The Molecular And Physiological Basis Underlying Acquisition Of Nitrogen And Phosphorus In Fine Roots Of Poplars

Roots play a crucial role during the process of nutrients uptake by plants.Nitrogen(N)and phosphorus(P)are the two essential macronutrients for plants growth and development.Populus are one of the main plantation tree species in north China,because of their wide distribution,strong adaptation ability and fast growth.While,poplar plantations are often cultivated on marginal land where the soil is N and/or P limited,which greatly limit their economic and ecological value.However,limited information is available about the physiological and transcriptional regulation mechanisms of P and N assimilation in poplars in response to P,N or both(NP)deficiency.Also,the konwledge about the effects of N on P uptake by poplars is rarely.At present,the molecular and transcription regulation of poplars adaptation to P spatial heterogeneity are unclear.This study investigated the defference in absorption and utilization of N and P among seven poplar genotypes;subsequently,the two genotypes with contrasting growth characteristics were chosen to elucidate the physiological and molecular acclimation mechanisms of both poplars in response to P and/or N starvation,and further analyzed which mechanisms controlling P uptake during N limitation;Finally,we investigated the mechanisms underlying morphological and physiological acclimation of poplar to the spatial heterogeneity of P by transcriptome sequencing.The main results obtained are as follows:Cutting seedlings of seven poplar genotypes were treated with NH4NO3(8 or 0 mM)and KH2PO4(2 or 0 mM)for 56 days.Populus × euramericana had lower P and N concentrations but highest P and N amounts in roots.And the root biomass and parameters of P.× euramericana were greatest than other poplar genotypes.Populus simonii had greater P and N concentrations and lower P and N amounts than other six poplar genotypes.The contrasts growth characteristics were significant between this two poplar genotypes.Therefore,P.× euramericana and P.simonii were used to elucidate the physiological and transcription regulation mechanism in respond to P,N or NP starvation.To elucidate the P and N physiology of woody plants during acclimation to P and/or N starvation,we exposed saplings of the slow-growing P.simonii and the fast-growing P.× euramericana to complete nutrients or starvation of P,N or both elements(NP).We found that the responsiveness of the transcriptional regulation of key genes encoding transporters for phosphate,ammonium and nitrate was stronger in P.× euramericana than in P.simonii,which promote faster growth compared with P.simonii.In both poplars,P or NP starvation caused significant decreases in the P concentrations and increases in PUE.Nitrogen or NP deficiency resulted in lower N concentrations,amino acid levels and higher NUE in both poplars.Thus,in P.simonii and P.× euramericana,the mRNA levels of PHT1;5,PHT1;9,PHT2;1,AMT2;1 and NR increased in the roots,while PHT1;9,PHO1;H1,PHO2,AMT1;1 and NRT2;1 increased in the leaves during acclimation to P,N or NP deprivation.These results suggest that both poplars change P/N uptake,mobilization and assimilation during acclimation to P,N or NP starvation.To investigate the role of N on P uptake by poplars fine roots,we exposed saplings of P.simonii and P.× euramericana to either normal(2 mM)or low KH2PO4(0 mM)and either 0,1 or 8 mM NH4NO3 for 64 days.N can affects P uptake through physiological and molecular mechanisms in poplars.Low N stress could limit P absorption through change the structure of root system,reduce the activities of APs,PEPC and MDH,reduce the concentrations of malic acid,citric acid,amino acid and the synthesis of protein.Aslo,N limitation can induce the expression of nitrogen limitation adaptation(NLA)and PHO2,and reduce the mRNA levels of PHT1;1 and PHT1;4 to limit the uptake,transport and utilize of P by poplars.The transcription factor,PHR1 plays a critical role during the poplar acclimation to low P stress.The transcription and expression of PHT1;1,PHT1;4,PHT1;9,PAP and SQD2 were induced and the expression of PHO2 and NLA were inhibited by PHR1 in the fine roots of P.simonii and P.× euramericana,hence,the capacity for P acquisition from soil and recycle were enhanced in both poplars.To investigate the mechanisms underlying morphological and physiological acclimation to the spatial heterogeneity of poplar,we exposed saplings of P.simonii to either homogeneous P(2 mM or 0.2 mM)or heterogeneous P(half is 2 mM P and half is 0.2 mM P).When compared with homogeneous P(2 mM or 0.2 mM)treatments,there were 3345 and 1477 differentially expressed genes in the fine roots of P.simonii gowth in heterogeneous P treatment normal P area and low P area,respectively.And most of this genes comprised a co-expression networks,and participated in the co-regulation to the heterogeneous P treatment in P.simonii fine roots.Gene ontology(GO)terms reveal that,many GO terms related to abiotic stress,organic acid synthesis and transport,regulation of gene expression and signal transmission.As the important components of the co-expression and regulatory networks,the transcription factors,Myb domain protein,WRKY family transcription factor and basic helix-loop-helix(bHLH)DNAbinding superfamily protein,playing important roles in the regulation of metabolic adaptations to heterogeneous P and the translocation and distribution of P in P.simonii.Under low P stress,a series of phosphorus starvation response genes in P.simonii can be actived by the TFs,such as induction expression of phosphorus transporter PHT1;4 and acid phosphatase gene PAP29,resulting in increased the absorption of phosphorus in the soil by the fine roots of P.simonii.