Mitigation Effect And Regulation Mechanism Of Exogenous Spermidine (Spd) On Maize Under Drought Stress

Drought stress is one of the main abiotic stresses that cause the reduction of crop yield.Research on improving drought tolerance of crops has attracted wide attention.Polyamines(PAs)are a class of low-molecular-weight aliphatic nitrogenous bases with strong biological activity that are ubiquitous in plants.They have the functions of regulating basic metabolism,affecting growth and development,controlling morphogenesis and delaying aging.They mainly include putrescine(Put),spermidine(Spd)and spermine(Spm),among which Spd has the closest relationship with plan t stress resistance.Previous studies have shown that the appropriate concentration of exogenous Spd could alleviate the damage caused by drought stress to a certain extent and promote the growth of plants under drought stress.However,the mechanism of photoinhibition protection,nitrogen metabolism and ascorbate-glutathione cycle of Spd under drought stress is still unclear,and there is still no in-depth study.Maize(Zea mays L.)is one of the most important cereal crops in the world.It is not only an important food and feed,but also has a wide range of industrial uses.It has become the world's largest food crop.As a rain-fed crop,maize has a long growth period and a large water requirement,which is one of the crops sensitive to water stress.In th is paper,(1)a hydroponics experiment,using two different drought-tolerant types of maize ?Xianyu 335‘(drought insensitive type)and ?Fenghe 1‘(drought sensitive type)as experimental materials,setting normal nutrient solution cultivation and PEG-6000 simulated drought cultivation,combined with exogenous Spd application,to investigate the effects of drought on plant growth,photosynthetic characteristics,xanthophyll cycle,chloroplast ultrastructure,antioxidant protection system,osmotic adjustmen t system,nitrogen metabolism,polyamine metabolism and endogenous phytohormone contents;(2)A pot experiment was conducted,with ?Xianyu 335‘ as the experimental material,and setting normal water treatment(water content is 75±5% of the maximum water holding capacity in the field)and moderate water deficit(water content is 50±5% of the maximum water holding capacity in the field).)treatment during maize flowering,combined with exogenous Spd application,studied the effect of drought on root growth,root activity,root injury flux and chlorophyll content(SPAD),net photosynthetic rate,leaf area,dry matter accumulation after anthesis,yield and yield components.To explore the physiological regulation function of exogenous Spd in improving drought tolerance of maize.At the same time,transcriptome analysis was carried out on the leaves of maize seedlings treated with exogenous Spd and drought stress in order to preliminarily reveal the regulatory basis of exogenous Spd on gene expression of maize in response to drought stress.The main results are as follows:(1)Drought stress inhibited the growth of two varieties of maize seedlings,and decreased the net photosynthetic rate(Pn),stomatal conductance(gs),transpiration rate(Tr),photosynthetic pigment content,PSII photochemical quantum efficiency(Fv/Fm),photosynthetic electron transfer rate(ETR),and photochemical quenching coefficient(q P),while increased the non-photochemical quenching coefficient(NPQ).Among them,?Fenghe 1‘ is more affected by stress.On the fouth day of drought stress,the decrease of Pn in ?Fenghe 1‘ mainly due to non-stomatal factors,and the decrease of Pn in ?Xianyu 335‘ is mainly due to stomatal factors.Exogenous Spd can promote the growth of maize seedlings under drought stress,increase the content of photosynthetic pigments,slow down the inhibition of photosynthetic electron transport induced by drought stress,improve the photochemical activity of PSII reaction center,and effectively alleviate the stomatal limitation and non-stomatal limitation of photosynthesis of maize seedling leaves under drought stress,and the mitigation effect on the drought-sensitive variety ?Fenghe 1‘ is more obvious.(2)Drought stress induced photoinhibition of photosynthesis in maize seedlings and caused a certain degree of photodamage to photosynthetic systems.Exogenous Spd treatment increased the total xanthophyll cycle pool(VAZ)of maize seedlings,enhanced the de-epoxidation of xanthophyll cycle(DEPS),dissipated excitation ene rgy in the form of heat,and protected photosynthetic organs from photodamage and stabilize d the chloroplast and thylakoid membrane structure of maize seedlings,thus alleviated the inhibition of drought stress on photosynthesis and plant growth of maize s eedlings.(3)Drought stress significantly reduced leaf water content and leaf water potential of maize seedlings,and significantly reduced root water conductivity(Lp),which induced the accumulation of osmotic regulators such as soluble protein,soluble sugar and proline.On the one hand,exogenous Spd can enhance root water uptake by promoting root growth and increasing root water conductivity(Lp);on the other hand,it can improve the water uptake capacity of cells by inducing the further synthesis or accumulation of osmotic regulators,thereby improving the water status of maize seedlings and improving the adaptability of maize seedlings to drought stress.(4)Under drought stress,the photosynthetic electron transport in maize seedlings was disturbed,and the production rate of superoxide anion(O2?