The Role Of Cell Wall In Aluminum Toxicity And Tolerance Of Wheat And Their Cellular Mechanisms

Aluminum (Al) toxicity is an important factor determining the distribution of plant species and ecotypes in natural habitats and crop productivity on acid mineral soils. In China, 21% of the total cultivated land is acidic, extending over 15 provinces in southern China. Aluminum primarily affects root growth by interfering with processes critical for the regulation of growth in the root apex. Despite a large research effort for the last 30 years, the mechanism of Al-induced inhibition of root growth and the reasons for the spatial differences in Al sensitivity between apical root zones are still not well understood. A number of studies have shown that inhibition of root growth occurs rapidly (minutes to hours) after exposure to Al, while inhibition of cell division requires 6-24 h to occur, so the inhibition of root growth is due to interference with cell elongation. Cell elongation is related to cell wall components, cell wall enzymes, and cell wall extension, and so on. Over the last years, evidence has accumulated supporting the hypothesis that the root apoplast (cell wall) plays an important role in the expression of Al toxicity and Al resistance. However, how cell wall-binding sites and cell properties such as polysaccharides, enzymes play a role in Al expression and resistance is not clear. The object of the present study is to investigate the relationships between the response of wheat (Triticum aestivum L.) roots to Al toxicity and either cell wall composition in different root segments or Al adsorption/desorption by root apex cell walls, the effects of Al on the amount of cell wall polysaccharides and PME activity in the root apex cell wall of Al-resistant (Xiaimai-1) and Al-sensitive (Fu-84) cultivars of wheat (Triticum aestivum L.). The main results of this study are as follows:(1). Al inhibits elongation of wheat seeding roots in time-dependent and dose-dependent manner. Exposure to 30 μmol/L A1C1₃ solution for 30 h, the root growth of Xiaimai-1 was 63.8% of control, compared to Fu-84 was merely 30.2 %. Exposure to 10 umol/L A1C1₃ hardly inhibited the root elongation in Xiaimai-1, while significantly inhibiting the root elongation in Fu-84. When exposure to 30 umol/L A1C1₃ for 24 h, the relative root elongation was 66.7% in Xiaimai-1, while only 29.1% in Fu-84. The results indicated that Fu-84 was more sensitive to Al than Xiaimai-1.(2). The 0-10 mm root segment accumulated more Al than 10-20 mm root segment in both genotypes. But in 0-10 mm root segment, the Al content in Fu-84 was 1.3 times of that in Xiaimai-1, which suggests that the root tip is the most Al-sensitive part of the root. The root tipof Al-sensitive genotype accumulated more Al than Al-resistant genotype is related to their Al toxicity expression and resistance.(3). Cell walls (root apex) were extracted for the kinetics of Al adsorption and desorption studies, the results shown that the amount of Al adsorbed by the cell walls of 0-10 mm root segments was significantly higher than that of 10-20 mm, while in 0-10 mm root segments the amount of Al adsorbed by the cell walls of Al-sensitive genotype (Fu-84) was significantly higher than that of Al-resistant (Xiaimai-1) genotype ( 20.9 %) when 20 μmol/L Al treatment. It indicated that the capacity of cell wall adsorbed Al determines the Al accumulation in root tip that related to their Al phytotoxicity expression and phytotolerance.(4). The results of the uronic acids content in cell wall shown that pectin uronic acid content in cell walls of 0-10 mm root segments was significantly higher than those of 10-20 mm, while in 0-10 mm, pectin uronic acid content in Fu-84 was significantly higher than that in Xiaimai-1. After decreasing the DM (degree of pectin esterification) of pectin by NH₃·H₂O treatment, the total amount of Al absorbed was decreased in both genotypes, indicating that pectin content, DM of pectin plays an important role in Al phytotoxicity and phytotolerance.(5). Chemical analysis of cell wall polysaccharides from 0-10 mm root segments revealed that Al increased pectin, hemi...