The Molecular And Physiological Basis Underlying Nitrogen Uptake Characteristics And Wood Formation Of Populus × Canescens In Acclimation To Changes In Nitrogen And Light

The growth of trees not only includes the root growth,but also includes the height growth and secondary growth.Root growth can absorb water and nutrients for trees.Height growth allows trees to get enough light for photosynthesis.Secondary growth(also termed as wood formation)of trees refers to the thickening of stems,and can produce wood(also termed as secondary xylem)for human beings.Wood formation not only manufactures channels(vessels or tracheids)for transporting water and nutrients to the aerial parts,but also produces fibers to provide mechanical support for trees.Wood formation is regulated by genetic factors and environmental factors such as nitrogen(N)and light.In the wild,trees are usually distributed in the marginal lands where N is often limited.Secondary growth was severely repressed by low N supply.Trees absorb NH₄⁺and NO₃^-from the soil through fine root,and synthesize various N-containing compounds to support the secondary growth further.Currently,the characteristics of NH₄⁺and NO₃^-fluxes among different root segments,and transcriptomic and metabolomic networks underlying secondary growth in acclimation to low N availability are still unclear.In practice,N fertilization and increasing light intensity are often applied to improve the wood yield.However,the physiological and proteomic regulation mechanism underlying wood formation in acclimation to fluctuating N levels and light intensities are still unknown.As woody model plants,Populus is one of the main plantation tree species in northern China for its fast-growing.N and light management are critical to improve the productivity of poplar plantations.Therefore,saplings of hybrid poplars(Populus×canescens,syn.P.tremula×P.alba)were selected as materials,and the transcriptomic regulation mechanism of NH₄⁺/NO₃^-spatial absorption characteristics in the root of poplar were studied.Then,the physiological,transcriptomic,and metabolomic underlying secondary growth of poplars in acclimation to low N were investigated.Finally,the physiological and proteomic mechanism underlying secondary growth of poplars in acclimation to fluctuating N levels and light intensities were explored.These results will provide theoretical basis for selecting and breeding improved cultivator of trees with high N use efficiency.In addition,these results will be benefit for orientation-breeding timber forest plantation to enhance productivity and quality of wood.The main results are as follows:It was found that net uptake rate of NH₄⁺in segment II(35-70 mm far from apical tip)was lower than that in segment I(0-35 mm far from apical tip).NH₄⁺concentration,activity of glutamine synthase(GS),and concentrations of most amino acids,and organic acids involved in tricarboxylic acid cycle(TCA)in segment II were decreased when compared with those in segment I.Consistently,genes such as Arabidopsis H⁺-ATPase 2(AHA2)and AHA6 which encode plasma membrane(PM)H⁺-ATPase,GSR1 which encode glutamine synthetase,and delta1-pyrroline-5-carboxylate synthase 1(P5CS1)which involved in proline synthesis,and citrate synthase 4(CSY4)which implicated in citrate synthesis were downregulated in segment II.Overall,these results suggest that decreased NH₄⁺uptake rate in root segment far from apical tip may be related to the reduced N assimilation and TCA,as well as downregulated of related genes such as AHA2 in poplar.Low N resulted in decreased xylem width and cell layers of the xylem(the number of cells counted along the ray parenchyma on the stem cross section),narrower lumina of vessels and fibers,greater thickness of double fiber walls(the walls between two adjacent fiber cells),more hemicellulose and lignin deposition,and reduced cellulose accumulation in poplar wood.In line with these anatomical and chemical changes,a number of m RNAs,long noncoding RNAs(lnc RNAs),and micro RNAs(mi RNAs)were significantly differentially expressed.Mi RNA-lnc RNA/circ RNA-m RNA regulatory networks were identified in the wood of low N-treated poplars.Noticeably,up-regulated circ RNA392 caused increased transcriptional level of NFYA1-A/1-B/10 via the modulation of mi R169 members,probably leading to reduced xylem width and cell layers in low N-supplied poplar wood.Downregulation of MSTRG.4094.1 brought about decreased m RNA level of TIP1;3 via the mediation of MIR5021 family members,probably contributed to narrower cell lumina of vessel and fibers in the wood of low N-supplied P.tremula×P.alba.Overall,these results indicate that mi RNA-lnc RNA/circ RNA-m RNA networks are involved in decreasing cell layers of xylem and cellulose concentration,narrowing vessel element lumen,and increasing concentrations of hemicellulose and lignin in secondary xylem of poplars in acclimation to low N availability.The influences of N fertilization on wood anatomical and chemical properties of Populus tremula×P.alba were generally the opposite compared to the effects of low N on wood properties.Additionally,high light led to narrower vessel element lumen,thicker double fiber walls,longer fiber length,more cellulose and hemicellulose deposition,and decreased lignin accumulation in poplar wood.Proteomics analysis revealed that a number of proteins were differentially expressed in response to N fertilization and/or high light intensities.For instance,laccase 4(LAC4)and LAC5 involved in lignin biosynthesis were downexpressed,probably repressed the lignin monomer polymerization and resulted in decreased lignin concentration in N fertilization-supplied poplar wood.Phosphomannomutase(PMM)which implicated in hemicellulose biosynthesis was upregulated,probably improved the hemicellulose biosynthesis and resulted in increased hemicellulose concentration in poplar wood when treated with high light.Overall,these results indicate that N fertilization resulted in reduced lignin accumulation and thinner fiber wall,which are related to up-regulation of LAC4 in the wood of Populus tremula×P.alba.Hight light led to higher hemicellulose accumulation and thicker cell walls,which are related to up-regulation of PMM in the wood of Populus tremula×P.alba.These results suggest that in N poor soil,application of N fertilization can not only improve the productivity of wood,but also reduce lignin accumulation.In addition,reducing planting density can enhance light intensity in N fertilization soil.Raising light intensity can also improve the wood productivity,and can increase thickness of fiber cell wall and cellulose concentrations.Thicker fiber cell wall can not only increase the mechanical support for trees,but also enhance stress resistance of trees.Wood with higher cellulose and lower lignin concentrations is benefit for paper-making and second generation bioethanol.