Rhizosphere, humic substances

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Other peculiar conditions of the rhizosphere, such as ion concentration, can influence the molecular complexity of humic substances, due to the possibility of the formation of interchain bonds producing insoluble macromolecules (Sequi et al., 1975). This latter aspect underlines the need of a dynamic evaluation of the processes occurring in the rhizosphere in relation to the activity and the physiological status of the plant. 

On the other hand, at a pH value of 5.0 there were no large differences in the strength of bonds between humic acids and metals such as Ca, Mg, Mn, Co, Ni, and Zn, whereas bonds with Pb, Cu, and Fe were stronger than with other metals (Schnitzer and Kahn, 1972) this behavior indicates that at different pH values, metal humic substance complexes of different stability are formed in the soil. This aspect is of particular relevance in an environment such as the rhizosphere, where dynamic pH gradients are present mainly due to the availability of nutrients and to their selective uptake by roots. With regard to plant availability, great importance lies in the molecular dimension and solubility of humic substances (Briimmer and Herms,... 

Stimulation of active H+ extrusion from roots (Cesco, 1995 Pinton et al., 1997 Table 9.1) and transmembrane potential hyperpolarization (Slesak and Jurek, 1988) indicated the involvement of the PM H+-ATPase in the increased nutrient uptake generally observed in the presence of humic substances. Direct proof of an interaction between humic molecules and the PM H+-ATPase has been obtained by Vara-nini et al. (1993), who demonstrated that low-molecular-weight (<5kDa) humic molecules at concentrations compatible with those present in the rhizosphere can stimulate the phospho-hydrolytic activity of this enzyme in isolated PM vesicles (Table 9.1). Further proof of the action of humic molecules on PM FT-ATPase activity and on nutrient uptake mechanisms was obtained when studying the effect of these molecules on NO3 uptake. Transport of this nutrient is a substrate-inducible process and involves FT co-transport. At higher uptake rates, the levels and activity of root PM FT-ATPase increased (Santi et al., 1995). The short-term (4h) contact... 

Grasso, M., D Orazio, V., and Senesi, N. (2004). Spectroscopic characterization of soil organic matter fractions from rhizosphere and bulk soils cultivated for tomato and artichoke. In Proceedings, 12th Meeting of the International Humic Substances Society, Sao Pedro, Brazil, pp. 368-371. 

Pinton, R., Cesco, S., Schmidt W., and Varanini, Z. (2006). Role of humic substances as rhizospheric signals affecting root growth and mechanisms of nutrient acquisition. In Proceedings, 13th Meeting of the International Humic Substances Society, Karlsruhe, vol. 45-1, pp. 45 18. 

Varanini, Z., and Pinton, R. (2001). Direct versus indirect effects of soil humic substances on plant growth and nutrition In The Rhizosphere Biochemistry and Organic Substances at the Soil-Plant Interface, Pinton, R., Varanini, Z., and Nannipieri, eds., Marcel Dekker, New York, pp. 141-157. 

Varanini, Z., and Pinton, R. (2006). Plant-soil relationship Role of humic substances in Iron Nutrition. In Iron Nutrition in Plants and Rhizospheric Microorganisms, Barton, L. L., and Abadfa, J., eds., Springer-Verlag, Heidelberg, pp. 153-168. 

---