RETRACTED: Use of Ore-Derived Humic Acids With Diverse Chemistries to Elucidate Structure-Activity Relationships (SAR) of Humic Acids in Plant Phenotypic Expression

Abstract

We report the results of a structure-activity relationship study that was undertaken to identify humic substance chemistries that drive the plant biostimulant response. The effects of seven extensively chemically characterized, ore-derived humic acids (HA) on corn seedling biomass and root and shoot morphological parameters were investigated. Chemometric analyses were then conducted to identify correlations between HA chemical features and plant biomass and morphological characteristics. The primary chemical driver of plant biomass and morphology was the ratio between HA electron accepting capacity (EAC) and electron donating capacity (EDC). The HA electron accepting capacity is found in quinones and semiquinone free radicals, while the HA electron donating capacity is found in polyphenolics and glycosylated polyphenolics. Based on our results, we propose a mechanism of action for ore-derived HA plant biostimulation that involves the interplay of pro-oxidants, in the form of quinones and semiquinone radicals, and antioxidants, in the form of polyphenols and possibly glycones and carbohydrates. The quinones/semiquinones initiate an oxidative stress response via the stimulation of transmembrane electron flow that results in both reactive oxygen species (ROS) production (i.e., an oxidative burst) and membrane depolarization, the latter of which allows Ca2+ flux from the apoplast into the cytoplasm. Based on the magnitude of depolarization, a specific cytoplasmic Ca2+ signature is produced. As a secondary messenger Ca2+, via binding to Ca2+− sensor proteins, transmits the signature signal, resulting in specific intracellular responses that include changes to plant morphology. The greater the EAC, the greater the ROS production and magnitude of plasma membrane depolarization and resulting stress response. The HA antioxidants are able to scavenge and quench the ROS and thus modulate the intensity and extent of the stress response to greater or lesser degrees, based on their concentrations and radical scavenging efficiencies, and thus modify the Ca2+ signature and ultimately the intracellular molecular responses.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

“…HS could support the initial formation of the extracellular matrix of polysaccharides and lipids required for colony formation and flocculation [ 169 ], reducing the metabolic costs of colony formation [ 170 ]. The surfactant agents, such as linear alkyl benzene (LAB) [ 171 ], benzalkonium bromide (BZK) [ 172 ], sodium dodecyl sulfate (SDS) [ 172 , 173 ], and nonionic surfactant polyoxyethylene (40) nonylphenol ether (NPE) [ 172 ] enhance microalgae colony formation in the presence of smaller amounts of grazers natural cues or even in their absence [ 171 ]. Such effect should be considered in terms of ecotoxicological risk assessment, but also for its biotechnological potential related to microalgae harvesting.…”

Organic matter in the pest and plant disease control: a meta-analysis

“…Humic substances (HS) consist of up to 80% of organic materials in soil and sediments whose reactivity and dynamics have pivotal importance on microbial activity and plant productivity [16,17,71,83]. Their effects on plant physiology depend on their chemical nature and the availability of biologically active compounds, concentration, type and age of plant species and are correlated with the apparent molecular mass distribution [16,22,[83][84][85][86][87]. Indirectly, the effect of HS can be associated with the effects of other organic matter sources, such as V, C and teas in the control of pests and diseases in plants, since HS are ubiquitous in all natural organic material.…”