Symbiotic symphony: The role of ABC-transporters in facilitating rhizobia-legume symbiosis

Authors: SHARMA, REENA - Brookhaven National Laboratory; CHAKRABORTY, SANHITA - Texas A&M University; BHAT, ADITI - Brookhaven National Laboratory; Curtin, Shaun; Paape, Timothy - Tim

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 12/8/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Legumes form symbiosis with nitrogen-fixing rhizobia, resulting in root organs called nodules. Rhizobia in the nodules reduce atmospheric nitrogen, providing the host-plant with a biologically derived source of nitrogen. Maintaining ion and hormone homeostasis is crucial for proper nodulation and symbiotic efficacy. The role of ABC-transporters in metal ion transport and hormone signaling is well studied in the model system Arabidopsis thaliana which does not form symbiosis with rhizobia, so their significance in nodule formation in legumes is poorly understood. We found that a mutation in the ABCG36 gene resulted in significantly reduced nodule number. Our study explores this function of this gene and its importance in symbiosis.

Technical Abstract: Legumes form symbiosis with nitrogen-fixing rhizobia, resulting in root organs called nodules. Rhizobia in the nodules reduce atmospheric nitrogen, providing the host-plant with a biologically derived source of nitrogen. Maintaining ion and hormone homeostasis is crucial for proper nodulation and symbiotic efficacy. The role of ABC-transporters in metal ion transport and hormone signaling is well studied in the model system Arabidopsis thaliana which does not form symbiosis with rhizobia, so their significance in nodule formation in legumes is poorly understood. In the present study, we identified an ortholog of the A. thaliana ABCG36 gene (also called PDR8 or pen3-like) in the model legume Medicago truncatula, which is known to be involved in cadmium (Cd) tolerance and auxin signaling in A. thaliana roots. We found that the Mtabcg36 loss-of-function mutant displayed a significant reduction in nodule count, fresh root and shoot biomass compared with wild-type (WT). Given the established role of AtABCG36 in Cd tolerance, the mutant (Mtabcg36) and R108 (WT) were exposed to Cd treatment. The mutant exhibited pronounced phenotypic alterations with effect on nodule number and biomass under Cd stress compared to WT plants. X-ray Fluorescence Imaging (XRF) of nodule sections revealed significant reduction in the distribution and abundance of the micronutrient iron (Fe), a vital element for nodulation. Dual transcriptomics was used to identify regulatory changes in nodule cells of WT and Mtabcg36 host-plants in control conditions, and also in response to Cd or zinc (Zn) treatments, and the responses in the Sinorhizobium meliloti rhizobia strain Sm2011. Analysis of differentially regulated genes revealed significant changes in the expression of WT MtABCG36 in response to Cd-induced stress, along with several other ABC-transporters, some of which are associated with heavy metal detoxification. This suggests correlated or compensatory changes in expression patterns due to the mutation. Cd but not Zn treatment had a stronger effect on the plant transcriptome. The expression of well-characterized plant symbiotic genes such as AMN1, ChOMT1, ENOD12 and NOD25 were differentially regulated in the mutant under control conditions and in response to Cd treatment in the wild-type but not the mutant. Differential expression was also observed in several nod and nif genes that are necessary for nitrogen fixation by the rhizobia. The genes identified in the dual-transcriptomic approach will help to model the relationship between ABC-transporters, symbiotic genes in the host-plant, and nitrogen-fixation genes in the rhizobia that may have all contributed to impaired nodule development in mutant. In summary, the mutation in MtABCG36 may have had polygenic effects that resulted in reduced nodule number and compromised nitrogen fixation, affecting plant biomass.