Kinetin was the first cytokinin to be isolated from the DNA of herring sperm, and zeatin was the first plant cytokinin isolated from coconut liquid endosperm. Since then it has been reported from maize and several other plants. Cytokinins influence cell division and shoot formation. These have a synergistic effect with auxins and ratios of these two hormones affect plant growth. They counter apical dominance induced by auxins and along with ethylene promote leaf, flower, and fruit abscission. Cytokinins are used in tissue culture for cell differentiation, shoot proliferation, shoot morphogenesis and formation of adventitious buds. With several more, such as Isopentenyl adenine (IPA) and dihydrozeatin being identified, Benzyl aminopurine (BAP), fufuryl aminopurine (kinetin), and Thidiazuron (TDZ) are used in plant-tissue culture. TDZ is effective at low concentrations to stimulate shoot formation. Cytokinins have been shown to increase biosynthesis of berberine and condensed tannins and rhodoxanthin in in vitro cultures of their source plants.

Gibberellins

The first gibberellins was isolated form a fungus—Gibberella fugikuroi. At present about 90 gibberellins are reported from plants and they are named GA₁, GA₂, GA₃, etc. They are synthesized by leaves and stored in immature seeds and fruits. Gibberellins cause internodal cell elongation and induce seed germination and seedling establishment. Gibberellins have been extensively studied with respect to their effects on many medicinal plant species such as Eucalyptus, Hops, Citrus, Coriander, Datura and Nicotiana, Vince, Tea, Digitalis, Sienna, and Buck wheat. In plant-tissue culture, they are used rarely and GA₃ when used is reported to induce plantlet generation form adventive embryos in culture.

Ethylene

It is a gaseous growth regulator and emitted especially from storing plant-tissues and ripening fruits (produced by all higher plants in almost all tissues). It is known to influence leaf abscission and fruit ripening and is associated with the triple response of shoot growth, root growth, and differentiation. Ethylene has limited solubility in water and hence diffuses out of cells after being formed, into the plants’ environment. Some young tissues like seedlings make more ethylene much faster than it can be diffused out. Hence there will be inhibition of leaf expansion with further growth, exposure to light, ethylene production decreases, thus promoting leaf expansion. It is used routinely for enhancing flow of rubber latex. Ethylene and related compounds such as 2-chloroethyl phosphonic acid are investigated for their effects on plant-tissue culture. It is reported with variable effect on cell morphogenesis in vitro.

Growth Inhibitors

Several plant compounds are reported with growth inhibitory effect in plants. These affect leaf and fruit abscission, dormancy, seed germination, and bud opening. Abscisic acid isolated from a fungus Cenospora rosicola, is used in embryo culture and to promote morphogenesis.

Brassinosteroids, jasmonates, stringolactones, karrikins, and polyamines are some of the other identified plant growth regulators.