Mechanisms Of The Invasive Plant Xanthium Strumarium In Response To High Levels Of Nitrate

Nitrogen（N）is an important nutrient resource in soil.High level of N often promotes exotic plant invasions.However,few study considers the effect of different forms of N on exotic plant invasions.Xanthium strumarium is a noxious invasive plant in Liaoning and other places in China,It grows faster when adding nitrate relative to ammonium nitrogen,while its co-occurring native plant X.sibiricum grows faster under ammonium nitrogen.However,the mechanisms underlying this difference is still unknown.In this study,we compared growth and N accumulation of X.strumarium and X.sibiricum under different NO₃^-concentrations,and determined the nitrate concentration,under which the interspecific differences were the most.Then,we determined leaf photosynthetic rate,and the activities of nitrate reductase and glutamine synthetase in the two Xanthium species under low（control）and high（with the maximum interspecific difference）nitrate levels,and analyzed the physiological bases of the differences of the two Xanthium species in response to high nitrate level.Transcriptome sequencing was performed on the roots and shoots of these two Xanthium species under high and low nitrate levels,and we analyzed the molecular bases of the differences of the two Xanthium species in response to high nitrate level.The main results are as follows:（1）With the increase of nitrate level,growth（biomass,diameter）and N accumulation increased significantly in both X.strumarium and X.sibiricum,reached the maximum values when the concentration of nitrate is 50 mmol L^-1,and then decreased with the increase of nitrate concentration.Under different nitrate levels,growth and N accumulation were significantly higher in X.strumarium than in X.sibiricum.Compared with X.sibiricum,X.strumarium showed a stronger plastic response to nitrate addition.（2）Under high relative to low nitrate level,X.strumarium had higher photosynthetic rate and shoot glutamine synthetase activity,while with similar root glutamine synthetase activity.However,there were no significant differences in these traits for X.sibiricum.The chlorophyll a content of X.strumarium was significantly higher than that of X.sibiricum under either high or low nitrate level.Under high nitrate level,shoot nitrate reductase activity of X.sibiricum was significantly higher than that of X.strumarium,while root nitrate reductase activity of X.strumarium was higher than that of X.sibiricum.（3）Transcriptome sequencing found:gene expressions of nitrate transporter 1/peptide transporter family 6（NPF6）and nitrate reductase were induced by high level of nitrate in the root of X.strumarium,increasing nitrate absorption and reduction.Gene expression of glutamine synthetase were induced by high level of nitrate in shoot of X.strumarium,increasing nitrate assimilation.The expression levels of many genes coding chlorophyll binding proteins were induced by high nitrate level in shoot of X.strumarium,promoting the ability of light-harvesting.Similar results were not found in X.sibiricum.Many auxin signal transduction related-genes were highly expressed in the shoot and root of X.strumarium under high nitrate level.Auxin signal may be involved in the utilization of high level of nitrate.In conclusion,compared with the coexisting native congener X.sibiricum,X.strumarium could better use nitrate,showing stronger plastic response to nitrate addition.These interspecific differences may be associated with the higher expression level of genes related to nitrogen uptake and assimilation,and chlorophyll binding proteins,and the expressions of these genes may be regulated by auxin signal transduction related genes.Our study revealed the reasons why the invasive plant X.strumarium prefers to invade hibitats with high nitrates from physiological and molecular levels,and provided new ideas for its prevention and control,which has important scientific significance and application potential.