Mechanism Of Boron And RALF On Root Elongation Of Arabidopsis

Both boron deficiency and rapid alkalization factor(RALF)have remarkable inhibitory effects on root elongation,however whether they are similar in the mechanisms of elongation regulation,or there are interactions between boron deficiency and RALF are unclear.The effects and mechanisms of boron deficiency and RALF in restraining root elongation were stduied in Arabidopsis thaliana(wild type Col-0).The main results from apoplastic pH,rhizophere p H,surface pH,intracellular pH and H2O2 production in different root zones under boron deficiency and RALF were shown as follow.1.Both boron(B)deficiency and RALF have remarkable inhibitory effects on the elongation of main root and root cells in Arabidopsis thaliana,and the effects aggravate with time.The original culture is 1/2 MS with 12.5 ?mol/L B,and after transferring four days seedlings to B deficiency treatment 12 h,root cells LEH decreased significantly,and LEH of B deficiency was only 71% to that of B supply;However,root length presented the evident difference only through direct seeding under B deficiency and supply,which indicated that LEH is the better indicator to study the effect of B deficiency.The effects on root elongation inhibition was remarkable after 24 h under RALF,and root length of RALF deficiency was only 75% to that of RALF after 96 h.B deficiency and RALF first inhibited the elongation of root cells,and then the root elongation.2.There is a spatio-temporal difference for the regulation of intracellular and extracellular pH of different root zones under B deficiency and RALF.The highest intracellular and extracellular pH appeared on the transition zone and the lowest on the maturation zone,which correlated with the growth status and functions of cells in root zones.Apoplastic,rhizosphere and root surface all present the acidification under B deficiency,while B supply causes alkalization with an increase apoplastic pH,escepially the transition zone alkalizes at most.A persistent effect caused by RALF for alkalizing the apoplastic pH and rhizophere pH,and the transition zone and the elongation zone have the most prominent difference,which implied the mechanism of th regulation of root cells elongation was related to the alkalization of the transition zone and the elongation zone under B deficiency and RALF.3.Both B deficiency and RALF increased H2O2 production of the root apex notably.H2O2 fluorescence intensity showed a spatio-temporal difference of different root zones,where the meristematic zone displays weaker and the other three zones stronger,which showed that the generation of H2O2 was related to cell elongation or the preparation of elongation,and irrelevant to cell division.B supply could maintain a certain amount of H2O2 production in the transition zone to function as the signal to promote the elongation,while B deficiency stimulates the escessive production of H2O2 to restrain the root and cells elongation.RALF could increase the content of root apex H2O2,especially the transition zone,therefore restrain the elongation of root and cells obviously,which reflects the dual roles of H2O2.4.Compared with RALF deficiency,RALF inhibits the elongation of primary root and root cells LEH strikingly that appeared on the second day under B supply,and the root cells depression effect was more obvious than the root length,that were 89% and 93% respectively.However RALF inhibits the elongation of primary root and root cells LEH remarkably appeared on the third day under B deficiency,which manifested that RALF suppresses root elongation significantly and rapidly only under the normal B supply.RALF enhances root surface pH and the H2O2 production of the transition zone and the elongation zone,therefore inhibits the root elongation.To sum up,both B deficiency and RALF inhibit the elongation of the primary root and root cells significantly,and the inhibitory effect mainly comes from root cells elongation inhibition through the overproduction of H2O2 to induce the toxicity of the elongation growth zones(the other three zones apart from the merismetastic zone),the acidification caused by B deficiency in the transition zone,and the alkalization in the elongation zone induced by RALF.Therefore,RALF has a slow inhibitory effect on the root and root cells elongation under B deficiency.RALF enhances root surface pH and the H2O2 production of the transition zone and the elongation zones,therefore inhibits the root and root cells elongation only under the normal B supply.The inhibition effect of RALF is connected with B,and the specific mechanism needs further research.