Cytokinins (CKs) are plant hormones affecting numerous developmental processes. Zeatin and its derivatives are the most important group of isoprenoid CKs. Zeatin occurs as two isomers: while trans-zeatin (transZ) was found to be a bioactive substance, cis-zeatin (cisZ) was reported to have a weak biological impact. Even though cisZ derivatives are abundant in various plant materials their biological role is still unknown. The comprehensive screen of land plants presented here suggests that cisZ-type CKs occur ubiquitously in the plant kingdom but their abundance might correlate with a strategy of life rather than with evolutionary complexity. Changing levels of transZ and cisZ during Arabidopsis ontogenesis show that levels of the two zeatin isomers can differ significantly during the life span of the plant, with cisZ-type CKs prevalent in the developmental stages associated with limited growth. A survey of the bioassays employed illustrates mild activity of cisZ and its derivatives. No cis↔trans isomerization, which would account for the effects of cisZ, was observed in tobacco cells and oat leaves. Differences in uptake between the two isomers resulting in distinct bioactivity have not been detected. In contrast, cisZ and transZ have a different metabolic fate in oat and tobacco. Analysis of a CK-degrading enzyme, cytokinin oxidase/dehydrogenase (CKX), reveals that Arabidopsis possesses two isoforms, AtCKX1 expressed in stages of active growth, and AtCKX7, both of which have the highest affinity for the cisZ isomer. Based on the present results, the conceivable function of cisZ-type CKs as delicate regulators of CK responses in plants under growth-limiting conditions is hypothesized.
Cassava (Manihot esculenta Crantz) sheds its leaves during growth, especially within the tropical dry season. With the production of SAG12-IPT transgenic cassava we want to test the level of leaf retention and altered cytokinin metabolism of transgenic plants via the autoregulatory senescence inhibition system. After confirmation of transgene expression by molecular analysis and phenotype examination in greenhouse plants, two transgenic plant lines, 529-28 and 529-48, were chosen for further investigation. Detached mature leaves of 529-28 plants retained high levels of chlorophyll compared with wild-type leaves after dark-induced senescence treatment. Line 529-28 showed significant drought tolerance as indicated by stay-green capacity after drought stress treatment. Field experiments proved that leaf senescence syndrome was significantly delayed in 529-28 plants in comparison with wild-type and 529-48 plants. Physiological and agronomical characterizations of these plants also revealed that the induced expression of IPT had effects on photosynthesis, sugar allocation and nitrogen partitioning. Importantly, the 529-28 plants accumulated a high level of trans-zeatin-type cytokinins particularly of corresponding storage O-glucosides to maintain cytokinin homeostasis. Our study proves the feasibility of prolonging the leaf life of woody cassava and also sheds light on the control of cytokinin homeostasis in cassava leaves. Zhang P, Wang WQ, Zhang GL, Kaminek M, Dobrev P, Xu J, Gruissem W (2010) Senescence-inducible expression of isopentenyl transferase extends leaf life, increases drought stress resistance and alters cytokinin metabolism in cassava.
Regulation of cytokinin oxidase (CKX) activity in relation to enzyme glycosylation and secretion was studied in wild‐type (WT) and transgenic conditionally isopentenyltransferase gene (ipt)‐expressing (IPT) tobacco (Nicotiana tabacum L. cv. Wisconsin 38) cell suspensions, calli and leaves. An increase in endogenous cytokinin content due to the tetracycline (Tc)‐induced derepression of the ipt gene transcription or surface application of N6‐benzylaminopurine (BA) resulted in significant enhancement of CKX activity in all these plant materials. As revealed by Concanavalin A‐Sepharose 4B chromatography the cytokinin‐induced enhancement of CKX activity was associated predominantly with the N‐glycoform of the enzyme (10‐ to 15‐fold increase) in calli and leaves. Application of BA to the culture media of WT and IPT cell suspensions and the derepression of the ipt by Tc substantially enhanced endogenous levels of isoprenoid cytokinins and CKX activity in both cells and the culture medium. Most CKX activity in control, BA‐ and Tc‐treated cells was associated with the non‐glycosylated form of the enzyme, whereas the majority of CKX activity in the culture media was due to the glycosylated form. The pH optimum of CKX in cells (pH 8.5) differed considerably from that in the culture medium (pH 6.0). No significant differences were found in apparent Km(iP) values of CKX between control, BA‐ and Tc‐treated IPT cells and media or between purified glycosylated and non‐glycosylated CKX. These results suggest that cytokinins induce changes in the proportions of glyco‐ and non‐glycoforms of the enzyme in multicellular calli and leaves, and influence its secretion to the cell exterior.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.