Mechanism Underlying Water And Nitrogen Regulating Spikelet Development And Grain Filling Of Rice

Spikelet degeneration(or sterile)and poor grain filling of inferior spikelets of rice not only seriously reduce grain yield,but also decrease water and nutrient(N)use efficiencies.How to decrease spikelet degeneration or sterile and increase grain filling of inferior spikelets is not only an important scientific issue,but also an important technical problem.Both water and N are two key factors determining crop yield,and also the most frequently manipulated and influential environment factors that affect crop growth.Although there are many studies on the effects of water or N fertilizer on rice yield and its physiological mechanism,the research on the new phytohormones of brassinosteroids(BRs)and polyamines(PAs)which respond to water and N management and regulate spikelet development and grain filling of rice is very scarce.Using different rice genotypes as materials,this study investigated the mechanism underlying water and nitrogen regulating spikelet development and grain filling,with focus on changes in edogenous BRs and PAs levels.The main results are as follows:1.Polyamines and ethylene in young panicles in response to soil drought during panicle differentiation and their relationship with spikelet development in riceTwo rice cultivars,Yong You-2640(YY-2640,a japonica/indica hybrid)and Yang Dao-6(YD-6,an indica inbred),with a large difference in panicle size,were grown in pots.Three treatments,well watered(WW),moderate soil drought(MD),and severe soil drought(SD),were imposed from the onset of panicle initiation to the pollen completion stage.The MD treatment significantly increased spikelet differentiation,spikelet number per panicle,fully filled grain percentage,and grain yield,and decreased the percentage of degenerated spikelets,sterile spikelets and partially filled grains,compared to the WW treatment.The SD treatment showed opposite effects on the above variables.The MD increased the contents of free spermidine(Spd)and free spermine(Spm),and the ratio of free putrescine(Put),free-Spd or free-Spm to 1-aminocylopropane-l-carboxylic acid(ACC),but decreased the ethylene(ETH)evolution rate and ACC content in young panicles.The SD treatment exhibited the opposite effects.The free-Spd and free-Spm contents were increased significantly,whereas ETH and ACC levels,the percentage of degenerated spikelets and sterile spikelets were decreased significantly,when Spd or an inhibitor of EHT synthesis,aminoethoxyvinylglycine(AVG),was applied to young panicles.The results were reversed when ACC or an inhibitor of Spd and Spm synthesis,methylglyoxal-bis(guanylhydrazone),was applied.The results suggest that antagonistic interactions between free-PAs(Spd,Spm)and ETH respond to soil drought,and consequently,mediate spikelet development.2.Effects of soil drought during panicle differentiation on root traits and spikelet development and its physiological mechanism in riceTwo rice cultivars YY-2640 and YD-6 were grown in field which was equipped with a rain shelter.Three treatments,well watered(WW,control),moderate soil drought(MD),and severe soil drought(SD),were imposed from the onset of panicle initiation to the pollen completion stage.The MD treatment significantly increased spikelet differentiation,spikelet nurmber per panicle,fully filled grain percentage,and grain yield,and decreased the percentage of degenerated spikelets,sterile spikelets and partially filled grains,compared to the WW treatment.The SD treatment showed opposite effects on the above variables.The photosynthetic rate and dry matter accumulation did not decrease significantly under MD treatment.The MD treatment significantly increased root biomass,root oxidation activity(ROA),active absorption root surface(ABS),BRs content,and relative expression levels of OsAMT1;1(high-affinity ammonium transporter)and OsNRT2;1(high-affinity nitrate transporter)in roots.The SD treatment showed opposite effects.The abscisic acid(ABA)content and relative expression levels of DEEPER ROOTING 1(DRO1),and the distribution ratios of root biomass,ROA,ABS and BRs in the deep roots(below 10 cm),and the distribution ratios of nutrient in panicles and roots were significantly increased with the increase of soil-drying.The results suggest that the MD treatment during the panicle differentiation stage promotes