Plant cuttings (left) can form new plants in a matter of weeks (right).

Plants: the green starfish of the world

Published in

Plant Cell Extracts

5 min readOct 27, 2017

By Patrice A. Salomé

By and large, most animals cannot grow back missing limbs or organs. Only a few creatures on Earth can regenerate severed organs: some starfish can regrow a severed arm; a salamander can grow a new tail, or even a new arm, when given time.

Most plants, on the other hand, perform this feat of nature without giving it a thought. How do they do it? It turns out that most plant cells can turn back the differentiation clock and make new plants from a root or a leaf cell. New work by Ohbayashi and colleagues show that this ability relies on a plant hormone and protein synthesis. Let me introduce you to this amazing hormone called auxin, derived from the greek word “to grow”.

Auxin and Charles Darwin, after the Beagle

Charles Darwin dabbled in plant biology research decades after his tour of the Galapagos Islands, in his book “The power of movement in plants” published in 1880. He worked with coleoptiles from grass: coleoptiles emerge first from the germinating seed and encase the young and tender first leaf, acting as a protective sheath against rough soil particles when pushing their way to the surface.

Darwin used grass coleoptiles to study the growth-promoting effects of an “influence” originating from the tip of the sheath. Later botanists chose similar plants to further describe this “influence”, which we now know as the phytohormone auxin.

Coleoptiles grow towards the light, very much like sunflowers. This curving is caused by cell elongation on the side of the coleoptile that is facing away from the light. Darwin noticed that only the tip, and not the sheath of coleoptiles detected the direction of light, by either cutting off or covering the tips with an aluminum “hat”. Darwin concluded that some “influence” moved from the coleoptile tips down and caused the sheath to bend in the direction of light.

Agar: Jell-O for scientists

Other scientists picked up where Darwin left off. The Danish scientist Peter Boysen-Jensen showed in the 1910s that Darwin’s “influence” was water-soluble, as it was able to go through a thin block of agar placed between a freshly-cut coleoptile tip and the coleoptile sheath. Agar itself is inert, and looks and behaves just like Jell-O, except without food coloring or flavor.

In the 1920s, the dutch botanist Fritz Warmolt Went placed coleoptile tips on blocks of agar to allow the “influence” to diffuse and accumulate into the gel, and then transferred the blocks onto new freshly decapitated coleoptiles. No curving happened when the blocks were placed in the center of the sheath, but Went was able to observe bending if he placed the block of agar (infused with the “influence”) to the side. Not only that, but cell elongation causing the curvature in the coleoptile happened on the side in contact with the block of agar. F.W. Went later developed an assay, based on this elongation response, to quantify the growth substance, which we now know as auxin.

Auxin and human pee

Auxin was technically first identified in bacterial growth medium in 1885 by the german biochemist Ernst Salkowski, but was not recognized as a growth-promoting hormone. It was only in 1931 that the dutch chemists Fritz Kögl and Arie Jan Haagen-Smit purified auxin — from human pee.

Their idea was brilliant and yet simple: one might purify a plant hormone from plant extracts, but it would come laced with sugars, proteins and amino acids in far greater quantities than auxin. They needed a good purification method that would remove sugars and amino acids, and they found it in the human kidney. We eat plants, but are immune to the effects of their hormones, which go right through the kidneys for disposal; or at least that’s the theory I would like to believe. An alternative explanation for the presence of auxin in human urine is based on the conversion of the amino acid tryptophan into a compound very close to auxin by some bacteria in the gut; much less interesting in my opinion.

Auxin as the master of plant growth

Research on auxin has come a long way from cutting off the tips of coleoptiles, or collecting human urine. We now know the identity of the genes responsible for the biosynthesis and perception of the hormone inside of plant cells. We also know a great deal about what auxin does in plants, and how critical it is to agriculture and amateur horticulturists alike.

Low auxin concentrations in the growth medium induce the formation of roots from stem cuttings The mutant cannot make as much protein synthesis, and cannot propagate new roots. Higher auxin induces new shoots to emerge.

With their new study, Sugiyama and colleagues probe the mechanisms by which detached plant organs can regenerate into a whole plant, or part of a plant, essentially the same way you might try and get plant cuttings to root at home: they placed plant cuttings onto growth medium containing auxin, and waited for the formation of new roots or shoots. They found that protein synthesis is critical for making new organs. If protein synthesis is decreased or blocked by mutations, plants will abort the formation of new organs.

These results provide the evidence that amateur horticulturists had noticed for decades: healthy cuttings are critical to propagation efforts. Sugiyama’s results show that plants perform a balancing act of checks and balances; only instead of bank accounts and money, they trade in proteins.