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.
Highlights
Accepted: 7 April 2021For many years, soil scientists have endeavored to define the chemical features and the molecular structure of humic substances (HS), and discover how they can modify the growth and development of plants
The biological activity of HS in soil and plants, which is responsible for plant growth promotion, becomes relevant in the context of sustainable agriculture that claims for solutions to address major issues of environmental pollution and economic costs related to fertilizer inputs, while preventing crop yield and quality trade-off
The results demonstrated that the chemical composition of low molecular size (LMS) fractions and soil type are important predictors in modifying nitrogen metabolism (Figure 5), and pinpointed the importance of root exudates in the separation of the plant’s active biological components endowed with hormone-like activity
Summary
Soil scientists have endeavored to define the chemical features and the molecular structure of humic substances (HS), and discover how they can modify the growth and development of plants. Molecules 2021, 26, 2256 fertilizers), attention is paid to low-impact agriculture approaches aimed to increase plant nutrient use acquisition and efficiency. Among these strategies, the use of biostimulants is relevant, of which HS category is. Biostimulants by definition substances that promote plant growth, nutrition and metabolism through modes of action that are challenging toare decipher, but definitely and metabolism through modes of action that are challenging different from those related to fertilizers [4] They are supplied to plants at very low dos- to decipher, but definitely different fromthereby those related to fertilizers [4]. Their mode of action by complementing the dated literature with novel studies
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