Humic acids modify root architecture in Arabidopsis through H+-ATPase-dependent target of rapamycin activation in concert with Ca2+ and ROS signaling
Mirella Pupo Santos, Milan Župunski, Hiarhi Monda, Jason Gralian, Aletia James, Guido Großmann, Richard T. Lamar, Daniel Basílio Zandonadi
Abstract
Abstract Background Humic acids (HA) function as plant biostimulants, enhancing plant metabolism by activating the primary proton transport system, which promotes root growth. This study investigated the effects of HA on root growth, H + extrusion, Ca 2+ signaling, and reactive oxygen species (ROS) production, examining how HA might integrate nutrient sensing with growth regulation through plant hormone transport. Results HA rapidly increased cytosolic Ca 2+ and ROS, and altered root architecture in Arabidopsis lines Columbia-0 (Col-0) and G548 TOROE (overexpressing TARGET OF RAPAMYCIN). In Col-0, HA exposure increased total, primary, and lateral root lengths, while in TOROE plants, only primary root length changed. HA also doubled the expression of transcripts in Col-0 roots, including those for PM H + -ATPase ( AHA2 ), TOR kinase, ROS-related RBOHC , and auxin transporters LAX3 and PIN3 . Only AHA2 and RBOHC were upregulated in TOROE plants. Findings indicate that HA promotes rhizosphere acidification and plasma membrane potential regulation via AHA2 and RBOHC , linked to auxin transporters and calcium signaling. Conclusions The data suggest HA, rich in compounds like quinones and flavonoids, stimulates root development by triggering Ca 2+ waves, NADPH oxidase and H + -ATPase activities. These findings advance our understanding of TOR and H + -ATPase roles in root architecture. Graphical abstract