Mechanism Of Regulation Of Erect Panicle Type On Rice Yield And Eating Quality Under High Nitrogen Condition

Over the past 30 years,super rice has played an important role in boosting rice yield.The phenotype of erect panicle(Ep)architecture controlled by dense and erect panicle 1(dep1)is the typical characteristic of super rice has been used in rice breeding.Although the Ep type plays an important role in promoting the yield of northern japonica rice,there is room for improvement in the eating quality(EQ)of Chinese rice to match Japanese high-quality rice.In recent years,breeders have paid increasing attention to the eating quality of rice.Maintaining high yield while improving inferior quality has become the main problems to be solved in this rice-growing region.We constructed a pair of rice NIL lines denoted as NIL-Ep,NIL-non Ep,in the LG5 and AKI backgrounds.The main causes affecting the yield and quality of Ep type under high-yield cultivation mode and the effects on nitrogen metabolism pathway were deeply analyzed.The main research results are as follows:1.Relationship between panicle type and yield and taste quality of riceTo investigate the effects of Ep on the grain yield and taste quality under different N fertilizer treatments,we used two nitrogen fertilizer treatments as low(L)and high(H).The yield and taste quality traits were investigated over four years.Under L treatment,the LG5 yield was significantly higher than the AKI yield in 2018,2019,and 2021 but not significantly different from in 2020.No significant difference was observed between the two NILs in L treatment.Under H treatment,LG5 yield was significantly higher than AKI in all four years(Fig.1A-D),and the NILs showed the same pattern as their parents.For taste quality,under L treatment,there was no significant difference between LG5 and AKI.Under H treatment,the taste quality of LG5 was significantly lower than that of AKI,and the quality of AKI did not decrease with the higher nitrogen fertilizer.The taste quality of the NILs showed the same pattern as that of their parents under the same treatment.However,NIL-non Ep showed significantly lower taste quality under H treatment,compared with L treatment.2.Source analysis of yield and taste quality differencesThere was no significant difference in yield under L treatment,but there was a significant difference in yield and its components between the NILs under H treatment.Under the different treatments,the yield and yield component traits of the NILs were consistent with their parents,AKI and LG5.The average panicle number per square metre(PNP)and grain number per panicle(GNP)in NIL-Ep than in NIL-non Ep(Fig.2B).In order to clarify the source of differences in GNP,we assigned the rice panicles into 3 parts namely top(top)(panicle positions 1-1 to 4-2),middle(mid,panicle positions 5-1 to 8-2)and bottom(bot,panicle positions 9-1 to 12-2).Compared with the NIL-non Ep,grain number was significantly higher in NIL-Ep,mainly in the middle and bottom locations.The panicle weight ratio of each part differed,and the panicle weight ratio in the middle to bottom part was21.5% and 18.7% higher in the Ep line.Although Ep showed a lower thousand-grain weight(TWG)because the grain length was significantly lower,the extremely significantly higher GNP and PNP was the key factor leading to the significantly higher yield;the contribution rates of the three factors were-7.1%,29.5% and 19.9% respectively.To accurately and objectively measure the EQ of the tested materials,we adopted two sets of evaluation systems: artificial tasting and machine evaluation.The two evaluation systems showed the same results.Under L treatment,there was no significant difference in the EQ between NILs;under H treatment,the EQ of NIL-Ep was significantly lower than that of NIL-non Ep under H treatment.The texture of rice showed that the hardness was significantly higher in Ep while the viscosity and elasticity were lower.We assessed the EQ of the top,middle and bottom panicle parts,and found that the EQ of the middle to the bottom part of NIL-Ep was significantly lower than in NIL-non Ep,which was the key factor affecting the overall EQ.For RVA characteristics,the middle and bottom locations of NIL-Ep showed significantly lower breakdown,higher final viscosity and setback values than those of NIL-non Ep.Our results confirmed that the EQ of middle and bottom position grains of NIL-Ep was lower compared with NIL-non Ep.The amylose content and protein components of grains in different panicle locations under H treatment were measured to analyze the key factors causing the lower eating quality.There was no significant difference in amylose content among different panicle positions,but there was a significant difference in nitrogen content of middle and bottom grains,which was significantly higher in NIL-Ep than that of NIL-non Ep.We tested the protein content in 24 panicle positions,and the results showed that,under H treatment,the grain protein content in the middle and bottom parts of NIL-EP was significantly higher than the top part,while there was no significant difference among panicle positions of NIL-non Ep.Subsequent analysis of the protein components showed that the difference in nitrogen accumulation was the result of the significantly higher prolamin and glutelin content.3.Nitrogen-use efficiency and grain protein accumulationIn the two years of repeated experiments in 2019 and 2020,under the high nitrogen condition,the yield of NIL-Ep was 30.6% and 50.4% higher than that of NIL-non Ep.In H treatment,the nitrogen recovery efficiency and physiological NUE were significant higher in Ep than non Ep.There was no significantly difference in the yield and NUE under L treatment.We examined the nitrogen transport in different organs at harvest stage under the condition of high nitrogen.Nitrogen accumulation of NIL-Ep was significantly higher than that of NIL-non Ep in all organs,so that the total nitrogen accumulation of NIL-Ep was significantly higher at harvest.From booting to full heading stage,the nitrogen content in leaves still maintained an upward trend in NIL-Ep,while in NIL-non Ep,this phenomenon only occurred in flag leaves and second leaves.In addition,it is interesting that 80-100 d after transplanting,the leaf nitrogen content of NIL-Ep showed a sharp downward trend,and the same trend also appeared in the stem and sheath organs.We further analyzed the dynamic changes of the glutelin and prolamin contents of the two genotypes during the grain filling stage,and the found there were marked significant differences in the contents of the two protein components between different panicle locations in NIL-Ep while there was little difference among different locations in NIL-non Ep.We measured the activities of these enzymes in the two genotypes during grain filling.Significant differences were detected between the two genotypes in glutamine synthetase and NADH-GAGOT glutamate synthase activity throughout the grain filling stages.The activity of asparagine synthetase and dehydrogenase increased significantly during the periods 4-28 d and 22-44 d after flowering respectively.To clarify the differences in grain nitrogen accumulation derived from different organs and exogenous nitrogen from full heading to harvest stage,we calculated the nitrogen accumulation in grains of NILs and the proportions contribution by each plant part.The total nitrogen accumulation in NIL-Ep grains was significantly higher than that in NIL-non EP grains,and the origin of grains nitrogen derived from various organs suggested that the ability of NIL-Ep to absorb nitrogen was significantly higher than that of NIL-non Ep at the grain-filling stage under the high nitrogen condition.4.Response of NILs’ transcriptome to nitrogen conditionsThe content of carbon and nitrogen elements,enzyme activities related to nitrogen metabolism and gene expression of rice leaves were significantly different under different nitrogen levels.Transcriptome analysis showed that compared with NIL-non Ep,40 differentially expressed genes were obtained in NIL-Ep under low nitrogen conditions,of which 22 were up-regulated and 18 were down-regulated.A total of 1,148 differentially expressed genes were obtained under high nitrogen conditions,of which 826 were up-regulated and 322 were down-regulated.KEGG enrichment analysis of differential genes showed that carbon and nitrogen metabolic pathways were significantly enriched.At the same time,based on the targeting relationship between transcription factors and differential genes related to carbon and nitrogen metabolism,a carbon-nitrogen metabolism co-expression regulation network for rice in response to nitrogen was constructed.It provides a theoretical basis for regulating carbon and nitrogen metabolism and improving eating quality of Ep type.