Browsing by Author "Li, Hong"

Bark tissue of Populus × canescens can hyperaccumulate cadmium, but microstructural, transcriptomic, and physiological response mechanisms are poorly understood. Histochemical assays, transmission electron microscopic observations, energy-dispersive x-ray microanalysis, and transcriptomic and physiological analyses have been performed to enhance our understanding of cadmium accumulation and detoxification in P. × canescens. Cadmium was allocated to the phloem of the bark, and subcellular cadmium compartmentalization occurred mainly in vacuoles of phloem cells. Transcripts involved in microstructural alteration, changes in nutrition and primary metabolism, and stimulation of stress responses showed significantly differential expression in the bark of P. × canescens exposed to cadmium. About 48% of the differentially regulated transcripts formed a coregulation network in which 43 hub genes played a central role both in cross talk among distinct biological processes and in coordinating the transcriptomic regulation in the bark of P. × canescens in response to cadmium. The cadmium transcriptome in the bark of P. × canescens was mirrored by physiological readouts. Cadmium accumulation led to decreased total nitrogen, phosphorus, and calcium and increased sulfur in the bark. Cadmium inhibited photosynthesis, resulting in decreased carbohydrate levels. Cadmium induced oxidative stress and antioxidants, including free proline, soluble phenolics, ascorbate, and thiol compounds. These results suggest that orchestrated microstructural, transcriptomic, and physiological regulation may sustain cadmium hyperaccumulation in P. × canescens bark and provide new insights into engineering woody plants for phytoremediation.

Populus × euramericana (Pe) displays higher stable carbon isotope composition (δ13C) and intrinsic water use efficiency (WUEi) than Populus cathayana (Pc) under unlimited water conditions, rendering us to hypothesize that Pe is better acclimated to water deficiency than Pc. To examine this hypothesis, saplings of Pc and Pe were exposed to drought and subsequently re‐watered. Pc and Pe exhibited distinct anatomical, physiological and transcriptional responses in acclimation to drought and re‐watering, mainly due to stronger responsiveness of transcriptional regulation of genes encoding plasma membrane intrinsic proteins (PIPs), higher starch accumulation, δ13C, stable nitrogen isotope composition (δ15N) and WUEi, and lower reactive oxygen species (ROS) accumulation and scavenging in Pe. In acclimation to drought, both poplar genotypes demonstrated altered anatomical properties, declined height growth, differential expression of PIPs, activation of ABA signaling pathway, decreased total soluble sugars and starch, increased δ13C, δ15N and WUEi, and shifted homeostasis of ROS production and scavenging, and these changes can be recovered upon re‐watering. These data indicate that Pe is more tolerant to drought than Pc, and that anatomical, physiological and transcriptional acclimation to drought and re‐watering is essential for poplars to survive and grow under projected dry climate scenarios in the future.

Selection of poplar species with greater Cd tolerance and exploiting the physiological mechanisms involved in Cd tolerance are crucial for application of these species to phyto-remediation. The aim of this study is to investigate variation in Cd tolerance among the six poplar species and its underlying physiological mechanisms. Cuttings of six Populus species were cultivated for 10 weeks before exposure to either 0 or 200 μM CdSO4 for 20 days. Gas exchange in mature leaves was determined by a portable photosynthesis system. Cd concentrations in tissues were analyzed by a flame atomic absorbance spectrometry. Subsequently, Cd amount per plant, bio-concentration factor (BCF) and translocation factor (T f) were calculated. Nonenzymatic compounds and activities of antioxidative enzymes in tissues were analyzed spectrophotometrically. Cd exposure caused decline in photosynthesis in four poplar species including Populus cathayana (zhonghua 1). Among the six species, P. cathayana (zhonghua 1) displayed the highest Cd concentrations in tissues, the largest Cd amount in aerial parts, the highest BCF in aerial parts and T f under Cd exposure. Under Cd stress, increases in total soluble sugars in roots but decreases in starch in roots, wood, and leaves of P. cathayana (zhonghua 1) were found. Induced O 2 •− and H2O2 production in roots and leaves, and increases in free proline, soluble phenolics, and activities of antioxidative enzymes were observed in P. cathayana (zhonghua 1). Based on results of this pot experiment, it is concluded that P. cathayana (zhonghua 1) is superior to other five species for Cd phyto-remediation, and its well-coordinated physiological changes under Cd exposure confer the great Cd tolerance of this species.

