Coba de la Peña T, Frugier F, McKhann H I, Bauer P, Brown S, Kondorosi A, Crespi M
Institut des Sciences Végétales, Centre National de la Recherche Scientifique, Gif-sur-Yvette, France.
Plant J. 1997 Mar;11(3):407-20. doi: 10.1046/j.1365-313x.1997.11030407.x.
Under nitrogen starvation, Rhizobium meliloti is able to induce nitrogen-fixing nodules on alfalfa roots. Certain alfalfa cultivars spontaneously develop pseudonodules in the absence of bacteria. A transcript, Msca1, expressed in spontaneous and R. meliloti-induced nodules, that codes for a carbonic anhydrase (CA), an enzyme catalyzing the hydration of CO2 has been identified. This is the first CA gene cloned from a non-photosynthetic tissue in plants. Msca1 was activated initially in all cells of the bacterium-induced nodule primordium and was also induced by cytokinin treatment of alfalfa roots. The presence of CA enzymatic activity in different nodule types was demonstrated. Thus, Msca1 is a new early nodulin gene with a function possibly related to the increased amyloplast deposition of the dividing cortical cells. Msca1 transcripts were subsequently found mainly in a peripheral envelope of cells in developing and mature nodules. This novel pattern of gene expression is controlled by the presence of the bacterium inside the nodule. Sucrose synthase and phosphoenol pyruvate carboxylase (PEPC), other genes of the carbon fixation metabolism, were expressed in the same peripheral cells and even more strongly in the nitrogen-fixing region. Analysis of expression patterns of these genes indicated that early CA function may not be related to carbon fixation through PEPC. CA might be acting in pH regulation and/or CO2/HCO3-transport during nodule initiation. Thus, carbonic anhydrase may play different roles at several stages of nodule development and function.
在氮饥饿条件下，苜蓿根瘤菌能够在苜蓿根上诱导形成固氮根瘤。某些苜蓿品种在没有细菌的情况下会自发形成假根瘤。已鉴定出一种在自发形成的和苜蓿根瘤菌诱导形成的根瘤中均有表达的转录本Msca1，它编码一种碳酸酐酶（CA），该酶催化二氧化碳的水合作用。这是从植物非光合组织中克隆得到的首个CA基因。Msca1最初在细菌诱导的根瘤原基的所有细胞中被激活，并且也可通过用细胞分裂素处理苜蓿根来诱导。已证明不同类型根瘤中存在CA酶活性。因此，Msca1是一个新的早期结瘤素基因，其功能可能与分裂的皮层细胞中淀粉质体沉积增加有关。随后发现Msca1转录本主要存在于发育中和成熟根瘤的细胞外周包膜中。这种新的基因表达模式受根瘤内细菌的存在所控制。蔗糖合酶和磷酸烯醇式丙酮酸羧化酶（PEPC），即碳固定代谢的其他基因，在相同的外周细胞中表达，并且在固氮区域表达更强。对这些基因表达模式的分析表明，早期CA功能可能与通过PEPC进行的碳固定无关。CA可能在根瘤起始过程中参与pH调节和/或CO2/HCO3转运。因此，碳酸酐酶可能在根瘤发育和功能的几个阶段发挥不同作用。
