Abstract
Corn has shown different degrees of positive response to inoculation with the nitrogen- fixing bacteria of the genera Azospirillum. Part of it has been attributed to the plant genotypic variation, including the root exudates, that are used by the bacteria as energy source. In this study, we grew two corn hybrids that differ for their response to Azospirillum, to investigate the effect of different exudates profiles on the bacteria growth and nitrogenase activity. Employing high performance liquid chromatography, we identified nine amino acids (asparagine, aspartic acid, serine, glutamic acid, valine, phenylalanine, threonine, tryptophan and alanine), six sugars (glucose, sucrose, xylose, arabinose, fructose and galactose) and four organic acids (citrate, malate, succinate and fumarate). The less responsive corn genotype showed reduced plant growth (root volume, shoot dry mass and shoot N content), a lower concentration of Azospirillum cells within the root tissues, a higher content of asparagine and glucose and a reduced amount of metabolites that serve as bacterial energy source (all organic acids + five sugars, excluding glucose). The genotypes did not interfere in the ability of Azospirillum to colonize the substrate, but the metabolites released by the less responsive one reduced the nitrogenase activity.
Highlights
Symbiosis is a biological phenomenon involving changes in the genome and metabolism of organisms from different species, usually with benefits to one or both [1]
We identified nine amino acids, six sugars and four organic acids in the exudates of the tested genotypes
Profile (Appendix B), with the less responsive ones showing a reduced concentration of both and, of chemotactic compounds, i.e., metabolites that serve as chemoattracts and an energy source for Azospirillum
Summary
Symbiosis is a biological phenomenon involving changes in the genome and metabolism of organisms from different species, usually with benefits to one or both [1]. While the first mentioned interaction is probably the most widespread in the ecosystem, the plant association with diazotrophic bacteria has been the more exploited symbiotic relationship in plant production [1,2]. Diazotrophic bacteria comprehend groups of free living, associated and nodule forming species capable of enzymatically reducing the atmospheric nitrogen (N) into plant bioavailable N compounds and, in the agriculture, have been shown capable of enhancing crop yield, while reducing the environmental impacts caused by mineral fertilizers [3,4], such as the production of phytohormones, Plants 2020, 9, 923; doi:10.3390/plants9070923 www.mdpi.com/journal/plants. Azospirillum, a free living diazotrophic bacterium of the Spirillaceae family, stands out as one of the best characterized and exploited plant-growth promoting microorganism in the agricultural production [3]. It is in the corn production that A. brasilense stands out as inoculant [6], because it allow for a 25% reduction in the need for nitrogenous fertilizer, and for being capable of increasing the grain yield up to 30% [7]
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