Plants run on hormones without a single gland to make them
Animals make hormones in dedicated glands. Plants have none, yet almost any plant cell can produce the signalling chemicals that control growth, flowering, defence and dormancy. They work at vanishingly small concentrations, which is why scientists only began untangling them properly in the late 1970s, after nearly a century of study.
Plant hormones, once called phytohormones, a term Went and Thimann put in the title of their 1937 book, are simple molecules that move through the plant by several routes. Within cells they ride cytoplasmic streaming; between neighbours they diffuse slowly; over longer distances they travel in the phloem, which carries sugars down from the leaves, or the xylem, which lifts water up from the roots. They are not nutrients. They act at roughly one millionth to one hundred thousandth of a mole per litre, and cells respond only at particular stages of growth, so a plant must switch signals on and off precisely by storing, inactivating, breaking down or diluting them.
The original five classes were abscisic acid, auxins, gibberellins, cytokinins and ethylene, and brassinosteroids, jasmonates, salicylic acid and strigolactones have since joined the list. Hormones often work together: auxins and cytokinins in combination boost cell division and organ formation more than either achieves alone.
Abscisic acid is a brake. It was studied under two different names before researchers realised they had the same compound, and was named for its abundance in freshly fallen leaves, though it turned out not to cause leaf drop after all. Its real job is dormancy: it keeps buds and seeds from sprouting in a warm spell mid-winter only to be killed by the next frost, and it stops seeds germinating inside ripe fruit. When roots run short of water, a relay of signals ends with abscisic acid causing guard cells to lose pressure and close the leaf pores.
Auxins, the first growth regulators discovered and first described by the Dutch biologist Frits Warmolt Went, loosen cell walls so cells can stretch, suppress buds lower on the stem so the tip dominates, and trigger root formation, which is why gardeners dip cuttings in synthetic auxins. In high doses they are toxic, especially to broad-leaved plants, a property exploited by herbicides such as 2,4-D. Brassinosteroids, the only steroid hormones in plants, were found in rapeseed pollen in 1979.
Source: Plant hormone
