules below the size exclusion limit, typically ~1 kDa under resting conditions. Plant viruses encode movement proteins that bind viral RNA and dilate plasmodesmata to ~50 kDa or larger, enabling cell-to-cell spread. Plasmodesmata also traffic transcription factors (SHORT-ROOT, KNOTTED1), small RNAs, and the systemic RNA interference signal — they are the plant analog of gap junctions but built from completely different molecular parts.
- Channel diameter~50 nm outer, ~2.5 nm sleeve
- Default SEL~1 kDa (mature tissue)
- Dilated SEL~50 kDa with viral MP
- Density~1-15 per µm² cell wall
- DiscoveredTangl 1879, named Strasburger 1901
- RegulatorCallose at neck (CALS3/CALS8)
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Why plasmodesmata matter
- They make plants multicellular in a way animals are not. Every living cell in a leaf, stem, or root is wired to its neighbors through plasmodesmata, sharing cytoplasm in a way no animal tissue does. A typical mature mesophyll cell has ~10 plasmodesmata per square micrometer of shared wall, totaling roughly 1e4 to 1e5 connections per cell. The plant body operates as a cytoplasmic syncytium for everything below the size exclusion limit.
- Phloem loading depends on them. In symplastic loaders (most trees, many tropical plants), sucrose synthesized in mesophyll cells diffuses through plasmodesmata into the phloem companion cell and sieve element — no membrane transport required. In apoplastic loaders (Arabidopsis, most temperate crops), sucrose is dumped to the apoplast and re-imported via SUC2 transporters. The two loading modes correlate with leaf architecture and have major implications for engineering crop sugar transport.
- Viral diseases cost ~30 percent of global crop yield. Every successful plant virus must traffic its genome through plasmodesmata, mediated by movement proteins. Engineering crops to disrupt MP-plasmodesmata interaction is an active disease-resistance strategy; Beachy's coat-protein-mediated resistance in tobacco was the prototype, deployed commercially in papaya rescue from ringspot virus.
- RNAi spreads systemically through plasmodesmata. The discovery that double-stranded RNA injected into a single cell silenced gene expression throughout the plant (Hamilton, Baulcombe 1999) revealed plasmodesmata as conduits for the RNA silencing signal. This is the basis of host-induced gene silencing (HIGS) in agricultural pest control — feeding plants engineered dsRNA targets specific insect or pathogen genes systemically.
- Stem cell maintenance uses symplastic signaling. SHORT-ROOT (SHR) transcription factor in Arabidopsis root is made in the stele and traffics one cell layer outward through plasmodesmata to specify endodermal identity (Helariutta, Benfey 2000). KNOTTED1 in maize moves from internal layers to the meristem epidermis. The mobility is sequence-specific, not bulk diffusion — there is selective trafficking machinery.
- Callose dynamics regulate immunity. Pathogen perception triggers callose synthesis at plasmodesmal necks within minutes, closing the symplastic doors and isolating infected cells. CALS1, CALS3, CALS8 mutants in Arabidopsis show altered cell-to-cell movement and altered immunity to viral and bacterial pathogens — plasmodesmal closure is part of the innate immune response.
- Embryo isolation depends on plasmodesmal closure. During seed maturation, plasmodesmata between the embryo and the surrounding maternal tissue close (callose-mediated), severing the symplastic supply line and forcing the seed into developmental arrest and dormancy. Reactivation at germination requires beta-1,3-glucanases to clear the callose plugs.
Common misconceptions
- Plasmodesmata are open holes. They are not. The cytoplasmic sleeve between desmotubule and plasma membrane is only ~2.5 nm wide and partially packed with proteins forming spoke-like extensions. The effective channel diameter is far smaller than the ~50 nm outer dimension would suggest. Diffusion through the sleeve is constrained, not free.
- The desmotubule is just a passive ER strand. The desmotubule is highly compressed ER, and although it lacks an obvious lumen at the standard EM resolution, it is functionally connected to the bulk ER of the cells on each side. ER-membrane proteins traffic along it, and recent cryo-ET suggests a continuous luminal connection of ~5-10 nm — possibly a separate transport route alongside the cytoplasmic sleeve.
- All plasmodesmata are equivalent. They are not. Primary plasmodesmata form during cytokinesis as ER strands are entrapped in the new cell plate. Secondary plasmodesmata form de novo across pre-existing walls between non-clonal cells. Branched (compound) plasmodesmata develop from primary ones over time and have higher SEL. Pit fields cluster many plasmodesmata together in primary walls.
- Movement proteins create new pores. They do not. They modify the pores that already exist by binding callose synthase regulators, dilating the cytoplasmic sleeve, and chaperoning cargo through. The plasmodesma is the highway; the movement protein is the truck driver who can move oversized loads.
- Plasmodesmal SEL is constant. Far from it. SEL increases dramatically in shoot apical meristems (up to ~50 kDa for stem cell signaling), drops to near zero during seed dormancy, varies with developmental stage, and is rapidly modulated by stress, immunity, and circadian time. Different plant tissues at the same moment can have SELs differing by orders of magnitude.
