Cytokinins, Ethylene and Abscisic Acid

Cytokinin's discovery grew out of plant tissue-culture research led by F. Skoog and his co-workers, who were searching for a chemical factor able to induce cultured plant cells to divide (undergo cytokinesis) reliably. They found that autoclaved (heat-sterilised) herring sperm DNA promoted exactly this kind of cell division, and went on to isolate and crystallise the active compound responsible, a modified adenine derivative they named kinetin; kinetin itself does not occur naturally in plants, but a closely related, genuinely natural cytokinin, zeatin, was subsequently isolated from corn (maize) kernels and is also abundant in coconut milk, which is part of why coconut milk has long been used as an effective, if empirically discovered, growth supplement in plant tissue culture media. Cytokinins are synthesised chiefly in regions of active cell division -- root apices, developing shoot buds, and young, developing fruits and seeds. Their central physiological effect, as their discovery history suggests, is to promote cell division; beyond this, cytokinins actively mobilise nutrients toward the tissue in which they are applied and, in doing so, delay the onset of senescence in that tissue -- a phenomenon known as the Richmond-Lang effect, after the researchers who first demonstrated that a detached leaf treated with cytokinin stays green and metabolically active for markedly longer than an untreated detached leaf. Cytokinins also help overcome apical dominance (working in this respect against auxin), promoting the growth of lateral shoots, and they promote the formation of new adventitious shoots in tissue culture, which is one reason cytokinin-auxin ratios are so carefully manipulated in micropropagation protocols.

Ethylene is set apart from the other four plant growth regulators by being the only one that occurs naturally as a simple gas, and its striking effects on plants -- most obviously on fruit ripening -- were noticed long before the compound responsible for them was correctly identified, including the once-puzzling observation that fruit stored near leaking coal-gas pipes or in the presence of already-ripening fruit ripened unusually fast. Ethylene is produced particularly abundantly in ripening fruit and in senescing (ageing) tissues generally, as well as at nodal regions of the stem, and its best-known effect is to trigger and accelerate fruit ripening, driving the sharp rise in respiration rate -- the respiratory climacteric -- characteristic of climacteric fruits such as banana, mango and tomato as they ripen. Ethylene also breaks seed and bud dormancy in a number of species, initiating germination in peanut seeds and triggering the sprouting of stored potato tubers; it promotes rapid elongation of internodes and petioles in deep-water rice varieties, helping the leaves and growing tip stay above rising floodwater; it accelerates senescence generally and promotes the abscission of leaves, flowers and fruits; and it promotes the formation of predominantly female flowers in cucumber and related cucurbits, a horticulturally useful effect for increasing fruit yield, and is used commercially to synchronise fruit-set in pineapple plantations. Because pure ethylene gas is inconvenient to apply directly in the field, growers commonly use ethephon, a liquid compound that is absorbed by plant tissue and then breaks down within the plant to release ethylene gas slowly, achieving the same physiological effects in a practical, sprayable form. …