A plant is a complex organism, but unlike animals, its cells are fixed in place by rigid walls. To coordinate their activities, plant cells are connected by microscopic channels called plasmodesmata. These structures pass through the cell walls of adjacent cells, creating a network for direct communication and transport. This system allows for the movement of nutrients and signals necessary for growth and development.
The Structure of Plasmodesmata
Each plasmodesma is a channel lined by the plasma membrane, the outer boundary of a plant cell. This creates a continuous tube that directly connects the cytoplasm of one cell to its neighbor. This shared cytoplasm within the channel is called the cytoplasmic sleeve, which is an active environment for molecular passage.
Running through the center of this channel is a thin, rod-like structure called the desmotubule. The desmotubule is a modified part of the endoplasmic reticulum, an organelle involved in protein and lipid synthesis, that extends from one cell to the next. This central rod provides structural stability to the channel.
The space for transport is the area between the desmotubule and the plasma membrane. Globular proteins are embedded within the plasma membrane and associated with the desmotubule’s surface. These proteins help recognize molecules and regulate what passes through the channel. This organization creates a channel between 30 and 50 nanometers in diameter.
Regulating Molecular Traffic
Transport through plasmodesmata is a regulated process. These channels act as molecular sieves, governed by the Size Exclusion Limit (SEL), which defines the maximum size of a molecule that can pass freely. For many plasmodesmata, this limit is small, allowing only water, ions, and small metabolites to move between cells.
Plants can dynamically alter the SEL of their plasmodesmata in response to developmental or environmental signals. A primary mechanism for this regulation involves the polysaccharide callose. When a plant needs to restrict transport, enzymes deposit callose at the neck of the plasmodesma, constricting the channel and reducing its SEL to isolate cells.
Conversely, to increase transport, other enzymes break down the deposited callose. One such enzyme is a β-1,3-glucanase, which hydrolyzes callose and widens the channel. This ability to rapidly modify the channel’s aperture allows the plant to control intercellular communication, transforming plasmodesmata from simple pores into responsive structures.
Role in Plant-Wide Communication and Development
The regulation of molecular traffic through plasmodesmata coordinates a plant’s growth and development. By enabling the passage of signaling molecules like hormones and transcription factors, these channels allow the plant to function as an integrated organism. This movement ensures that different parts of the plant, such as roots and leaves, develop in a coordinated manner.
This interconnectedness creates symplastic domains, which are groups of cells connected by open plasmodesmata that share a common cytoplasm. Within these domains, developmental signals move freely, directing processes like cell fate decisions. For example, the KNOTTED1 protein, a transcription factor, was one of the first proteins discovered to travel between cells through plasmodesmata.
The configuration of these channels changes as the plant matures. Younger, developing tissues have simpler plasmodesmata that permit more extensive communication. As tissues mature, the plasmodesmata may become more complex and branched, and their permeability may decrease. This developmental regulation of communication helps orchestrate the plant’s body plan.
Viral Movement and Plant Defense
The channels important for plant communication also represent a vulnerability. Plant viruses have evolved to exploit plasmodesmata to spread throughout the plant. To establish a systemic infection, viruses must move from the initial infection site to other cells by hijacking these intercellular conduits.
To achieve this, many viruses produce specialized movement proteins. These proteins interact with the plasmodesmata, manipulating them to increase the channel’s Size Exclusion Limit. The movement protein facilitates the passage of the viral genome, often as a nucleic acid-protein complex, through the dilated channel. Other viruses can modify the plasmodesma so extensively that entire virus particles can move through.
In response, plants have developed defense mechanisms centered on plasmodesmata. Upon detecting a viral attack, a plant can close the plasmodesmata surrounding the infection site. This is accomplished by the rapid deposition of callose, which quarantines the infected cells and prevents the virus from spreading. This process makes plasmodesmata a site where the plant’s immune system fights to restrict pathogen movement.