the root growth,water and nutrient absorption and utilization thus increased grain yield of rice.3.ABA and BRs in rice grains in response to soil drought during grain filling periodTwo rice cultivars Lianjing 7 and Huaidao 5(both japonica inbreds)were grown in the field which was equipped with a rain shelter.Three treatments,well watered(WW,control),moderate soil drought(MD),and severe soil drought(SD),were imposed from 7 days after anthesis to maturity.Compared with WW treatment,the activities of enzymes related to sucrose conversion and starch synthesis,grain filling rate and grain weight of inferior spikelets were significantly increased under MD treatment,and they showed no significant differences in superior spikelets between WW and MD treatments.The SD treatment decreased them in both inferior and superior spikelets.The ABA and BRs contents in inferior spikelets were significantly increased under the MD treatment,and they showed no significant differences in superior spikelets between WW and MD treatments.BRs content was significantly decreased,and ABA content was significantly increased in both inferior and superior spikelets under the SD treatment when compared to the WW treatment.ABA or BRs content was very significantly and positively correlated with grain filling rate and starch biosynthetic ability.Application of a lower concentration of ABA or BR to plants under WW treatment imitated the results under the MD treatment,and applying with a higher concentration of ABA,or brassinazole(BRZ,an inhibitor of BRs),showed the effect of the SD.The results suggest that both BRs and ABA respond to soil drought,and moderate increase of ABA content and BRs content increase grain filling rate and grain weight of inferior spikelets of rice under MD treatment.4.Effects of alternate wetting and dry during the whole growing season on carbon remobilization and grain filling and its physiological mechanism in riceTwo rice cultivars YY-2640 and Huaidao 5(a japonica inbred)were grown in the field which was equipped with a rain shelter,and alternate wetting and moderate soil drying(AWMD,re-watered when soil water potential reached-15 kPa)was imposed from 10 days after transplanting to maturity,with continuous flooding irrigation(CF)as control.Compared with CF,AWMD significantly increased the photosynthesitic rate and shoot dry matter,reduced the maximum number of tillers,and significantly increased the rate of productive tillers,the effective leaf area per stem,non-structural carbohydrate(NSC)content in stems and the radio of spikelets number to amount of NSC at heading and NSC translocation to grain during grain filling,grain yield and irrigation water productivity.The AWMD treatment also significantly accelerated the reduction of starch in stems,promoted the reallocation of prefixed 13C from the stems to grains,shortened the grain filling period,and increased the grain filling rate.More soluble sugars including sucrose and trehalose were accumulated in the stems at the heading time under AWMD than under CF.Activities of ?-amylase,sucrose phosphate synthase in stems and granule bound starch synthase and soluble starch synthase in inferior spikelets,and the relative expression levels of SUT1(RICE SUCROSE TRANSPORTER)and TPP7(TREHALOSE-6-PHOSPHATE PHOSPHATASE)were enhanced by the AWMD treatment.The results suggest that the AWMD treatment can increase assimilate accumulation and carbon remobilization from stems to grains,and then increase grain weight of inferior spikelets and grain yield.5.Endogenous BRs in response to nitrogen and its relationship with spikelet development in rice5.1.Effect of nitrogen rate on spikelets development of riceTwo rice cultivars of YY-2640 and YD-6 were pot grown with three nitrogen(N)rates including 2,4,and 6 g pot-1,which were represented low amount(LN),medium amount(MN),and high amount of N(HN),respectively.The results showed that total spikelet number per pot was increased with the increase of N rates,but grain yield was the highest at MN,followed by HN,and the lowest at LN.The increase in grain yield under MN and HN treatments was mainly due to the increase in total spikelet number when compared with that under LN treatment.The promoted spikelet development was closely associated with higher N accumulation in stems and the increase in 24-Epicastasterone(24-epiCS)and 28-Homobrassinolide(28-homoBL)biosynthesis