To investigate influences of forest plantations on soil nutrient properties, biomass accumulation, major nutrient elements (NPK) and their stoichiometric couplings in different tissues and aged plants, and correlations between major nutrient contents in soils and in foliage of plants, 5-, 10-, 15- and 20-year-old plantations of black locust (Robinia pseudoacacia L.) and farmland were selected. Black locust plantations increased soil organic carbon (SOC) and N stocks by 23–327 and 23–119 %, respectively, in the 0–10 cm top soil layer compared to those in farmland. Soil C:N, C:P, C:K, N:P, N:K and P:K ratios were 10.1, 22.9, 0.7, 2.2, 0.7 and 0.03, respectively. These ratios were higher in the 0–10 cm soil layer than those in the 10–20 cm soil layer and increased under older plantations. Higher C contents in stem, N contents in leaf, the largest C pools in stem and N pools in root in 20-year-old plantation were observed. Correspondingly, the highest C:N, C:P and C:K and the lowest N:P and N:K ratios in stem, decreased C:N and C:P ratios in older trees were found. No strong correlations were observed between element contents in soils and in leaves of black locust trees. These results suggest that black locust plantations can increase soil nutrient concentrations, SOC and N stocks resulting in changes in element stoichiometric relations. CNPK contents and their stoichiometries vary with tissues and tree ages of black locust. No strong coupling relations exist between major nutrient element contents in the top soil and in foliage of black locust.

Ectomycorrhizas (EMs), which are symbiotic organs formed between tree roots and certain fungi, can mediate cadmium (Cd) tolerance of host plants, but the underlying physiological and molecular mechanisms are not fully understood. To investigate EMs mediated Cd tolerance in woody plants, Populus × canescens was inoculated with Paxillus involutus (strain MAJ) to establish mycorrhizal roots. Mycorrhizal poplars and non‐mycorrhizal controls were exposed to 0 or 50 μM CdSO4. EMs displayed higher net Cd2+ influx than non‐mycorrhizal roots. Net Cd2+ influx was coupled with net H+ efflux and inactivation of plasma membrane (PM) H+‐ATPases reduced Cd2+ uptake of EMs less than of non‐mycorrhizal roots. Consistent with higher Cd2+ uptake in EMs, in most cases, transcript levels of genes involved in Cd2+ uptake, transport and detoxification processes were increased in EMs compared to non‐mycorrhizal roots. Higher CO2 assimilation, improved nutrient and carbohydrate status, and alleviated oxidative stress were found in mycorrhizal compared to non‐mycorrhizal poplars despite higher Cd2+ accumulation. These results indicate that mycorrhizas increase Cd2+ uptake, probably by an enlarged root volume and overexpression of genes involved in Cd2+ uptake and transport, and concurrently enhance Po. × canescens Cd tolerance by increased detoxification, improved nutrient and carbohydrate status and defence preparedness.

A greenhouse experiment was conducted to study whether exogenous abscisic acid (ABA) mediates the responses of poplars to excess zinc (Zn). Populus × canescens seedlings were treated with either basal or excess Zn levels and either 0 or 10 μm ABA. Excess Zn led to reduced photosynthetic rates, increased Zn accumulation, induced foliar ABA and salicylic acid (SA), decreased foliar gibberellin (GA3) and auxin (IAA), elevated root H2O2 levels, and increased root ratios of glutathione (GSH) to GSSG and foliar ratios of ascorbate (ASC) to dehydroascorbate (DHA) in poplars. While exogenous ABA decreased foliar Zn concentrations with 7 d treatments, it increased levels of endogenous ABA, GA3 and SA in roots, and resulted in highly increased foliar ASC accumulation and ratios of ASC to DHA. The transcript levels of several genes involved in Zn uptake and detoxification, such as yellow stripe‐like family protein 2 (YSL2) and plant cadmium resistance protein 2 (PCR2), were enhanced in poplar roots by excess Zn but repressed by exogenous ABA application. These results suggest that exogenous ABA can decrease Zn concentrations in P. × canescens under excess Zn for 7 d, likely by modulating the transcript levels of key genes involved in Zn uptake and detoxification.

