Discriminating the short-term action of root and foliar application of humic acids on plant growth: emerging role of jasmonic acid

Humic substances (HS, fulvic and humic acids) are widely used as fertilizers or plant growth stimulants, although their mechanism of action still remains partially unknown. Humic substances may be applied either directly to the soil or as foliar sprays. Despite both kind of application are commonly used in agricultural practices, most of the studies regarding the elicited response in plants induced by HS are based on the root-application of these substances. The present work aimed at discriminating between the mechanisms of action of foliar application versus root application of a sedimentary humic acid (SHA) on plant development. For this purpose, six markers related to plant phenotype, plant morphology, hormonal balance and root-plasma membrane H+-ATPase were selected. Both application strategies improved the shoot and root growth. Foliar applied- and root applied-SHA shared the capacity to increase the concentration of indole-3-acetic acid in roots and cytokinins in shoots. However, foliar application did not lead to short-term increases in either abscisic acid root-concentration or root-plasma membrane H+-ATPase activity which are, however, two crucial effects triggered by SHA root-application. Both application modes increased the root concentrations of jasmonic acid and jasmonoyl-isoleucine. These hormonal changes caused by foliar application could be a stress-related symptom and connected to the loss of leaves trichomes and the diminution of chloroplasts size seen by scanning electron microscopy. These results support the hypothesis that the beneficial effects of SHA applied to roots or leaves may result from plant adaptation to a mild transient stress caused by SHA application

Root ABA and H+-ATPase are key players in the root and shoot growth-promoting action of humic acids

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))

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

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.Las sustancias húmicas (SH) constituyen una de las formas más abundantes de materia orgánica de la biosfera. Muchos estudios anteriormente publicados describen que los ácidos húmicos (AH) extraídos previamente de las SH poseen la capacidad de afectar al desarrollo de plantas de diversas especies. Sin embargo, la comprensión del mecanismo de acción que subyace a este efecto es escasa y no está integrada en una visión holística sobre el tema. Con este fin, el objetivo principal de este trabajo fue estudiar en detalle el mecanismo de acción de un ácido húmico específico de origen sedimentario (AHS) aplicado radicularmente en plantas de pepino (Cucumis sativus L. cv Ashley). Concretamente se estudió la participación potencial de la conductividad hidráulica (Lpr) unido a la síntesis de ácido abscísico (ABA) en raíz junto a otras relaciones hídricas tales como la conductancia estomática (Gs) o la síntesis de ABA en hoja sobre la actividad promotora del AHS en el desarrollo vegetal de la parte aérea. Además, se investigó la relación funcional entre Lpr y ABA en raíz con la regulación molecular de las acuaporinas de la membrana plasmática en raíz (CsPIPs). Con la intención de integrar el citado mecanismo radicular del AHS en respuestas activadas por AHS y descritas anteriormente, se estudió la implicación relativa de la activación de la ATPasa de la membrana plasmática (PM-H+-ATPase) y de la Lpr sobre la estimulación del desarrollo de la raíz y parte aérea en plantas de pepino. También se estudió la regulación de los genes de la raíz por acción del AHS, en plántulas de Arabidopsis thaliana. Finalmente, con el objetivo de estudiar la relación estructura química de AHS- actividad biológica ejercida en planta, se llevó a cabo un fraccionamiento secuencial del AHS denominado “humeómica” y se estudió la actividad biológica derivada de la aplicación de ciertas fracciones en plantas de pepino. Los resultados mostraron que el efecto beneficioso mostrado por AHS en el crecimiento de la parte aérea estaba unido a un incremento significativo de Lpr y Gs. Este aumento de Lpr mostró ser dependiente de ABA-radicular y estar regulado por ciertos hómologos de las acuaporinas de la membrana plasmática de células radiculares. En relación al estudio de la relevancia ejercida por AHS en la activación de PM-H+-ATPase o Lpr como respuesta al crecimiento de la raíz y/o parte aérea, los resultados mostraron que la activación de la PM-H+-ATPase mediada por AHS jugaría un papel secundario en el beneficio del desarrollo de la parte aérea, siendo más relevante la activación de Lpr. Además, nuestros resultados también indicaron que probablemente la estimulación de PM-H+-ATPase sería un factor más relevante que el incremento de Lpr sobre el crecimiento de la raíz. En relación al estudio de estructura-actividad biológica realizada aplicando el fraccionamiento “humeómico”, los resultados mostraron que el 62.5% del material extraído correspondía a la fracción no enlazada (ORG 1). Para la determinación de la actividad biológica, se estudiaron dos fracciones: la mencionada fracción ORG 1, y el remanente del fraccionamiento (RES 1). La principal diferencia estructural entre las dos fracciones fue que la fracción RES 1 presentaba un carácter más aromático que la fracción ORG 1, cuyo carácter fue más alifático. La aplicación de la fracción RES 1 en planta incrementó Lpr, el crecimiento de la raíz y parte aérea en comparación al control y al AHS integral. Sin embargo, la fracción ORG 1, no presentó diferencias significativas en Lpr o crecimiento de la parte aérea en comparación al control o al AHS integral. La respuesta de la aplicación de ORG 1 en el crecimiento radicular fue similar al mostrado por el AHS integral. Con respecto a la regulación de genes radiculares ejercida por la aplicación de AHS en Arabidopsis thaliana, los resultados mostraron que entre los genes sobre-expresados, algunos codificaban para respuestas relacionadas con la estimulación del desarrollo radicular, como el gen AT3G15370.1, identificado como un miembro de la familia de los genes de Alpha-Expansinas, o el gen AT2G14960.1, que codifica para una proteína similar a IAA-amino synthetase, el cual mantiene la homeostasis de auxinas. Además, también fueron identificados otros genes sobre-expresados que relacionaban respuestas previamente descritas por AHS en planta por otros autores

