Humic acids trigger the weak acids stress response in maize seedlings

Abstract

Background Plants primed by humic acids showed physiological and molecular response against different abiotic stresses without the presence of stressor agents (salinity, drought, heavy metal toxicity). It is plausible that humic acids themselves can act as chemical priming substances in plants. We hypothesized that humic acids can trigger the weak acids stress response in cell plants acidifying the cytosol and thus eliciting the transduction signalling response cascade. Methods The dose–response curves of maize seedlings roots with different concentrations of humic, acetic and salicylic acids determined the most active and inhibitory concentration. These data were further used to evaluate changes on intracellular pH using BCECF-AM probe (2,7-bis(2-carboxyethyl)-5(and 6)-carboxyfluorescein, acetoxymethyl ester) and differential transcription level of genes related to weak stress response in plants by qPCR real time. Results Humic acids like short chain organic acids decrease the intracellular pH showed by the increased fluorescence of BCECF probe. The drop in cytosolic pH promoted by humic acids was not transient. We observed a high level of protein kinases related to cell energy-sensing and transcription factors associated to transduction of stress signalling. Conclusion The humic acids can be considered as a chemical priming agent, since in the appropriate concentration they can induce the typical plant abiotic stress response of weak acids inducing plant acclimation and enhancing the abiotic stress tolerance.

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

“…These fractions can target the cell receptors at the surface of the root or enter the root cells and induce biological activity. Nardi et al [72][73][74] demonstrated through in vitro experiments that organic acids (e.g., fumaric and succinic acids) from maize exudates were able to modify the molecular size of HS, shifting them from high to low molecular weight.…”

“…The LMS obtained from these exudates showed differences in C, N, and organic acid contents, as evident in Table 1. Nardi et al [72][73][74] demonstrated through in vitro experiments that organic acids (e.g., fumaric and succinic acids) from maize exudates were able to modify the molecular size of HS, shifting them from high to low molecular weight.…”

“…Very recently, Baia et al [74] demonstrated that the addition of organic acids typically found in the rhizosphere increased drought tolerance in crops (maize, rice, and wheat) by eliciting the activation of the jasmonate (JA)-signaling pathway. In addition, HS Nardi et al [2] therefore postulated that HS may act as signaling molecules in the rhizosphere releasing hormones and hormone-like substances or promoting their production at plant/microorganism level.…”

“…Very recently, Baia et al [74] demonstrated that the addition of organic acids typically found in the rhizosphere increased drought tolerance in crops (maize, rice, and wheat) by eliciting the activation of the jasmonate (JA)-signaling pathway. In addition, HS at a suitable concentration, can act as a chemical triggering agent that leads to plant acclimatization and increased tolerance to abiotic stress.…”

Transcriptomics of Biostimulation of Plants Under Abiotic Stress

“…Using maize seedlings under weak acids stress (acetic and salicylic acids), the application of humic acids extracted from vermicompost produced with cattle manure was studied (Baía et al, 2020). Humic acids decrease the intracellular pH and produce high level of SnRK2.2 and MDH genes, and low level of WRKY TFs family.…”