The topical challenge of modern agriculture is to combine the maintenance of crop yields with lower energetic inputs within changing environmental conditions. In this scenario, the abiotic stresses become prevalent limiting factors that need to be faced with the development of sustainable approaches to protect soil and plants.
The combination of humic substances and plant growth-promoting bacteria has produced a biostimulant that has been systematically tested on an experimental scale (1) and recently used in large areas of tropical crops.
The increase of microbiological processes such as biological nitrogen fixation, phosphate solubilization, and the biostimulation process is particularly significant given the magnitude of the increasing production costs and environmental impacts. The indirect (most soluble complex) and the direct effect of humic substances on nutrient uptake have been widely reported, and its effect on ion fluxes is one of the most studied issues (2). The stimulation of plasma membrane proton pumps has a central role in ion uptake. These enzymes provide an electrochemical gradient necessary to energize ion transport for cell uptake and induce cell growth by a mechanism known as acid growth, in which H+ acts as the intermediate between auxin and cell wall loosening (3).
The practical results of foliar applying of the biostimulant based on humic substances and plant growth-promoting bacteria have been especially significant on crop production when some type of limitation imposed by abiotic stress are present such as low natural soil fertility (4) or drought (5). Moreover, the strength of the stress factor in the last data meta-analysis of experiments considering the root and shoot growth induced by humic substances cannot be disregarded (6).
The scientific literature concerning the effect of humic substances on the mitigation of abiotic stress damage in plants is abundant, and some relatively recent reviews are available (7, 8, 9, 10). Here we would like to introduce a different aspect considering the biostimulant as tools for crop management strategy, i.e. plant preparation for rapid response against abiotic stress. Chemical priming is a promising field in plant stress physiology and crop stress management. Preconditioning strategy can be utilized to stimulate the adaptation mechanism and enhance the crop resilience capacity and thus limiting the negative impacts on yield quality and productivity. Priming means to elicit and anticipate plant tolerance against environmental challenges by improving physiological performance via the application of chemical or biotic agents (11). Humic acids have been found to act as active priming agents against plants’ abiotic stress in a typical hormesis response (12). Hormesis can be defined as a biphasic response in which high doses of a toxic agent could cause inhibition while a low dose of the same toxic agent can cause stimulation (13). It is characterized by a phenomenon by which a low dose of the stressor anticipates the cellular stress response, including secondary metabolites production, in order to help organisms establish adaptive responses. Humic acids can show a typical hormesis dose-response, i.e. a biphasic dose-response characterized by low-dose stimulation and high-dose inhibition (Fig 1).
[Figure 1]: Typical dose-response curve of humic acids (mg L-1) isolated from vermicompost of cattle manure and maize seedlings root growth (fresh weight) and theoretic dose-dependent changes in response in case of a monophasic, biphasic or triphasic dose-response pattern and quantitative thresholds to separate the low-dose range from a high-dose exposure. This is an adaptation of the original image from Belz and colleagues (14)
Hormesis mechanisms in plants induced by humic substances were recently highlighted (Fig 2). Humic acids and plant growth-promoting bacteria can be used as a chemical priming plant defence agent since seedlings showed typical hormesis response with biphasic dose-response. The prime state induced by humic substances showed a significant transcription level of genes encoding stress perception and cell signalization, including kinases, phosphatases proteins, and functional and regulatory (transcription factors) proteins, which are involved in gene response against abiotic stress (12). The further exposition of chemically primed maize seedling to abiotic stress agents resulted in an apparent increase of plant tolerance against abiotic stress, including osmotic stress (Fig 3).
[Figure 2]: Hormesis mechanisms induced by humic acids related to signal transduction network in plant cells. Coloured steps indicate that signals were altered by humic treatments producing changes in membrane potential, occuping and activating plasma membrane receptors, or modifing protein kinases. The Ca2+ cytosolic pulse was previously measured by an ion-selective vibrating probe system and the Real-Time quantitative PCR to measure the differential expression of voltage-dependent channels and kinases (15). Other second messengers induced by humic acids were previously described by García and colleagues (16) that observed regulation of oxygen reactive species production at a cellular level. The activation (down or upregulation) of different transcription factors and the high transcription level of specific abiotic gene response in the prime state induced by humic acids isolated from vermicompost were also found (12). The figure is an adaptation of the original scheme proposed by Trewavas and Malhó (17) to describe the signal transduction network in plant cells.