Polygenic adaptation to heavy metals in a legume host plant and structural adaptation in a symbiotic rhizobia partner

Authors: BHAT, ADITI - Brookhaven National Laboratory; SHARMA, REENA - Brookhaven National Laboratory; CHAKRABORTY, SANHITA - Texas A&M University Institute For Advancing Health Through Agriculture; MISRA, ANKITA - Texas A&M University Institute For Advancing Health Through Agriculture; TIFFIN, PETER - University Of Minnesota; PUEYO, JOSE - Institute Of Agricultural Sciences; Paape, Timothy - Tim

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 5/15/2024
Publication Date: 7/7/2024
Citation: Bhat, A., Sharma, R., Chakraborty, S., Misra, A., Tiffin, P., Pueyo, J.J., Paape, T.D. 2024. Polygenic adaptation to heavy metals in a legume host plant and structural adaptation in a symbiotic rhizobia partner [abstract]. Annual Meeting of the Society for Molecular Biology and Evolution. July 7-11, 2024; Puerto Vallarta, Mexico. Poster presentation.

Interpretive Summary:

Technical Abstract: The heavy metals cadmium (Cd) and mercury (Hg) are highly toxic to most organisms including plants and their associated root microbes. In the Medicago-Sinorhizobium system, both partners possess ion detoxification mechanisms that are essential for free-living and in symbiotic life stages. We used a combination of GWAS, genome assembly of heavy metal adapted rhizobia, dual-transcriptomics of host-plants and rhizobia in a single tissue (nodules), and X-ray fluorescence microscopy to identify ion distributions in root nodules. Genome assemblies of Sinorhizobium medicae with high variation in Cd and Hg tolerance showed structural differences between strains, including a Mer operon in the most Hg-tolerant strains. While independent mercury reductase A (merA) genes are prevalent in Sinorhizobium, complete Mer operons are rare and often vary in their gene organization. Differentially expressed genes (DEGs) in S. medicae inside of nodules showed that rhizobia and host-plant tolerance affected the number of DEGs. Aside from Mer operon genes, nif genes which are involved in nitrogenase activity in S. medicae showed significant up-regulation in the most Hg-tolerant strain in nodules of the most Hg-accumulating host-plant, indicating a genotype-by-genotype interaction that influences nitrogen-fixation under stress conditions. Transfer of the Mer operon to low-tolerant strains resulted in an immediate increase in Hg tolerance, indicating that the operon is solely necessary to confer hypertolerance to Hg, despite paralogous merA genes present elsewhere in the genome. This study demonstrated a pivotal role of the Mer operon in Hg hypertolerance in nitrogen-fixing rhizobia, with benefits conferred to host-plants from tolerant rhizobia.