Involvement Of Hormones In Salt Resistance For Rice Plants And The Exogenous Regulating Mechanism

Rice(Oryza sativa L.)is more sensitive to salinity stress than other cereals,which varies from one growth stage to another.Under salinity stress hormones imbalance arrested the plant growth and development at germination to reproductive stage.Involvement of ethylene in salt resistance for rice plants and the exogenous regulating mechanism was studied.Further,role of exogenous application of Abscisic acid(ABA)and Brassinosteroids(BRs)on rice resistance to salt stress was also studied.The major findings of the study are briefed in the followings,providing more insights into the hormonal imbalance interactions with growth,physiological traits,and yield.This will be helpful for the future researches1.The accumulation of ethylene under salt stress is the main factor affecting the growth and development of rice.Therefore,maintaining ethylene homeostasis is very important for rice to survive under salt stress.Ethylene biosynthesis can be promoted or inhibited by the application of exogenous chemicals.At present,most studies have shown that priming rice seeds using these chemicals before seed germination is a simple and effective method to improve the salinity adaptability of rice.1-methylcyclopropene(1-MCP)can reduce ethylene release in plants by inhibiting ethylene binding to receptors.Ethephon is a compound that releases ethylene,leading to an increase in plant ethylene production.Therefore,the hydroponic culture experiment under completely randomized design(CRD)factorial was conducted to study the effects of seed priming with two contrasting chemicals 1-MCP and Ethephon with respect to ET production under salinity stress levels 0(control,CK)and 25 mM NaCl(salinity stress,SS)on cultivar Zhongzheyou 1(indica)with five replications.Results revealed that the use of 1-MCP and Ethephon improved overall rice seed germination and growth as compared to the unprimed one under salinity stress.Early germination rate was found to be higher in the Ethephon primed seeds.However,the late germination rate,overall seedling vigor index(SVI),total plant dry biomass(higher as 37.8 and 42%in 1-MCP-primed seeds,and 28.1 and 21.3%higher in Ethephon-primed seeds than no primed seeds under both salinity levels),other agronomic traits,and SPAD value were found to be higher in the 1-MCP primed seedlings.Further,the production of reactive oxygen species was comparatively higher in the Ethephon primed seedlings than the 1-MCP-primed seedlings.In addition,Ethephon priming resulted in elevated ET concentration and upregulated ET signaling genes,including OsACS1 and OsEIN2.Overall,these results have shown that priming rice seeds with suitable chemicals can minimize the negative effects of salt stress on germination and early growth of rice2.The production of 1-aminocyclopropyl carboxylic acid(ACC),the precursor of ethylene synthesis induced by salt,is the main factor leading to ethylene accumulation.ACC deaminase is a pyridoxal 5'-phosphate(PLP)-dependent enzyme that can relieve salt stress by cleaving ACC and thereby reducing the production of ethylene.However,the effects of exogenous PLP on the growth and physiological characteristics in relieving rice salt stress were still unclear.Thus in the present study,two japonica rice cultivars,jinyuan85(JY85)and Nipponbare(NPBA),with three levels of salt stress(control(0 mM,CK),low salinity(50 mM,LS),and heavy salinity(100 mM,HS)and two PLP levels(No PLP and 25 mM PLP)were used to study the effects of PLP application on the growth and morphological traits of rice cultivars under different salinity levels.The results showed that,compared with no-PLP,exogenous application of PLP significantly inhibited ACC and ethylene production in the roots,leaves and panicles of both cultivars under salinity stress,and PLP was effective in improving the physiological characteristics(Pn,Tr,gs,and SPAD value)of both cultivars under salinity stress.Further,root morphological traits and pollen viability were triggered in the PLP treatment compared to the no-PLP treatment under various salinity levels.ACC production inhibited by PLP was useful for improving the 1000-grain weight,grain yield per plant,and total plant biomass under the CK,LS and HS treatments in both rice cultivars.These results revealed that PLP,as an ACC deaminase cofactor,is a key tool for mitigating ethylene-induced effects under salinity stress and for enhancing the agronomic and morphological traits of rice under saline conditions.3.Hormone imbalance caused by salt stress,such as abscisic acid(ABA)and BRs(BRs)is the main factor affecting spikelets development and yield reduction of rice.However,the effects of ABA and BRs on spikelets development under salt stress have not been studied.So,to solve this problem,the effects of ABA and BRs on rice growth characteristics and development of spikelets under different salinity levels was explored.Three salinity levels,as 0(Control,CK),2 dS/m(Low Salt Stress,LS),4 dS/m(Heavy Salt Stress,HS),were conducted.Additionally,treatments as single application of two types of hommones or combination application of ABA and BRs,with two application levels as ABA(0 and 25?M),BRs(0 and 5?M),and ABA+BB16(0+0,25?M+5?M)were conducted.The hormones were applied at time of transplanting of rice seedlings and at rice booting stage.The results showed that exogenous application of ABA,BRs,and ABA+BRs improved the agronomic traits,net photosynthesis rate,SPAD value,pollen viability,1000-grain weight,and grain yield per plant,and reduce the spikelet sterility under different salt stress levels(CK,LS,and HS)compared with no hormone application at same salinity levels.This study will resolve the problems of poor rice spikelets development under salt stress and will also supply scientific support for wise use of hormones in improving rice grain yield under salt stress and enhance effective utilization of salt-affected soils.