Veach Yeonjin K, Martin Ruth C, Mok David W S, Malbeck Jiri, Vankova Radomira, Mok Machteld C
Department of Horticulture and Center for Gene Research and Biotechnology, Oregon State University, Corvallis, Oregon 97331-7304, USA.
Plant Physiol. 2003 Mar;131(3):1374-80. doi: 10.1104/pp.017210.
trans-Zeatin is a major and ubiquitous cytokinin in higher plants. cis-Zeatin has traditionally been viewed as an adjunct with low activity and rare occurrence. Recent reports of cis-zeatin and its derivatives as the predominant cytokinin components in some plant tissues may call for a different perspective on cis-isomers. The existence of a maize (Zea mays) gene (cisZOG1) encoding an O-glucosyltransferase specific to cis-zeatin (R.C. Martin, M.C. Mok, J.E. Habben, D.W.S. Mok [2001] Proc Natl Acad Sci USA 98: 5922-5926) lends further support to this view. Results described here include the isolation of a second maize cisZOG gene, differential expression of cisZOG1 and cisZOG2, and identification of substantial amounts of cis-isomers in maize tissues. The open reading frame of cisZOG2 has 98.3% identity to cisZOG1 at the nucleotide level and 97.8% at the amino acid level. The upstream regions contain common and unique segments. The recombinant enzymes have similar properties, K(m) values of 46 and 96 microM, respectively, for cis-zeatin and a pH optimum of 7.5. Other cytokinins, including N(6)-(delta(2)-isopentenyl)adenine, trans-zeatin, benzyladenine, kinetin, and thidiazuron inhibited the reaction. Expression of cisZOG1 was high in maize roots and kernels, whereas cisZOG2 expression was high in roots but low in kernels. cis-Zeatin, cis-zeatin riboside, and their O-glucosides were detected in all maize tissues, with immature kernels containing very high levels of the O-glucoside of cis-zeatin riboside. The results are a clear indication that O-glucosylation of cis-zeatin is a natural metabolic process in maize. Whether cis-zeatin serves as a precursor to the active trans-isomer or has any other unique function remains to be demonstrated.
反式玉米素是高等植物中一种主要且普遍存在的细胞分裂素。顺式玉米素传统上被视为一种活性较低且很少出现的辅助成分。最近有报道称顺式玉米素及其衍生物是某些植物组织中主要的细胞分裂素成分，这可能需要对顺式异构体有不同的看法。玉米（Zea mays）中存在一个编码顺式玉米素特异性O-葡萄糖基转移酶的基因（cisZOG1）（R.C. Martin，M.C. Mok，J.E. Habben，D.W.S. Mok [2001] Proc Natl Acad Sci USA 98: 5922 - 5926），这进一步支持了这一观点。此处描述的结果包括第二个玉米cisZOG基因的分离、cisZOG1和cisZOG2的差异表达以及玉米组织中大量顺式异构体的鉴定。cisZOG2的开放阅读框在核苷酸水平上与cisZOG1的同一性为98.3%，在氨基酸水平上为97.8%。上游区域包含共同和独特的片段。重组酶具有相似的性质，对顺式玉米素的K(m)值分别为46和96微摩尔，最适pH为7.5。其他细胞分裂素，包括N⁶-(Δ²-异戊烯基)腺嘌呤、反式玉米素、苄基腺嘌呤、激动素和噻苯隆抑制该反应。cisZOG1在玉米根和籽粒中表达较高，而cisZOG2在根中表达较高但在籽粒中表达较低。在所有玉米组织中都检测到了顺式玉米素、顺式玉米素核苷及其O-葡萄糖苷，未成熟籽粒中含有非常高水平的顺式玉米素核苷O-葡萄糖苷。这些结果清楚地表明顺式玉米素的O-糖基化是玉米中的一个自然代谢过程。顺式玉米素是否作为活性反式异构体的前体或具有任何其他独特功能仍有待证明。
