Effects Of High Temperature And Drought Stress On Pollen Viability Of Summer Maize Were Studied By Transcriptomics And Metabolomics

The increasing global temperature has undoubtedly increased the degree and frequency of extreme weather such as heat and drought.The robust development of tassel is an important basics to provide high-quality pollen,maintain synchronous pollination and increase the maize yield.The period from trumpet to tasseling is the dominant stage of maize pollen development,which was more susceptible to environmental changes.However,the metabolite variations and molecular mechanism of maize tassel and pollen are deficiently understood by high temperature and drought in the dominant stage.Therefore,high temperature and drought treatments were carried out 15 days before tasseling,including normal condition(CK),drought stress(DS),high temperature stress(TS),and combined temperature and drought stress(TDS).In the experiment,the growth and development of tassel,pollen viability,pollen quantity and physiological indexes during the shedding stage were measured,and the mechanism of pollen response to TDS was further studied by transcriptomics and metabolomics.The important results were summarized as follows:(1)The effects of stresses on the growth and development of tassel were as follows: TDS>DS>TS>CK.In addition to the number of spikelets per plant,different treatments had different effects on the morphology of tassel.The most important effects were the rate of dry matter accumulation and dry matter,which might lead to energy shortage and lower pollen quality during pollen development.Additionally,the anthesis period of tassel was delayed 1-2 days under stress treatments.(2)Pollen quantity and vitality affected by stresses were consistent with the response trend of tassel.Although stresses had not significant difference in pollen dispersion duration,it had a negative effect on pollen viability and pollen quantity,particular in pollen viability.On the third day after pollen shedding,TS in the early stage did not result in a significant decline in pollen viability,but on the fourth day,pollen viability was significantly reduced.Therefore,TS ultimately affected the effectiveness of pollen grains.However,both DS and TDS caused a significant decrease in pollen viability and quantity,and the decline trend was more severe on the fourth day.Under stress,the pollen showed shrinking,cell wall uneven thickening,starch grains gradually close to cell wall and the number of starch grains decreased sharply.This indicated that the dry matter accumulation of tassel in the early stage was insufficient,which could not provide sufficient energy for the pollen development,leading to the decrease of pollen effective quantity.(3)Transcriptomics and metabolomics of the pollen in the third shedding day results showed that differential expression genes(DEGs)and differential metabolites(DMs)under the TDS condition was maximum compared to TS or DS.In order to explore the key genes and metabolites in response to TS and DS,13 DEGs and 121 DMs in three treatment groups were synthesized,including TS_VS_CK,DS_VS_CK and TDS_VS_CK,among which DMs included 8 free sugars.Subsequently,Pearson correlation analysis showed that most DEGs were significantly correlated with pollen viability and pollen weight.Among them,5 free sugars were significantly correlated with pollen viability,and 3 sugars were significantly correlated with pollen quantity.In addition,the changes of β-glucosidase,β-galactosidase,sucrose phosphate synthase and sucrose synthase activities were consistent with those of glucose,galactose and sucrose.(4)To reveal the molecular mechanism of pollen response to stress,WGCNA(a weighted gene co-expression network analysis)package was used to construct a weighted co-expression analysis of all expressed genes with pollen viability,pollen weight and 8 free sugars.Totally 58 gene expression modules were obtained.Since pollen viability was significantly positively correlated with pollen weight,and most free sugars.Dark green module(0.75,p=0.005)with the strongest positive correlation with pollen viability and brown(-0.93,p=1E-05)module with the strongest negative correlation were selected for gene network visualization.The front 4-5 genes with the highest connectivity in the screening module were selected as the hub genes,and Zm00001d021706(histone H2A)and Zm00001d042481(cysteine protease)were finally selected according to the principle of high basal expression level and significant change of expression level under stress.These two genes were positively correlated with D-ribose,β-D-glucose and D-Fructose(p<0.001).We speculate that Zm00001d021706 and Zm00001d042481 may co-regulate the response of maize pollen to high temperature and drought together with non-structural carbohydrates.In conclusion,the combined stress of high temperature and drought intensified the adverse effects of single stress of high temperature and drought on the growth and development of male panicle and pollen activity of maize,which will help us to further study the molecular mechanism of pollen response to high temperature and drought,and provide a theoretical basis for the establishment of maize stress-resistant cultivation and stress-resistant germplasm resources.