Mechanisms Of Arabidopsis Thaliana Responses To Nitrogen Nutrition Stresses

Nitrogen nutrition plays a very important role in the stage of the vegetative and the reproductive growth which affect the crop yield and quality.(NH₄⁺-N)and(NO₃^--N)are the main forms of nitrogen absorption by plants.Excessive application of nitrogen fertilizer in agricultural production and soil damage to the farmland will cause the high concentration of ammonium(NH₄⁺)in the soil and produce ammonium stress on the plant to inhibit the growth of the primary root and the aerial part of the plant.Nitrogen deficiency in soil can significantly affect plant growth and development.Low nitrogen stress can properly promote plant root growth in order to obtain more nutrients that meet plant growth.Nitrate reductase(NR)affects plant growth by mediating nitrate metabolism in plants.However,the physiological mechanisms which transcription factors mediate plant responses to ammonium toxicity and nitrate reductase(NR)regulating plants to respond to low nitrogen stress is still pretty vague.In this study,Arabidopsis thaliana L.wild type(Col-0)and mutants(wrky23and nia1nia2)were used as materials on ammonium stress(10 m M NH₄⁺)and low nitrate stress(0.5 m M NO₃^-)in the growth of plant seedlings which aimed at elucidating:(1)WRKY23 mediates the response physiological mechanism of Arabidopsis thaliana L.ammonium stress;(2)NIA1 and NIA2 mediates the physiological mechanism of Arabidopsis thaliana L.response to low nitrogen stress.The main results are as follows.1.WRKY23 mediates NH₄⁺uptake in Arabidopsis thaliana L.roots and responds to plant ammonium stress.Under ammonium stress the root NH₄⁺concentration of wrky23 mutant was significantly higher than in the Col-0,while under control conditions(10 m M KNO₃),there was no significant difference in the NH₄⁺concentration of wrky23 and Col-0 roots.The relative expression of wrky23 root AMT1;2 under ammonium stress was significantly higher than that of control conditions,but the expression of GLN family genes,GS enzyme activity and free amino acid concentration were not significantly affected in ammonium stress,so the concentration of NH₄⁺in wrky23 roots increased under ammonium stress.2.WRKY23 regulates the synthesis and transport of auxin in Arabidopsis thaliana L.and responds to plant ammonium stress.The auxin influx transport vector genes AUX1,LAX1,and LAX2 in the roots of wrky23mutants were strongly induced by NH₄⁺stress,while the expression of PIN family genes in the auxin efflux transport vector of wrky23 mutant roots was significantly inhibited by ammonium stress treatment.Under NH₄⁺stress,the expressions of wrky23×Pro DR5::GUS and wrky23×Pro DR5::GFP were significantly higher than the control conditions.The concentration of auxin(IAA)in wrky23 mutant under NH₄⁺stress was significantly higher than the control conditions.3.Nitrate reductase genes(NIA1 and NIA2)mediate NO and auxin in Arabidopsis thaliana L.in response to low nitrogen stress.Under low nitrate stress the primary root length of the nia1nia2 mutant did not change significantly,but the elongation of primary root in Col-0 was significantly induced which indicate that the primary root growth adaptive mechanism of Arabidopsis to low nitrogen stress was disappear after mutating the nitrate reductase gene.Under low nitrate stress,the NO content of the roots of Col-0 and nia1nia2 mutants was significantly lower than that of the control(2.25 m M KNO3),and the NO content of the roots of nia1nia2 mutants was significantly lower than that of Col-0.Under low nitrate stress,exogenous addition of 20?M NO,the root NO content of the Col-0 and nia1nia2 mutants increased significantly,but the increase in NO content in the roots of Col-0 was significantly higher than that in the roots of the nia1nia2 mutants,excessive NO inhibited the main root growth of Col-0 under low nitrate stress and exogenous addition of 20?M NO.Under low nitrate stress,nia1nia2mutant auxin transport to the root tip was affected.The nia1nia2 mutant root auxin content was significantly lower than Col-0,which affected the main root growth of the mutant.Under low nitrate stress,exogenous addition of 20?M NO,the content of auxin in the root of Col-0 did not change significantly,and the content of auxin in the root of nia1nia2 mutant significantly increased which promoted the main root elongation of nia1nia2 mutant.The results of our study indicate that the expression of AMT1;2 in the root of Arabidopsis thaliana after WRKY23 mutation under ammonium stress was significantly higher than that in control conditions,but NH₄⁺is not affected by ammonium assimilation in roots which leads to NH₄⁺accumulation and inhibits Arabidopsis root growth.Meanwhile,after mutating WRKY23 caused excessive accumulation of auxin in the roots tips of plants and inhibited the growth of primary roots under ammonium stress.In our study,we clarified that the nitrate reductase gene mediates Arabidopsis primary root growth adaptability to low nitrogen stress from a physiological level by regulating the concentration of NO and auxin in the plant.This study provides a theoretical basis for the study of plant response to nitrogen nutrition stress and optimization of soil nitrogen nutrient management.