Acclimation with humic acids enhances maize and tomato tolerance to salinity

Abstract

Background: Salinity is one of the major environmental threats to agriculture, limiting plant growth and reducing crop yield. The use of humic substances to alleviate salt stress in plants is well reported, but the mechanisms remain unclear. This work aimed to apply humic acids on seedlings to acclimate plants to tolerate further salt stress exposition as a pre-treatment. Materials and methods: Two independent experiments with mono (maize) and dicot (tomato) seedlings were carried out. Maize was primed by humic acids (4 mM C) and further submitted to moderate salinity exposition (60 mM NaCl). The acclimation period of maize seedlings was characterized by ion balance and transcriptomic analysis of salt response genes. The tomato seedlings were also primed by humic acids (4 mM C) and exposed further to salinity (200 mM NaCl), and we measured only physiological aspect, including the activity of plasma membrane proton pumps and net photosynthesis rate. Results: Seedlings primed by humic acids minimized the salinity stress by changing ion balance, promoting plasma membrane proton pumps activity and enhancing photosynthesis rate and plant growth. We showed for the first time that maize seedlings treated with humic acids had a high transcription level of salt responsive genes and transcription factors even before the salt exposition. Conclusion: Humic acids previously activate cellular and molecular salt defence machinery, anticipating the response and reducing salinity stress. This is a key knowledge to manipulate manufactured biostimulants based on humic substances towards a maximized crop protection. Graphic abstract: [Figure not available: see fulltext.]