Roles Of Salicylic Acid In The Polar Distribution Of Auxin In Arabidopsis

Auxin is a kind of weak acids,and functions as a phytohomone in controlling variable aspects of plant growth,development,and physiology.Indole-3-acetic acid(IAA)is a main form of functional auxin in plants.Auxin has the distinct feature of polar distribution from other phytohormones.The polar distribution are the results of polar auxin transport(PAT),the activity conducting the transportation of auxin at morphological upper to the lower end of various tissues.The PAT are coordinatedly fullfilled by auxin transporters,which includes influx carriers and efflux carrier.The polar distribution of auxin not only regulates the growth and development,such as the development of hypocotyl apical hook and root hair,but also physiological functions,such as the phototropism of the hypocotyl and gravitropism of the root.Dicotyledon plants form the apical hook to protect the cotyledons and hypocotyls from damage during the seedlings break through the soil.Soon after the plants spear out,the hypocotyl directs toward the light to get more light for higher photosynthesis.The gravitropism confers the roots to grow toward and firmly stand in the soil.The hair roots are the key tissues for roots to absorb nutrients from the soil.These aspects are irreplaceable for plant to conduct its normal growth and development.However,wether the changes of these aspects happen when plants are exposed to environmental stress,especially in the pathogenic bacteria invasion,1s not clear.Salicylic acid(SA)is a phenolic compound with small molecular weight and widely exists with the free and bound forms in plants.SA,as a plant hormone,plays roles in plant growth,development,ripening and senescence,as well as in physiological process induced by abiotic stresses,such as salt and drought,and by pathogen infection.Specailly,SA is an endogenous signaling molecule in plant allergic responses(hypersensitive response,HR)and SAR acquired resistance(systemic acquired resistance).Therefore,SA enhances the tolearance of plants to pathogen invasion.But so far,there has been no report on the effect of salicylic acid on the development of the apical hook,the response of hypocotyl to unilateral light,the gravitropism of root and the normal formation of root hair.In this study,we used Arabidopsis thaliana as the experimental material to test the relationship between SA and auxin with the combined methods including plant physiolog,plant molecular genetics,fluorescent protein molecular positioning methods.We demonstrate the effects of SA in the development of the apical hook,the response of hypocotyl to unilateral light,the gravitropism of root and the normal formation of root hair.The main results are shown as follows:1.NAA can alleviate the inhibition of exogenous salicylic acid on plants.Compared with the normal culture condition,Arabidopsis thaliana becomes short with the treatment of the exogenous SA.NAA,a kind of auxin analogue,can alleviate the inhibition to the development of plants resulting from the SA,and this alleviating performance has a positive correlation with NAA concentration.These results show that SA has the relationship with auxin in the inhibition to the growth of plants.2.The high concentration of salicylic acid can inhibit the formation of Arabidopsis apical hook and the cotyledon of phototropic response through changing the polar distribution of auxin.Arabidopsis etiolated seedling can't form the apical hook and there were some defects in the response of Arabidopsis hypocotyls to light.The treatment with SA leads to transgenetic seedling with the auxin markers(DR5::GFP and PIN3::GFP)having lower fluorescence distribution in the inner of apical hook.Also,the result from SA has the similar result with NPA,a kind of auxin transport inhibitor,in the DR5::GFP transgenetic seedling when treated with a single light.These results show that SA maybe control the formation of apical hook and the cotyledon of phototropic response through regulating PIN3 and then changing the polar distribution of auxin.3.The high concentration of salicylic acid can inhibit the root gravitropic response and the normal formation of root hair.SA can reduce the numbers and the length of Arabidopsis root hair,and root gravitropic response has disappeared.The fluorescence intensity in the root of the DR5::GFP transgenetic seedling has weakened,which indicates that SA can reduce the accumulation of auxin in root.Through the treatment with SA to a series of transgenetic seedlings with different auxin transporters(PIN1::GFP,PIN2::GFP,PIN3::GFP,AUX1::GFP),we found that the content of the transporters are decreased,and there are some endocytosis bodies in root cell,indicating that SA changes the polar distribution of auxin in root and then results in the abnormal information of root hair and the gravitropic response dispeared.·through lowering the distribution of auxin transporters in membrane.In conclusion,this thesis reveals that SA can change the location in membrane of auxin transporters,and effect the polar distribution of auxin in plants tissues and organs in order to control the formation of apieal hook and phototropism in hypocotyls,gravitropic response of root and the formation of root hair.This finding helps to explore the relationship with SA and auxin in the response of disease resistance response.