Ethylene and cytokinins

Most species of angiosperms produce hermaphroditic flowers. The unisexuality in some plants most often results from developmentally programmed abortion or selective reduction in sex organ primordia. In dicots, higher levels of auxins, cytokinins, and ethylene usually correlate with female sex expression and in most of them the femaleness is mainly promoted by ethylene. ... 

Addition of large amounts of these growth hormones tend to suppress ethylene production. However, after ethylene production starts and accelerates, only cytokinin consistently suppresses ethylene production. This suggests a special antagonism between cytokinins and ethylene production. This antagonism is consistent with the well known retardation effect of cytokinins on loss of chlorophyll and protein in aging leaves (19). 

A final exciting approach is to introduce into plants the genes for overproduction of a hormone, or antisense genes for an enzyme involve in hormone synthesis. These transgenic plants have already provided us with important information about the biological roles of auxins, cytokinins and ethylene [32]. 

In most cases, a detectable induction of mRNA levels is observed within 15-30 min following auxin exposure. The levels of three soybean mRNAs (detected by the cDNA clones 6, lOA and 15) are induced by 2.5 min of treatment of excised elongating soybean hypocotyl sections with the synthetic auxin 2,4-D [8]. These mRNAs and others identified in soybean [5, 16], pea [13] and tobacco [14] are specifically induced by auxins such as lAA and NAA, and not induced by nonauxin analogs. With some exceptions, these auxin-induced sequences do not accumulate in response to other plant growth regulators such as GA, ABA, cytokinins and ethylene, or to environmental stresses such as heat shock or cold... 

Figure 3, Influence of 1AA and cytokinins on ethylene production by postclimacteric avocado tissue slices (16)...

There also have been clear indications that interactions of ethylene with auxins, cytokinins, gibberellins and ABA are involved in both ethylene production and action. Generally the effects of ethylene tend to antagonize those of auxins, cytokinins and gibberellins, and tend to reinforce those of ABA, depending, however, on tissue systems involved. Reinforcement of ethylene by ABA and vice versa occurs more frequently in senescence. 

Brassinosteroids are reported to stimulate overall plant growth and development, especially under stress conditions, to enhance auxin-induced growth as well as auxin-induced ethylene production (5, 6). Brassinosteroids interact with most of the phytohormones, such as cytokinins and gibberellins, and in particular with auxin. 

Until the discovery of brassinolide by USDA scientists in 1979, it was thought that only five groups (indole auxins, gibberellins, cytokinins, abscisic acid, and ethylene) of hormones were responsible for regulating plant growth and development. Following this discovery, a number of compounds similar to brassinolide both in structure and physiological activity were isolated from different parts of plants. On the basis of published... 

Other hormones gibberellins, cytokinins, methyl jasmonate, and ethylene... 

Molecular approaches have contributed considerably to our current understanding of phytohormone action and signal transduction. Putative receptors have been identified for ethylene, cytokinin and brassinosteroids, whereas for hormones such as auxin and GAs components acting more downstream in signal transduction pathways have been identified. However, many steps in phytohormone signalling are still unclear and also the cross-talk between hormones is a field that has hardly been touched. 

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