Life's a Gas - if you're a plant
Command and control with gases?!
While nitrogen is an essential element in plant nutrition it also has an important role in plants’ internal communication systems as one half of a gas molecule, nitric oxide, that is involved in many plant signalling systems. This role for nitric oxide is remarkable because it is a highly toxic chemical that quickly damages living tissues and yet scientists have recently found that the controlled production of nitric oxide is important in controlling plant growth, development, and defence. Nitric oxide seems to have a particularly significant role in defence against pathogen attack where it stimulates genes involved in defence reactions and is involved in the Hypersensitive Response (HR) and Systemic Aquired Resistance (SAR). HR is the programmed rapid death of cells at and around pathogen infection sites. This pre-programmed cell suicide is controlled by the plant and activated in response to attack to restrict infection. SAR is the result of a chemical signal, or signals, being transmitted around the plant in response to infection by a pathogen or attack by a pest. The alarm signal activates defence systems in cells remote from the initial site of infection or attack and, by ‘priming’ defences throughout the plant, inhibits the spread of disease and protects against further infection. Nitric oxide is involved in plant responses to stress, such as drought, high temperature and UV, it also has a role in regulating seed germination, root development and several cellular metabolic processes.
The sweet smell of new mown grass is in fact an alarm signal from the wounded leaf cells that switches on the grasses’ defence systems. The ‘green grass’ smell is predominantly one aldehyde (trans-2-hexenal), with other aldehydes and alcohols adding subtler notes. Grasses are an example of plants (including trees, tomatoes and sagebrush) that produce volatile chemicals when attacked or damaged. These signals spread through the plant and activate the plant’s defences. In tomato the volatile chemical methyl jasmonate (the compound responsible for the smell of jasmine) is produced in response to mechanical wounding or insects feeding. Methyl jasmonate is important in defence signalling and in stimulating the activity of genes that produce various protective chemicals. The chemical arsenal that is activated by these alarm gases is extensive and includes compounds that discourage insects from feeding, inhibit fungal and bacterial growth or attract predators and parasites that attack pests feeding on the plant. In some cases the signals themselves are part of the plant’s chemical protection, for example trans-2-hexenal is an antimicrobial compound. There is mounting evidence that plants use volatile chemicals, produced in response to damage, not only as internal signals but as alarms to warn their neighbours. Work with maize, alder and tomatoes suggests that undamaged plants are ‘primed’ to go on the defensive by gases given off by neighbouring plants being attacked by insects or that have been mechanically damaged. It has been suggested that both methyl jasmonate and trans-2-hexenal could act as external alarm signals.
Plants also use the odourless gas ethylene, as a hormone to control their
development. Among its many effects ethylene stimulates growth of dormant
potato buds, germination of grain, and aging of flowers and leaves. Its action
is most easily seen in the ripening of some fruits, for example tomatoes and
bananas, where the maturing fruits produce ethylene that stimulates the ripening
process. The gardener’s old trick of putting unripe tomatoes in a plastic
bag or drawer with ripe tomatoes or bananas is using the ethylene given off
by the ripe fruit to accelerate ripening of the unripe tomatoes.