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

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.Las sustancias húmicas (SH) constituyen una de las formas más abundantes de materia orgánica de la biosfera. Muchos estudios anteriormente publicados describen que los ácidos húmicos (AH) extraídos previamente de las SH poseen la capacidad de afectar al desarrollo de plantas de diversas especies. Sin embargo, la comprensión del mecanismo de acción que subyace a este efecto es escasa y no está integrada en una visión holística sobre el tema. Con este fin, el objetivo principal de este trabajo fue estudiar en detalle el mecanismo de acción de un ácido húmico específico de origen sedimentario (AHS) aplicado radicularmente en plantas de pepino (Cucumis sativus L. cv Ashley). Concretamente se estudió la participación potencial de la conductividad hidráulica (Lpr) unido a la síntesis de ácido abscísico (ABA) en raíz junto a otras relaciones hídricas tales como la conductancia estomática (Gs) o la síntesis de ABA en hoja sobre la actividad promotora del AHS en el desarrollo vegetal de la parte aérea. Además, se investigó la relación funcional entre Lpr y ABA en raíz con la regulación molecular de las acuaporinas de la membrana plasmática en raíz (CsPIPs). Con la intención de integrar el citado mecanismo radicular del AHS en respuestas activadas por AHS y descritas anteriormente, se estudió la implicación relativa de la activación de la ATPasa de la membrana plasmática (PM-H+-ATPase) y de la Lpr sobre la estimulación del desarrollo de la raíz y parte aérea en plantas de pepino. También se estudió la regulación de los genes de la raíz por acción del AHS, en plántulas de Arabidopsis thaliana. Finalmente, con el objetivo de estudiar la relación estructura química de AHS- actividad biológica ejercida en planta, se llevó a cabo un fraccionamiento secuencial del AHS denominado “humeómica” y se estudió la actividad biológica derivada de la aplicación de ciertas fracciones en plantas de pepino. Los resultados mostraron que el efecto beneficioso mostrado por AHS en el crecimiento de la parte aérea estaba unido a un incremento significativo de Lpr y Gs. Este aumento de Lpr mostró ser dependiente de ABA-radicular y estar regulado por ciertos hómologos de las acuaporinas de la membrana plasmática de células radiculares. En relación al estudio de la relevancia ejercida por AHS en la activación de PM-H+-ATPase o Lpr como respuesta al crecimiento de la raíz y/o parte aérea, los resultados mostraron que la activación de la PM-H+-ATPase mediada por AHS jugaría un papel secundario en el beneficio del desarrollo de la parte aérea, siendo más relevante la activación de Lpr. Además, nuestros resultados también indicaron que probablemente la estimulación de PM-H+-ATPase sería un factor más relevante que el incremento de Lpr sobre el crecimiento de la raíz. En relación al estudio de estructura-actividad biológica realizada aplicando el fraccionamiento “humeómico”, los resultados mostraron que el 62.5% del material extraído correspondía a la fracción no enlazada (ORG 1). Para la determinación de la actividad biológica, se estudiaron dos fracciones: la mencionada fracción ORG 1, y el remanente del fraccionamiento (RES 1). La principal diferencia estructural entre las dos fracciones fue que la fracción RES 1 presentaba un carácter más aromático que la fracción ORG 1, cuyo carácter fue más alifático. La aplicación de la fracción RES 1 en planta incrementó Lpr, el crecimiento de la raíz y parte aérea en comparación al control y al AHS integral. Sin embargo, la fracción ORG 1, no presentó diferencias significativas en Lpr o crecimiento de la parte aérea en comparación al control o al AHS integral. La respuesta de la aplicación de ORG 1 en el crecimiento radicular fue similar al mostrado por el AHS integral. Con respecto a la regulación de genes radiculares ejercida por la aplicación de AHS en Arabidopsis thaliana, los resultados mostraron que entre los genes sobre-expresados, algunos codificaban para respuestas relacionadas con la estimulación del desarrollo radicular, como el gen AT3G15370.1, identificado como un miembro de la familia de los genes de Alpha-Expansinas, o el gen AT2G14960.1, que codifica para una proteína similar a IAA-amino synthetase, el cual mantiene la homeostasis de auxinas. Además, también fueron identificados otros genes sobre-expresados que relacionaban respuestas previamente descritas por AHS en planta por otros autores

