Sequencing the Gene Space of the Model Legume, Medicago Truncatula

豆科植物有2万多个物种，是世界上最重要的两个农作物家族之一。在种植的植物中，豆类通过与根瘤菌共生来固定大气中的氮的能力是独一无二的。由于豆类不受氮的限制，它们的蛋白质水平非常高，这一特性在生物学和农业上都具有重要意义。近33%的营养氮来自豆类，豆类是整个发展中国家最重要的营养蛋白质来源。豆类还合成了一系列令人印象深刻的二次化合物，具有抗癌和促进健康的作用。不足为奇的是，豆科植物在几乎所有的种植制度中都发挥着核心作用，被普遍认为是安全和可持续的粮食生产所必需的。在豆科植物中，与紫花苜蓿关系密切的紫花苜蓿(Medicago Truncatula)被广泛认为是基因组研究的卓越模式。紫花苜蓿基因组紧凑，约4.7亿碱基对，孟德尔遗传简单，世代时间短，转化效率较高，表型突变体和自然生态型广泛。优秀的细胞遗传学研究表明，紫花苜蓿基因组被组织成独立的基因富集区和基因贫乏区，实际上，紫花苜蓿基因组中基因丰富部分的基因密度几乎与拟南芥一样高。为了对Medicago的基因空间进行测序，一组美国实验室将对Medicago的八条染色体中的六条进行完整的基因测序。这项工作将与欧盟的同事密切协调，他们将对剩余两条紫花苜蓿染色体的基因空间进行排序。在这个过程中，可能包含基因的序列将被识别，这些信息将与现有的关于Medicago的基因组信息整合在一起。Medicago基因组序列最重要的受益者将是豆类研究人员--从基因学家到育种者--他们最终将拥有一个代表大多数豆类特有特性的参考基因组序列。有了Medicago序列，研究人员将可以详细研究豆科植物特有的基因家族、发育过程和生化途径，基于比较基因组学的基因克隆，发现泛豆科植物遗传标记，以及理想的与拟南芥进行进化比较的基因组系统。

With more than 20,000 species, legumes are one of the two most important crop families in the world. Among cultivated plants, legumes are unique in their ability to fix atmospheric nitrogen through symbiosis with bacteria known as Rhizobia. Since they are not limited for nitrogen, legumes have remarkably high levels of protein, a property that is both biologically and agriculturally significant. Nearly 33% of all nutritional nitrogen comes from legumes, and legumes are the single most important source of nutritional protein throughout the developing world. Legumes also synthesize an impressive array of secondary compounds with anti-cancer and health promoting effects. Not surprisingly, legumes play a central role in nearly all cropping systems and are universally viewed as essential for secure and sustainable food production.Among legumes, Medicago (Medicago truncatula), a species closely related to alfalfa, is widely considered the preeminent model for genomic research. Medicago has a compact genome of approximately 470 million base pairs, simple Mendelian genetics, short generation time, relatively high transformation efficiency, and extensive collections of phenotypic mutants and naturally occurring ecotypes. Excellent cytogenetic research demonstrates that the Medicago genome is organized into separate gene-rich and gene-poor regions.Indeed, gene density in the gene-rich portions of the Medicago genome is nearly as high as in Arabidopsis. Thus, the Medicago genome is favorably organized for genome sequencing likely to capture nearly all of its gene-space.To sequence the gene-space of Medicago, a group of U.S. laboratories will completely sequence the gene-rich regions of six of Medicago's eight chromosomes. This work will be closely coordinated with colleagues in the European Union, who will sequence the gene-space of the remaining two Medicago chromosomes. In the process, sequences likely to contain genes will be identified and this information will be integrated with existing genomic information about Medicago. The most important beneficiaries of the Medicago genome sequence will be legume researchers - from genomicists to breeders - who will finally have a reference genome sequence representing most legume-specific properties. With the Medicago sequence in hand, detailed studies of legume-specific gene families, developmental processes, and biochemical pathways, gene cloning based on comparative genomics, discovery of pan-legume genetic markers - plus a genomic system ideally positioned for evolutionary comparison with Arabidopsis - will be available to researchers.