Abstract Background ATPase family AAA-domain containing protein 3A (ATAD3A) is a nuclear-encoded mitochondrial membrane-anchored protein involved in diverse processes including mitochondrial dynamics, mitochondrial DNA organization, and cholesterol metabolism. Biallelic deletions (null), recessive missense variants (hypomorph), and heterozygous missense variants or duplications (antimorph) in ATAD3A lead to neurological syndromes in humans. Methods To expand the mutational spectrum of ATAD3A variants and to provide functional interpretation of missense alleles in trans to deletion alleles, we performed exome sequencing for identification of single nucleotide variants (SNVs) and copy number variants (CNVs) in ATAD3A in individuals with neurological and mitochondrial phenotypes. A Drosophila Atad3a Gal4 knockin-null allele was generated using CRISPR-Cas9 genome editing technology to aid the interpretation of variants. Results We report 13 individuals from 8 unrelated families with biallelic ATAD3A variants. The variants included four missense variants inherited in trans to loss-of-function alleles (p.(Leu77Val), p.(Phe50Leu), p.(Arg170Trp), p.(Gly236Val)), a homozygous missense variant p.(Arg327Pro), and a heterozygous non-frameshift indel p.(Lys568del). Affected individuals exhibited findings previously associated with ATAD3A pathogenic variation, including developmental delay, hypotonia, congenital cataracts, hypertrophic cardiomyopathy, and cerebellar atrophy. Drosophila studies indicated that Phe50Leu, Gly236Val, Arg327Pro, and Lys568del are severe loss-of-function alleles leading to early developmental lethality. Further, we showed that Phe50Leu, Gly236Val, and Arg327Pro cause neurogenesis defects. On the contrary, Leu77Val and Arg170Trp are partial loss-of-function alleles that cause progressive locomotion defects and whose expression leads to an increase in autophagy and mitophagy in adult muscles. Conclusion Our findings expand the allelic spectrum of ATAD3A variants and exemplify the use of a functional assay in Drosophila to aid variant interpretation.

Nitrogen (N) starvation and excess have distinct effects on N uptake and metabolism in poplars, but the global transcriptomic changes underlying morphological and physiological acclimation to altered N availability are unknown. We found that N starvation stimulated the fine root length and surface area by 54 and 49%, respectively, decreased the net photosynthetic rate by 15% and reduced the concentrations of NH4+,NO3− and total free amino acids in the roots and leaves of Populus simonii Carr. in comparison with normal N supply, whereas N excess had the opposite effect in most cases. Global transcriptome analysis of roots and leaves elucidated the specific molecular responses to N starvation and excess. Under N starvation and excess, gene ontology (GO) terms related to ion transport and response to auxin stimulus were enriched in roots, whereas the GO term for response to abscisic acid stimulus was overrepresented in leaves. Common GO terms for all N treatments in roots and leaves were related to development, N metabolism, response to stress and hormone stimulus. Approximately 30–40% of the differentially expressed genes formed a transcriptomic regulatory network under each condition. These results suggest that global transcriptomic reprogramming plays a key role in the morphological and physiological acclimation of poplar roots and leaves to N starvation and excess.

