LAUSR

Background and aimsThe adaptation of plants to land ecosystems involves complex rhizosphere interactions between organic matter and microbial communities. Border cells (BC) constitute the first living boundary in plant-soil ecosystems and play an important role in environmental sensing and signaling in response to different biotic and abiotic conditions. In this study, we evaluate the effect of humic acid on the release of BCs and its impact on the colonization of Herbaspirillum seropedicae at maize root tips.MethodsMaize seedlings (1.0 ± 0.05 cm root length) were immersed for 48 h in solutions with different concentrations of humic acid (0, 12, 42, 143 and 500 mg L−1). Light and scanning electron microscopy were used to evaluate the structural interaction between border cells and H. seropedicae at the root tips.ResultsThe release of BCs from root tips was significantly increased by humic acid (HA) application and exhibited a bell-shaped dose-response curve; the highest release of BCs occurred at 143 mg HA L−1 and was confirmed by microscopy. The colonization of roots by H. seropedicae strain RAM10 (tagged with green fluorescent protein, GFP) was monitored by epifluorescence microscopy with and without exogenous humic acid (143 mg L−1). Increased BC release resulted in a high density of diazotrophic bacteria at root tips, and bacteria sometimes aggregated with mucilage and humic acid particles, thus enhancing their viability. Increased BC numbers in response to humic acid might explain previous studies showing a concomitant increase in H. seropedicae populations in the rhizosphere, rhizoplane, and endosphere of grasses.ConclusionsThe population of H. seropedicae strain RAM10 colonizing root caps and BCs increased in response to exogenous humic acids.