Effects Of Nitrogen On The Stem Strength In Rice And Its Physiological Mechanisms

Studies have demonstrated that the stem strength,one of the crucial factors in determining the lodging resistance,is determined by the genetic characteristics of the varieties,environmental factors and cultivation measures.Application of an appropriate amount of nitrogen fertilizer is an effective way to achieve high yield of rice,but excessive nitrogen fertilizer usually causes over-grwoth of plants and reduces the stem strength.Therefore,it is important to figure out the effects of nitrogen on the formation of rice stem strength and its physiological mechanism,since it has theoretical guiding significance for optimizing nitrogen fertilizer management and coordinating the contradiction between high yield and lodging.In this experiment,two indica rice varieties,Yliangyou2 and IIyou084,and two japonica rice varieties,Wuyunjing23 and W3668,were selected.The characteristics of the varieties and main factors affecting the stem strength were determined by comparing the differences in the stem strength between varieties and their responses to interannual changes.A field experiment was carried out by using Wuyunjing23(lodging-resistant)and W3668(lodging-susceptible).The effects of nitrogen on the stem strength and the formation of cell wall components were studied by applying different levels of top-dressing nitrogen fertilizer.A pot experiment was conducted by using W3668.The molecular mechanism of nitrogen regulation of cell wall formation in the basal internode was studied by transcriptomic and proteomic analyses.Meanwhile,the gibberellin pathway of nitrogen-regulated cell wall formation was explored.The main results are as follows:1.The reduction of the cell wall components significantly decreased the stem strengthBy comparing varieties from 2012 to 2014,it can be found that the stem strengths of Yliangyou2 and Wuyunjing23 were significantly higher than those of ?you084 and W3668,respectively.The difference in cellulose and lignin contents between varieties is the key factor leading to the difference in the stem strength.The mechanical stem strengths of indica rice varieties were more sensitive to the interannual variability.The interannual changes of the breading strength and bending stress were mainly due to the significant decrease of the lignin content in the internodes.2.High nitrogen rate inhibits the biosynthesis of cell wall componentsThe comparison of sclerenchyma tissues in the basal internode showed that high nitrogen treatment significantly inhibited the accumulation of the cell wall in sclerenchyma tissues,and also reduced the thickness of sclerenchyma tissues.Under high nitrogen conditions,the expression of lignin biosynthetic genes were significantly inhibited,resulting in a decrease in the lignin content in the basal internodes.The cellulose content was not reduced in W3668,which indicated that high nitrogen treatment mainly reduced the lignin content.Carbon content in the stem is an important factor affecting the formation of the cell wall components.Under high nitrogen conditions,the NSC content of leaves and internodes decreased significantly and the carbon content in stems was determined by that of leaves.3.Nitrogen significantly regulated the cell wall formation in the stemDifferentially expressed genes(DEGs)and proteins(DEPs)in the second basal internode were analyzed under nitrogen treatments at 3 days and 9 days after the elongation through RNA-seq and iTRAQ.The results showed that a total of 216 DEGs and 330 DEPs were screened at 3 days after jointing,and 1620 DEGs and 763 DEPs at 9 days after jointing.It was proved that a large number of DEGs and DEPs were involved in the pathways related to carbohydrate metabolism and cell wall formation by performing KEGG pathway enrichment analysis.High nitrogen treatment generally down-regulated the expression of genes and proteins in the lignin biosynthetic pathway and also significantly affected the expression of genes and proteins in the cellulose and hemicellulose biosynthetic pathway.Carbohydrate metabolism,which supplies a carbon source for cell wall formation,was significantly suppressed under high nitrogen conditions,including carbon fixation,starch and sucrose metabolism,and glycolysis.In addition,the abundance of cell wall modifying proteins was significantly increased under high nitrogen conditions,which promoted cell wall loosening and decreased the cell wall stiffness.Nitrogen may inhibit lignin biosynthesis by up-regulating the expression of the transcription factor OsOFP2.4.The regulation of nitrogen on the stem strength and the formation of cell wall components is not dependent on gibberellinIn the RNA-seq study,nitrogen significantly regulated the expressions of GA biosynthetic and signalling genes in the stem.Therefore,we wondered whether gibberellin is involved in the alteration of the cell wall components regulated by nitrogen fertilizer.In the field experiment,rice plants were subjected to two different top-dressing nitrogen levels and a GA3 treatment to investigate the potential relationship between nitrogen and gibberellin functioning in the cell wall formation.Both high nitrogen and GA significantly increased the plant height,but decreased the breaking strength,dry weight and NSC content of the second basal internode compared with low nitrogen treatment.However,the reduction of breaking strength under high nitrogen and GA treatments were due to a decrease in bending stress(BS)and section modulus(Z),respectively.Additionally,GA homeostasis was not greatly affected by nitrogen treatments.The accumulation of cell wall components,especially lignin,was inhibited by HN by suppressing expressions of lignin biosynthetic genes,while no significant effect was observed for GA.In conclusion,the regulation of nitrogen on the stem strength and the formation of cell wall components is not dependent on gibberellin.