The role of natural humic substances on plant cell phosphate metabolism was investigated in tobacco
BY-2 suspension cell cultures grown in the presence of different humic samples (HS). These HS were
characterized by CPMAS-NMR spectroscopy and on-line pyrolysis-gas chromatography-mass
spectrometry. After incubation of suspension cell cultures for 7 days, either in the presence or absence
of HS, cells were facing Pi starvation. The most hydrophilic and smallest in molecular size among
fractions of HS (fraction III) induced a partial relief from Pi starvation, increasing total cell phosphate
amount, ATP and glucose-6-phosphate levels, as well as the activity of secreted acid phosphatases.
Fraction III induced also a decrease of KCN-insensitive respiration, evaluated in both suspension cells
and isolated mitochondria. The molecular characteristics of fraction III are conducive to a flexible
conformational structure that may induce an efficient release of Pi from organic sources (e.g. nucleic
acids), released in the media by damaged or dead cells, exerting a sort of positive effect on either the
production or activity of extracellular Pi hydrolytic enzymes.