-)and the content of hydrogen peroxide(H2O2)were significantly increased.The accumulation of excessive reactive oxygen species(ROS)leads to the increase of membrane lipid peroxidation,the increase of malondialdehyde(MDA)content and the enhancemen t of membrane lipid permeability.Exogenous Spd further enhanced the activities of superoxide dismutase(SOD)and catalase(CAT)in maize seedling under drought stress,ascorbic acid peroxidase(APX)and glutathione reductase(GR)in ascorbate-glutathione cycle(As A-GSH),and slowed down the decrease in the activities of monodehydroascorbic reductase(MDHAR)and dehydroascorbic acid carboxylase(DHAR)under drought stress.And increased the contents of reduced ascorbic acid(As A)and reduced glutathione(GSH),decreased the contents of oxidized ascorbic acid(DHA)and reduced glutathione(GSSG),and improved the ratios of As A/DHA and GSH/GSSG.The high activity of antioxidant enzymes and the content of As A and GSH help to maintain the stability of As A-GSH cycle,effectively remove ROS,reduce the degree of membrane lipid peroxidation,maintain the integrity of cell membrane structure and function,and improve the drought tolerance of plants.(5)Under drought stress,the contents of NH4+ and free amino acids in leaves of maize seedlings increased significantly,while the contents of NO3-and the activities of nitrate reductase(NR),glutamine synthase(GS),glutamate synthase(GOGAT),glutamate dehydrogenase(GDH),glutamic oxalate transaminase(GOT),glutamic alanine transaminase(GPT)decreased,while exogenous Spd could effectively alleviate these effects induced by drought stress.The effect on ?Fenghe 1‘ was more obvious.This indicated that exogenous Spd could promote the assimilation of excessive NH4+ in drought-stressed plants by enhancing the activities of GS/GOGAT and GDH,and transaminase(GOT and GPT),effectively alleviate the ammonia toxicity and nitrogen metabolic disorder induced by drought stress,thus promoting plant growth.(6)Under drought stress,the content of PAs in leaves of two varieties of maize seedlings increased significantly.And the increases in the contents of free,bound and bound Spd and Spm,and the activities of S-adenosylmethionine decarboxylase(SAMDC)and diamine oxidase(DAO)in leaves of ?Xianyu 335‘ under drought stress were higher than that of ?Fhenghe 1‘.Under drought stress,exogenous Spd treatment reduced the content of free Put in leaves of two maize seedlings,and further increased the content of free,bound and bo und Spd and Spm.This indicated that exogenous Spd treatment could promote the transformation of free Put to free Spd and Spm,and the accumulation of bound and bound PAs in drought-stressed plants.At the same time,exogenous Spd treatment increased the a ctivities of ornithine decarboxylase(ODC),SAMDC and DAO in the leaves of maize seedlings under drought stress,decreased the activities of polyamine oxidase(PAO),and the effects were more obviously on ?Fenghe 1‘.This indicated that exogenous Spd could enhance drought tolerance of maize seedlings by regulating the metabolism of endogenous PAs.(7)Under drought stress,the contents of abscisic acid(ABA),salicylic acid(SA)and methyl jasmonate(Me Ja)in the leaves of Maize Seedlings of the two varieti es increased significantly,and the increase in ?Fenghe 1‘ was greater than that in ?Xianyu 335‘;the contents of auxin(IAA),zeatin nucleoside(ZR)and gibberellin(GA3)in the leaves of maize seedlings of the two varieties decreased significantly.The d ecrease of ?Fenghe 1‘ is larger than that in ?Xianyu 335‘.The application of exogenous Spd could alleviate the decrease of IAA,ZR and GA3 contents in Maize Seedling leaves,reduce the increase of SA and M e JA contents,but had no significant effect on ABA contents.The regulation of endogenous hormone balance by Spd may be closely related to the delayed senescence of maize leaves and the protection of photosynthetic function under drought stress.(8)The gene expression of maize seedlings under exogenous Spd and drought stress were analyzed.Statistical and cluster analysis of differentially expressed genes among the treatments showed that the gene expression of CK group was the closest to that of S pd group,followed by PEG+Spd group with exogenous Spd unde r drought stress,and the difference was the greatest between CK group and PEG group.There were 2084 differentially expressed genes in PEG group compared with CK group,which involved 4 4 metabolic pathways.The differentially expressed genes of porphyrin and chlorophyll metabolism,photosynthesis-antenna protein metabolism pathway,photosynthesis metabolism pathway,carbon fixation metabolism pathway,nitrogen metabolism pathway,arginine and proline metabolism pathway were clustered.Cluster analysis show ed that exogenous Spd alleviated the effects of drought stress on porphyrin and chlorophyll metabolism,photosynthesis-antenna protein metabolism,photosynthesis metabolism,carbon fixation metabolism,nitrogen metabolism,arginine and proline metabolism in maize seedlings.(9)Under pot experiment conditions,exogenous Spd significantly promoted root growth,increased root length,root surface area and root volume,increased root activity and root bleeding flow,thus improving plant water and nutrient upta ke and utilization.At the same time,application of exogenous Spd could increase chlorophyll content(SPAD)and photosynthetic rate of maize leaves under drought condition,increase the leaves area,delay leaf senescence,increase dry matter accumulation after anthesis,increase grain weight and grain number per ear,and ultimately increase maize yield.