Effects of a sedimentary humic acid on plant growthstudy of the general mechanisms of action upon root application in cucumber plants (Cucumis Sativus l. Cv Ashley).

Resumen

HS (humic substances) constitute one of the most abundant forms of organic matter in the biosphere. Many studies reported the ability of humic acids (HAs) isolated from HS to affect plant the plant development of diverse plant species. However, the mechanism of action that is behind these events is scarcely understood and it is not integrated into a holistic view. To this end, the aim of this work was to study the mechanism of action of an specific sedimentary humic acid (SHA) under roots application in cucumber plants (Cucumis sativus L. cv Ashley) in detail. More specifically, the potential role of hydraulic conductivity (Lpr) linked to root abscisic acid (ABA) concentration and some of water relations such us stomatal conductance (Gs) and leave ABA concentration in the SHA shoot growth promotion activity. Indeed, the functional relationship between Lpr with root-ABA biosynthesis and root plasma membrane aquaporins (CsPIPs) molecular regulation was also investigated. With the aim to integrate the studied SHA root mechanism into previously SHA enhanced potential response, the relative role of SHA mediated PM-H+-ATPase activity and Lpr on shoot and/or root growth promotion of cucumber plants was also assessed. Indeed, the SHA-mediated regulation of root genes associated with root growth was also studied in Arabidopsis thaliana seedlings. Finally, with the purpose of studying the relationship between the chemical structure of SHA and the biological activity exerted in plants, a sequential fractionation process named �humeomics� was applied to SHA, and the behavior of some of the obtained fractions was assessed in cucumber plant biological activity. The results showed that the beneficial effect of SHA on shoot growth was associated with a significant increase of Lpr and Gs. The SHA mediated increase in Lpr was root-ABA dependent and regulated by some of the root aquaporin homologues. Regarding the discriminating role of SHA enhancing Lpr activation or PM-H+-ATPase activity in both shoot and/or root growth, results showed that the SHA-mediated enhancement of root PM-H+-ATPase activity might play a secondary role in the improvement of shoot growth caused by SHA, Lpr activity being more relevant. Furthermore, our results indicated that probably the stimulation of root PM-H+-ATPase activity plays a more relevant role than the increase of Lpr in the mechanism of action of SHA on plant root growth. In relation to the structure-activity relationship using humeomic fractionation approach in cucumber, results showed that 62.5% of the extracted material corresponded to the unbound fraction (ORG 1). In the determination of biological activity, two fractions were studied: the mentioned ORG 1 fraction and the remained residue (RES 1). The main structural difference between both samples was that RES 1 fraction showed higher aromatic character than ORG 1, which was more aliphatic. RES 1 fraction increased Lpr, root and shoot growth compared to control and bulk SHA treated plants. ORG 1 fraction, instead, did not show significant differences on Lpr or shoot growth compared to control or SHA treated plants. The response in root growth was similar to SHA bulk treatment. As for the SHA regulated root gene expression in Arabidopsis thaliana, results showed that there were several up-regulated genes encoding root promotion ability functions, such us AT3G15370.1 gene function, identified as a member of an Alpha-Expansin Gene Family or AT2G14960.1 gene that encodes a protein similar to IAA-amino synthetase, which maintains the auxin homeostasis. Moreover, some other up-regulated genes that could be related with SHA responses that other authors previously reported were also identified.