Search

Increasing attention has been devoted in the last few years to the mechanisms by which humic substances (HS) influence plant growth. Nevertheless, amongst the biologically active compounds that might be present in HS, only indoleacetic acid has been identified. An enzyme-linked immunosorbent assay (ELISA) was applied to detect the presence of isopentenyladenosine (IPA), a cytokinin, in two lignosulphonate-humates, leonardite humic acid and in a high molecular weight humic fraction extracted from earthworm faeces. The IPA concentration ranged from 34 to 145 pmol mg− 1 humic C and was estimated by using a linear dose–response curve between the carbon (C) concentration and percentage of binding of an anti-IPA monoclonal antibody. The cytokinin-like activity of HS was evaluated by weighing small radish (Raphanus sativus L.) cotyledon leaves and measuring chlorophyll and protein contents, the activities of ATP sulfurylase and O-acetylserine sulfhydrylase on maize plants. It is noteworthy that, amongst the four HS, those from earthworm faeces showed the highest amount of IPA and the strongest cytokinin-like activity in both plants. The presence of IPA confirmed that HS influence plant metabolism at different levels. Our results open new perspectives in the plant–HS relationship and shed new light on the active components of HS.

Humic substances (HS) have positive effects on plant physiology, but the molecular mechanisms underlying these events are only partially under-stood. HS exert auxin-like activity, but data supporting this hypothesis are under debate. To investigate the auxin-like activity of HS, we studied their biological effect on lateral root initiation in Arabidopsis thaliana. To this aim we characterised HS by means of DRIFT and 13C CP⁄MAS NMR spectroscopy, and measured their endogenous content of IAA. We then utilised a combination of genetic and molecular approaches to unravel HS auxin activity in the initiation of lateral roots. The data obtained using specific inhibitors of auxin transport or action showed that HS induce lateral root formation mostly through their ‘auxin activity’. These findings were furthersupported by the fact that HS used in this study activated the auxin synthetic reporter DR5::GUS and enhanced transcription of the early auxin responsive gene IAA19

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.