Cells grow. Whether they are a part of an antelope, an azalea or an amoeba, they expand, divide and expand again. This poses few problems for animal cells, but for plant cells encased in a tough, inflexible cell wall, things are not so simple, Daniel J. Cosgrove of Pennsylvania State University explains in Nature1.

Plant cell walls have defined shape, strength and rigidity, making them a straightjacket to growth. So plants produce some remarkable proteins -- 'expansins' -- which loosen cell walls allowing them to expand.

The plant cell wall is a highly versatile composite material that is the basis for products from wood and paper to paint thickeners and explosives. It is a meshwork of insoluble cellulose fibres -- which give it tensile strength -- imbedded in a matrix of gelatinous sugar polymers known as glycans. It is analogous to modern materials such as fibreglass and mylar.

When under tension, plant cell walls very slowly lengthen owing to slippage of the cellulose fibres against one another. Placing the cells in an acidic solution greatly enhances this creep.

In the early 1990s researchers worked out that this 'acid growth' is not an inherent property of the cell wall, but depends on expansins. These proteins were first isolated from cucumbers, but were soon spotted throughout the plant kingdom.

How expansins alter the mechanical properties of the cell wall is far from clear. Apparently they unravel the crosslinks that matrix glycans form between cellulose fibres, by allowing them to slide more easily over each other.

Expansins are also implicated in the ability of plants such as maize to survive periods of drought, by helping maintain the growth of their roots. In fruits such as tomato, strawberries and melons, expansins are involved in ripening -- softening the fruits and improving their texture while other enzymes break down their complex polysaccharides to produce their sweet taste.

Grasses have highly specialized expansins -- ðb-expansins, found abundantly in their pollen -- which can act only on the cell walls produced by this family of plants. They aid fertilization by softening the stigmas on which the pollen lands, helping it grow along tube down through the flower's style to the ovary.

ðb-expansins have been known since the early 1980s, long before their activity was uncovered, as the source of misery for millions of people. They are the major allergen in pollen, and hence the chief cause of hay fever and seasonal asthma.

The plant cell wall is unique to the plant kingdom, so it is perhaps not surprising that expansins are found only in plants. Despite the wealth of sequence data now becoming available, no expansin genes have been found in animals or even fungi.

Except, that is, in the common garden snail Helix pomatia, which has expansin-like proteins in its digestive tract. Presumably these help the snail digest plant matter. It is not yet clear whether the snail synthesizes these proteins itself, or whether microorganisms living in its gut produce them.

Expansins are prime targets for many branches of biotechnology: in genetic engineering to improve plants' growth, drought tolerance, fertility and fruit texture; as tools in the lumber, paper and textile industries; and perhaps as a spur to develop new ways of working and shaping what we consider 'modern' composite materials.

Above all, these curious proteins are yet another example of just how alien plants are from animals. By adopting such different lifestyles they have encountered unique problems which evolution has solved in equally unique and intriguing ways.