Effects Of Aluminum On Hydrogen Peroxide Content And Cell Wall Peroxidase Activity In Root Tips Of Wheat (Triticum Aestivum L.) Genotypes Differing In Aluminum Tolerance

Acid soils, which are soils with a pH of 5.5 or below, are distributed worldwide. It is estimated that approximatedly 30% of the world's arable land area consists of acidic soils, and as much as 50% of the world's potentially arable lands are acidic. In China, 21% of the total land area (about 2.03 million km²) is acidic, extending over 15 provinces in southern China. Aluminum (Al) toxicity is the primary factor limiting crop production on acidic soils, which severely influences crop productivity. The primary and earliest symptom of Al toxicity is a rapid inhibition of root growth, and then the inhibition of the uptake of water and nutrients. Root apex is the major zone which perceives and responds to Al toxicity. Although a great many studies have been carried out to elucidate the mechanisms of Al toxicity and tolerance, the mechanisms of Al-induced inhibition of root growth 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 24 h or more to occur, thus the inhibition of root growth is probably due to the interference with cell expansion and elongation. Cell wall peroxidase (POD) is related to cell expansion and elongation, and reactive oxygen species (ROS) and antioxidative enzymes are also related to root growth under Al stress. Over the last years, evidences have accumulated supporting the hypothesis that root cell wall POD, anxioxidative enzymes and Al-induced ROS play an important role in the expression of Al toxicity and Al resistance in plants. However, the relationships between root apex cell wall POD activity, anxioxidative enzyme activity, Al-induced ROS and the Al toxicity expression and tolerance between plant genotypes differing in Al tolerance are still not clear. The objective of the present study is to investigate the relationships between the response of wheat (Triticum aestivum L.) roots to Al toxicity and cell wall POD activity, anxioxidative enzyme activity and Al-induced ROS in the root apex cell wall of Al-resistant(Jian-864) and Al-sensitive (Yangmai-5) wheat (Triticum aestivum L.) cultivars when exposed to different conditions of Al stress. The main results of the study are as follows:(1) The dose-dependent experiment of 24 h exposure to 0, 10, 20, 30, 40, 50 μmol/L AlCl₂ indicated that root elongation of Jian-864 and Yangmai-5 was inhibited with the increasing Al concentrations. There was no significant difference in relative root elongation (RRE) between the two genotypes at the concentration of 10 μmol/L AlCb. However, the RRE decreased remarkably and more inhibition was found in Yangmai-5 with the increasing Al concentrations. Meanwhile, the time-dependent experiment of 0, 3, 6, 12, 24h exposure to 30 umol/L showed that there was no significant difference in root elongation of Jian-864 after 3h and 6h exposure to Al, whereas the root elongation of Yangmai-5 was distinctly inhibited after 6 h exposure to Al. The RRE decreased markedly with the time extending and more inhibition was found in Yanmgai-5. The greatest difference was observed at the concentration of 30 umol/L AlCb, with the RRE 71% of Jian-864 and only 37% of Yangmai-5. The results indicated that Yangmai-5 was more sensitive to Al than Jiang-864.(2) The dose-dependent experiment indicated that the Al content in 0-10 mm root apex of Jian-864 and Yangmai-5 increased with the increasing Al concentration, and the Al content in Al-sensitive genotype Yangmai-5 was significantly higher than that in Al-tolerant genotype Jian-864. There was a little difference between two genotypes at the concentration of 10 umol/L AlCb with the Al content in Yangmai-5 a bit higher than that in Jian-864. However, the Al content in Yangmai-5 was more than two times as much as that in Jian-864 at the concentration of 30 umol/L AlCb, with the content of 2.83 nmol/root apex and 1.16 nmol/root apex, respectively. In the time-dependent experiment, the Al content in both genotypes increased with the time extending and there was a little difference between two genotype after 3 h exposure to 30 umol/L AlCb with the Al content in Yangmai-5 a bit higher than that in Jian-864. Nevertheless, the Al content in Yangmai-5 was more than two times as much as that in Jian-864 after 24 h exposure to 30 umol/L AlCb, with the content of 3.01 nmol/root apex and 1.02 nmol/root apex, respectively. More accumulation in the root tip of Al-sensitive genotype than that of Al-resistant genotype is related to their Al toxicity expression and resistance.(3) At the concentrations of 10, 20, 30, 40, 50 umol/L AlCb, the soluble guaiacol peroxidase (GPX) and coniferyl alcohol peroxidase (CAPX) activity in root tips of both lines had no significant changes, while cell wall-ionically bound GPX and CAPX activity were greatly enhanced and more conspicuous changes were found in 0-10 mm root tips Yangmai-5. The changes in H2O2 content were similar to those of GPX and CAPX activity in root tips of Al-stressed seedlings. In conclusion, it appears that the increase in cell wall-ionically GPX, CAPX and H2O2 content may lead to the enhancement of lignification and sbuerisation and H2O2 dependent peroxidase-catalyzed formation of cross-link among cell wall polymers, and thus increase the rigidity of cell wall, thereby reducing cell expansive ability and inhibiting root elongation.(4) With the increasing time exposed to 30 u,mol/L A1C13, there was no significant difference in the soluble GPX and CAPX activity in 0-10 mm root tips of both genotypes,but both the enzyme activity in Yan'gmai-5 was higher than that in Jian-864. In contrast, cell wall-ionically bound GPX and CAPX increased markedly with the time extending. There was no significant difference between Jian-864 and Yangmai-5 in the two enzyme activity after 3 h exposed to Al. However, cell wall-ionically bound GPX and CAPX activity in Yangmai-5 were significantly higher than that in Jian-864. Besides, there was no remarkable difference in H2O2 content and cell death in root tips of both genotypes without Al whereas H2O2 content and cell death increased significantly when exposed to 30 umol/L AICI3, and the difference became more obvious after 6 h, with H2O2 content and cell death more higher in Yangmai-5. In the experiment, Al-induced the enhancement of cell wall-bound GPX and CAPX activity and H2O2 content may lead to lignin deposition, and then the inhibition of cell elongation that results in root inhibition. The cell wall-bound peroxidase activity and H2O2 content in Al-sensitive genotype were more higher, so the lignification and cell death were more severe. As a result, the root inhibition in Al-sensitive genotype was more serious.(5) The superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activity in 0-10 mm root tips of both lines increased with prolonged time exposed to 30 umol/L AlCb. There was no significant difference in the three enzyme activity of both genotypes after 3h exposure to Al. In contrast, SOD activity in Yanmgai-5 was distinctly higher than that in Jian-864 while CAT activity and APX activity in Jian-864 were markedly higher than those in Yangmai-5 after 6 h exposure to Al. Malondialdehyde (MDA) content in 0-10 mm both genotypes increased with time extending. There was no remarkable difference in MDA content of both genotypes after 3 h exposure to Al whereas MDA content in root tips of Yangmai-5 was significantly higher than that of Jian-864 after 6 h. H2O2 content was also increased with the increasing time of Al treatment, with more accumulation in Yangmai-5. In conclusion, the SOD avtivity in root tips of Al-sensitive genotype(Yangmai-5) was much more higher and CAT and APX activity were much more lower. Therefore, H2O2 was more accumulated and then leaded to oxidative stress and severe lipid peroxidation, thereby significantly inhibiting root elongation.