Involvement of Hormone- and ROS-Signaling Pathways in the Beneficial Action of Humic Substances on Plants Growing under Normal and Stressing Conditions

Figure 1

Main hormonal-related signaling pathways affected by humic substances obtained from compost (CHS) or natural sediments (SHS) and some of their biochemical and physiological effects at whole plant level. The metabolic effects of CHS (in red) are associated with stimulation of metabolic pathways belonging to the stimulation of H⁺-ATPase (PM H⁺-ATPase), Ca⁺⁺ transport, and signaling pathways of auxin-nitric oxide (AUX-NO). Therefore, the morphological effects are related to stimulation of lateral roots and improved root architecture. The metabolic effects of SHS (in blue) in plants occur by stimulating the metabolic pathways belonging to the stimulation of H⁺-ATPase enzymes, signaling through pathways of auxin-nitric oxide abscisic acid (Aux-NO-ABA), expression of responsive genes for membrane aquoporins (PIPs) and hydraulic conductivity (Lpr). Therefore, the morphological effects are related to stimulation of lateral root, root architecture improvements, and increased root biomass. These root physiological effects are associated with events in leaf related to stomatal conductance (gs), cytokinins (CKs), and nitrate (), finally setting off on effects on leaf growth, photosynthesis, and chloroplasts.