How Do Plant Cells Talk Through a Wall? They Drilled Holes in It

Each connection is a narrow tube passing through the shared wall between two plant cells. The membranes of both cells are continuous through it, so the outer boundaries join, and a strand of the internal membrane system also runs through the middle, leaving a thin ring of cytoplasm between the two. Material passes along that ring. Thousands of these connections may perforate the wall between a single pair of cells, and the effect is that the interiors of every living cell in a plant form one connected space.

The wall that makes plants possible also creates the problem:

• •A rigid wall lets a plant stand up without a skeleton
• •It also seals each cell off from its neighbours
• •Animal cells touch directly and can signal across the contact
• •Plant cells cannot, so a route through the wall is required
• •Nutrients, hormones and instructions all use these channels
• •So does the machinery that tells a cell what to become

The channels are actively controlled rather than fixed openings, which is the part that surprises people. A collar of a carbohydrate can be deposited around the neck of a channel to narrow or close it, and removed again to reopen it, so a plant can isolate a region of tissue within minutes. Closure happens in response to wounding, to infection and during normal development when a group of cells needs to follow a different programme from its neighbours. The size of molecule that can pass is therefore not a fixed property but something the plant adjusts.

Water and dissolved substances have two ways of crossing plant tissue and the distinction organises a great deal of plant physiology. One route goes through the connected interiors of the cells, passing from cell to cell through these channels and never crossing a membrane again after the first entry. The other soaks through the walls and the spaces between cells without entering any cell at all, which is faster and entirely unselective. At a particular layer in the root the second route is blocked by a waterproof band, which forces everything through a membrane and gives the plant control over what gets in.

A connected network is an opportunity for anything trying to spread, and plant viruses have taken it comprehensively. Most plant viruses are far too large to pass through a channel at its normal setting, so they carry dedicated proteins whose only job is to widen the opening and escort the viral genome through, which is how an infection moves from cell to cell without ever going outside. Identifying those proteins explained why plant viruses behave so differently from animal ones. Some parasitic plants tap into a host's network directly, and aphids feeding on sap exploit the same connected system.

Narrow tubes through the shared wall join the interiors of neighbouring plant cells, with both membranes continuous through them, so all living cells in a plant form one connected space. A carbohydrate collar opens and closes them within minutes, isolating tissue after wounding or during development. Plant viruses carry proteins whose only job is to widen them and pass through.