| Aluminum(Al) toxicity is an important factor for crop production and quality in acid soil;it has become hot spots in plant stress physiology research.Al primarily affects root growth by interfering with processes critical for the regulation of growth in the root apex. The exclusion mechanism that excludes Al from entering root apex is considered one of the most important mechanisms of Al resistance.Mucilage layer is high molecular complex secreted from root cap and root border cells(BC),which is considered to play a major role to suffer the stress of Al.In this paper,two significantly different Al genotypes of soybean (Glycine max L.) were selected as material,such as Zhechun No.3(Al sensitive genotype) and Zheqiu No.2(Al tolerant genotype) which were respectively Al-tolerant genotype and Al-sensitivity genotype.Mucilage was collected from root tips of soybean which was grown in aeroponic culture for 24 h with Al supply.Monosaccharide content of polysaccharide,the mucilage secretion quality and its primary components were examined to evaluat its capacity in binding Al.Besides,the main effective compound and characteristic of root physiological were also analysed,in order to reveal the function and mechanism of mucilage on Al resistance.Explore some new pathways for Al toxicity, Al-resistant mechanism and genetic improvement.The main results were as follows:(1)A novel gas chromatography method was developed to determine six kinds of monosaccharide units hydrolyzed by trifluoroacetic acid from polysaccharide in pea root cap mucilage.The baseline separation of the monosaccharide units were achieved with a medium polar capillary column OV-17(35 m×0.32 mm×0.33μm).The total analysis time was 42 min.Retention times were used to identify the individual sugar in the chromatogram and external standard method was used for quantitative analysis.The linear range of the method was 0.0025～2.5 mg/mL and the correlation coefficient were over 0.9991.Minimum detectable limit at a signal-to-noise of 2:1 was 0.0216～0.1578μg/mL. The recovery ration of the six kinds of standard mono sugars was 96.7%～107.4%.The results indicated that this method was effective and quick in analyzing the components of sugars in the mucilage collected from plant root tips under adversity stress.(2) Mucilage was collected from soybean through areoponic culture to study its. characteristics.Results shown as follows,Al obviously enhanced mucilage secretion, especially in Zheqiu No.2.Abundant of sugar,protein,amino acid and organic acid composed main components of mucilage.All these were much higher in Zheqiu No.2 than in Zhechun No.3.The protein,amino acid and organic acid contents of Zhechun No.3 and Zheqiu No.2 were 4.7 and 3.9,8.0 and 5.2,7.0 and 3.9 times of control,respectively. Mucilage presented weak alkali and didn't show much varity under Al stress.These results revealed that sugar shown significant change under Al toxicity,which might be the main Al-response compound in mucilage.(3) Sugar composed almost 90%of soluble components in mucilage.Total sugar content in Zhechun No.3 and Zheqiu No.2 increased for 215.2%and 88.1%more than control under 400μmol/L Al treatments,respectively.The polysaccharides of sugar were mainly composed of six monosaccharides,which were the same as some other kinds of species.The monosaccharides variation of polysaccharide in mucilage might be characteristic reaction under Al stress.Xylose and glucose were the primary component and significantly response to Al toxicity which could use to be symbol monosaccharide to calculate Al tress grade.(4) Relative root elongation ratio,PME activities and Al content in root,mucilage and root cell wall of soybean were detected,which were cultred by earoponic culture.Relative root elongation ratio of Zhechun No.3 and Zheqiu No.2 increased for 15.8%and 21.6%on average when mucilage was kept on root.Mucilage also debased the increasing of PME activity which was much higher in Al-sensitive genotype.Most of Al was blocked outside the root by mucilage,49.5%of total Al was held back by mucilage of Zheqiu No.2,while its only 14.2%in Zhechun No.3.Monomeric Al was most toxicity form which shown only 0.06%～1.16%of total Al in mucilage,despite the large content of total Al in muciage. Morin fluorescence stain and cell wall Al content both shown that Al content siginificantly increased when mucilage was wipped off.Al binding capacity of mucilage could prevent Al from entering root tip and meristem of root cap,and then alleviated Al toxicity to root growth.(5) Root cell wall compositions and ATPase were also detected.Erase root mucilage significantly stimulate pectin and hemicelluloses production,especially in Zheqiu No.2. Pectin,hemicelluloses 1 and hemicelluloses 2 content reached 239.4%,194.8%and 759.3%that of control in Zheqiu No.2.If mucilage was wipped off,H+-ATPase viability in Zhechun No.3 and Zheqiu No.2 reduced for 15.9%and 31.1%,respectively,Ca2+-ATPase activity decreased 34.8%and 54.3%.Results indicated that Mucilage obviously alleviated the polysaccharide increasing of cell wall caused by Al stress,which decreased combination structure in cell wall,reduced Al content in cell wall.Meanwhile,mucilage stabilized ATPase activity which related to root antioxidant system.(6) Al bingding properties such as pectin and hydroxybenzene were also detected. Pectin content in Zhechun No.3 and Zheqiu No.2 were respectively 293.5%and 195.7% higher than control under 400μmol/L treatments,while hydroxybenzene increased 215.6% and 214.0%.Acid phosphatase activity(APA) also increased for 8.9 and 10.3 times more than control in Zhechun No.3 and Zheqiu No.2(200μmol/L).APA decreased when Al concentration kept rising.The results above indicated that the increasing of pectin and hydroxybenzene enhanced Al binding ability.At the same time,APA increased root Pi metabolize.The production phosphate can also bind Al.The results indicated that pectin and hydroxybenzene may be the main component which enabled mucilage binding ability, and AP also played important role in Al binding.
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