Genetic determination of symbiotic compatibility between Lotus species and rhizobia strains

Lin, Yen-Yu (2022): Genetic determination of symbiotic compatibility between Lotus species and rhizobia strains. Dissertation, LMU München: Fakultät für Biologie

Abstract

Legume crops greatly reduce the need for synthetic nitrogen fertilizers and thus have been indicated as central for sustainable agricultural practices. This results from a symbiosis with nitrogen-fixing rhizobia that provide legumes with nitrogen in exchange for carbohydrates in root organs known as nodules. In nature, legumes encounter the dilemma of whether to be selective on rhizobia symbiont but risk starvation or broaden the selectivity but increase the chance to be infected by ineffective rhizobia, which fix little to no nitrogen. Symbiosis between legumes and rhizobia is initiated after flavonoids in the root exudates induce the production of rhizobial Nod factors, which are perceived by the legume receptor complex. This molecular interaction determines the symbiotic compatibility between host and rhizobia species and triggers downstream rhizobia infection and root nodule organogenesis. Variation in the symbiotic compatibility between legumes and rhizobia is observed between and within species. Here we investigate a Rhizobium leguminosarum strain Norway (Rl Norway) that nodulates different Lotus accessions without nitrogen fixation. Thus, it is considered a sub-compatible symbiont of Lotus. The most striking phenotype difference is between L. burttii and L. japonicus Gifu, as Rl Norway induces white nodules on L. burttii but fails to nodulate L. japonicus Gifu. A region associated with this variation in nodulation phenotype between L. burttii and L. japonicus Gifu was identified by quantitative trait locus (QTL) mapping, but the gene(s) responsible for the various symbiotic compatibility remained unknown. This study aimed to characterize candidate genes in the QTL region and reveal the putative regulatory mechanism mediating the symbiotic compatibility. To achieve this, phenotypic observation, transcriptomic sequencing, and genetic analysis were integrated. We observed variation in symbiotic compatibility between Lotus accessions and Ensifer and Allorhizobium strains in addition to Rl Norway. Moreover, the substrate moisture affected the symbiotic compatibility between L. japonicus Gifu and Rl Norway, resulting in the nodulation of L. japonicus Gifu in high substrate moisture. Transcriptome analyses revealed that several genes involved in the flavonoid biosynthesis were downregulated in L. japonicus Gifu grown in high moisture. We hypothesized that the accumulation of intermediates in the flavonoid biosynthesis pathway alters the symbiotic compatibility in high moisture. Naringenin, a flavonoid compound that is predicted to accumulate in high moisture was applied to roots and its effect on nodule formation and rhizobia colonization was evaluated. Transcriptomic analyses also showed that Rl Norway activated the symbiosis response in L. burttii but not in L. japonicus Gifu. Phenotyping the F1 progeny of a cross between L. burttii and L. japonicus Gifu indicated that the nodulation phenotype of L. burttii is dominant. Four genes encoding receptor-like proteins (RLPs) in the QTL region were identified as candidates that contribute to the symbiotic compatibility, named RLP1 to RLP4. An additive effect between RLP2 and RLP4 on nodule formation was observed by trans-complementing RLP2 and RLP4 from L. burttii into L. japonicus Gifu. Furthermore, mutant lines of the RLPs were generated by the CRISPR-Cas12a gene-editing method for future studies. Altogether, we identified candidate genes that contribute to the different symbiotic compatibility between L. burttii and L. japonicus Gifu and showed the variation in symbiotic compatibility between L. japonicus Gifu and Rl Norway is altered by substrate moisture.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Fakultät für Biologie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Juni 2022
• 1. Bericht­erstatter:in: Marín, Macarena