To investigate how N-fertilization affects the growth, carbon and nitrogen (N) physiology, and wood properties of poplars with contrasting growth characteristics, slow-growing (Populus popularis, Pp) and fast-growing (P. alba×P. glandulosa, Pg) poplar saplings were exposed to different N levels. Above-ground biomass, leaf area, photosynthetic rates (A), instantaneous photosynthetic nitrogen use efficiency (PNUEi), chlorophyll and foliar sugar concentrations were higher in Pg than in Pp. Foliar nitrate reductase (NR) activities and root glutamate synthase (GOGAT) activities were higher in Pg than in Pp as were the N amount and NUE of new shoots. Lignin contents and calorific values of Pg wood were less than that of Pp wood. N-fertilization reduced root biomass of Pg more than of Pp, but increased leaf biomass, leaf area, A, and PNUEi of Pg more than of Pp. Among 13 genes involved in the transport of ammonium or nitrate or in N assimilation, transcripts showed more pronounced changes to N-fertilization in Pg than in Pp. Increases in NR activities and N contents due to N-fertilization were larger in Pg than in Pp. In both species, N-fertilization resulted in lower calorific values as well as shorter and wider vessel elements/fibres. These results suggest that growth, carbon and N physiology, and wood properties are more sensitive to increasing N availability in fast-growing poplars than in slow-growing ones, which is probably due to prioritized resource allocation to the leaves and accelerated N physiological processes in fast-growing poplars under higher N levels.

To characterize the dynamics of Cd2+ flux in the rhizosphere and to study cadmium (Cd) plant‐internal partitioning in roots, wood, bark and leaves in relation to energy metabolism, reactive oxygen species (ROS) formation and antioxidants, Populus × canescens plantlets were exposed to either 0 or 50 µM CdSO4 for up to 20 days in the nutrient solution. A strong net Cd2+ influx in root apex was observed after Cd exposure for 24 h, even if net Cd2+ influx decreased gradually in roots. A large amount of Cd was accumulated in roots. Cd ions were uploaded via the xylem to leaves and further transported to the phloem where significant accumulation was detected. Cd accumulation led to decreased photosynthetic carbon assimilation but not to the depletion in soluble carbohydrates. Increased levels of ROS were present in all tissues, except the bark of Cd‐exposed poplars. To combat Cd‐induced superoxide and hydrogen peroxide, P.×canescens appeared to rely mainly on the formation of soluble phenolics as these compounds showed the highest accumulation in the bark and the lowest in wood. Other potential radical scavengers such as proline, sugar alcohols and antioxidant enzymes showed tissue‐ and exposure time‐specific responses to Cd. These results indicate a complex pattern of internal Cd allocation in P.×canescens resulting in higher ROS stress in wood than in bark and intermediate responses in roots and leaves, probably because of differential capacities of these tissues for the production of protective phenolic compounds.

Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH4 +) and/or nitrate (NO3 −) to improve yields. However, little is known about net NH4 +/NO3 − fluxes and their relation with H+ fluxes in poplar roots. In this study, net NH4 +/NO3 − fluxes in association with H+ fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH4 + and NO3 − fluxes was found along root tips of P. popularis. The maximal net uptake of NH4 + and NO3 − occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH4 + uptake was induced by ca. 48 % with provision of NO3 − together, but net NO3 − uptake was inhibited by ca. 39 % with the presence of NH4 + in poplar roots. Furthermore, inactivation of plasma membrane (PM) H+-ATPases by orthovanadate markedly inhibited net NH4 +/NO3 − uptake and even led to net NH4 + release with NO3 − co-provision. Linear correlations were observed between net NH4 +/NO3 − and H+ fluxes in poplar roots except that no correlation was found between net NH4 + and H+ fluxes in roots exposed to NH4Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH4 +/NO3 − uptake and that net NH4 +/NO3 − fluxes and the interaction of net fluxes of both ions are tightly associated with H+ fluxes in poplar roots.

