| Food security is the most important thing for a country,and ensuring food security is related to the country’s destiny and people’s livelihood.Land saline-alkalization has posed a serious obstacle to food security.The area of saline-alkali land in the world has exceeded 833million hectares and more than 1.5 billion people are facing major challenges in growing food crops due to land saline-alkalization.The saline-alkalized land area is over 500 million mu in China,posing a major hidden danger to our food security.Saline-alkali-tolerant crops can improve saline-alkalized land and increase crop productivity in low-and medium-yield saline-alkalized land,which is of great significance to ensure food security.Broomcorn millet（Panicum miliaceum L.）is a resistance pioneer crop and a future smart crop.The adaptation to harsh environment may have promoted the evolution of a unique alkali-tolerant mechanisms in broomcorn millet,however,current researches on broomcorn millet focused on tolerance to barrenness,drought and neutral salt stress,and there are no reports on the mechanism of alkali resistance in broomcorn millet.It is of considerable practical significance for breeding of alkali-tolerant crops,improving crop production capacity in saline-alkalized land with low-and medium-yield fields,and ensuring food security to revealing the physiological response and molecular mechanism of alkali tolerance in broomcorn millet.In this study,296 broomcorn germplasm resources were used as experimental materials,and alkali stress was simulated with mixed alkali solution（molar ratio Na HCO3:Na2CO3=9:1）.The alkali resistance of broomcorn millet was identified and evaluated by investigating germination parameters and seedling growth indicators,and the physiological response of broomcorn millet to alkali stress in germination and seedling stages was studied.According to the identification results of alkali tolerance at germination and seedling stages,one alkali-tolerant（T289）and one alkali-sensitive（S223）varieties were selected to study the response mechanism of broomcorn root,leaves and grains to alkali stress.The main results are summarized as follows:（1）The evaluation system of alkali resistance in broomcorn millet germination stage and seedling stage was established,the 80 mmol·L-1 and 40 mmol·L-1 mix alkali-concentrations were determined to be the most suitable alkali-concentrations for the identification of alkali-tolerance in the germination and seedling stages,respectively.The 296 broomcorn millet germplasm resources were identified for alkali-tolerance during germination,the germination index,root fresh weight,and shoot length were determined to be the most important loading factors in evaluation system during germination,which can be used as the main index for identification and evaluation of broomcorn millet alkali-tolerance.Identification at the seedling stage showed that the green leaf area is the most visual indicator of alkali-tolerance and can be used as a direct indicator for broomcorn millet alkali-tolerance evaluation at seedling stage.12 alkali-tolerant germplasm resources and 41 alkali-sensitive germplasm resources were screened out,which can be used to study the mechanism of broomcorn millet adaptation to alkali stress.（2）Under alkali-stress,alkali-tolerant broomcorn millet exhibited stronger antioxidant defenses,andα-amylase activity of germinating seeds was positively correlated with germination rate.Alkali stress suppressed GA concentration but promoted ABA concentration in germinating seeds,and GA/ABA ratio was higher in alkali-tolerant broomcorn millet than in alkali-sensitive broomcorn millet.Real-time polymerase chain reaction analysis showed that alkali stress downregulated GA synthesis genes but upregulated GA inactivation and ABA synthesis genes.Physiological response study of broomcorn millet seedlings showed that alkali-tolerant broomcorn millet had lower malondialdehyde content and electrolyte leakage rates and more integrated leaf stomata and root structure than alkali-sensitive broomcorn millet.Alkali-stress inhibited mineral uptake and reduced biomass accumulation in broomcorn millet.（3）Alkali-stress caused 9,306 and 3,624 genes to be differentially expressed in S223（alkali-sensitive）and T289（alkali-tolerant）roots,respectively,and these differentially expressed genes（DEGs）were significantly enriched in pathways such as plant hormone signal transduction,MAPK signaling,phenylpropanoid biosynthesis,ABC transporter,and ion transport（ion binding,ion transport,transmembrane transport）.Joint analysis of transcriptome and metabolome showed that broomcorn millet upregulated the gene expression levels of phenylpropanoid biosynthesis-related enzymes（4CL,CCR,CAD,POD,C3’H）and flavonoid biosynthesis-related enzymes（F3’5’H,F3’H,ANR）to resist alkali-stress,while the accumulation of related metabolites were inhibited.（4）Alkali-stress induced 21,113 and 12,151 DEGs in S223（alkali-sensitive）and T289（alkali-tolerant）leaves,respectively,which were mainly involved in photosynthesis,carbohydrate metabolism,kinase activity and other pathways.Broomcorn broom adapts to alkali stress by sacrificing growth.Under alkali stress,BGL and EG related genes were activated and highly expressed in broomcorn millet leaves.The genes related to auxin regulation pathway and the auxin content of T289 were higher than those of S223,which contribute to better growth state under alkali-stress.Under alkali-stress,the expression levels of photosynthesis-related genes in T289 were higher than those in S223,especially PSⅠ,PSⅡand ATP synthase-related genes.Additionally,the expression level of chl G-related genes was higher in T289 than S223 under alkali stress,resulting in higher chlorophyll content in T289.Under alkali stress,BR increased the antioxidant enzyme activity,promoted the maintenance of ion homeostasis,and maintained physiological structure and photosynthetic properties of broomcorn millet leaves.Meanwhile,BR significantly reduced the transcriptional response induced by alkali-stress and promoted the accumulation of effective metabolites such as biliverdin,L-glutamate,and phosphoric acid in broomcorn millet.（5）Alkali stress inhibited the development of broomcorn millet grain.Under alkali stress,the grain length,1000-grain weight,and number of grains per plant of T289（alkali-tolerant）were higher than those of S223（alkali-sensitive）.Alkali stress promoted the accumulation of total phenols,total flavonoids,γ-VE and other antioxidant substances in broomcorn millet grains.Alkali stress caused differential accumulation of 114 and 89 non-volatile metabolites and 16 and 20 volatile metabolites in S223 and T289 grains,respectively,involving the phenylpropanoid,flavonoids,flavonoids and flavonols,valine,leucine and isoleucine biosynthesis pathways,and the arginine,proline,tryptophan,and ascorbic acid metabolism pathways.Broomcorn millet grains activate phenolic acid,flavonoid,and ascorbic acid-related biosynthesis,while promoting the accumulation of amino acids and organic acids to resist alkali-stress.In summary,this study integrated physiology and multi-omics to reveal the mechanism of broomcorn millet response to alkali stress from multiple perspectives,providing a theoretical basis for molecular breeding of alkali-tolerant plants and improvement of saline-alkali land,and helping to ensure national food security... |
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