Occurrence and Functions of Mucilage

A Model of a Mucilage-Supported Diazotrophic Microbiota

Disassembly of the Complex Mucilage Polysaccharide

Sugar Utilization to Support Nitrogenase Activity

Reduction of Oxygen Levels in the Mucilage Environment

Maintaining Low Nitrogen Levels in the Mucilage Environment

Concluding Remarks

Acknowledgement

References

Role for Nitrogen Fixation in Cereal Crops

Nitrogen-fixing microbial associations with nonlegumes, especially cereals, have been a topic of intense interest for more than a century, since such associations could reduce the requirement for nitrogen fertilizers [1–۴]. For instance, sugarcane associates with diazotrophic endophytes that contribute to its nitrogen nutrition in some environments [5–۷] and a study based on 15N dilution experiments in Miscanthus × giganteus concluded that this bioenergy feedstock could acquire about 16% of its nitrogen from the air [8]. These examples indicate that some monocots have the potential to associate with diazotrophs and acquire small, but significant, amounts of fixed nitrogen from the atmosphere. It has been shown that the model C4 grass Setaria viridis can obtain most of its fixed N from associative nitrogen fixation following inoculation with diazotrophs [9]. The possibility that cereal crops obtain a significant proportion of total N by associative nitrogen fixation has also been suggested by a 50-year assessment of global nitrogen budgets in maize (Zea mays), rice (Oryza sastiva), and wheat (Triticum aestivum), which concluded that up to 24% of the total nitrogen in these crops was derived from nonlegume symbiotic nitrogen fixation [10]. Interestingly, mucilage on aerial roots of sorghum (Sorghum bicolor) with an inset micrograph of Azospirillum brasilense was highlighted on the cover page of the proceedings of a conference on cereal nitrogen fixation held in India in 1984 [11], suggesting that it was suspected more than 35 years ago that sorghum mucilage harbors a diazotrophic microbiota. However, no report on nitrogen fixation in sorghum mucilage was published in the 1984 proceedings volume or elsewhere.

Occurrence and Functions of Mucilage

A key feature of the Sierra Mixe maize landrace mucilage is the abundance of sugars that potentially serve as a source of energy for the diazotrophs. The monosaccharide composition indicated that the mucilage primarily comprises fucose (41%), galactose (36%), arabinose (14%), xylose (3%), glucuronic acid (3%), and mannose (3%) [12]. This composition is similar to the mucilage reported for maize underground roots [15,16]. Osborne et al. found that underground root mucilage contained fucose (61.0%) and glucose (31.4%), while Chaboud found the mucilage to contain galactose (30.7%), fucose (19.3%), glucose (18.5%), xylose (15.2%), and arabinose (13.4%). Both of these previous studies showed high amounts of fucose, xylose, arabinose, and galactose, which agrees qualitatively with the analysis of the Sierra Mixe aerial root mucilage. This monosaccharide composition is not commonly found in plant cell wall polysaccharides and may play a role in signaling associative diazotrophic bacteria that can degrade the mucilage complex polysaccharide and use the released monosaccharides to support growth and nitrogen fixation.