Mechanisms Of Premature Flag Leaf Senescence In Wheat Under Water Deficit At Grain Filling Stage And Its Regulatory Response To Exogenous ABA

Drought during grain filling stage causes premature senescence of leaves in wheat,which leads to poor synthesizing and transporting ability of photosynthetic assimilates.As a result,a significant reduction in crop yield and quality was found.Functional leaves senescence process of wheat regulated by genotypes is also sensitive to environmental changes and cultivation measures.Therefore,conjoint-analysis of physiological,biochemical indicators and gene expression level of leaf senescence was used to screen hub modules and genes.A comprehensive and systems-level view on mechanisms of premature flag leaf senescence was clarified.Meanwhile,analyzing the hormones homeostasis changes caused by exogenous chemical control substances paves the way to explore the cultivation ways and techniques for drought resistance and delaying senescence of wheat.We aimed to provide a theoretical basis for screening specific exogenous regulators.Field experiments were conducted at Shandong Agricultural University Farm during two wheat growing seasons?2016-2018?.The plot was arranged in split-split plot design.The main plots were two wheat varieties,Fu287?F287,a water sensitive genotype?and Shannong20?SN20,a drought resistant genotype?.The subplots were water deficit treatments,which were set as below,well-watered?WW?,the target relative soil content in 0-30 cm soil layer was 75%-80%of field capacity?FC?;mild water deficit?MiWD?,65%-70%of FC;moderate water deficit?MoWD?,50%-60%of FC;severe water deficit?SWD?,30%-40%of FC.The sub-sub plots were exogenous abscisic acid treatments.Results were shown as follows:1.Mechanisms of premature flag leaf senescence under water deficit in wheat at grain filling stageWater deficit significantly decreased the weight of superior,inferior grains,and total grains.The results also indicated that inferior grains weight decreased more than that of superior grains when suffered from water deficit stress.Furthermore,total grain weight and inferior grain weight of F287 were much more sensitive to water deficit than those of SN20.The relationship between senescence related parameters and grain weight of wheat was analyzed,implying that flag leaf senescence process had considerable impact on weight of superior,inferior and total grains.Besides,the regulation effects of leaf senescence process on superior,inferior and total grains weight were different.Water deficit at grain filling stage significantly influenced leaf senescence process.Compared with F287,SN20 had longer total duration of chlorophyll(Chl_total),rapid loss phase(Chl_loss),larger initial senescence rate?r₀?,the average senescence rate(r_aver),and inflection points accumulated temperature?M?,but lower the max senescence rate(r_max).Moreover,the data also showed that under WW conditions,persistence phase(Chl_per)of SN20 was longer than that of F287,contrarily,it was shorter than that of F287 when exposed to water deficit stress,which demonstrated that compared with F287,SN20 had longer photosynthetic active duration?PAD?with sufficient water irrigation.Conversely,SN20 held shorter PAD but higher photosynthetic productivity than that of F287 under water deficit stress.Therefore,compared with F287,SN20 could accumulate adequate photosynthetic assimilates at early grain filling stage.Besides,leaf rapid senescence maintained for an extended period,which was advantageous for the translocation of assimilates from source organs to grains at grain filling stages.Extending period of Chl_perer by cultivation or chemical control measures could further employ the potential of drought survivability and high productivity of SN20.Moreover,data also showed that r₀ and r_averver were enhanced while M,Chl_per,and Chl_totalotal were reduced with increasing water stress.The above results made clear that water deficit could initiate leaf rapid senescence prematurely as well as accelerate senescence process,which lead to premature senescence and shorten the PAD of leaf.As a result,grain weight of wheat saw obvious decreases.Water stress at grain filling stage affects leaf senescence by regulating homeostasis of endogenous hormones in wheat.The correlation between the ratios of hormones and senescence parameters were shown in the present study.The results showed that grain mass of wheat was significantly negatively