To investigate N metabolism of two contrasting Populus species in acclimation to low N availability, saplings of slow-growing species (Populus popularis, Pp) and a fast-growing species (Populus alba × Populus glandulosa, Pg) were exposed to 10, 100, or 1000 μM NH4NO3. Despite greater root biomass and fine root surface area in Pp, lower net influxes of NH4+ and NO3– at the root surface were detected in Pp compared to those in Pg, corresponding well to lower NH4+ and NO3– content and total N concentration in Pp roots. Meanwhile, higher stable N isotope composition (δ15N) in roots and stronger responsiveness of transcriptional regulation of 18 genes involved in N metabolism were found in roots and leaves of Pp compared to those of Pg. These results indicate that the N metabolism of Pp is more sensitive to decreasing N availability than that of Pg. In both species, low N treatments decreased net influxes of NH4+ and NO3–, root NH4+ and foliar NO3– content, root NR activities, total N concentration in roots and leaves, and transcript levels of most ammonium (AMTs) and nitrate (NRTs) transporter genes in leaves and genes involved in N assimilation in roots and leaves. Low N availability increased fine root surface area, foliar starch concentration, δ15N in roots and leaves, and transcript abundance of several AMTs (e.g. AMT1;2) and NRTs (e.g. NRT1;2 and NRT2;4B) in roots of both species. These data indicate that poplar species slow down processes of N acquisition and assimilation in acclimation to limiting N supply.

Overexpression of bacterial γ‐glutamylcysteine synthetase in the cytosol of Populus tremula × P. alba produces higher glutathione (GSH) concentrations in leaves, thereby indicating the potential for cadmium (Cd) phytoremediation. However, the net Cd2+ influx in association with H+/Ca2+, Cd tolerance, and the underlying molecular and physiological mechanisms are uncharacterized in these poplars. We assessed net Cd2+ influx, Cd tolerance and the transcriptional regulation of several genes involved in Cd2+ transport and detoxification in wild‐type and transgenic poplars. Poplars exhibited highest net Cd2+ influxes into roots at pH 5.5 and 0.1 mM Ca2+. Transgenics had higher Cd2+ uptake rates and elevated transcript levels of several genes involved in Cd2+ transport and detoxification compared with wild‐type poplars. Transgenics exhibited greater Cd accumulation in the aerial parts than wild‐type plants in response to Cd2+ exposure. Moreover, transgenic poplars had lower concentrations of O2˙− and H2O2; higher concentrations of total thiols, GSH and oxidized GSH in roots and/or leaves; and stimulated foliar GSH reductase activity compared with wild‐type plants. These results indicate that transgenics are more tolerant of 100 μM Cd2+ than wild‐type plants, probably due to the GSH‐mediated induction of the transcription of genes involved in Cd2+ transport and detoxification.

Phosphorus (P) and nitrogen (N) are the two essential macronutrients for tree growth and development. 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 Populus simonii Carr (Ps) and the fast-growing Populus × euramericana Dode (Pe) to complete nutrients or starvation of P, N or both elements (NP). P. × euramericana had lower P and N concentrations and greater P and N amounts due to higher biomass production, thereby resulting in greater phosphorus use efficiency/N use efficiency (PUE/NUE) compared with Ps. Compared with the roots of Ps, the roots of Pe exhibited higher enzymatic activities in terms of acid phosphatases (APs) and malate dehydrogenase (MDH), which are involved in P mobilization, and nitrate reductase (NR), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH), which participate in N assimilation. The responsiveness of the transcriptional regulation of key genes encoding transporters for phosphate, ammonium and nitrate was stronger in Pe than in Ps. These results suggest that Pe possesses a higher capacity for P/N uptake and assimilation, which promote faster growth compared with Ps. In both poplars, P or NP starvation caused significant decreases in the P concentrations and increases in PUE. Phosphorus deprivation induced the activity levels of APs, phosphoenolpyruvate carboxylase and MDH in both genotypes. Nitrogen or NP deficiency resulted in lower N concentrations, amino acid levels, NR and GOGAT activities, and higher NUE in both poplars. Thus, in Ps and Pe, 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 suppress P/N uptake, mobilization and assimilation during acclimation to P, N or NP starvation.