- Sieve elements have no plasmodesmata. They have specialized branched plasmodesmata called pore-plasmodesma units (PPUs) connecting them to companion cells, but mature sieve elements lose nuclei and many organelles. The companion cell supplies all metabolic functions through PPUs, which have very high SEL — companion cell-to-sieve element movement of macromolecules is essentially unrestricted.
How plasmodesmata are built and operated
A primary plasmodesma forms during cell division, when ER tubules become trapped in the developing cell plate as the phragmoplast lays down new wall between daughter nuclei. The trapped ER is compressed against the encircling plasma membrane to a diameter of ~15 nm, becoming the desmotubule, while the cytoplasmic sleeve — typically 2 to 4 nm wide — runs around it. Spoke-like proteins (still partly uncharacterized at atomic resolution) connect desmotubule to plasma membrane and likely partition the sleeve into ~3 nm channels through which solutes diffuse. The cell walls on either side of a plasmodesma are thinner than elsewhere, and a callose collar reinforces the neck region.
Operation is governed by the cytoplasmic sleeve geometry and the callose collar. Increasing callose narrows the neck and reduces SEL; reducing callose widens it. CALS3 (callose synthase 3) generates callose at plasmodesmata; beta-1,3-glucanases (beta-1,3-glucanase BG_PPAP and others) hydrolyze it. Movement proteins — TMV 30K, the most studied — bind to a plasmodesmal receptor, recruit chaperones, displace the spoke proteins, and dilate the sleeve. They simultaneously bind viral RNA cooperatively, packaging it into a transit-competent ribonucleoprotein. The whole unit walks along the desmotubule (likely using motor proteins or actomyosin tracks) to the next cell. Recovery is fast: SEL returns to baseline within minutes after the movement protein dissociates.
Symplast vs apoplast vs gap junctions
| Feature | Symplast (via plasmodesmata) | Apoplast | Animal gap junction |
|---|---|---|---|
| Spatial domain | Continuous cytoplasm + ER across cells | Cell walls + extracellular spaces | Continuous cytoplasm across animal cells |
| Channel diameter | ~2.5 nm cytoplasmic sleeve | Variable cell-wall pores ~5-20 nm | ~1.4 nm pore through connexins |
| Size exclusion limit | ~1 kDa (default), up to ~50 kDa (regulated) | Limited mostly by cell wall + Casparian strip | ~1 kDa, regulated by phosphorylation/Ca2+ |
| Selectivity | SEL-based + sequence-specific MP trafficking | None until Casparian strip; bulk flow | Charge-selective; gated by voltage and Ca2+ |
| Driving force | Diffusion + cytoplasmic streaming | Transpiration tension + osmotic gradients | Diffusion |
| Molecular building block | Plasma membrane + ER (desmotubule) | Cellulose, lignin, pectin, hemicellulose | Connexin 26/43 hexamers (vertebrate) |
| Regulator | Callose deposition at neck (CALS3) | Cell wall composition, suberin in Casparian strip | Phosphorylation, pH, voltage, Ca2+ |
| Pathogen exploitation | Plant viruses (movement proteins) | Bacteria (Pseudomonas, Xanthomonas) | Some animal viruses, but rare |
| Discovered | Tangl 1879; named Strasburger 1901 | Munch 1930s pressure flow theory | Revel and Karnovsky 1967 (electron microscopy) |
Famous experiments and case studies
- Tangl 1879 — first description. Eduard Tangl observed thin connecting strands between adjacent cells in onion endosperm, drawing them in Sitzungsberichte der Akademie der Wissenschaften Wien. He proposed they functioned as cytoplasmic bridges enabling intercellular communication, decades before electron microscopy could resolve them.
- Wolf, Deom, Beachy 1989 — TMV movement protein. Showed that the TMV 30K protein localizes to plasmodesmata, increases SEL from ~1 kDa to ~50 kDa as measured by microinjected fluorescent dextrans, and is necessary and sufficient for cell-to-cell viral spread. Established the paradigm of viral movement proteins as plasmodesmal modulators.
- Helariutta and Benfey 2000 — SHR cell-to-cell movement. Genetic analysis in Arabidopsis showed SHORT-ROOT mRNA is made only in stele cells, but SHR protein moves one cell layer outward through plasmodesmata to the endodermis, where it activates SCARECROW and specifies the cell layer's identity. The first clean demonstration of selective plasmodesmal trafficking of a transcription factor.
- Vaten et al. 2011 — CALS3 dominant mutants. Identified gain-of-function CALS3 alleles in Arabidopsis that increase callose at plasmodesmata and block phloem-to-root symplastic communication, dramatically reducing root growth. Provided direct genetic evidence that callose deposition at plasmodesmal necks regulates symplastic flux.
