High molecular size humic substances enhance maize (Zea mays L.) secondary metabolic pathways

Humic substances (HS) promote plant growth by modifying the membrane permeability for the entry of nutrients, and induce C and N metabolisms by increasing the activity of such enzymes working in glycolisis, Krebs cycle and N assimilation (1). However, no information is available on the interactions between HS and secondary metabolic pathways, whose products are involved in plant defence. Phenolic compounds are products of phenylpropanoid metabolism, with the phenylpropanoids being derived from phenylalanine (Phe) and tyrosine (Tyr) (2). The enzyme phenylalanine (tyrosine) ammonia-lyase catalyzes the first committed step in the biosynthesis of phenylpropanoids by converting Phe (PAL) to trans-cinnamic acid and Tyr (TAL) to p-coumaric acid (2). Since HS can stimulate plant C and N primary metabolic pathways, and these in turn cross-talk with phenylpropanoid metabolism, it may be hypothesized that HS might enhance the synthesis of PAL. To verify this hypothesis, a high molecular size HS was supplied to maize plants and its effects on PAL and TAL activity, gene expression, content of amino acids and phenolic compounds was evaluated.