Chemical Structure and Biological Activity of Humic Substances Define Their Role as Plant Growth Promoters

Abstract

Humic substances (HS) are dominant components of soil organic matter and are recognized as natural, effective growth promoters to be used in sustainable agriculture. In recent years, many efforts have been made to get insights on the relationship between HS chemical structure and their biological activity in plants using combinatory approaches. Relevant results highlight the existence of key functional groups in HS that might trigger positive local and systemic physiological responses via a complex network of hormone-like signaling pathways. The biological activity of HS finely relies on their dosage, origin, molecular size, degree of hydrophobicity and aromaticity, and spatial distribution of hydrophilic and hydrophobic domains. The molecular size of HS also impacts their mode of action in plants, as low molecular size HS can enter the root cells and directly elicit intracellular signals, while high molecular size HS bind to external cell receptors to induce molecular responses. Main targets of HS in plants are nutrient transporters, plasma membrane H⁺-ATPases, hormone routes, genes/enzymes involved in nitrogen assimilation, cell division, and development. This review aims to give a detailed survey of the mechanisms associated to the growth regulatory functions of HS in view of their use in sustainable technologies.

Keywords: auxin; biological activity; growth promoters; hormone-like activity; humic substances; hydrophily; hydrophobicity; nutrition.

Figure 1

Associations between mineral colloids and humic substances are characterized by a variety of interactions and chemical bonds that make these structures stable in soils.

Figure 2

Typical FT–IR spectrum of a soil humic substance. The main oxygenated functional groups are reported in the spectrum.

Figure 3

(A) Typical chemical structure of phenolic acids. These compounds are considered major components of soil humic substances. (B) Quinones (left) are groups that accept electrons and are reduced to hydroquinones (right).

Figure 4

Root exudates contain substances, including low molecular weight organic acids (OA) that may influence the solubility of soil HS (bulk HS) by inducing their disaggregation to produce LMS and HMS fractions.

Figure 5

NO₃⁻ and NH₄⁺ uptake (left) and nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities (right) in P. sylvestris seedlings treated with low-molecular-size organic fraction extracted from Eutric Cambisol, EC and Rendzic Leptosol, RL by maize (cultivar Mytos and Sandek) or forest root exudates [73]. P. syl = P. sylvestris; P. ab = P. abies; San = Sandek¸ My = Mytos; W = water.

Figure 6

Central cylinder of wheat roots untreated (A) and treated (B) with HS. Xylem vessels of untreated roots exhibit lower differentiation degree and thinner cell walls compared to HS-treated roots. TEM micrograph of wheat roots untreated (C) or treated (D) with HS. In the treated samples, root cells have cell wall (cw) with high thickness. ed: endodermis; mt: mitochondria; nu: nucleus; v: valcuole; *: provacuoles.

Figure 7

SEM micrographs of the 0–20 mm region behind the root tips of wheat seedlings surface. Plants were grown in Hoagland solution and treated for the last 2 days with HS. (A) plant treated with HS on the left, untreated plant on the right; (B) = untreated plant; (C) = plant treated with HS. rp: root primordia; rh: root hair.

Figure 8

Mechanism of action of HS at root level to induce lateral root emergence and stimulate root hair density and length, and metabolic-physiological targets in plants. HMS interact with membrane receptors to induce cascade signaling inside root cells (1); LMS enters the root cells (2). Both HMS and LMS stimulate the activity of the root membrane H⁺-ATPase (3); and the auxin polar flux (4) to promote auxin and NO accumulation at the pericycle cells to enhance lateral root emergence (5). HMS and LMS induce accumulation of auxin in the root epidermal cells, resulting in increased root hair formation and cell elongation (6).