Humic acids and Herbaspirillum seropedicae change the extracellular H+ flux and gene expression in maize roots seedlings

Background: The use of plant-based biostimulants to increase crop yield and enhance protection has grown in popularity. We previously manufactured an inoculant combining humic acids with plant growth-promoting bacteria and observed a significant effect on crop yields. However, electrophysiology studies elucidating the mechanisms responsible for enhanced nutrient and water uptake are scarce and are generally restricted to in vitro models. Materials and methods: In this study, a suspension of humic acids isolated from vermicompost along with Herbaspirillum seropedicae was applied to maize seedlings. We measured the root H+ flux with an ion-selective vibrating probe system. Furthermore, the transcription of plasma membrane H+ ATPase, aquaporin and high- or low-affinity nitrate transporters was measured. Results: Inoculation activated the extracellular H+ flux, thus changing the pH and membrane potential of maize root cells and altering the electrochemical potential generated by P-H+-ATPase at the biochemical and molecular level. The overexpression of aquaporins was also observed; however, nitrate transporters were repressed by the inoculants. Conclusion: We demonstrate an increase in the H+ efflux in maize root seedlings inoculated with H. seropedicae and concomitant changes in membrane voltage. The inoculation of H. seropedicae in the presence of humic acids decreased the H+ flux and surface acidification without changes in the aquaporin transcription level. However, the H+ flux in root seedlings in inoculated plants (with or without humic acids) was larger in respect to control plants. These results support the increased water and nitrogen efficiency in plants inoculated with humic acids and H. seropedicae.