NO and IAA Key Regulators in the Shoot Growth Promoting Action of Humic Acid in Cucumis sativus L.

Previous studies have reported that a purified sedimentary humic acid (PHA) was able to increase the concentration of nitric oxide (NO), indole-acetic acid (IAA) and ethylene in cucumber roots. Here, we investigated if these effects are functionally related to the ability of PHA to improve shoot growth. The effect of specific inhibitors of NO, IAA and ethylene functionality and signaling on PHA-induced shoot growth was studied. Likewise, the effect of these inhibitors on the synthesis and activity of the phytoregulators concerned by PHA action in cucumber roots was also explored. The results show that shoot growth promoted by PHA is due to an increase of IAA concentration in the root through both a NO-dependent and a NO-independent pathway. In addition, the increased ethylene production in the root is regulated by an IAA-dependent pathway. Finally, results also showed that the increase of ABA concentration in the root is regulated through both IAA- and ethylene-dependent pathways. In summary, the shoot growth promoting action of PHA involves a complex hormonal network. On one hand, the PHA action is functionally linked to increments in NO and IAA concentration in roots. And on the other hand, PHA action also increases ethylene and ABA root concentration mediated by NO-IAA dependent pathways.