Molecular Mechanism Of Salicylic Acid Regulated Lateral Root Growth And Development In Arabidopsis

Salicylic acid(SA)plays an important role during the entire lifespan of the plant.It has been demonstrated that plants can quickly synthesize SA through ICS/PAL pathway when attacked by pathogens.With the increased SA level,cytoplasmic oligomeric NPR1 proteins becole monomers.NPR1 monomers enter into nucleus through nuclear melbranes and interact with transcription factors such as thioglycolic acid(TGA).and thus activate the expression of SA responsive genes to resist pathogens.Likewise,when plants are subject to abiotic stress,SA can respond to these stresses by inducing relevant gene expression to protect plants.Recent researches suggest that in addition to participating in plant's disease resistense responses,SA is also involved in diverse processes during plant growth and development,such as seed germination,photosynthesis,flower formation,senescence,and so on.Our study found that SA can regulate lateral root growth and development of Arabidopsis,haliana.Lateral root emergence(LRE)density and total lateral root(LR)density both decreased after exogenous SA treatment.Moreover,the analysis of two key SA responsive genes Pathogenesis related gene 1(PR1)and Pathogenesis related gene 2(PR2)indicated that SA represses LR development mainly through PR1 and PR2.For plants,root is one of the important nutritive organs and plays a curial role in absorbing Water and inorganic salt.anchoring plant.transferring nutrition.The primary root forms during embryogenesis.and the lateral rods orig,inate trom pericyele founder cells that located opposite to the xylem poles.Auxin is crucial lor the lateral root formation and paieicipates almost in every stag of lateral root growth and development.Previous studies found that auxin response factor ARF7 and ARF19 positively regulate lateral root formation.ARF7 and ARF19 can lediate LATERAL ORGANBOUNDARIES.DOMAIN 16/ASYMMETRIC LEAVES2-LIKE18(LBD16/ASL18)expression by interacting with INDOLE-3-ACETIC ACID INDUCIBLE 14(IAA14)to regulate lateral root growth and development.In our study,we treated arf7 mutant with exogenous SA and found arf7 mutant is more sensitive to SA than wild type(WT),suggesting that SA and auxin may have interaction during lateral root development.Furthermore.we found that the expression of SA responsive genes PR1 and PR2 in arf7 mutant is higher than Col,and decreased in transgenic plants 35S::ARF7.illustrating that lateral root growth and development regulated by ARF7 partly come true through inhibiting the expression of PR1 and PR2.However.the protoplast transient assays and yeast one hybrid experiments deonstrated that the transcription factor ARF7 doesn't interact with PR1 and PR2 directly,indicating that ARF7 regulating PR1 and PR2 expression by indirectly process.In addition,ethylene is also an important plant hormone that regulates plant developlent processes.including seed gerlination,primary and lateral root development,fruit ripening etc.Sole studies found that different ethylene level have different inpacts on lateral root development.1-aminocyclopropane-1-carboxylic acid(ACC)is the ethylene precursor,high ACC concentration inhibits lateral root developnent.but low ACC concentration promotes lateral root development.Our experiments confirmed the above results.Meanwhile,I-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE(ACS)mutants and ETHYLENE-INSENSITIVE3(EIN3).ETHYLENE-INSENSITIVE3-LIKE 1(EILI)double mutant ein3/eill are insensitive to SA treatment wlen compared with WT.These results Suggest that SA may influence ethylene biosynthetic pathway.QRT?PCR experiments showed that SA repressed ACS5 and ACS9 expression.However.the expression of ACS5 and ACS9 in PR1 and PR2 overexpression transgenic lines didn't change obviously.illustrating that SA repression on ACS5 and ACS9 doesn't rely on SA response factor PR1 and PR2.In conclusion,our research preliminary indicated the mechanism of SA influencing lateral root development.SA repress lateral root development through PR1 and PR2 and auxin response factor ARF7 can relieve this repression by inhibiting PR1 and PR2 expression;on the other hand,SA can also indirectly influence lateral root development by repressing ethylene biosynthesis.Our study illustrates that SA regulates laterral root growth and development may through intercting with auxin and ethylene signaling.