Effects Of Low-pH And Aluminum On The Nodulation Signal Transduction In Medicago Sativa L

To elucidate the mechanisms responsible for the transduction of nodulation signal in symbiotic nitrogen-fixation system, the effects of low-pH and aluminum (Al) on the root hair deformation in Medicago sativa L. (alfalfa), which is sensitive to Al, was investigated in this study. After the roots had been exposed to 3, 6 or 9 mol/L A1Cl3 solution (pH 4.5) for 24 h, primary root elongation was notably inhibited. Similarly, the rate of root hair deformation decreased significantly with decreased pH or elevated Al concentrations. After the roots had been exposed to 4, 5 and 6 mol/L A1C13 in the presence of R. meliloti for 24 h, root hair deformation was depressed by 44.59%, 71.43% and 80.09%, respectively. Although the growth of Rhizobia was restricted by low-pH and Al, the depressing effect of low-pH and Al on the deformation could also be observed after exposing the roots to the low-pH (pH 4.5) solution with sterilized Nod factor (NF, isolated from the medium with R. meliloti and the exudates of alfalfa seeds) or to which combined Al (3, 6 or 9 mol/L AlCl3) with sterilized NF for 24 h. The rate of root hairdeformation was depressed by 45.92%, 44.59%, 71.43% and 80.09%, respectively.These results suggested that low-pH and Al would inhibit the transduction of nodulation signal in symbiotic nitrogen-fixation system. Primary root elongation, root hair deformation and nodulation in alfalfa were suppressed by low-pH and Al, depressing effect of low-pH and Al was strengthened with decreased pH or elevated Al concentrations. One hand, low-pH and Al inhibited the growth of alfalfa and R. meliloti. On the other hand, nodulation signal transduction in symbiotic nitrogen-fixation system was depressed by low-pH and Al. Therefore, the nodulation and the growth of alfalfa in low-nitrogen soils were inhibited by low-pH and Al.