Plasma membrane carriers, modulation

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. 

IP3 acts as the second messenger for Ca2+ mobilisation from the endoplasmic reticulum while DAG stimulates the Ca2+ sensitive protein kinase C (which modulates EGF receptor) and activates the plasma membrane Na+/H+ exchange carrier. In this function DAG is mimicked by phorbol esters which have a tumour promoting function. The effect of stimulating Na+/H+ exchange is to increase the intracellular pH and K+ concentration, conditions which are necessary to induce or maintain the proliferative response in quiescent cells (Rozengurt, 1986 Wakelam, 1989). 

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