related with r₀ and r_aver,however,it was notably positively correlated with r_max,Chl_total,Chl_per,and M.What was more,there was a significantly negative correlation between Z/GA₃ and M as well as Chl_total,whereas M and Chl_totalotal were significantly positively relative with grain weight.Besides,Spm/Put was significantly positively correlated with r₀ and r_aver,but it had remarkably negative correlation with Chl_per.Moreover,r₀ and r_averver were inversely associated with grain weight,but the association between Chl_perer and grain weight was opposite.Spm/Spd was significantly negatively related to r_maxax that was observably positively related with grain weight.The results indicated that there was a decline in grain weight of wheat,caused by increasing the ratios of Z/GA₃,Spm/Spd and Spm/Put in wheat leaves under water deficit stress at grain filling stage.Genes with low expression level were filtered out,and then 65535 genes were considered for the weighed gene coexpressionn network analysis?WGCNA?.A total of 33 distinct modules were identified for leaf senescence induced by drought.We evaluated the correlation between modules and our traits of interest.7 modules were selected as specific gene modules for drought resistance or specific gene modules involved in leaf senescence.What follows was to construct gene networks,candidate hub genes related to drought resistance and leaf senescence were screened out,which included the genes encoding sucrose-6-phosphatase,chitinase-8 and E3 ubiquitin protein ligase related to drought resistance as well as genes encoding glutamyl-tRNA reductase and glucose 4,6-dehydratase relevant to leaf senescence.The results could provide a reference for breeding,which may contribute to enhancing drought tolerance and improving productivity of crop plants.2.Regulation effects of exogenous ABA on wheat under water stress at grain filling stageHormone is an important signal that regulates leaf senescence under drought stress.It was shown that exogenous ABA enhanced the ratio of Spm/Put,but decreased the ratios of Z/GA₃and Spm/Spd.Therefore,hormone homeostasis in wheat leaves was changed.Exogenous ABA could alter the expression levels of genes encoding sucrose-6-phosphate synthase,chitinase,serine/threonine protein phosphatase,peroxidase and histone H2A in flag leaves of wheat.As the core genes,they could guide the biosynthesis of secondary metabolites and amino acids in flag leaves.Besides,they could influence the activity of antioxidant enzyme directly.Under the treatments of WW,MiWD and MoWD,spraying ABA could significantly enhance the accumulation of proline?Pro?.Moreover,the activity of superoxidase dismutase?SOD?,peroxidase?POD?,Catalase?CAT?and ascorbate peroxidase?APX?was improved at 5 to 15 days after anthesis?persistence phase of chlorophyll?.Conversely,the level of Pro was decreased when suffered from SWD.The activity of antioxidant enzyme?SOD,POD,CAT,and APX?was decreased at 20 to 30 days after anthesis?rapid loss phase of flag leaf?.The above results revealed that under different water deficit treatments,the obvious changes of antioxidant enzyme activity and pro accumulation significantly influenced the senescence process of flag leaves in wheat.Consequently,r₀,Chl_total,Chl_loss,and M of wheat flag leaves were decreased,whereas,Chl_perer was enhanced.As a result,spraying ABA decreased r₀,Chl_total,Chl_loss,and M,instead,Chl_perer was enhanced by ABA.Exogenous ABA showed positive regulation effects on grain weight under WW,MiWD,and MoWD treatments.Firstly,spraying ABA could decrease the ratios of Z/GA₃,Spm/Spd and SA/ABA,which could lead to prolonging the PAD by stimulating high level of Pro and enhancement of antioxidant enzyme activity?SOD,POD,CAT,and APX?at persistence phase of flag leaves.So,photosynthetic assimilates were accumulated more.Simultaneously,rapid senescence phase was sped up and the transport rate from assimilates to grains was accelerated,which led to high grain weight,yield and water use efficiency?WUE?in wheat.Contrarily,exogenous ABA would lead to low level of Pro and antioxidant enzyme activity?SOD,POD,CAT,APX?of flag leaves at rapid loss phase under severe water deficit.The wheat plants would accelerate aging process.The duration of assimilates transported to grains was short,thereby making photosynthetic assimilates redundant.Therefore,exogenous ABA would lead to the reduction of grain weight,yield and WUE in wheat when suffered from SWD.