Genetic analysis of tolerance to Boron toxicity in the legume Medicago truncatula

Abstract

BackgroundMedicago truncatula Gaertn. (barrel medic) is cultivated as a pasture legume for its high protein content and ability to improve soils through nitrogen fixation. Toxic concentrations of the micronutrient Boron (B) in agricultural soils hamper the production of cereal and leguminous crops. In cereals, the genetic analysis of B tolerance has led to the development of molecular selection tools to introgress and maintain the B tolerance trait in breeding lines. There is a comparable need for selection tools in legumes that grow on these toxic soils, often in rotation with cereals.ResultsGenetic variation for B tolerance in Medicago truncatula was utilised to generate two F2 populations from crosses between tolerant and intolerant parents. Phenotyping under B stress revealed a close correlation between B tolerance and biomass production and a segregation ratio explained by a single dominant locus. M. truncatula homologues of the Arabidopsis major intrinsic protein (MIP) gene AtNIP5;1 and the efflux-type transporter gene AtBOR1, both known for B transport, were identified and nearby molecular markers screened across F2 lines to verify linkage with the B-tolerant phenotype. Most (95%) of the phenotypic variation could be explained by the SSR markers h2_6e22a and h2_21b19a, which flank a cluster of five predicted MIP genes on chromosome 4. Three CAPS markers (MtBtol-1,-2,-3) were developed to dissect the region further. Expression analysis of the five predicted MIPs indicated that only MtNIP3 was expressed when leaf tissue and roots were assessed. MtNIP3 showed low and equal expression in the roots of tolerant and intolerant lines but a 4-fold higher expression level in the leaves of B-tolerant cultivars. The expression profile correlates closely with the B concentration measured in the leaves and roots of tolerant and intolerant plants. Whereas no significant difference in B concentration exists between roots of tolerant and intolerant plants, the B concentration in the leaves of tolerant plants is less than half that of intolerant plants, which further supports MtNIP3 as the best candidate for the tolerance trait-defining gene in Medicago truncatula.ConclusionThe close linkage of the MtNIP3 locus to B toxicity tolerance provides a source of molecular selection tools to pasture breeding programs. The economical importance of the locus warrants further investigation of the individual members of the MIP gene cluster in other pasture and in grain legumes.

Breeding for boron tolerance in lentil (Lens culinaris Medik.) using a high‐throughput phenotypic assay and molecular markers

“…The QTL region identified in this study is accurate and confirms the single locus identified in a previous study of B tolerance in lentil (Kaur et al., ). This finding is consistent with the outcomes of studies of B tolerance in other legume species such as pea (Sudheesh et al., ) and Medicago truncatula (Bogacki, Peck, Nair, Howie, & Oldach, ), which also reported single‐gene models. Using the dense genetic map developed, a more closely linked marker (Lc_0002525) was found for this B tolerance gene as compared to the markers identified from ILL2024 × ‘Cassab’.…”

“…The QTL region identified in this study is accurate and confirms the single locus identified in a previous study of B tolerance in lentil (Kaur et al, 2014). This finding is consistent with the outcomes of studies of B tolerance in other legume species such as pea and Medicago truncatula (Bogacki, Peck, Nair, Howie, & Oldach, 2013), which also reported single-gene models.…”

“…Kaur et al (2014) discussed the aim to find a candidate B transporter gene like those isolated in Arabidopsis (Miwa, Aibara, & Fujiwara, 2014;Miwa & Fujiwara, 2011). Studies in Arabidopsis, wheat, rice and M. truncatula have revealed genes for efflux-type B transporters and major intrinsic protein (MIP) family controlling B toxicity tolerance (Bogacki et al, 2013;Takano et al, 2002Takano et al, , 2006. At the genomic location on LcChr2, one candidate was a lentil MIP gene (Lc09014) located in proximity (790 kb) to one of the QTL flanking SNP markers (Lc_0002525).…”

Generation and Characterisation of a Reference Transcriptome for Lentil (Lens culinaris Medik.)

“…Previous studies in A. thaliana and M. truncatula have identified genes for efflux-type B transporters and members of the MIP family as controlling B toxicity tolerance [54,55,56,57]. The majority of known B tolerance-related genes described in model legume M. truncatula were shown to be present in the Cassab-derived transcriptome, validating integrated approach for large-scale identification of candidate genes for a given biological process.…”

“…To identify the candidate genes associated with tolerance to B toxicity [54,55,56,57] and time to flowering [58,59] in lentil, a text-based search of the Cassab transcriptome BLAST analysis data (against M. truncatula CDS, Nr and UniRef100) was performed. Two SNP-based linkage maps [38,39] which were used to precisely map QTLs for B toxicity tolerance and time to flowering were selected in this study.…”