Characteristic Of Pectin Polysaccharide Of Cell Wall In Root Border Cells And Root Of Pea (Pisum Sativum) Under Aluminum Toxicity At Different Boron Levels

Root border cells are recently defined alive population sloughed from root tips and have especial biological activity.It plays a crucial role in rhizosphere.Reports showed that the root tip was rapidly affected by Al toxicity and B deficiency,and the cell wall pectin was the target of Al toxicity and B deficiency.It had been proved that B could alleviate Al toxicity,but the mechanism was not clear.Features of pectin polysaccharide of cell wall in root border cells and root of pea under aluminum toxicity at different boron levels were studied in order to prob the mechanism of B alleviate Al toxicity as well as roles of border cells.Mist culture was adopted in the developing of root border cells to study effects of B in alleviating of Al toxicity in the aspects of pectin content,pectin polysaccharide composition and its molecular weight,the degree ofmethylation(DM).The results were:1.Border cells viability reduced(p＜0.05) and numbers increased significantly(p＜0.01) at Al stress.It indicated that Al toxicity induced cell death,but broke the regulation of border cells production.Border cell viability increased(p＜0.05) and its number reduced after B addition, indicating B could alleviate Al toxicity by inhibiting A1 induced cell death.2.At Al stress,Al content in border cells wall and pectin 1 and 2 was higher than root segaments,indicating that border cells was the barrier of Al to root apices.Al was found mainly in pectin 2,especially for border cells.Al content of pectin decreased after B addition under Al exposure,which indicated that B inhibited combination of aluminum and cell wall pectin in root tip.3.Pectin uronic acid content in cell walls of root border was significantly higher than those of root segment 0-10mm and 10-20mm.Al increased the content of the total sugar and uronic acids,contrary to B addition only,so B might alleviate Al toxicity via decreasing pectin content.Further,KDO content increased significantly in border cells (p＜0.01) by B addition under Al exposure.KDO content of border cells was higher than root segments,indicating that border cells had higher capability for binding B.4.Pectin Polysaccharide was separated and the molecular weight was determined by gel chromatography(sepharcy^TMS-200).Polysaccharides of cell wall pectin 1 was separated to two fractions,a high and a lower molecular mass polysaccharide,and polysaccharides of cell wall pectin 2 had only one high molecular weight fraction in border cells and root segments of pea.The molecular weight of the high mass polysaccharides was 79.55KD regardless of Al and B exposure.The peak time advanced and the molecular weight of the lower increased from 3.32 KD to 4.42KD in border cells and 0-10mm root segment and from 1.86KD to 3.13KD in 10-20mm root segment after exposed to Al.Then B addition,the molecular weight in root segment did not change,but the area of peak increased,which meaned increase of pectin content.It changed significantly in border cells.The time of peak were 1.8h and 2.3h,and the molecular weight were 14.05KD and 3.32KD.The area of peak 3 was bigger than peak 2.Those results indicated interaction of Al and B increased low molecular weight polysaccharide and content of pectin 1.The time of peak delayed in B treatment,thus the molecular weight decreased.5.A1 stress can increase the degree of methylation(DM) of border cells,root segment 0-10mm and 10-20mm 26%,16%and 17%.DM was declined significantly in border cells(p＜0.05) after B addition under Al stress,so B alleviated Al toxicity by decreasing free carboxyl or packing it under Al toxicity.In conclusion,the uronic acids content and DM increased significantly at Al stress, resulting in more free carboxyls,thus more Al content was bound in cell wall pectin.The molecular mass of cell wall pectin 1 polysaccharide increased after Al exposure indicated that the formation of polysaccharide-Al-polysaccharide complex which led to more rigid cell wall inhibiting the elongation of cells as well as root.The total sugar and uronic acids content in pectin of border cell was significantly higher than root segments,and DM increased more than root segments at Al stress which would inhibit Al entering into root cell.The amount of unmethylated pectin lessen significantly after B addition at Al toxicity which reduced pectin content and DM as well as Al target in pectin,B addition also leading to higher KDO which might maintain the stability of pectin net structure, thus B alleviated Al toxicity.B could alleviate Al induced borders cell death.The KDO content in pectin of border cell was significantly higher than root segment,thereby pectin RGⅡof border cells had more stronger cross-linking with B,thus maintain stability of cell wall,At the same time, root border cells could protect root from Al toxicity by binding more Al in its cell wall pectin.