High molecular size humic substances enhance phenylpropanoid metabolism in maize (Zea mays L.)

In this study a humic fraction at high molecular weight (> 3,500Da) was chemically characterized by using DRIFT and 1H NMR spectroscopy, and was supplied to Zea may L. plants to evaluate its biological effect on the phenylpropanoid metabolism. To this aim, the activity and gene expression of phenylalanine (tyrosine) ammonia-lyase (PAL/TAL), as well as the content of phenols and their amino acid precursors (Phe and Tyr), were assayed. The maximum induction of PAL/TAL activity and of gene expression was obtained at the concentration of 1 mg C/l. Concomitantly, the content of Phe and Tyr significantly decreased (-16% and -22%, respectively) as a consequence of their utilization for the synthesis of phenylpropanoid derivatives, including flavonoids. The higher amount of phenol compounds in HS treated plants could be partly ascribed to the presence of auxin (27 nmol indoleacetic acid) in the humic fraction. Our results suggest that HS are able to induce changes in the phenylpropanoid metabolism significantly modulating the synthesis of phenolic acids. At our knowledge, this is the first study that shows interaction between HS and the phenylpropanoid pathways, and opens the way for further research on HS and plant secondary metabolic routes.