The Auxin-like Activity of Humic Substances is Related to Membrane Interactions in Carrot Cell Cultures

Abstract

A detailed characterization of two humic fractions was performed: One with low relative molecular mass (LMr<3,500 Da) and one with high relative molecular mass (HMr>3,500 Da). Distinct (1)H NMR spectroscopic patterns were observed for the two fractions. HMr showed an aromatic proton region, an intense and broad region (3.0-5.0 ppm) attributed to sugar-like and polyether components, and an intense doublet at 1.33 ppm (identified as protons of the beta-CH(3) in lactate). In contrast, LMr did not show resonances due to aromatic protons and was characterized by a broad unresolved region, assigned to sugar-like components. The (13)C NMR spectra showed that the LMr humic fraction was richer in carboxylic and aliphatic C groups compared to HMr fraction. These substances were fluorescein-labeled [fluorescein isothiocyanate (FITC)], and their interaction with carrot cells in culture was monitored for 10 d, and compared to FITC-indole-3-acetic acid (IAA) to clarify their mechanisms of biological activity. After different incubation times, fluorescein staining of carrot cells and decrease of fluorescein concentration in the culture medium were evaluated. Fluorescent membrane staining was only present in IAA and the LMr humic fraction treated cell cultures. A consequential decrease of fluorescein concentration in the culture media was also observed. Pretreatment of carrot cells with unconjugated IAA or LMr humic fraction markedly reduced fluorescein staining of both FITC-IAA and FITC-LMr humic fraction. Blocking tests gave indirect evidence of possible binding of the LMr humic fraction to IAA cell membrane receptors. These results indicate that the two humic fractions behave differently. Only LMr humic fraction, like IAA, interacts with cellular membranes in carrot cell cultures.

Root growth of tomato seedlings intensified by humic substances from peat bogs

“…Isso torna a planta capaz de explorar um volume maior de solo e representa um processo importante na adaptação das plantas a ambientes com baixo conteúdo de nutrientes disponível ou submetidos a estresse hídrico (Fitter, 1991). Muscolo et al (2007) postularam que a interação entre a matéria húmica e o sistema radicular é possível na rizosfera quando moléculas húmicas em solução tem tamanho suficiente para permitir seu fluxo no apoplasto e atingir as membranas celulares. Dessa forma, uma série de trabalhos (Piccolo et al, 1992;Nardi et al, 2002;Quaggiotti et al, 2004;Muscolo et al, 2007;Nardi et al, 2007) consideraram os AF como a fração de SH de maior bioatividade.…”

“…Muscolo et al (2007) postularam que a interação entre a matéria húmica e o sistema radicular é possível na rizosfera quando moléculas húmicas em solução tem tamanho suficiente para permitir seu fluxo no apoplasto e atingir as membranas celulares. Dessa forma, uma série de trabalhos (Piccolo et al, 1992;Nardi et al, 2002;Quaggiotti et al, 2004;Muscolo et al, 2007;Nardi et al, 2007) consideraram os AF como a fração de SH de maior bioatividade. Entretanto, nesse trabalho foi observado que, independente do estádio de decomposição da matéria orgânica das turfeiras, os AH foram o material húmico de maior bioatividade.…”

Biological Activities of Humic Substances

“…Another line of evidence supports the hypothesis that LMS fraction could interact with these transport proteins ( " fi ne " regulation), leading to a modulation of NO 3 − uptake. This contention is reinforced by the observation that LMS fraction can reach the apoplast and interact with the plasma membrane of roots and cultured carrot cells (Muscolo et al, 2007a ). Other evidence is reported in the paper of Canellas et al (2002) , in which a stimulation of the plasma membrane H + -ATPase activity took place, apparently associated with an ability to promote expression of this enzyme, as confi rmed by western blot analysis.…”

Mutual influence of soil basidiomycetes and white mustard plants on their enzymatic and catabolic activities

“…Root uptake is not postulated. Uptake of non-specific colloids by intact roots is restricted to molecular sizes of ≤3 kDa [67]. Five of the saprobes but neither A. bisporus nor the ectomycorrhizal fungi increased the production of extracellular laccases in response to normal (and stressrelated?)…”