in young panicles at suitable N fertilizer application.When 24-epiCS or 28-homoBL were applied to the young panicles,the percentages of degenerated spikelets and sterile spikelets were significantly decreased,and spikelets per panicle and percentage of fully filled grains were significantly increased,and opposite effects were observed when BRZ,an inhibitor of BRs biosynthesis,was applied.The results suggest that 24-epiCS and 28-homoBL plays an important regulatory role in enhancing spikelet development and fertility of rice through responding to N application.5.2.Effects of application time of panicle N fertilizer on spikelets development of riceTwo rice cultivars YY-2640 and YD-6 were field-grown with five N application treatments imposed during panicle differentiation:no N application(CK),N application at panicle initiation(PI),at spikelet primordium differentiation(SPD),at the onset of pollen mother cell meiosis(PMC),and at PI,SPD,and PMC.Compared with CK,the most effective N application timing for increasing number of spikelet differentiation was at PI,for decreasing the percentage of degenerated spikelets was at SPD,and for increasing pollen fertility of the spikelets was at PMC.Spikelet differentiation number and pollen fertility were closely associated with the concentrations of 24-epiCS and 28-homoBL in rice panicles.Enhanced BR biosynthesis and signal transduction by N application increased spikelet differentiation and reduced spikelet degeneration,leading to an increase in grain yield.Changes in the levels of antioxidant system(AOS),proton-pumping ATPase activity,ATP concentration and energy charge were consistent with the changes in 24-epiCS and 28-homoBL concentrations.The contents of hydrogen peroxide(H2O2)and malondialdehyde(MDA)were opposite to the levels of 24-epiCS and 28-homoBL.Application of 24-epiCS or 28-homoBL to young panicles markedly increased the endogenous concentrations of 24-epiCS or 28-homoBL,energy charge and AOS levels,decreased the contents of H2O2 and MDA,spikelet differentiation and panicle weight.The opposite effects were observed when BRZ,an inhibitor of BR biosynthesis,was applied.The results suggest that BRs mediate the effect of N application on rice spikelet differentiation and play a vital role in promoting spikelet development though enhancing AOS and energy charge during the panicle development period.6.Interaction between N rates and irrigation regimes on spikelet development and grain filling of riceA rice cultivar YY-2640 was grown in field which was equipped with a rain shelter,and two irrigation regimes,continuous flooding(CF)and alternate wetting and moderate drying irrigation(AWMD),and three N rates,LN(120 kg N ha-1),MN(240 kg N ha-1),HN(360 kg N ha-1)were designed.Irrigation regimes and N rates showed significant interaction on spikelet development,grain filling and grain yield.At the same N rate and compared with CF,AWMD significantly decreased the percentage of degenerated spikelets,sterile spikelets,and increased spikelet differentiation number,spikelet number per panicle,grain physiology activity and grain yield.In the CF regime,the spikelet differentiation number,spikelet number per panicle and grain yield were the highest at MN,followed by HN and the lowest at LN.The yield at HN was lower than that at MN also due to the lower percentage of filled grains and lower 1000-grain weight.In the AWMD regime,the spikelet differentiation number,spikelet number per panicle and grain yield showed no significance between MN and HN,and both MN and HN exhibited a higher spikelet differentiation number,spikelet number per panicle and grain yield than LN.Among all the treatment combinations of irrigation regimes and N rates,the combination of AWMD+MN showed the highest grain yield.The reason for the high yield under AWMD+MN was the synergetic increase in total number of spikelets,the percentage of filled grains and 1000-grain weight.In conclusion,BRs,PAs and ABA can respond to water and nitrogen management and mediate the effect of water and N on spikelet development and grain filling.Adoption of moderate soil drought or alternate wetting and moderate drying irrigation combined with moderate amount of nitrogen fertilizer can enhance biosynthesis of BRs,PAs,and ABA,and consequently,effectively promote spikelet development and grain filling,and improve grain yield and water and N use efficiency.