Effects Of Salinity Stress On The Interactions Between Rice Plant(Oryza Sativa L.)and Brown Planthopper(Nilaparvata Lugens Stal)

Salinization is one of the most critical abiotic stress factors for crops and a rising setback in agro-ecosystems.Changes in weather,land usage and the salinization of irrigation water are increasing soil salinity of many farmlands.Salinity stress triggers changes in plant morphology,physiology,and molecular responses,which can subsequently influence plant-insect interactions;however,these consequences remain poorly understood.Salinity stress affects nutrient uptake and assimilation,sugar and amino acid accumulation,hormonal balance,and defense system in rice(Oryza sativa L.)plant.On the other side,host plant quality plays a significant role in the performance and fitness of the brown planthopper(BPH),Nilaparvata lugens,which is one of the major biotic constraints of rice.We postulated that increased soil salinity might modulate the interactions between plant and herbivore.Initially,we examined the bottom-up effect of salinity stress on population parameters of the BPH through rice plants.The results revealed that salinity interfered with egg hatching of BPH.The nymphal development period,adult longevity,and oviposition were also influenced by salinity.Notable differences appeared in the intrinsic growth rate(r_m),the finite increase rate(?),and the net reproduction rate(R₀)of the BPH population,and a concentration-dependent effect was detected.Although salinity adversely affected BPH development,our study predicts that BPH could become a worse problem in lower and medium salinity areas and that those planthoppers may exacerbate the adverse effects of salinity for rice production.Since plant abiotic tolerances as well as defense responses against herbivores varied with different rice cultivars,we adopted a broader approach to investigate the impact of salinity stress on plant-herbivore interactions by analyzing the plant biomass and insect population growth rates on different cultivars.Considering the plant resistance to abiotic factor,salinity-susceptible cultivar TN1 showed better resistance to BPH feeding under high saline conditions(10 d S/m),whereas moderately salinity resistant cultivars(IR64,TPX,and HHD)exhibited variable resistance to BPH.Nevertheless,there was no significant difference in biomass losses between control and low saline conditions(5 d S/m)in all tested cultivars.On the other hand,BPH-resistant cultivar,TPX somewhat sacrifice the herbivore resistance,and IR64 showed no effect in herbivore resistance under high saline conditions.Besides,BPH-sensitive cultivar,TN1 showed improved herbivore resistance,and HHD exhibits no difference in herbivore resistance under high saline conditions.This cultivar-specific effect was reflected in BPH population growth rates on the corresponding plants.To elucidate the effect of salinity stress on BPH feeding behavior,we investigated the response of BPHs feeding on the different cultivars in terms of host preference,honeydew excretion,and salivary flange production.We also compared the electrical penetration graph(EPG)recordings to observe the feeding of BPHs in real-time.The feeding activities of BPH showed that xylem feeding was significantly increased on high salinity TN1 and IR64 plants.Accordingly,phloem ingestion was inhibited on those plants.In contrast,BPHs showed improved feeding efficiency on stressed TPX plants.Nevertheless,BPHs feeding ability was not affected on HHD plants when the plants were treated with salt.Further,salivary flange quantification and BPH settlement behavior results were consistent with EPG and honeydew measurement data.Taken together,the effect of salinity stress on BPH feeding behavior and preference varied with rice genotypes.In further investigation of the underlying mechanism of plant-herbivore interactions,we examined the impact of salinity stress on herbivore-induced plant defense response via the abscisic acid(ABA),salicylic acid(SA),and jasmonic acid(JA)signaling pathways.Quantification of ABA,SA,and JA signaling transcripts indicated that salinity-induced down-regulation of ABA signaling increased SA-dependent defense in TN1.While the up-regulation of ABA-related genes in salinity stressed TPX resulted in a decrease in SA-signaling genes.Nevertheless,salinity had little effect on the JA-related genes expression in both cultivars upon BPH infestation.Hormonal cross-talk regulates the plant responses to abiotic and biotic stresses;therefore,we tried to dissect the hormonal interactions occurring under simultaneous salinity stress and herbivory.Measurement of phytohormone contents demonstrated that salinity stress had little impact on ABA production and SA-mediated defense response in the case of TN1.While salinity induced up-regulation of ABA contents might be contributed to the down-regulation of SA concentration in TPX plants.However,BPH feeding resulted in no significant increase in the JA and JA-Ile contents in either genotype or salinity treatment.Thus,JA-independent ABA-SA antagonism might be a key element in the interaction between BPH and salinity stress.Taken together,we concluded that plant-BPH interactions are markedly shaped by salinity,and the interaction might be cultivar specific.