The Role Of Abscisic Acid In Photosynthesis And Sucrose Metabolism And Transport In Rice

Abscisic acid?ABA?is one of the most important hormones involving in regulating the plant growth irrespectively whether under non-stress or abiotic stress condtions.However,its effect on the photosynthesis,sucrose metabolism and transport in rice plants and the underlying mechanism is unclear.Thus,in this study,two rice genotypes,Zhefu802 and its near-isogenic line fgl were used as materials to:i)reveal the mechanism underlying ABA affect photosynthesis of leaf in rice plants;ii)the role of ABA in sucrose metabolism and transport in spikelet of rice at pollen mother cell meiotic stage;iii)The effect of the interaction between ABA and sucrose on the grain yield and quality at the grain filling of rice.The main results are listed bellow:1.About 45 day old seedlings of these two genotypes were selected to study the effect of ABA on the photosynthesis of leaf in rice.The results showed that the chlorophyll content of isogenic line fgl were significantly lower than that of recurrent parent Zhefu802,but the actual fluorescence quantum efficiencyY?II?,net photosynthetic rate?Pn?and stomatal conductance?Cond?were found signficanly higher in the former than the latter.Compared with Zhefu802,the ABA and H₂O₂ levels in leaf of fgl were signficanly lower,and accordingly the antioxidant enzymes such as CAT,POD and APX were significantly higher.Thus,it was inferred by the authors that ABA might be the main factor resulting in lower Pn in Zhefu802.This hypothesis was confirmed by the results of rice plants extrogenouly treated with ABA or H₂O₂,which could reduce the Pn and Cond in leaf of rice,while enhancement were found in the treatment of fluridone?ABA syhthesis inhibitor?or dimethylthiourea?H₂O₂ scavenger?.Additionally,rice plants sprayed with catalase?CAT?or ascorbic acid?ASA?could enhance the Pn and Cond of leaf accompanied with not only increase in the stomatal apertures and antioxidant capaicities but also the rubisco enzyme activities.Thus,enhancements in antioxidant capaicity could increase Pn of leaf in rice plants via reducing H₂O₂ and enhancing the rubisco acivity.2.The transcriptome analysis results also showed that there are 653 differentially expressed genes?DEGs?in fgl when compared to Zhefu802 plants.Among these 653DEGs,314 were up-regulated as well as 339 down-regulated.The results of gene ontology?GO?were classified three items:molecular function,biological process and cellular component.Molecular function mainly included protein binding,ATP binding,protein serine/threonine kinase activity,kinase activity and so on.Biological process mainly included regulation of transcription,oxidation-reduction process,and protein phosphorylation,and defense response,response to biotic and abiotic stresses and so on.Cellular component mainly included nucleus,plasma membrane,cytoplasm,chloroplast,mitochondrion,golgi apparatus and so on.The KEGG pathway results showed that the DEGs mainly involved in the pathways of carbohydrate metabolism,transcription and translation,response to biotic and abiotic stresses,lipid metabolism,amino acid metabolism,biosynthesis of other secondary metabolites,signal transduction,photosynthesis metabolism and so on.Focused on the photosynthesis pathway,we found that there were three DEGs;two of them were up-regulated,with the description ferredoxin from photosynthetic electron transport and alpha subunit of ATP synthase.The description of the rest down-regulated one was oxygen-evolving complex from the photosystem II,which might be the main factor resulting higher photosynthesis showed in fgl than Zhefu802.3.Heat stress at the pollen mother cell?PMC?meiotic stage leads to pollen sterility in rice,in which the reactive oxygen species?ROS?and sugar homeostasis are always adversely affected.This damage is reversed by ABA,but the mechanisms underlying the interactions among the ABA,sucrose metabolism,ROS and heat shock proteins?HSPs?in rice spikelets under heat stress are not clear.These two rice palnts,Zhefu802 and fgl were subjected to heat stress of 40?after pre-foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC.The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress.This was achieved through various means,including:increased levels of soluble sugars,starch and non-structural carbohydrates;markedly higher relative expression levels of heat shock proteins?HSP24.1 and HSP71.1?and genes related to sucrose metabolism and transport,such as sucrose transporters?SUT?genes,sucrose synthase?SUS?genes,and invertase?INV?genes;increased antioxidant activities;increased content of ATP and endogenous ABA in spikelets.In short,exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis,thus ABA contributed to heat tolerance in rice.4.The interactions effect between ABA and sucrose on grain yield and quality was conducted with Zhefu802,and exogenously applied with ABA,sucrose,ABA and sucrose after flowering.Combination application of ABA and sucrose significantly increased the seed-setting rate,kernel weight and grain yield.The percentage of milled rice,head milled rice,and protein content were significantly improved.The proportion of dry matter accumulation and non-structure carbohydrate in panicles were significantly increased under combination application of ABA and sucrose.The starch content,activities and gene expression of enzymes involved in starch synthesis in panicles were significantly increased,while changes of soluble sugar contents showed no obvious tendency.Furthermore,endogenous phytohormones including ABA,IAA,GA,and ZRs were significantly increased.Trehalose content and gene expression of TPS1,TPP7,which participated in the biosynthesis of trehalose,were significantly increased under combination application of ABA and sucrose.Result of this study revealed that grain yield and quality were improved by the interactions between ABA and sucrose,and mainly through the sugar conversion pathway including starch synthesis and trehalose metabolism.