Abstract
Abstract Background Co-inoculation of maize with Herbaspirillum seropedicae and humic substances increases the sizes of plant-associated bacterial populations and enhances grain yields under laboratory and field conditions. Root exudation is a key mechanism in the regulation of plant-bacterial interactions in the rhizosphere; humic matter supplementation is known to change the exudation of H+ ions and organic acids from maize roots. Our starting premise was that H. seropedicae and humic acids would modify maize seedling exudation profiles. We postulated that a better understanding of these shifts in exudate profiles might be useful in improving the chemical environment to promote better performance of plant growth-promoting bacteria delivered as bioinoculants. Thus, root exudates of maize were collected and analyzed by gas chromatography-mass spectrometry (GC-MS) and proton nuclear magnetic resonance (1H NMR). Results Nitrogenous compounds, fatty acids, organic acids, steroids, and terpenoid derivatives were the main structural moieties found in root exudates. Significant changes in exudation patterns occurred 14 days after the initiation of experiments. Quantities of fatty acids, phenols, and organic acids exuded by seedlings treated with humic acids alone differed from the quantities exuded in other treatments. Seedlings treated with H. seropedicae or H. seropedicae in combination with humic acids exuded a diversity of nitrogenous compounds, most of which had heterocyclic structures. Twenty-one days after initiating the experiment, seedlings treated with H. seropedicae alone exuded elevated quantities of steroids and terpenoid derivatives related to precursors of gibberellic acids (kaurenoic acids). Conclusions Changes in root exudation profiles induced by our treatments became most marked 14 and 21 days after initiation of the experiment; on those days, we observed (i) increased fatty acid exudation from seedlings treated only with humic acids and (ii) increased exudations of nitrogenated compounds and terpenes from seedlings treated only with H. seropedicae. Improved knowledge on the effects of bacterial inoculants and supplementation with humates on plant exudate composition may contribute substantially to improved understanding of plant metabolic responses and lead to new approaches in the use of selected compounds as additives in bioinoculant formulations that will modulate the cross-talk between bacteria and plants, thereby improving crop yields.
Highlights
Co-inoculation of maize with Herbaspirillum seropedicae and humic substances increases the sizes of plant-associated bacterial populations and enhances grain yields under laboratory and field conditions
We examined main changes in exudation profiles of maize seedling roots induced under laboratory conditions by (i) single applications of humic acids or (ii) H. seropedicae or (iii) combinations of the bacteria and humic acids
When cell shape in the pellicle material differed from that of H. seropedicae, we identified the microbes as native bacteria associated with maize roots
Summary
Co-inoculation of maize with Herbaspirillum seropedicae and humic substances increases the sizes of plant-associated bacterial populations and enhances grain yields under laboratory and field conditions. Root exudates function in processes of plant adaptation. They have roles in nutrient cycling in the rhizosphere and in responses to pathogens and symbiotic microorganisms [1]. The quantities of organic compounds exuded by roots are variable, but they are frequently a significant proportion of the carbon fixed photosynthetically by plants [2]. The plant rhizosphere modulates microbial community structure and function, primarily through the release of chemical compounds [5]. Like fatty acids, sterols, enzymes, vitamins, and plant growth regulators (e.g., auxins, gibberellins, and cytokinins), are generally released in only very small quantities [7]. Only small quantities of secondary metabolites are exuded by roots, their plant role in signaling to the microbial community is substantial [8]
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