Mucilage Produced by Sorghum (Sorghum bicolor) Aerial Roots Hosts Diazotrophs that Provide Significant Amounts of Nitrogen to the Plant

Abstract

SummarySorghum (Sorghum bicolor) is gaining popularity as a sustainable energy crop due to its high biomass and potential for biofuel production. Some rare sorghum accessions develop many aerial roots that produce viscous carbohydrate-rich mucilage after rain.This aerial root mucilage is strikingly similar to that observed in specific landraces of maize (Zea mays) from southern Mexico, which have been previously shown to host nitrogen-fixing bacteria (diazotroph). The landraces displaying these traits can reduce nitrogen-based fertilizer input, mitigating their negative environmental impacts.In this study, we characterized the aerial root development of several sorghum accessions and successfully isolated more than 103 distinct diazotrophs from the sorghum mucilage.Using acetylene reduction and15N gas enrichment assays, we confirmed that sorghum plants acquire nitrogen from the atmosphere through the diazotrophic associations in the mucilage.This sorghum symbiotic relationships with diazotrophs offer a promising avenue for nitrogen fixation, potentially diminishing reliance on synthetic fertilizers and promoting sustainable agricultural practices.

Identification of genetic and environmental factors influencing aerial root traits that support biological nitrogen fixation in sorghum

“…Environmental factors contributing to phenotypic differences amongst locations include soil type, nutrient availability, the soil microbiome, temperature, day length, rainfall, and humidity. In particular, high humidity has been shown to stimulate aerial root formation ( Venado et al 2023 ). During the summer months, the average relative humidity in Live Oak, Florida ranges from 80 to 85%, compared to 65 to 77% at Madison, Wisconsin.…”

“…In addition to aerial root formation, aerial root diameter is positively correlated with the ability to produce increased volumes of mucilage that harbors diazotrophs in both maize and sorghum ( Pankievicz et al 2022 ; Venado et al 2023 ). The GWAS evaluating aerial root diameter detected 2 SNPs on chromosome 3, S3_1871493 and S3_11242543 ( Fig.…”

“… Venado et al (2023) demonstrated that sorghum aerial root mucilage is necessary to support BNF. Analysis of monosaccharide composition indicated minimal differences between genotypes and fertilizer treatments ( Table 5 ; Supplementary Table 6 ).…”

“…From past studies of the sorghum minicore, which includes sorghum landraces from 57 different countries ( Upadhyaya et al 2009 ), we were aware that some accessions could form aerial roots with mucilage, ( Fig. 1 ) and Venado et al (2023) recently showed they could support diazotrophs like the Sierra Mixe maize. The genetic basis underlying the production of aerial roots is not well-documented, yet it is plausible that the ability to form aerial roots with mucilage is evolutionarily conserved given that maize and sorghum diverged from the most recent common ancestor 12 MYA ( Swigoňová et al 2004 ),…”