Regulation Mechanism Of Exogenous Tryptophan On Root Elongation Of Sorghum Seedlings Under Low Nitrogen Stress

Sorghum is an important grain crop in north China,but sorghum is generally planted in barren marginal land,which will seriously affect the absorption of nitrogen by sorghum,and then affect the growth and yield formation of sorghum.When crop is subjected to low nitrogen stress,its root system will extend to absorb more nitrogen to maintain growth.Therefore,exploring the response mechanism of root system to low nitrogen is a key link in the study of crop nitrogen absorption.However,less attention has been paid to the physiological mechanism of sorghum root elongation under low nitrogen.In this experiment,100 sorghum inbred lines were screened for low nitrogen tolerance and obtained low nitrogen tolerance inbred line 398B and low nitrogen sensitive inbred line CS-3541.398B and CS-3541 were cultured under low nitrogen(0.05 mmol/L)and normal nitrogen(5 mmol/L)conditions to study the effects of low nitrogen stress on sorghum root elongation,and identify the key pathways and key genes of root elongation in response to low nitrogen stress.The effects of exogenous tryptophan on root elongation of sorghum seedlings under low nitrogen stress were studied by foliar spraying with 50 mg/L tryptophan,and the physiological and molecular regulation mechanism of exogenous tryptophan on root elongation of sorghum seedlings under low nitrogen stress was revealed.The main results of the study were as follows:1.Under low nitrogen stress,root elongation and root tip cell length of sorghum seedlings increased.Under low nitrogen stress,exogenous tryptophan could promote root elongation of 398B and CS3541,which had a greater promotion effect on root elongation of398B.2.Under low nitrogen stress,auxin synthesis related genes and auxin transduction related genes in dependent tryptophan pathway were highly expressed in sorghum seedling roots,indicating that root elongation of sorghum seedlings under low nitrogen stress was related to auxin synthesis and signal transduction,and the change of auxin content was regulated by the tryptophan synthesis pathway.3.Under low nitrogen stress,nitrogen content and activities of nitrogen metabolism-related enzymes in sorghum seedling roots decreased.Exogenous tryptophan can increase the activities of nitrite reductase,nitrite reductase,glutamine synthase,glutamate synthase,glutamate dehydrogenase and other key enzymes of nitrogen metabolism,and increase the root nitrogen content.Exogenous tryptophan can enhance the nitrogen metabolism of 398B under low nitrogen stress.4.The contents of starch and soluble sugar in sorghum seedling roots increased under low nitrogen stress;Under low nitrogen stress,exogenous tryptophan increased the contents of starch and soluble sugar in sorghum seedling roots,and activated the activities of sucrose synthase,sucrose phosphate synthase and invertase,and improved the sucrose synthesis and transformation ability of sorghum seedling roots.5.Under low nitrogen stress,sorghum seedling roots can increase auxin synthesis through the tryptophan pathway,activate the activity of H~+-ATPase in plasma membrane,reduce p H value,acidify cell wall,and facilitate cell elongation.Meanwhile,exogenous tryptophan increased the activities ofα-ketoglutarate dehydrogenase,6-phosphoglucose dehydrogenase,pyruvate kinase and isocitrate synthase to provide energy for root cell elongation.