LIPM, Université de Toulouse, INRAE, CNRS, Castanet-Tolosan 31320, France.
Genes (Basel). 2020 Mar 23;11(3):339. doi: 10.3390/genes11030339.
Rhizobia, the nitrogen-fixing symbionts of legumes, are polyphyletic bacteria distributed in many alpha- and beta-proteobacterial genera. They likely emerged and diversified through independent horizontal transfers of key symbiotic genes. To replay the evolution of a new rhizobium genus under laboratory conditions, the symbiotic plasmid of was introduced in the plant pathogen , and the generated proto-rhizobium was submitted to repeated inoculations to the host, L. This experiment validated a two-step evolutionary scenario of key symbiotic gene acquisition followed by genome remodeling under plant selection. Nodulation and nodule cell infection were obtained and optimized mainly via the rewiring of regulatory circuits of the recipient bacterium. Symbiotic adaptation was shown to be accelerated by the activity of a mutagenesis cassette conserved in most rhizobia. Investigating mutated genes led us to identify new components of virulence and symbiosis. Nitrogen fixation was not acquired in our short experiment. However, we showed that post-infection sanctions allowed the increase in frequency of nitrogen-fixing variants among a non-fixing population in the system and likely allowed the spread of this trait in natura. Experimental evolution thus provided new insights into rhizobium biology and evolution.
根瘤菌是豆类的固氮共生体，是分布在许多α-和β-变形菌属中的多系细菌。它们可能通过关键共生基因的独立水平转移而出现和多样化。为了在实验室条件下重现新根瘤菌属的进化，将 的共生质粒引入植物病原体 中，并将产生的原根瘤菌反复接种到宿主 上，即 。该实验验证了关键共生基因获得和基因组重排的两步进化情景，这是在植物选择下进行的。通过对受体菌调控回路的重新布线，主要获得和优化了结瘤和根瘤细胞感染。研究表明，大多数根瘤菌中保守的诱变盒的活性加速了共生适应性。研究突变基因使我们能够鉴定 的毒性和 共生的新成分。在我们的短期实验中没有获得固氮作用。然而，我们表明，感染后制裁允许在 系统中非固氮群体中增加固氮变体的频率，并且可能允许这种性状在自然状态下传播。实验进化因此为根瘤菌生物学和进化提供了新的见解。
