Mechanisms For Enhanced Resistance To The Brown Planthopper With Silicon Amendment To Rice

The brown planthopper（BPH）,Nilaparvata lugens（St?l）,is a destructive sucking insect pest of rice and cause a severe threat to rice production in China and other Asian countries.Improving cultivation measures is a key component in integrated pest management.It has been reported that the application of silicon（Si）can enhance the resistance of rice to brown planthopper,but the mechanisms of Si-mediated rice plant resistance to the brown planthopper（BPH）are still unclear.In the present study,we tested the effects of Si addition on rice plant resistance to BPH by employing the susceptible rice cultivar Taichung Native 1（TN1）.Effects of Si application on BPH performance and population growth,BPH feeding behaviors,plant antiherbivore defense responses,and plant histological characteristic were investigated.The results of the current study may shed light on reasonable cultivation measures for BPH regulation.1.Effects of Si addition on BPH performance and population growth.Si treatments were established using 25-d old potted rice plants treated with Si at 0,0.16（low level）or 0.32（high level）g Si/kg soil.When the seedlings was 40-d old,the plants were infested with BPH and then performance of the insects was observed.Results showed that Si addition to rice plants at both the high and low rates significantly extended nymph duration,reduced nymph survival rate,and reduced adult fecundity,sex ratio and longevity in BPH.Altogether,intrinsic rate of increase,finite rate of increase and net reproduction rate of BPH population were all reduced at both the high and low Si addition rates and population doubling time extended at both the high and low Si addition rates.Thus Si addition enhanced resistance of rice plants to BPH and suppressed BPH population growth rate.2.Effects of Si addition on BPH feeding behavior.This experimental setup was the same as that for the effects of Si addition on BPH performance and population growth.Feeding amount was measured as honeydew excretion using a parafilm sachet method.Feeding behaviors of brown planthopper adults was monitored using electrical penetration graph（EPG）.We recorded antixenosis effect of Si-amended rice plants on BPH,as indicated by deterrence of BPH settlement on plants with the high Si addition rate.BPH feeding amounts on rice plants amended with Si at both the low and high rates were remarkably reduced.The detailed feeding behaviors of BPH were different between Si treatments and untreated monitored by EPG.Si addition at the high rate extended the total duration of np and Nc and reduced the total duration of N4a and N4b.Si addition at the high rate also extended duration of first np per insect and time to the first N4a.Si addition to rice plants at both the high and low rates evidently extended total duration of N4a followed by sustained N4b.These results indicate that Si addition has an adverse effect on the feeding behavior of brown planthopper.3.Effects of Si application on antiherbivore defense responses of rice.In this study,aquacultured rice plants with Si amendment at 112 mg Si/kg nutrient solution or 0 mg Si/kg nutrient solution（control）were used.Ten-day old seedlings was treated by Si,thirty-day old seedlings was infested with BPH.The rice seedling was sampled at 0,24,48,72,or 96 h post-infestation（hpi）for the determination of various plant physiological indices.With BPH infestation,CAT activities were down-regulated all the way until 72 hpi in+Si plants and until 48 hpi in–Si plants.A sharp increase in CAT actitity was observed at 96 hpi in+Si plants while a further decrease was evident in–Si plants.Between Si treatments,CAT activities were higher in+Si than in–Si plants at 24,48 and 96 hpi,indicating that Si amendment retards the decrease in CAT activity due to BPH infestation.With BPH infestation,SOD activities showed a steady increasing pattern in+Si plants while a gradual decreasing pattern in–Si plants.Significant differences in SOD activities were observed between+Si and–Si plants at 48,72 and 96 hpi.POD activities were characterized by similar patterns of temporal changes in+Si and–Si plants.POD activeties differed significantly only at 48 hpi,higher in+Si plants than in–Si plants.PPO activities in+Si plants underwent a significant increase at 24 from 0 hpi and thereafter a significant decrease at 48 hpi.In contrast,PPO in–Si plants experienced no significant temporal changes in activity.Like POD,PAL activities were also characterized by similar patterns of temporal changes in+Si and–Si plants.Between the treatments,PAL activeties differed significantly at48 and 72 hpi,higher in+Si plants than in–Si plants.+Si and–Si plants showed similar patterns of temporal changes in the activities ofβ-1,3-glucanase.Between+Si and–Si plants,β-1,3-glucanase activeties were higher at 24,48 and 72 hpi in the former than in the latter.MDA concentrations maintained at a relatively flat level while in–Si plants.Between the Si treatments,MDA concentrations were higher in–Si plants than in+Si plants at 48,72 and 96 hpi.H₂O₂ concentrations also responded positively to BPH attack in both+Si and–Si plants,peaked at 48 hpi.H₂O₂ concentrations were higher in+Si plants than in–Si plants at 24,48 and 96 hpi.The results show that Si is involved in the activation of the antiherbivore defense responses indued by BPH attack..4.Effects of Si application on plant histological measurements.This experimental setup was the same as for that for effect of Si application on plant antiherbivore defense responses.Morphological characteristics of silica cells in rice leaf sheath of 96 h post BPH infestation were observed by scanning electron microscope,and callose deposition on sieve plates in rice leaf sheath of 24,48,72,or 96 hpi was investigated using paraffin section and staining technique.Further,we quantified the relative expression levels of the callose synthase-encoding gene（OSGSL1）and callose hydrolyzing enzymeβ-1,3-glucanase gene（Gns5）in leaf sheath samples of 0,3,6,12,24,48,72,or 96 hpi by real-time RT-PCR.The results showed that application of Si significantly enhanced Si content in the stem and silicification of rice leaf sheath.The number of callose deposition on the sieve plates was increased in response to BPH infestation.At 24 and 48 hpi,callosic sieve plates were more in Si treatment than the control.The dynamic callose deposition is regulated by the genes encoding callose synthase and the callose hydrolyzing enzymeβ-1,3-glucanase.The expression of encoding callose synthase gene OSGSL1 was triggered highly in+Si plants in relative to–Si plants,and the callose hydrolyzing enzymeβ-1,3-glucanase（Gns5）was highly in-Si plants in relative to+Si plants.Altogether,our results show that Si amendment enhances resistance of rice plants to BPH and suppress BPH population growth,which is mediated through increased Si content and intensified silisification of rice leaf sheaths,reduced feeding amount,impaired piercing behaviors,strengthened antiherbivore defense responses,and increased callose deposition due to differential modulation of callose synthase and hydrolase gene expression.