Humic substances stimulate maize nitrogen assimilation and amino acid metabolism at physiological and molecular level

Background: The effects of a humic substance (HS) extracted from a volcanic soil on the nitrate assimilation pathway of Zea mays seedlings were thoroughly examined using physiological and molecular approaches. Plant growth, the amount of soluble proteins and amino acids, as well as the activities of the enzymes involved in nitrogen metabolism and Krebs cycle, were evaluated in response to different HS concentrations (0, 1 and 5 mg C L-1) supplied tomaize seedlings for 48 h. To better understand the HS action, the transcript accumulation of selected genes encoding enzymes involved in nitrogen assimilation and Krebs cycle was additionally evaluated in seedlings grown for 2 weeks under nitrogen (N) sufficient condition and N deprivation. Results: HS at low concentration (1 mg C L-1) positively influenced nitrate metabolism by increasing the content of soluble protein and amino acids synthesis. Furthermore, the activity and transcription of enzymes functioning in N assimilation and Krebs were significantly stimulated. Conclusions: HS treatment influenced the gene expression of Zea mays plants at transcriptional level and this regulation was closely dependent on the availability of nitrate in the growth medium.