Both cytokinin (N6-benzyladenine [BA]) and auxin (2,4-dichlorophenoxyacetic acid [2,4-D]) stimulate the accumulation of an mRNA, represented by the cDNA pLS216, in Nicotiana plumbaginifolia suspension culture cells. The kinetics of RNA accumulation were different for the two hormones; however, the response to both was transient, and the magnitude of the response was dose dependent. Runoff transcription experiments demonstrated that the transient appearance of the RNA could be accounted for by feedback regulation of transcription and not by the induction of an RNA degradation system. The feedback mechanism appeared to desensitize the cells to further exposure of the hormone. In particular, cells became refractory to the subsequent addition of 2,4-D after the initial RNA accumulation response subsided. A very different response was observed when the second hormone was added to cells that had been desensitized to the first hormone. Under such conditions, BA produced a heightened response in cells desensitized to 2,4-D and vice versa. These findings support a model in which cytokinin further enhances the auxin response or prevents its feedback inhibition. The hormone-induced RNA accumulation was blocked by the protein kinase inhibitor staurosporin. On the other hand, the protein phosphatase inhibitor okadaic acid stimulated expression, and, in particular, okadaic acid was able to stimulate RNA accumulation in cells desensitized to auxin. This suggests that hormone activation involves phosphorylation of critical proteins on the hormone signaling pathway, whereas feedback inhibition may involve dephosphorylation of these proteins. The sequence of pLS216 is similar to genes in other plants that are stimulated by multiple agonists such as auxins, elicitors, and heavy metals, and to the gene encoding the stringent starvation protein in Escherichia coli. It is proposed that this gene family in various plants be called multiple stimulus response (msr) genes.

Cytokinins and auxins control the expression of a gene in Nicotiana plumbaginifolia cells by feedback regulation.

This review describes recent advances in the study of cytokinin metabolism. It highlights how plant development is influenced by cytokinin synthesis, conjugation and conjugate hydrolysis, and what has been learned of the enzymes that regulate these processes. Although cytokinin metabolism and physiology are complex issues, some of the key enzymatic players are now being identified. This holds out the prospect of rapid progress in the near future. Just as much of what we know about the control of animal cell proliferation was learned by studying the cellular counterparts of viral oncogenes, so important information about the control of plant development by phytohormones has come from studying the genes of bacterial pathogens that subvert host phytohormone metabolism to their own advantage. We will focus on what has been learned from the use of such genes, and describe progress in identifying their functional counterparts in plants.

Cytokinin metabolism : implications for regulation of plant growth and development

We have reviewed recent progress in research on plant growth regulator (PGR) receptors. For some growth regulators, no receptor protein has yet been identified, but promising new approaches are discussed. For other receptors, specific and sensitive probes have been developed and, in one case, the membrane-associated auxin-binding protein of maize, these have been used to study the function of the receptor. The maize receptor has been cloned and sequenced; cDNA probes will allow the expression of receptor genes in normal and transformed plants to be studied. PGR sensitivity mutants have been described and, in conjunction with biochemical probes, should prove valuable in elucidating the functions of receptors and the nature of subsequent signal transduction events.

Receptors for plant growth regulators: Recent advances

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2890%2980604-H

Isolation of a protein with cytokinin-receptor properties by means of anti-idiotype antibodies

Cytokinin signalling systems : From a whole plant to the molecular level

Purified fractions of soluble proteins from barley leaves have been shown to contain specific binding sites fortrans-zeatin, a natural cytokinin. Such binding is very strong in vitro in concentrated solutions of some salts (ammonium sulfate or potassium phosphate) with optimum at pH 7–8 and temperature within the range 0–20°C. The cytokinin-binding sites have high affinity for zeatin (Kd∼1.5·10−8 M) and low capacity corresponding to 1–1.5 pmol zeatin per milligram of initial soluble protein. Cytokinin binding is reversible; it is due to protein (or proteins) with molecular weight 40–45 kDa. This protein(s) does not bind3H-adenine and3H-abscisic acid. The ability of various compounds to displace3H-zeatin from its high-affinity binding sites is in strict accordance with their biological cytokinin activities. Other phytohormones as well as fusicoccin do not displace3H-zeatin from its binding sites. Specific zeatin binding is sensitive to heat, alkali, and pronase, but not to RNase treatment. The 150- to 200-fold purification of cytokinin-binding proteins was achieved by a combination of ammonium sulfate precipitation and Ultrogel AcA-54- and DEAE-cellulose chromatography. The zeatin-binding protein(s) from barley leaves is suggested to take part in cytokinin action in vivo.

V. Ya. Taran

Papers

Receptor-like cytokinin-binding protein(s) from barley leaves