Charles Darwin performed experiments on plant movements with his seventh child, Francis Darwin, which they wrote up as joint authors in 1880 (Francis was a young man at the time). Every year, thousands of children and budding scientists repeat these experiments:
- Shine a light on a plant from just one source, and observe as plants orientate their growth to ‘face’ the light.
- Adorn growing shoots with a ‘cap’ that is impermeable to light and observe as they grow straight, even as light hits the open stem.
The tip of the plant senses the direction of the light, and this information is transmitted to the growing stem to direct growth behaviour. That transmission is performed by auxins, the first of which was isolated in the 1930s.
By the early part of the 20th Century, as the chemistry of life was becoming clearer, people began to see that the way animals and plants transmitted information through their bodies must itself be chemical. In animals, the information-transfer network can make use of nerve structures. But in plants, the only sensible option is to use diffusible chemicals.
The first auxin to be identified was indole-3-acetic acid, isolated and characterised by Kenneth Thimann, a transplanted Brit at Harvard University. He literally wrote the book on plant hormones, together with a Dutch transplant to California, Frits Warmolt Went.
Plants are very amenable to physical manipulation. For example, one can cut shoots from the plant and re-grow them on porous substrates, such as agar jelly. Well before he joined forces with Thimann, Went knew that there were diffusible, growth-promoting substances he could ‘capture’ in that agar jelly and move around to promote growth in other parts of the plant. He coined the phrase “auxin” (from the Greek meaning ‘to increase’) to describe this property.
Auxins aren’t just used in light sensing– they are reused throughout plant development. They do not rely on ‘normal’ molecular diffusion through the plant material; rather, every plant cell helps set up particular auxin transport gradients.
By tightly organising sensing cells –light sensing at the tips of shoots and gravity sensing at the tips of roots – and connecting their auxin-producing components to the correct counterparts, the beautiful growth of plants can be controlled precisely. That allows plants to adapt to whatever environment it finds itself in.
It takes a village
Unsurprisingly, this chemical gradient can be subverted for other uses – both good and bad.
Plants do not live in isolation. Their roots generally co-exist with a complex web of microbes and fungi, which extend the root’s nutritional reach – so much so that in sterile soil, most plants will not flourish. This community of bacteria and fungi will often make auxins themselves to help stimulate the root growth, encasing themselves in a protective, sheltered environment.
An extreme example of this is legume plants, which develop ‘bacteria hotels’ to supply them with plentiful sugar and water. In exchange, the bacteria expend huge amounts of energy in capturing nitrogen from the atmosphere – the plant’s own, captured fertilizer factory.
Auxins are just one variety of chemical that can be swapped between bacteria guest and plant host, like tickets to a VIP lounge, ensuring the right bacteria are coming in. Other, not-so-friendly organisms have caught onto the power of auxins. For example, many nematodes produce auxin to trigger root growth around them, forming ‘giant cells’ and other super-structures for their own, exclusive benefit – and certainly no benefit to the plant
The constant need to innovate to secure the communication network, keeping the signals away from opportunistic parasites, is why there is such a diversity of auxins. Plants are constantly shifting their internal messaging system, keeping one step ahead of the game as parasites start to exploit them