Although the ability of humic (HA) and fulvic acids (FA) to improve plant growth has been demonstrated, knowledge about the mechanisms responsible for the direct effects of HA and FA on the promotion of plant growth is scarce and fragmentary. Our study investigated the causal role of both root PM H⁺‐ATPase activity and ABA in the SHA‐promoting action on both root and shoot growth. The involvement of these processes in the regulation of shoot cytokinin concentration and activity was also studied. Our aim was to integrate such plant responses for providing new insights to the current model on the mode of action of HA for promoting root and shoot growth. Experiments employing specific inhibitors and using Cucumis sativus L. plants show that both the root PM H⁺‐ATPase activity and root ABA play a crucial role in the root growth‐promoting action of SHA. With regard to the HA‐promoting effects on shoot growth, two pathways of events triggered by the interaction of SHA with plant roots are essential for the increase in root PM H⁺‐ATPase activity—which also mediates an increase in cytokinin concentration and action in the shoot—and the ABA‐mediated increase in hydraulic conductivity (Lp_r).

Some studies have reported that the capacity of humic substances to improve plant growth is dependent on their ability to increase root hydraulic conductivity. It was proposed that this effect is directly related to the structural conformation in solution of these substances. To study this hypothesis, the effects on root hydraulic conductivity and growth of cucumber plants of a sedimentary humic acid and two polymers—polyacrylic acid and polyethylene glycol—presenting a molecular conformation in water solution different from that of the humic acid have been studied. The results show that whereas the humic acid caused an increase in root hydraulic conductivity and plant growth, both the polyacrylic acid and the polyethylene glycol did not modify plant growth and caused a decrease in root hydraulic conductivity. These results can be explained by the different molecular conformation in water solution of the three molecular systems. The relationships between these biological effects and the molecular conformation of the three molecular systems in water solution are discussed.

Ascophyllum Nodosum (L.) Extracts (ANE), even at low concentrations, are reported to behave as plant growth promoters and inducers of resistance to diverse abiotic and biotic stresses. However, the mechanisms regulating ANE's actions on plant metabolism have not been fully described. This study has two main objectives: (1) to investigate the role of some of the main components of an ANE extract on the plant growth-promoting action of ANE in Arabidopsis thaliana. To this end, experiments have been carried out studying the effect on plant growth of some of the main components of ANE at the concentration contained in the application rate of ANE extract. This study will allow us to obtain some insights about whether the ANE´s action may be ascribed to one or several of its components or derives from the ANE`s action as a whole and (2) to investigate the role of the signaling pathways regulated by salicylic acid (SA), indolacetic acid (IAA), and cytokinins (CK) in the mechanism of action behind the plant growth-promoting action of ANE. To this end, three Arabidopsis thaliana lines affected by the action of SA were treated with ANE: two loss-of-function mutants, pad4 (Phytoalexin Deficient 4) and sid2-2 (Salicylic acid Induction Deficient), along with a transgenic line expressing a bacterial salicylate hydroxylase, nahg, that prevents SA accumulation. ANE was also tested in an IAA loss-of-function mutant (tir1) and two CK receptors loss-of-function mutants (ahk2 and ahk3). Thus, the research advanced in the ANE´s mediated shoot growth-promoting mechanism of action through the coordinated signaling network involving SA-, CK-, and IAA-signaling pathways. The results revealed that: (1) the plant growth-promoting action of ANE seems to be related to the interaction of its components rather than to the action of some of its components and (2) the nahg transgenic line and the sid2-2 mutation, blocked the capacity of ANE to stimulate shoot growth. However, the pad4 mutant was stimulated by ANE. The fact that the action of ANE is impaired in sid2-2 and nahg indicates that the shoot growth-promoting effect of ANE is dependent on the SA-regulated signaling pathway. Likewise, the application of ANE to tir1, ahk2, and ahk3 plant mutants also affected the ANE´s capacity to promote shoot growth. These results indicate that the ANE`s ability to stimulate shoot growth involves the coordinated activation of salicylic acid-, cytokinin-, and auxin-signaling pathways. The results obtained in the pad4 mutant indicate that the ANE´s action through the SA-signaling pathways that promote shoot growth differs from those regulated by the Enhanced Disease Susceptibility1/Phytoalexin Deficient4 (EDS1/PAD4) system. Finally, the action of ANE1 promoting plant growth seems to derive from the action of the extract as a whole.