Supramolecular arrangement of lignosulfonate-based iron heteromolecular complexes and consequences of their Interaction with Ca2+ at alkaline pH and fe plant root uptake mechanisms

Previous studies have shown that natural heteromolecular complexes might be an alternative to synthetic chelates to correct iron (Fe) deficiency. To investigate the mechanism of action of these complexes, we have studied their interaction with Ca2+ at alkaline pH, Fe-binding stability, Fe-root uptake in cucumber, and chemical structure using molecular modeling. The results show that a heteromolecular Fe complex including citric acid and lignosulfonate as binding ligands (Ls-Cit) forms a supramolecular system in solution with iron citrate interacting with the hydrophobic inner core of the lignosulfonate system. These structural features are associated with high stability against Ca2+ at basic pH. Likewise, unlike Fe-EDDHA, root Fe uptake from Ls-Cit implies the activation of the main root responses under Fe deficiency at the transcriptional level but not at the post-transcriptional level. These results are consistent with the involvement of some plant responses to Fe deficiency in the plant assimilation of complexed Fe in Ls-Cit under field conditions

The molecular conformation, but not disaggregation, of humic acid in water solution plays a crucial role in promoting plant development in the natural environment.

Many studies have shown the capacity of soil humic substances (HS) to improve plant growth in natural ecosystems. This effect involves the activation of different processes within the plant at different coordinated molecular, biochemical, and physiological levels. However, the first event triggered by plant root-HS interaction remains unclear. Some studies suggest the hypothesis that the interaction of HS with root exudates involves relevant modification of the molecular conformation of humic self-assembled aggregates, including disaggregation, which might be directly involved in the activation of root responses. To investigate this hypothesis, we have prepared two humic acids. A natural humic acid (HA) and a transformed humic acid obtained from the treatment of HA with fungal laccase (HA enz). We have tested the capacity of the two humic acids to affect plant growth (cucumber and Arabidopsis) and complex Cu. Laccase-treatment did not change the molecular size but increased hydrophobicity, molecular compactness and stability, and rigidity of HA enz. Laccase-treatment avoided the ability of HA to promote shoot- and root-growth in cucumber and Arabidopsis. However, it does not modify Cu complexation features. There is no molecular disaggregation upon the interaction of HA and HA enz with plant roots. The results indicate that the interaction with plant roots induced in both HA and laccase-treated HA (HA enz), changes in their structural features that showed higher compactness and rigidity. These events might result from the interaction of HA and HA enz with specific root exudates that can promote intermolecular crosslinking. In summary, the results indicate that the weakly bond stabilized aggregated conformation (supramolecular-like) of HA plays a crucial role in its ability to promote root and shoot growth. The results also indicate the presence of two main types of HS in the rhizosphere corresponding to those non-interacting with plant roots (forming aggregated molecular assemblies) and those produced after interacting with plant root exudates (forming stable macromolecules)