To elucidate the physiological and transcriptional regulatory mechanisms that underlie the responses of poplars to high temperature (HT) and/or drought in woody plants, we exposed Populus alba × Populus tremula var. glandulosa saplings to ambient temperature (AT) or HT under 80 or 40% field capacities (FC), or no watering. HT increased the foliar total carbon (C) concentrations, and foliar δ13C and δ18O. HT triggered heat stress signaling via increasing levels of abscisic acid (ABA) and indole‐3‐acetic acid (IAA) in poplar roots and leaves. After perception of HT, poplars initiated osmotic adjustment by increasing foliar sucrose and root galactose levels. In agreement with the HT‐induced heat stress and the changes in the levels of ABA and carbohydrates, we detected increased transcript levels of HSP18 and HSP21, as well as NCED3 in the roots and leaves, and the sugar transporter gene STP14 in the roots. Compared with AT, drought induced greater enhancement of foliar δ13C and δ18O in poplars at HT. Similarly, drought caused greater stimulation of the ABA and foliar glucose levels in poplars at HT than at AT. Correspondingly, desiccation led to greater increases in the mRNA levels of HSP18, HSP21, NCED3, STP14 and INT1 in poplar roots at HT than at AT. These results suggest that HT has detrimental effects on physiological processes and it induces the transcriptional regulation of key genes involved in heat stress responses, ABA biosynthesis and sugar transport and HT can cause greater changes in drought‐induced physiological and transcriptional responses in poplar roots and leaves.

To dissect the salt responsive genes in the roots of Populus x canescens after salt exposure and to characterize the conserved transcripts with differential expression in the roots of P. x canescens and Arabidopsis thaliana under salinity, P. x canescens grown in sandy soil was exposed to 150 mM NaCl for 18 days and the transcriptional profiling was analyzed in the roots by using a whole genome poplar array chip. The raw data of the Affymetrix Arabidopsis genome arrays were obtained from the online array data bank and statistically analysed as that of the poplar. In the roots of P. x canescens exposed to salinity, about 860 genes displayed significantly differential expression, including 647 up-regulated and 213 down-expressed genes. In the roots of A. thaliana,1292 genes were up- and 718 down-expressed. Among the differentially expressed genes between P. x canescens and A. thaliana, a set of common genes (128 genes) showed the same change pattern in response to salinity, including 114 induced and 14 repressed genes. A salt-responsive co-expression network was constructed with 98 common genes. Among the co-expressed genes, 22 genes were defined as hub genes which were involved in fundamental biological processes such as abiotic stimulus and signal transduction. Moreover, the cis-regulatory elements were found in the conserved motifs of hub genes. These results suggest that P. x canescens and A. thaliana possess conserved salt-responsive genes and that cis-elements in the conserved motifs of hub genes play a crucial role in coordinating the co-expression of the common genes underlying the physiological acclimation to salinity in the roots of P. x canescens and A. thaliana. (C) 2015 Elsevier B.V. All rights reserved.

Ectomycorrhizas (EMs) are mutualistic associations between certain soil fungi and higher plants. EMs can modulate the cellular, physiological and molecular processes of host plants, resulting in altered responses of the colonized plants to heavy metals. Progress in elucidating the role of EMs in modulating heavy metal tolerance of host trees is reviewed. In the last decade, a number of ectomycorrhizal fungal isolates and host plants have been characterized for their tolerance to heavy metals. Additionally, the cellular processes have been investigated with regard to heavy metal uptake, transport, distribution, toxicity and detoxification by ectomycorrhizal fungi and/or host plants. At the cellular level, mechanisms of heavy metal detoxification include (i) binding of heavy metals to cell wall and extracellular exudates, (ii) decreased uptake and/or pumping metal ions out of cytosol, (iii) chelation of metal ions in cytosol, (iv) compartmentation of metals in vacuoles or other subcellular structures, and (v) repair of damaged biomolecules. The efficiency of these protective measures is often increased by EMs, resulting in improved physiological status and rescued growth. While physiological and cellular responses to heavy metals have been well studied, experimental data on the underlying molecular mechanisms, especially those induced by the interaction of ectomycorrhizal fungi and hosts, are scattered. Progress in genome sequencing of EM partners has revealed the importance of metal transporters in mediating tolerance. A better understanding of the molecular mechanisms is essential for effective application of selected fungal isolates and hosts to improve the efficiency of bioremediation on heavy metal polluted sites.