Evaluation of Sorghum (Sorghum Bicolor L. Moench) Genotypes for Aluminium Toxicity Tolerance Using Morphological and Molecular Markers

Abstract

Aluminium (Al) toxicity is listed among the leading edaphic factors limiting production of sorghum in acidic soils (pH<5.0). It affects the apical root cell multiplication and elongation, hence, inhibiting the absorption of water and mineral elements which eventually leads to poor plant growth, yield and kernel quality. Liming, a most adopted remedy against Al toxicity has proved costly and unsustainable, however, identification and utilization of tolerant genotypes could sustainably aid in management of the constraint. Magnavaca solution screening was used in phenotypic evaluations of 14 selected lines for Al toxicity tolerance. Among the genotypes were the sensitive and the tolerant checks. Seeds for each genotype were pre-germinated in an incubator and initial seminal root lengths (ISRL) taken 4 days upon germination. Selected seedlings were then laid out in a completely randomized design (CRD) with 2 treatment levels of Al; 0 and 148 μM bearing pH levels of 4.3. Final seminal root lengths (FSRL) of the seedlings were taken 5 days after exposure to Aluminium and together with initial root lengths used to compute the net seminal root length (NSRL), relative seminal root length (RSRL), root tolerance index (RTI) and % response to Al that were applied in establishing the tolerance status for the genotypes in reference to the provided standard scales. Results from ANOVA showed that genotypes varied significantly in response to the Aluminium treatment. Genotypes exhibited significant (P<.001) variability in growth of roots under Aluminium stress. Genotypes Gadam and Wagita were found to be tolerant, Macia and Kiboko local 2 moderately tolerant while a remainder of 8 genotypes expressed sensitivity. To validate sorghum genotypes with Al tolerance genes, specific SSR markers linked to Al tolerance in sorghum were used. DNA was isolated from each of the genotypes following the CTAB protocol, quantified using a nanodrop spectrophotometer and subjected to polymerase chain reactions. DNA amplicons were detected through agarose gel electrophoresis where band patterns of the genotypes were analyzed in respect to the checks. Genotypes with band pattern identical to the tolerant check were categorized as in possession of Al tolerance genes and vice versa. Identification of tolerant genotypes was largely achieved through marker Xtxp34. Unlike markers Sb5_236, Sb6_342 and Sb6_34, the marker was polymorphic and specific to the targeted gene locus linked to Al tolerance. Band patterning due to the marker clearly discriminated between tolerant and sensitive genotypes and

strongly associated with genotypes’ tolerance status established via nutrient solution screening. Based on the marker (Xtxp34), genotypes Gadam, Wagita and Macia had identical band pattern to the tolerant check IS 41764 and were considered Al tolerant. The rest of the genotypes shared the same band pattern with the sensitive check Seredo hence categorized as Al sensitive. Contrary to nutrient solution screening, the applied markers did not present genotype Kiboko local 2 as tolerant indicating that there could be other gene variants for Al tolerance in this genotype that are yet to be ascertained. The observed existence of variability and potential for Al tolerance in sorghum germplasm provides basis for selection of parental lines for breeding against the stress.