IRD, Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR LSTM, Campus International de Baillarguet, F-34398, Montpellier, France.
LSTM, Univ. Montpellier, CIRAD, INRA, IRD, Montpellier SupAgro, Montpellier, France.
BMC Plant Biol. 2018 Apr 3;18(1):54. doi: 10.1186/s12870-018-1260-2.
Among semi-aquatic species of the legume genus Aeschynomene, some have the unique property of being root and stem-nodulated by photosynthetic Bradyrhizobium lacking the nodABC genes necessary for the production of Nod factors. These species provide an excellent biological system with which to explore the evolution of nodulation in legumes. Among them, Aeschynomene evenia has emerged as a model legume to undertake the genetic dissection of the so-called Nod-independent symbiosis. In addition to the genetic analysis of nodulation on a reference line, natural variation in a germplasm collection could also be surveyed to uncover genetic determinants of nodulation. To this aim, we investigated the patterns of genetic diversity in a collection of 226 Nod-independent Aeschynomene accessions.
A combination of phylogenetic analyses, comprising ITS and low-copy nuclear genes, along with cytogenetic experiments and artificial hybridizations revealed the richness of the Nod-independent Aeschynomene group with the identification of 13 diploid and 6 polyploid well-differentiated taxa. A set of 54 SSRs was used to further delineate taxon boundaries and to identify different genotypes. Patterns of microsatellite diversity also illuminated the genetic basis of the Aeschynomene taxa that were all found to be predominantly autogamous and with a predicted simple disomic inheritance, two attributes favorable for genetics. In addition, taxa displaying a pronounced genetic diversity, notably A. evenia, A. indica and A. sensitiva, were characterized by a clear geographically-based genetic structure and variations in root and stem nodulation.
A well-characterized germplasm collection now exists as a major genetic resource to thoroughly explore the natural variation of nodulation in response to different bradyrhizobial strains. Symbiotic polymorphisms are expected to be found notably in the induction of nodulation, in nitrogen fixation and also in stem nodulation. Subsequent genetic analysis and locus mapping will pave the way for the identification of the underlying genes through forward or reverse genetics. Such discoveries will significantly contribute to our understanding of the molecular mechanisms underpinning how some Aeschynomene species can be efficiently nodulated in a Nod-independent fashion.
在豆科含羞草属的半水生物种中，有些物种具有独特的特性，即其根和茎被缺乏产生结瘤因子所需的 nodABC 基因的光合慢生根瘤菌定殖。这些物种为探索豆科植物的共生进化提供了一个极好的生物系统。其中，含羞草属 evenia 已成为一种模型豆科植物，用于开展所谓的非结瘤共生的遗传剖析。除了对参考系上的结瘤进行遗传分析外，还可以对种质资源的自然变异进行调查，以揭示结瘤的遗传决定因素。为此，我们调查了 226 份非结瘤含羞草属材料的遗传多样性模式。
结合 ITS 和低拷贝核基因的系统发育分析，以及细胞遗传学实验和人工杂交，确定了非结瘤含羞草属组的丰富程度，共鉴定出 13 个二倍体和 6 个多倍体分化良好的分类群。利用 54 个 SSR 标记进一步划定了分类群的边界，并鉴定了不同的基因型。微卫星多样性模式也阐明了含羞草属分类群的遗传基础，发现所有分类群均主要为自交的，且预测其具有简单的二倍体遗传，这两个特征有利于遗传研究。此外，表现出明显遗传多样性的分类群，特别是含羞草属 evenia、A. indica 和 A. sensitiva，具有明显的基于地理的遗传结构以及根和茎结瘤的变化。
现在存在一个经过良好特征描述的种质资源收集，可作为彻底探索不同慢生根瘤菌菌株诱导的结瘤自然变异的主要遗传资源。共生多态性有望在结瘤的诱导、固氮和茎结瘤中得到发现。随后的遗传分析和基因座定位将为通过正向或反向遗传学鉴定潜在基因铺平道路。这些发现将极大地促进我们对一些含羞草属物种如何以非结瘤方式高效结瘤的分子机制的理解。
