| Nitrogen is one of the most important nutrient elements in rice growth and development.Due to the limited nitrogen resources in the soil,use of large amounts of nitrogen fertilizers,or application of transgenic means to increase the nitrogen using efficiency of plants are very popular.The former caused environmental pollution and energy waste,while the latter had low controllability and stability.Therefore,the adoption of a green method to increase nitrogen using efficiency in rice is imperative for the sustainable development of rice production.Endophytes in plants can form mutually beneficial symbiosis with plants and have the potential to promote plant growth and increase host nitrogen uptake and metabolism.In this article,a strain of Streptomyces albidoflavus OsiLf-2 with a disease resistance ability,which was isolated from our preliminary work,was used to study for promoting rice growth and nitrogen assimilation under nitrogen stress.The study was aimed to provide a basis for further understanding of the mechanism of efficient endophytes rice nitrogen biotransformation system and for application in agricultural production.First of all,the effect of OsiLf-2 on the growth of rice under low-nitrogen stress was examined.After germination,the shoots were transferred to a nitrogen-free and low-nitrogen-stress environment.The stress concentrations were 0 mmol/L,0.05 mmol/L,0.5 mmol/L(NH4+),and the stress time was 21 days.The results showed that the plant height,leaf width,stem width,leaf angle,root length,fresh weight,dry weight,chlorophyll content,and soluble sugar content in rice plants treated with OsiLf-2 under low-nitrogen-stress concentration were basically higher than that of the untreated rice,although the growth was still negatively affected by the nitrogen stress.It suggested that OsiLf-2 may promote the growth of rice by improving rice tolerance to low-nitrogen stress.Subsequently,the detection of activities of the key defense enzymes in rice,i.e.peroxidase(POD),polyphenol oxidase(PPO)and catalase(CAT),showed that almost all of them significantly higher than those in the control group under no nitrogen and low nitrogen stress.It can be seen that the endophytic actinomycetes OsiLf-2 can enhance the ability of rice to resist abiotic stress by increasing defensive enzyme activity.Finally,the activities of nitrogen assimilation related enzyme,such as glutamine synthetase(GS),glutamate synthetase(GOGAT)and nitrate reductase(NR),in rice treated with OsiLf-2 under low nitrogen stress were analyzed to clarify the mechanism of the enhanced rice tolerance to low nitrogen stress.The results showed that the activities of three enzymes in the OsiLf-2 treatment group were slightly higher than those in the control group.We further studied the expression ofthe NH4+ transporter gene OsAMT2;1 and the key genes in nitrogen assimilation pathway OsGSl;1,OsGS2,OsNADH-GOGAT2,OsFd-GOGAT.Real-time PCR results showed that the relative expression of these genes in OsiLf-2 treatment group was basically significantly higher than that in the control group,indicating that OsiLf-2 has the ability of improve the nitrogen transfer efficiency in rice under low nitrogen stress.In conclusion,under low nitrogen stress,rice endophytic actinomycete OsiLf-2 can increase the activities of defense enzymes in rice,up-regulate the expression of rice nitrogen assimilation genes,and enhance nitrogen assimilation related enzyme activities to improve rice nitrogen using efficiency,so as to enhance rice tolerance to low nitrogen stress,and ultimately promote the growth of rice.Endophytes and plants combination promote the efficient biotransformation of nitrogen and are environmentally friendly,thus provide new resources for the development of new biological fertilizers to reduce the abuse of chemical nitrogen fertilizer. |
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