Humic Substances: Determining Potential Molecular Regulatory Processes in Plants

Abstract

Humic substances (HSs) have considerable effects on soil fertility and crop productivity owing to their unique physiochemical and biochemical properties, and play a vital role in establishing biotic and abiotic interactions within the plant rhizosphere. A comprehensive understanding of the mode of action and tissue distribution of HS is, however, required, as this knowledge could be useful for devising advanced rhizospheric management practices. These substances trigger various molecular processes in plant cells, and can strengthen the plant’s tolerance to various kinds of abiotic stresses. HS manifest their effects in cells through genetic, post-transcriptional, and post-translational modifications of signaling entities that trigger different molecular, biochemical, and physiological processes. Understanding of such fundamental mechanisms will provide a better perspective for defining the cues and signaling crosstalk of HS that mediate various metabolic and hormonal networks operating in plant systems. Various regulatory activities and distribution strategies of HS have been discussed in this review.

Biostimulants research in some horticultural plant species—A review

“…Depending on their composition and expected results, biostimulants can be soil‐ or leaf‐applied (Kunicki, Grabowska, Sękara, & Wojciechowska, ). In the last two decades, special attention has been given to compounds of plant origin and to some other natural bioactive materials, such as humic and fulvic acids (Calvo, Nelson, & Kloepper, ; Canellas et al., ; Nardi, Pizzeghello, Schiavon, & Ertani, ; Shah et al., ). Their physiological effects occur after their entrance into plant tissues and cells, where these compounds are involved in the plant's metabolism, signaling, and hormonal regulation of growth and development.…”

Manure Fertilization Gives High-Quality Earthworm Coprolites with Positive Effects on Plant Growth and N Metabolism

“…In literature, HS are known to affect nutrient bioavailability by forming complexes with metallic ions, thus improving accessibility of micronutrient and macronutrients [54]. They can additionally favor plant nutrition by stimulating the activity of plasma membrane H + -ATPases, enhance the gene expression and activity of mineral nutrient transporters, and modify the root growth and architecture via hormone-like effects that promote the plant cellular division and elongation processes [8,14,24,49]. Recently, the effects of humic acids on root architecture, including the induction of lateral roots and the increase of biomass have been reported to be accompanied by changes in the energy metabolism-associated proteins [27].…”

“…Also, some authors highlighted the issue that HS might be artifacts forming during the alkaline extraction procedure [22], but this theory has been recently contested [23].Although the existence of these different viewpoints on HS molecular structure, some of their components are critical in the establishment of biotic relationships in the rhizosphere and can interact with plant cell receptors, while triggering plant physiological responses through mechanisms that, so far, are not yet completely elucidated [22].Humic acids (HA) are HS constituents produced via associations of hydrophobic compounds, such as polymethylene chains, fatty acids, phenolics and steroids, which are stabilized by hydrophobic forces at neutral pH [17]. HA isolated from earthworms' coprolites in particular, have been reported to stimulate root growth and plant defense systems against stress [24]. However, their bioactivity is a very complex and sometimes controversial issue.…”

“…Humic acids (HA) are HS constituents produced via associations of hydrophobic compounds, such as polymethylene chains, fatty acids, phenolics and steroids, which are stabilized by hydrophobic forces at neutral pH [17]. HA isolated from earthworms' coprolites in particular, have been reported to stimulate root growth and plant defense systems against stress [24]. However, their bioactivity is a very complex and sometimes controversial issue.…”

Correlation Biological Activity and Physicochemical Properties of Humic Acids in Aqueous Solutions

“…Thus interacting with living organisms, HS in small quantities stimulate plant growth [5,6], and are able to protect living cells from the toxic effects of natural and anthropogenic compounds [7]. HS not only regulates the processes of nutrition and plant development [8], but they themselves can serve as a source of [9][10][11]. That is, nature in its genius has come up with a completely unique product that combines the properties of both organic and inorganic fertilizers, being like a universal growth factor.…”