Secondary PlasmodesmataEdit
Secondary plasmodesmata are specialized cytoplasmic channels that form after cell division to fuse neighboring plant cells into a continuous symplast. They extend the connectivity provided by primary plasmodesmata, which originate during cytokinesis, and they enable intercellular movement of signals, metabolites, and macromolecules across existing cell walls. The formation and regulation of these channels are tightly linked to cell wall remodeling and endoplasmic reticulum dynamics, making them central to plant development, physiology, and defense.
Structure and formation
Origin and morphology
- Secondary plasmodesmata arise in walls that have already formed, creating new, trans-wall conduits that connect adjacent cells beyond the original network established during cytokinesis. They can be simple channels, or complex, branched structures that emanate from a single wall locus to multiple neighboring cells.
- A characteristic feature of plasmodesmal channels is the presence of an appressed endoplasmic reticulum strand, often termed a desmotubule, that traverses the channel and connects the ER of adjacent cells. This ER continuity is a defining structural element of many plasmodesmata, including secondary forms.
Desmotubule and membrane organization
- The pore is bounded by the plant plasma membrane, and the embedded desmotubule represents ER-derived membrane that runs through the pore. The arrangement of the PM and ER within secondary PD can affect channel geometry, including diameter and the distribution of cytosolic space available for transport.
Formation mechanisms
- The precise cellular steps by which secondary PD are formed remain an active area of research, but they are broadly understood to involve targeted remodeling of the cell wall followed by the ingress of membrane and ER-derived components to establish a stable channel through which cytoplasm can mix. This process can be accompanied by early localization of plasmodesmata-associated proteins that guide pore formation and stabilization.
- Once established, the opening of these channels is dynamically regulated, allowing or restricting movement depending on developmental cues or stress signals.
Regulation of transport and functional significance
Symplastic transport and gating
- Plasmodesmata, including secondary ones, regulate the passage of small metabolites and macromolecules between cells. The permeability—often described as the size-exclusion limit (SEL)—is modulated by local deposition or removal of callose, a polysaccharide that stiffens the wall around the channel and constrains flux.
- Enzymes that synthesize and degrade callose, such as callose synthases and β-1,3-glucanases, adjust SEL in response to developmental cues or stress. The dynamic balance of these activities determines how readily signaling molecules, RNAs, transcription factors, or nutrients move between cells through secondary PD.
Role in development and pattern formation
- Secondary PD contribute to coordinated development by enabling rapid, coordinated signaling across tissues. They support gradient formation, organ primordia formation, and the establishment of robust patterning in leaves, roots, and reproductive structures.
- The distribution and regulation of secondary PD are linked to tissue-specific demands for rapid communication, particularly in tissues where primary PD density is insufficient to sustain synchronized growth.
Defense and pathogen interactions
- In plant defense, plasmodesmata can be gated to restrict pathogen movement. Callose-mediated constriction of PD is a common defense response that limits access to the symplast for viral genomes or bacterial effectors.
- Pathogens and their movement proteins often manipulate PD to promote cell-to-cell spread, while plants counteract this by reinforcing walls around PD or by altering ER and PM organization at the pore. Secondary PD thus sit at an interface where developmental signaling, nutrient transport, and immune responses converge.
Molecular players and signaling
Plasmodesmata-located proteins (PDLPs)
- PDLPs are a family of proteins associated with plasmodesmata that influence pore function and regulation. They participate in signaling networks that modulate SEL and the structural integrity of channels.
Callose synthesis and degradation
- Callose synthases (CalS or callose synthases) and β-1,3-glucanases regulate the deposition and removal of callose around PD. This dynamic cycling directly affects the permeability of secondary PD and the plant’s ability to control intercellular traffic during development or stress.
Cargo and signaling molecules
- A variety of signaling molecules, including transcription factors, small RNAs, and metabolic intermediates, can move through secondary PD. The selective transport of these cargos contributes to coordinated responses across tissues and to developmental timing.
Interactions with endomembrane systems
- The formation and maintenance of secondary PD are connected to vesicular trafficking and the organization of the endomembrane network. ER continuity through the desmotubule and its connections to the cytoskeleton influence pore stability and transport capacity.
Research and perspectives
Comparative occurrence
- Across plant species and tissues, the prevalence and functionality of secondary PD vary. Some tissues rely more heavily on secondary PD to meet demands for rapid, long-distance intracellular signaling, while others depend primarily on primary PD and transport via specific transporter systems.
Open questions
- Key questions in the field include the exact molecular steps by which wall remodeling enables secondary PD formation, how cells target specific wall regions for pore insertion, and how environmental conditions influence PD dynamics at the molecular level.
- Ongoing work seeks to clarify how secondary PD integrate with other intercellular transport pathways, how they contribute to developmental robustness, and how their regulation interfaces with immune signaling.
Biotechnological implications
- Understanding secondary PD offers potential strategies to influence plant growth, nutrient distribution, and disease resistance by tuning symplastic connectivity. For example, the controlled modification of PD permeability could impact phloem loading, stress resilience, or the spread of viral pathogens in crops.