γ-Aminobutyric Acid Regulates The Growth And Adaptation Of Poplar Under Low Nitrogen

Nitrogen is one of important factors that limit the growth and production of plant,and deficient nitrogen had adversely influences on the plant growth and development.γ-Aminobutyric acid(GABA),a four carbon amino acid,is reported as an important inhibitory neurotransmitter in animals.In plants,GABA is considered to locate in the central of carbon and nitrogen metabolism,and involved in regulating the plant growth and response to the adverse environments.However,the functions of GABA under low nitrogen remain unclear.In this study,the roles of GABA in the growth and adaptation were investigated,combined the systematic analysis about the characteristics of the gene families in the GABA shunt,in the poplar(Populus alba×Populus.glandulosa“84K”)under low nitrogen conditions by the ways of bioinformatics,morphology and anatomy,physiology and molecular biology methods.1.The GABA shunt in poplar had three gene families:6 Pop GADs,2 Pop GABA-Ts and 2Pop SSADHs.The analysis of promotors showed that GAD and GABA-T gene families had regulatory elements involved in light response and stress environments;Quantitative expression of genes revealed that the expression of most GABA-shunt gene family members was higher in roots than in stems and leaves.These results indicated GABA shunt might play important roles in the growth and stress responses and in plants.2.Low nitrogen obviously inhibited poplar growth,and externally supplied GABA alleviated this inhibitory effect.Analysis of morphology and anatomy showed GABA promoted height,diameter,stem node numbers and biomass in leaves and stems,whereas it reduced the xylem thickness significantly,and increased the fiber length evidently.Analysis of photosynthetic performance showed GABA significantly increased the chlorophyll content and photosynthesis rate.Results of anti-oxidative system demonstrated that the increase in the content of MDA and the activities of POD and CAT,which were induced by low nitrogen,was significantly inhibited by the application of exogenous GABA.These results proved that exogenous GABA could promote the poplar growth under low nitrogen,might resulting from the enhanced ability to assimilate carbon by photosynthesis,or the reduction in the raw materials and energy consumption required for antioxidant enzyme biosynthesis by exogenous GABA,which may allocate more carbon and nitrogen flow into the growth.3.Low nitrogen significantly changed the balance of carbon and nitrogen metabolism in poplar.Exogenous GABA significantly reduced the starch contents in leaves,promoted the accumulation of total soluble sugars,fructose and sucrose in stems,reduced the contents of lignin and hemicellulose,and increased the contents of malate,citrate and succinate in stems.On the other hand,exogenous GABA promoted the accumulation of NO3~-as well as the activity of NR/Ni R and GS enzymes in leaves.Meanwhile,exogenous GABA also induced the activity of GOGAT enzyme in stems,but inhibited the activity of NR/Ni R and GS enzymes in stems.The protein contents in stems and leaves were also increased significantly by exogenous GABA.Analysis at the transcription levels showed that low nitrogen also affected the expression of key genes related to carbon and nitrogen metabolism,while the application of exogenous GABA could upregulated or downregulated their expression accordingly.These results showed that exogenous GABA can promote the fixation and transportation of photosynthetic carbon assimilation products and enhanced the ability of nitrogen assimilation in leaves under low nitrogen conditions,contributing to the tolerance of poplar.In summary,this study systematically identified the characteristics of three gene families GABA shunt in poplar for the first time.The promoter and gene expression analysis suggested that GABA plays an important role in poplar stress response and growth.The results then demonstrated that exogenous GABA may improve the ability to withstand low nitrogen stress and promote growth in poplar mainly by regulating the assimilation of carbon and nitrogen in leaves and their allocation.This study would provide a reference for the genetic improvement and cultivation of trees under the conditions of insufficient soil nitrogen supply.