He X Z, Dixon R A
Plant Biology Division, Samuel Roberts Noble Foundation, 2510 Sam Noble Parkway, Ardmore, Oklahoma 73401, USA.
Plant Cell. 2000 Sep;12(9):1689-702. doi: 10.1105/tpc.12.9.1689.
4'-O-Methylation of an isoflavonoid intermediate is a key reaction in the biosynthesis of the phytoalexin medicarpin in legumes. However, isoflavone O-methyltransferase (IOMT) from alfalfa converts the isoflavone daidzein to 7-O-methyl daidzein (isoformononetin) in vitro as well as in vivo in unchallenged leaves of transgenic alfalfa ectopically expressing IOMT. In contrast, elicitation of IOMT-overexpressing plants with CuCl(2) or infecting these plants with Phoma medicaginis leads to greater accumulation of formononetin (4'-O-methyl daidzein) and medicarpin in the leaves than does elicitation or infection of control plants, and no isoformononetin is detected. Overexpression of IOMT results in increased induction of phenylpropanoid/isoflavonoid pathway gene transcripts after infection but has little effect on basal expression of these genes. IOMT-overexpressing plants display resistance to P. medicaginis. The apparently different regiospecificities of IOMT in vivo and in vitro are discussed in relation to potential metabolic channeling at the entry point into the isoflavonoid pathway.
异黄酮中间体的4'-O-甲基化是豆科植物植保素美迪紫檀素生物合成中的关键反应。然而，来自苜蓿的异黄酮O-甲基转移酶（IOMT）在体外以及在异位表达IOMT的转基因苜蓿未受刺激的叶片中，都能将异黄酮大豆苷元转化为7-O-甲基大豆苷元（异芒柄花素）。相比之下，用CuCl₂处理过表达IOMT的植物或用苜蓿茎点霉感染这些植物，会导致叶片中芒柄花素（4'-O-甲基大豆苷元）和美迪紫檀素的积累量比处理或感染对照植物时更多，且未检测到异芒柄花素。IOMT的过表达导致感染后苯丙烷类/异黄酮途径基因转录本的诱导增加，但对这些基因的基础表达影响很小。过表达IOMT的植物对苜蓿茎点霉表现出抗性。本文结合异黄酮途径入口处潜在的代谢通道化，讨论了IOMT在体内和体外明显不同的区域特异性。
