Mechanism Analysis Of Methyl Jasmonic Acid Signalling In Aluminum-Inhibited Root Growth In Arabidopsis

Aluminum has abundant content in the crust and it is known as the third largest metal element in the earth's crust.When the pH of the soil is reduced,(pH ? 5.0)aluminum dissociates into A13+,A1(OH)2+ and A1(OH)+.The dissociation of positive ion is the main limiting factor of crop growth and produces toxic to plants,affecting crop yields.Research shows that the main effect of aluminum toxicity is in the distal part of the root-tip transition zone(TZ).Aluminum toxicity inhibit the growth of root and destroy the structure of root tip.The investigation of the molecular mechanism of aluminum toxicity stress will provide the theoretical basis for the improvement of agricultural production in acid soil.Jasmonic acid compounds(JAs),which includes jasmonic acid,methyl jasmonic acid and their derivatives,was found to play a very important role in the process of plant growth and development as well as in plant responses to biotic and abiotic.In response to stress,the induced JA biosynthesis leads to increased JA levels.JA activate transcription for MYC2 through the interaction with JA receptor COIl and degradation of JAZ repressors.JA-activated transcription factor MYC2 regulates the expression of down-stream target genes thus response to environmental cues.On the other hand,JA also regulates root growth and development in response to endogenous developmental cues.The previous studies have shown that aluminum stress could induce the expression of TAA1,which encodes a key auxin biosynthesis enzyme,in the root transition zone(TZ).The up-regulated TAA1 expression causes increased auxin levels in root TZ and root growth inhibition.This process is dependent on the ethylene signaling pathway.In this study,we investigate the molecular mechanism of JA regulated root growth inhibition in response A1 stress.The following results were obtained in this study.(1)JA regulates root-growth inhibition in response to Al stress.Using different concentrations of exogenous MeJA and aluminum treatment,the root growth phenotype analysis was done in wild type seedlings.As the concentration of MeJA increases,the inhibition rate of root was enhanced.Both JA signaling mutant coil-2 and JA biosynthesis mutant aos displayed highly increased tolerance to various concentration of Al.This result indicates that JA is involved in root growth inhibition in response to Al stress.(2)The high tolerance of the coil-2 mutant might be result from the reduced accumulation of Al than wild type controls.Morin staining showed that the Al accumulation in the root tips of coil-2 mutant was greatly reduced compared to that of WT seedlings,which was further confirmed by the determination of the of Al concentration in the intact roots.Fractional Al analysis showed that the Al concentrations were lower in both cell wall and cell sap of coil-2 mutant compared to the WT plants.This suggests that exclusion of Al accumulation from roots contributes to the COI1-regulated Al resistance(3)Al stress induces the up-regulation of JA signaling in root tip To address how JA regulates Al-induced inhibition of root growth,the expression of the JA receptor COI1 in the root tips was examined in response to Al stress using the transgenic line COIlp:VENUS.Either 10?M or 25 ?M AlCl3 treatment,highly induced the expression of COI1p:VENUS in the root tips especially in the root apex TZ.This result was also confirmed in the COI1p:GUS transgenic line.The expression of COI1p:GUS in the root tips including both the meristem zone(MZ)and TZ was highly induced under 10 ?M AICl3 treatment(4)Al stress induces the up-regulation of JA signaling in root tip dependent on ethylene signalingJA signaling has been reported to regulate root growth and development through the cross-talk with ethylene.In this study,we found that the Al-induced up-regulation of pCOIl:COI1-VENUS and pCOIl:GUS in the root apex especially in root TZ was clearly enhanced by ACC application while it was strongly repressed by AVG co-treatment.The phenotype analysis of ethylene defective mutant ein3eill—1 also showed that the JA-enhanced root-growth inhibition under Al stress was observed in both WT seedlings and the ein3eil1-1 mutant.These results indicated that JA act downstream of ethylene in Al-induced root growth inhibition.(5)Al stress induced up-regulation of auxin signaling(DR5-GFP)and TAA1-GFP in root transition zone are not affected in coilAlCl3 solution treatment can induce the expression of the DR5-GFP and TAA1-GFP in the root tip,when jasmonic acid signal way is blocked,AlCl3 inducing the expression of DR5-GFP and TAA1-GFP in root tip is not affected,it is concluded that in the process of aluminum toxicity,jasmonic acid signal does not affect auxin signal.Aluminum can induce the expression COIlp:VENUS in the root tip,NAA,PEO and Kyn can not effect the expression COIlp:VENUS induced by Aluminum.Under aluminum stress,the mutant coil-2/arf7/19,the mutant coil-2 and mutant arf7/19 are more patient than wild type.In the process of Aluminum toxin inhibiting the growth of the arabidopsis root,jasmonic acid and auxin is two separate signal pathway.Under aluminum stress,the two signaling pathways of jasmonic acid and auxin in the process of root growth inhibition relies on the ethylene signaling pathway.