Regulatory Mechanism Of Dwarf In Leymus Chinensis Induced By Long-term Overgrazing

The dwarf grassland plants caused by long-term overgrazing is one of the important factors leading to the degradation of natural grassland and the decrease of productivity.It is significant restoring and improving the productivity of grassland.At present,there are relatively few studies on the regulatory mechanism of the dwarf grassland plant.In this paper,Leymus chinensis as a dominant plant in the eastern margin of the Eurasian temperate grassland,was selected to study the differences in nitrogen uptake,transport and utilization through the nitrogen metabolic pathway between the normal and dwarf L.chinensis.The present study is able to reveal the effect of long-term overgrazing stress on the nitrogen metabolism of L.chinensis and its regulatory pathways,which will elucidate the molecular regulatory mechanism of dwarf grassland plants and expand our understanding of micro-evolution and the adaptation of grassland plants in the grazing system.In summary,the following main conclusions were obtained:（1）L.chinensis had a dwarf stress memory under long-term overgrazing stressDue to the effects of long-term overgrazing,L.chinensis plants showed a dwarf phenotype.We cultivated L.chinensis rhizome buds inside and common garden outside,and found that long-term overgrazing resulted in stress memory in individual dwarfed L.chinensis.Specifically,in the environment in sufficient water and nutrients,the overgrazing L.chinensis remained a certain degree of dwarf characteristics,mainly in plant height,stem length,leaf length and spike length and other phenotypic traits.The dwarf phenotype is ultimately reflected in the biomass that the dwarf L.chinensis upper ground biomass is 31%lower than that in the normal L.chinensis.（2）Long-term overgrazing stress significantly inhibited the nitrogen uptake process of L.chinensisUnder the condition of rhizome buds culture,the nitrogen accumulation,nitrogen uptake efficiency,nitrogen utilization efficiency and nitrogen utilization index of the dwarf L.chinensis were reduced by18.3%,18.5%,8.7%and 24.7%,respectively,compared with normal.This indicated that the decreased nitrogen uptake and utilization might be a key factor in the formation of the dwarf L.chinensis.In addition,we found that the activity of nitrate reductase（NR）was significantly decreased in roots of dwarf L.chinensis compared with the normal,which indicated that the reductive ability of NO₃^-in the root system was reduced.The elevated activity of glutamine（GS）and glutamic acid synthase（GOGAT）in the dwarf L.chinensis demonstrated that the assimilative capacity of NH₄⁺in the roots was enhanced.Above results showed that the primary pathway of nitrogen assimilation was changed from nitrate nitrogen to ammonium nitrogen.（3）Long-term overgrazing had a significant effect on the key genes expressions related to nitrogen metabolism in L.chinensisIn the early stage of the experiment,we performed transcriptome sequencing of L.chinensis under long-term overgrazing condition and long-term enclosure condition.Based on RNA-Seq results,we analyzed expressions of a group of key genes involved in the regulation of nitrogen metabolism using real-time quantitative PCR.The results showed that NO₃^-transporter genes associated with nitrogen uptake including NRT1.1 and NRT1.2,were down-regulated in dwarf L.chinensis,which was an important factor leading to the decrease of nitrogen uptake.Another NO₃^-transporter gene NRT1.7related to the nitrogen redistribution,was up-regulated,which can activate the nitrogen reuse in leaves of L.chinensis.Therefore,the expression of NO₃^-transporter genes was the key gateway to regulate the nitrogen metabolism in the dwarf L.chinensis.The key regulatory pathway of nitrogen metabolism in L.chinensis may be the decreased nitrogen transportability and the activation of nitrogen redistribution.