Physio-biochemical Mechanism Of Exogenous Ursolic Acid In Regulating Rice Growth Under Salinity Stress

In recent years,due to the effects of climate warming,industrial pollution,agricultural irrigation,the area of saline-alkali land around the world has increased year by year.Therefore,many studies have been devoted to the research of rice salinity tolerance in order to improve the status of saline-alkali land and increase rice yield under salinity stress.The treatment of various compounds to improve the salinity tolerance of rice has also been gradually applied.Ursolic acid as a pentacyclic triterpenoid has been proven beneficial in a variety of human diseases(such as cancer)and in mice,but its role in plants has been scarcely studied.Therefore,the purpose of this experiment is to study the effects of ursolic acid pretreatment on the growth,photosynthesis,morphology and physiology of different rice varieties(HZ,712,HAY)under different salinity gradients(0,40mM,80mM).The salinity-tolerance mechanism of three rice cultivars under salinity stress after pretreatment with ursolic acid was preliminarily explored,which provided new ideas for the further study of ursolic acid in plants and the breeding of rice with salinity tolerance.The main findings are as follows:1.The effects of different concerns of salinity treatment and ursolic acid pretreatment on physiological morphology and membrane metabolism of three rice varieties were studied.The results showed that compared with the control,salinity stress severely inhibited the growth of rice and the biomass was significantly reduced.And with the increase of salinity concentration,the dry and fresh weight of stems and leaves gradually decreased.Under high-concentration salinity stress(80 mM NaCl),the biomass of roots and stems of HAY variety reduced the least,while 712 variety showed a moderate salinity response compared to HZ and HAY.At the same time,salinity stress also caused a large accumulation of ROS in plants,causing oxidative damage to plants,and significantly increasing MDA,EL,and LOX contents.Among them,HAY variety had the lowest content of membrane metabolism-related indicators whether or not they were pretreated with ursolic acid,indicating that HAY variety had the highest tolerance to salinity stress.2.Electron microscopy results also showed that ursolic acid pretreatment can reduce the effects of salinity stress on it by improving cell chloroplast and mitochondrial structure.Ursolic acid pretreatment can relieve mitochondrial swelling,make mitochondrial morphology and size return to normal,and significantly reduce damage to the nucleus,nuclear membrane,and cell membrane.The recovery effect is most significant in HAY variety.Therefore,the salinity tolerance of the three tested rice varieties in order from low to high was:HZ<712<HAY.HAY is a salinity-tolerant variety,while HZ is a salinity-sensitive variety.3.The effects of salinity treatment and ursolic acid pretreatment on antioxidant enzyme system in three rice varieties were studied.The rapid increase in ROS level further resulted in increased antioxidant enzyme activity.Among them,the activities of SOD and POD enzymes were most significantly affected by salinity stress,and the activities of SOD and POD enzymes were highest in HZ variety,followed by 712 and HAY varieties.The effects of salinity stress on CAT and APX were relatively insignificant.In the GSH-AsA cycle,GSSG and GR activities were significantly affected by high salinity stress,while the effects of low salinity stress were relatively insignificant.Ursolic acid pretreatment can significantly increase the expression of antioxidant enzymes such as SOD,APX,POD,CAT,GR,and significantly increase the GR,GSH,AsA content and decrease the GSSG content in the GSH-AsA cycle.4.The effects of salinity treatment and ursolic acid pretreatment on nitrogen metabolism,Na~+/H~+ions transportation and related gene expression in three rice varieties were studied.In the nitrogen cycle,UA pretreatment significantly enhanced the NR and NOS levels and NO content in rice varieties under salinity stress.Salinity stress also resulted in increased Na~+accumulation and reduced K~+absorption in plants,while ursolic acid pretreatment resulted in reduced Na~+accumulation and increased K~+accumulation in rice varieties under salinity stress.Quantitative gene expression analysis confirmed that UA pretreatment significantly increased the transcriptional expression of the Na~+transporter genes OsHKT1;5 and the Na~+/H~+exchanger gene OsSOS1,while inhibiting the transcriptional expression of the non-selective cation transporter OsCNGC1,resulting in higher gene expression levels of OsNR1,OsNR2, OsGR and OsCAT,CuZnSOD,OsPOD,and OsCAT under salinity stress.Therefore,the pretreatment of ursolic acid enhances the tolerance of rice to salinity stress by increasing the content of antioxidant enzymes,GSH,NO,etc.,reducing the absorption of Na~+,and increasing the expression of related genes of Na~+transporters.5.We conducted principal component analysis on various physiological and biochemical indexes of rice under salt stress after ursolic acid pretreatment.The correlation degree of each index was explored,and the important role of NR,K~+,GR and other key indexes in the regulation of rice salt stress by ursolic acid was determined.