How Does a Plant Decide Anything? Chemical Signals Instead of Nerves

An animal responds to its environment by moving, which requires fast signals and a nervous system to carry them. A plant cannot move and responds instead by growing differently, which is slower and needs no nerves, so its coordination is chemical throughout. Signal molecules are produced in one tissue, travel through the plant by diffusion or in the transport streams, and change how cells elongate, divide or differentiate where they arrive. The same molecule frequently produces opposite effects in different tissues or at different concentrations, which is unlike the specific messages of most animal hormones and makes the system harder to describe simply. What matters is generally the ratio between several signals rather than the amount of any one.

A handful of classes account for most of what is understood:

• •Auxin, which drives cell elongation and is central to bending towards light and to root and shoot architecture
• •Gibberellins, which promote stem elongation, germination and flowering
• •Cytokinins, which promote cell division and delay ageing in leaves
• •Abscisic acid, which closes leaf pores under water stress and maintains seed dormancy
• •Ethylene, a gas, which triggers ripening, leaf fall and responses to damage
• •Several others including jasmonates and salicylic acid, which coordinate defence against attack

The response that started the field is worth following through, since it demonstrates the whole logic. A shoot illuminated from one side bends towards the light, and experiments beginning with Darwin and his son in the 1880s established that the tip detects the light while the bending occurs below it, which implied that something travelled down from the tip. Later work showed that a signal could pass through a permeable barrier but not an impermeable one, and eventually that the substance is auxin, which redistributes towards the shaded side. Cells on that side elongate more, and the differential elongation bends the shoot. Nothing decides anything and no tissue is instructed, since an asymmetric distribution of a growth-promoting molecule produces the bend as a mechanical consequence.

Some of these molecules leave the plant entirely, which raises questions about what counts as communication. A plant under attack by insects releases volatile compounds, and neighbouring plants exposed to those volatiles increase their own defensive chemistry before being attacked themselves, which has been demonstrated repeatedly in both laboratory and field conditions. Whether this is signalling in any meaningful sense is disputed, since the sender gains nothing obvious from warning a competitor, and the leading interpretation treats the volatiles as within-plant signalling that neighbours eavesdrop on. The same volatiles attract predators and parasites of the attacking insect, which does benefit the sender. Underground, connections through fungal networks have been proposed as a further route, and claims about the extent of that have run well ahead of the evidence.

These molecules and their synthetic analogues are used throughout agriculture and horticulture. Rooting powders contain auxin analogues that induce cuttings to form roots. Several important herbicides are auxin analogues that plants cannot break down, causing uncontrolled growth that kills broadleaved species while leaving grasses, including cereals, largely unaffected, which is why selective weedkilling works at all. Ethylene is applied to ripen fruit picked hard for transport, and it is excluded by controlled atmosphere storage to hold fruit unripe for months, which is why apples are available year round. Gibberellin applications increase berry size in grapes. Growth retardants that block gibberellin production keep ornamental plants compact. The commercial scale of all this is considerable and is largely invisible to consumers.

Plants coordinate by chemical signals rather than nerves, and respond by growing differently rather than by moving, with the ratio between signals mattering more than any single amount. Auxin redistributing towards the shaded side of a shoot makes those cells elongate, which bends it towards light mechanically. Auxin analogues are selective herbicides and ethylene control is why apples are available year round.