A fourth class of plant hormones is cytokines, so-called because stimulates cell division (cytokinesis).
Cytokines are produced in the roots and transported through the xylem to all parts of the plant. Embryos and fruits also produce cytokines.
The role of cytokines in plant development has been studied in tissue cultures. When a fragment of a plant, a piece of parenchyma, for example, is placed in a culture medium containing all the nutrients essential for its survival the cells can grow but not divide.
If we only add cytokine to this medium, nothing happens, but if we also add auxin, the cells begin to divide and can differentiate into various organs.The type of organ that arises in a plant tissue culture depends on the relationship between the amounts of cytokine and auxin added to the medium. When the concentrations of the two hormones are equal, the cells multiply but do not differentiate, forming a mass of cells called callus. If the auxin concentration is higher than that of cytokine, callus forms roots. If, on the other hand, the concentration of cytokine is higher than that of auxin, callus forms shoots.
Cytokines also act in association with auxins in the apical dominance control. In this case, both hormones have antagonistic effects. Auxins down the stem inhibit the development of lateral buds, while cytokines from the roots stimulate buds to develop. When the apical bud is removed, the action of auxins ceases and cytokines induce the development of lateral buds. Once started the development of the lateral buds can no longer be inhibited. The fact that the lower stem buds emerge from dormancy before the highest stem has to do with the fact that they are closer to the roots where the cytokines are produced.
The cytokines also slow aging of plants. Branches and flowers cut and placed in water age rapidly due to the lack of this hormone. Adding cytokine to the pot water makes the cut flowers last much longer. It is common practice in the plant trade to spray cytokine on harvested flowers in order to slow their aging.
Senescence is the set of changes that cause the deterioration and death of the plant cell. In multicellular plants, senescence occurs after juvenility (vegetative growth) and maturity (reproduction) and is rapid in perennials, according to the genetic program characteristic of each type of plant. Senescence is also sensitive to the influence of environmental factors such as short days, low light, low and high temperatures, low levels of essential nutrients and toxic salts in the soil.
Senescence and death may occur at approximately the same time throughout the plant, for annual plants (maize, soybean) and some perennial plants (agave, bamboo), which bloom once and die soon after, or may occur only in aerial part of the biennial and perennial herbaceous plants, where the underground parts remain alive and serve as reserves for next year's growth and can occur only in the leaves and fruits of perennial woody plants. These plants bloom every year and their total senescence and death take many years.
In plants with total senescence, it occurs soon after flowering and fruiting. The removal of flowers and fruits postpones senescence and causes a return to the rapid vegetative growth characteristic of the phase before flowering. In cereals (maize, wheat) senescence is easily observed as it happens almost simultaneously in millions of plants.
Senescence does not occur randomly and when it is total it is more related to internal factors of the plant than to environmental factors. The changes observed at this stage are part of a mechanism of nutrient transfer from parts of the plant such as the leaf to other parts such as fruits, seeds and stem. Often, the senescence of leaves and fruits is accompanied by their abscission.
A possible cause of plant senescence could be the large mobilization of nutrients and cytokines in the direction of fruits and seeds. The death of the vegetative part of the plant would be the consequence of this activity driven mobilization of the auxins produced by the fruits. The removal of flowers and fruits delays and may even prevent senescence. This mobilization could also be considered as an effect rather than a cause of senescence. The theory of mobilization does not explain why the presence of male flowers in male plants causes the onset of senescence while the removal of these flowers slows it down.
The application of growth retardants has the effect of accelerating flower initiation and flowering of certain plants. In pineapple, the application of ethylene, compounds that release ethylene in contact with the plant (ethephon) or auxins that induce the production of ethylene by the plant cause flower initiation and flowering. In cabbage, however, flowering is associated with rapid vegetative growth. This shows that the decrease in growth rate is not always closely associated with flower initiation, flowering and fruiting. In fact, apart from the fact that it is part of the plant's genetic program and is in some cases subject to environmental factors, little is known about the root causes of senescence.