Tripping with triffids- the sense of touch in plant cells

(left) Receptor potentials and action potentials in a cell at normal turgor pressure, and with turgor pressure reduced by adding 150 mM sorbitol (click for enlargement) (right) Sodium oscillations triggered by touch. Adding extra calcium helps the cell recover (click for enlargement)

In John Wyndham's classic science fiction novel "The Day of the Triffids", most of humanity is blinded by watching a meteor shower, and the triffids, motile human-hunting plants, pick off the survivors, one by one. Whilst it is doubtful that the roots of plants could be converted to triffid feet, there can be no doubt that plant cells do have a sense of touch. Why should plants, which have no eyes, need a sense of touch? We might ask why trees don't simply try to grow through brick walls, or how a vine knows where the fence ends, and it should stop climbing. In fact plants require a sense of touch for their very growth and differentiation, and this sensitivity to the mechanical stimulus of touch tells plant cells exactly where they are in the body, when they should divide or stop dividing. The sense of touch in plant cells is due to the presence of mechanosensory ion channels, literally ion channels that open and change the electrical properties of the cell membrane whenever the touch stimulus is received.

It is difficult to study mechanosensory ion channels in whole plant tissues because many cells contribute to an observed response. Charophyte cells are giant cells that enable us to look at the sense of touch in a single cell. In August 2000, Dr Shepherd was given an award from the Australian Academy of Sciences and Japan Society for Promotion of Science to visit the laboratory of Professor Teruo Shimmen, Himeji Institute of Science and Technology, Hyogo, Japan. Professor Shimmen has done ground-breaking work with a special device developed to cause repeatable mechanical stimulus to charophyte cells. The cells respond to touch stimulus with a series of receptor potentials, small flows of ions across the cell membrane. These ion flows are caused by activation of mechanosensory channels. Critical receptor potentials then set off action potentials. The research highlights included a clear demonstration that the receptor potentials are affected by changes in cell turgor pressure (how "pumped-up" the cell is), and that salinity stress in these cells may be due partly to an inability to recover calcium homeostasis following touch stimulus. These freshwater cells are different to salt-tolerant charophyte cells. Here, we find cells secrete charged "goo" that enables them to modulate the mechansoensory channels, and to live in extreme environments, either very salty, or fresh. Somewhere during evolution this capacity was lost by freshwater species.

Virginia Shepherd, Mary Beilby, Chris Cherry, Alan Walker