Root exudate profiling of maize seedlings inoculated with Herbaspirillum seropedicae and humic acids

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 (H-1 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.