- Hamilton and Baulcombe 1999 — RNAi systemic spread. Demonstrated that a transgene-derived silencing signal in tobacco moves systemically through plasmodesmata and phloem to silence cognate genes throughout the plant. Founded the field of mobile small RNAs and underpins the growing use of RNAi-based crop protection (Greenlight Biosciences, Bayer Calantha).
- Maule lab cryo-ET 2018-2024. Cryo-electron tomography of Arabidopsis plasmodesmata is producing the first high-resolution structures of the desmotubule, sleeve, and spoke architecture, revising classical models. Multiple papers from the Tilsner, Maule, and Bayer labs.
Frequently asked questions
What is the size exclusion limit of plasmodesmata?
Under resting conditions in mature tissues, plasmodesmata pass molecules up to roughly 1 kilodalton — small sugars, amino acids, ions, and the occasional small peptide. The cytoplasmic sleeve between desmotubule and plasma membrane is only ~2.5 nanometers wide, which sets the limit. The size exclusion limit (SEL) is regulated dynamically: callose deposition at the neck region narrows the channel and reduces SEL, while viral movement proteins, plant transcription factors with mobility signals, and developmental signals can dilate the pore to pass molecules of 50 kDa or larger. SEL also varies with tissue: shoot apical meristems have larger SEL (~50 kDa) to support symplastic signaling among stem cells.
How is the symplast different from the apoplast?
The symplast is the continuous cytoplasmic interior of the plant body, threaded together through plasmodesmata. The apoplast is the external space outside the plasma membranes — cell walls, intercellular spaces, and the lumens of dead xylem vessels. Water and small solutes move through both routes, with the relative contribution depending on tissue and conditions. Symplastic transport is selective and active, governed by SEL and active loading; apoplastic transport is bulk and passive but is blocked at the Casparian strip in roots, forcing all root-to-shoot solutes through symplastic loading at the endodermis. Sucrose loading into phloem can be apoplastic (in many crops including Arabidopsis) or symplastic (in trees and many tropical plants) — a major distinction in crop physiology.
How do plant viruses use plasmodesmata?
Plant viruses cannot leave a cell directly because they cannot cross the rigid cell wall. Instead, every successful plant virus encodes a movement protein (MP) that binds the viral genome and traffics it through plasmodesmata to the next cell. Tobacco mosaic virus 30K MP, identified by Roger Beachy in the 1980s, was the first MP characterized — it raises plasmodesmal SEL from ~1 kDa to ~50 kDa, binds viral RNA cooperatively, and walks the ribonucleoprotein complex along the desmotubule. Other viruses (potyviruses, geminiviruses) encode entire suites of movement-associated proteins that form tubules through plasmodesmata, large enough to pass intact virions. About 30 percent of crop yield loss to disease worldwide is virus-mediated and depends on plasmodesmal trafficking.
What is callose and how does it regulate plasmodesmata?
Callose is a beta-1,3-glucan deposited in the cell wall around plasmodesmal necks. Callose synthases (CALS3, CALS8 in Arabidopsis) build it; beta-1,3-glucanases break it down. Callose deposition narrows the cytoplasmic sleeve and reduces or abolishes molecular flux. Plants use callose to close plasmodesmata in response to pathogens (sealing off infected cells), wounding, salicylic-acid-mediated defense signaling, and during seed dormancy. Callose dynamics also pattern development: callose-mediated closure isolates the embryo from the maternal tissue, and callose levels in shoot apical meristems regulate the symplastic communication that maintains stem cell identity. Loss-of-function CALS3 mutants in Arabidopsis show enhanced cell-to-cell movement of fluorescent tracers.
How were plasmodesmata discovered?
Eduard Tangl described pit-field connections between adjacent plant cells in onion endosperm in 1879, naming them plasmaverbindungen. The term plasmodesmata was coined by Eduard Strasburger in 1901. Their tubular structure and the desmotubule were resolved by electron microscopy in the 1960s by Lopez-Saez and others. The functional symplastic connection — proving small molecules diffuse between cells — was established by Goodwin and coworkers in the 1980s using fluorescent dyes injected into single cells. Modern characterization of viral movement proteins (Wolf, Deom, Beachy 1989), regulated SEL (Maule lab), and the role of callose synthases (Vaten 2011) has built up a molecular picture, but high-resolution structures of an intact plasmodesma remain elusive — current cryo-electron tomography work in the Maule and Tilsner labs is the cutting edge.
Do animals have anything analogous to plasmodesmata?
Animal cells use gap junctions to achieve a similar function — direct cytoplasmic connection between adjacent cells — but the molecular architecture is completely different. Gap junctions are formed by hexamers of connexin proteins (or innexins in invertebrates) creating ~1.4 nm pores in the plasma membrane, with a similar size exclusion limit of ~1 kDa. Plasmodesmata are membrane-lined channels through cell walls with a central desmotubule of ER, an architecture animals do not have. Functionally both pass small metabolites, ions, and second messengers; both are regulated; both can be opened or closed in response to signals. The convergence on ~1 kDa SEL and the same need for selective communication between adjacent cells is striking — independent solutions to the same problem.