Soil–root cross‐talking: The role of humic substances

Abstract

The biological activity of humic substances (HS) has been elucidated in the last 40 years. Growth enhancement from HS has been demonstrated in several plants in the laboratory and the field. Morphogenesis effects have also been investigated and include induction of lateral root formation and root hair initiation in intact plants and stimulation of root and shoot development in treated cell calluses. HS enhance nutrient use efficiency, aiding assimilation of both macro and microelements and promoting plant growth by the induction of carbon, nitrogen, and secondary metabolism. The review aims are to: (1) shed light on the mechanism by which plants “talk” with soil through humic substances, (2) elucidate the plant responses to the stimulatory effects of HS, the regulatory circuits that allow plants to cope with humus, and the feedback between plant community structures, and (3) show (in light of recent debate about the alkaline extraction of soil humic substances) the plant capability to acquire biologically active substances from soil. It will be shown that plants modify soils, creating and maintaining favorable habitats for growth and survival. Therefore, organic substances exuded by roots are not a wasteful loss of carbon and energy. They represent an evolved strategy by which plants “talk” to the soil. The mobilization of bioactive organic/humic substances from bulk soil or bulk humus is critical to plant and soil health.

Plant growth promoting bacteria and humic substances: crop promotion and mechanisms of action

“…Finally, it is possible return and extend the conclusion of Nardi and colleagues [21] saying that plant co-inoculation with DEB and HS, besides induction of lateral root formation and root hair initiation, enhance nutrient use efficiency, aiding assimilation and promoting plant growth by the induction of carbon, nitrogen, and secondary metabolism.…”

“…2A). The morphological changes of the root system triggered by HS comprise increases of lateral root formation sites, root hair density and length [21], as well as overall surface area available for bacteria attachment (Fig. 2B).…”

“…The presence of organic acids, like those produced by root exudation, can modify the supramolecular arrangement, releasing bioactive molecules which can affect plant growth. The enhancement of organic acids exudation by maize due to HS treatment were independently demonstrated [19,20] and the cross-talk involving plant roots and humic matter was recently approached [21]. The physiological effects of HS is widely documented [22,23] and summarized as a result of enhance nutrient use efficiency, aiding assimilation of both macro and micronutrients and promoting plant growth by induction of carbon, nitrogen, and secondary metabolism [21].…”

The biostimulant manufactured using diazotrophic endophytic bacteria and humates is effective to increase sugarcane yield

“…Finally, in addition to stress response probably induced by biostimulant application the increase of sugarcane yield is a consequence of the well-known effect of humic substances and plant growth-promoting bacteria in plant physiology. Nardi et al [25] summarize the effect of humic substances on plant growth as a result of the enhancement of nutrient use efficiency, aiding the assimilation of both macro-and microelements and the induction of carbon, nitrogen, and secondary metabolism. Plant growth-promoting bacteria can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production, etc.…”

Humic Substances and Efficient Microorganisms: Elicitation of Medicinal Plants—A Review

“…The humic acids influence on the fruits, flowers and seeds development (M. A. Baldotto & E. B. Baldotto, 2015) that acts in several phases of the vegetable physiology such as growth, development, photosynthetic structures of respiratory activity (Costa et al, 2008); changes on biochemical pathways as primary and secondary metabolism (Trevisan et al, 2011;Aminifard et al, 2012;Nardi et al, 2017). Due to all these beneficial effects of elicitors on plant productivity, they have attracted the interest of farmers and companies.…”