Aluminum Induced Inhibition Of Root Elongation Of Cassia Tora L. Is Related To Lignin Synthesis

Aluminum induced inhibition of root elongation of C.tora was related to increased lignin content.Cell-wall-bound peroxidase is invovled in lignification by catalyzing oxidative polymerization of monolignols.We focused on Cell-wall-bound activities of coniferyl alcohol peroxidase(CAPX) and syringaldazine oxidase(SyrOx).Activities of cell-wall-bound CAPX and SyrOx were stimulated under Al stress.Plant cells needed H₂O₂ as a substrate in the cross-linking of monolignols.Majority of H₂O₂ in cell wall was produced by an NADH oxidase-like enzyme.With the aluminum concentration applied,the activities of cell-wall-bound NADH oxidase increased.Correspondingly,H₂O₂ production was shown to increase.These results suggest that accumulation of H₂O₂ was constantly regulated by cell-wall-bound NADH oxidase.While MeJA inhibited root elongation in the presence of aluminum in a manner of dose response,pretreatment with 0.4 mM NO donor,sodium nitroprusside(SNP) before Al exposure alleviated the Al-induced inhibition of root growth.Treatment with MeJA in the presence of Al enhanced activities of cell-wall-bound CAPX as compared to the treatment with Al alone.Pretreatment with NO donor sodium nitroprusside(SNP) with subsequent Al exposure reduced the activities of cell-wall-bound CAPX.However,these were not the case for cell-wall-bound SyrOx.The similar effect on cell-wall-bound NADH oxidase and H₂O₂ content in C.tora root tips was observed.The above results could allow us to make conclusion that MeJA and NO might differently regulate cross-linking of monolignals in the root tips of C.tora under Al stress.To understand the molecular regulation of Al-induced lignin synthesis,partial conserved sequences encoding PAL,4CL and F5H in the lignin biosynthetic pathway in the roots of C.tora.were cloned.We therefore used a candidate gene approach with semi-quantitative reverse transcription(RT)-PCR to investigate the expression of transcripts of the three genes in the roots of C.tora under Al stress.It was found that Al induced expression of the three genes.Treatment with MeJA in the presence of Al caused enhanced levels of PAL and F5H transcripts.Treatment with MeJA in the presence of Al could also elevate the level of 4CL transcripts as compared to MeJA treatment alone.Aluminum induced inhibition of root elongation of C.tora might be described as following process:Al induced the levels of PAL,F5H and 4CL transcripts which were invovled in lignin biosynthetic pathway,inhanced the activities of cell-wall-bound CAPX and SyrOx,increased the activities of cell-wall-bound NADHoxidase and H₂O₂ content. Increased lignin content decreased the cell wall plasticity and,therefore,reduced root elongation of C.tora.