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Abstract
Maize roots can be colonized by free-living atmospheric nitrogen (N2)-fixing bacteria (diazotrophs). However, the agronomic potential of non-symbiotic N2-fixation in such an economically important species as maize, has still not been fully exploited. A preliminary approach to improve our understanding of the mechanisms controlling the establishment of such N2-fixing associations has been developed, using two maize inbred lines exhibiting different physiological characteristics. The bacterial-plant interaction has been characterized by means of a metabolomic approach. Two established model strains of Nif+ diazotrophic bacteria, Herbaspirillum seropedicae and Azospirillum brasilense and their Nif- couterparts defficient in nitrogenase activity, were used to evaluate the impact of the bacterial inoculation and of N2 fixation on the root and leaf metabolic profiles. The two N2-fixing bacteria have been used to inoculate two genetically distant maize lines (FV252 and FV2), already characterized for their contrasting physiological properties. Using a well-controlled gnotobiotic experimental system that allows inoculation of maize plants with the two diazotrophs in a N-free medium, we demonstrated that both maize lines were efficiently colonized by the two bacterial species. We also showed that in the early stages of plant development, both bacterial strains were able to reduce acetylene, suggesting that they contain functional nitrogenase activity and are able to efficiently fix atmospheric N2 (Fix+). The metabolomic approach allowed the identification of metabolites in the two maize lines that were representative of the N2 fixing plant-bacterial interaction, these included mannitol and to a lesser extend trehalose and isocitrate. Whilst other metabolites such as asparagine, although only exhibiting a small increase in maize roots following bacterial infection, were specific for the two Fix+ bacterial strains, in comparison to their Fix- counterparts. Moreover, a number of metabolites exhibited a maize-genotype specific pattern of accumulation, suggesting that the highly diverse maize genetic resources could be further exploited in terms of beneficial plant-bacterial interactions for optimizing maize growth, with reduced N fertilization inputs.
Highlights
Cereals are the basis of most human food in the world, especially wheat, rice and maize
The two maize lines FV2 and FV252 were inoculated with the two Fix+ strains of A. brasilense FP2 and H. seropedicae SmR1 using the gnotobiotic system shown in Figure A (a) in S1 File
The results of the bacterial counting (Fig 1A) show that A. brasilense colonized the root surface of both maize lines 7 Days After Inoculation (DAI) and that the number of Colony Forming Units (CFU) per g FW was slightly higher in line FV252, but this was not statistically significant
Summary
Cereals are the basis of most human food in the world, especially wheat, rice and maize. Large amounts of N fertilizer are required to obtain the maximum yield of maize but its nitrogen use efficiency (NUE), i.e. the yield obtained per unit of available N in the soil (supplied by the soil + N fertilizer), is typically less than 50% [2,3]. Maize is considered as a model crop that exhibits specific phenologic characteristics with respect to leaf and reproductive organ structure and development [7] These characteristics are suited for the performance of detailed and integrated agronomic, physiological and molecular genetic studies during the developmental cycle of the whole plant [8]. Many tools are available for maize, such as mutant collections, a wide genetic diversity, recombinant inbred lines (RILs), straightforward transformation protocols, physiological, biochemical and “omics” data [7], as well as genome sequences and more recently genome-scale metabolic models [13]
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