Characterization and primary functional analysis of a bamboo NAC gene targeted by miR164b

Alternative Splicing of PheNAC23 from Moso Bamboo Impacts Flowering Regulation and Drought Tolerance in Transgenic Arabidopsis

NAC (NAM, ATAF, and CUC) transcription factors are essential in regulating plant stress response and senescence, with their functions being modulated by alternative splicing. The molecular mechanisms of stress-induced premature flowering and drought tolerance in Phyllostachys edulis (moso bamboo) are not yet fully understood. In this study, a novel NAC variant derived from PheNAC23, named PheNAC23ES, was isolated. PheNAC23ES exhibited distinct expression patterns compared to PheNAC23 during leaf senescence and drought stress response. Overexpression of PheNAC23 promoted flowering and reduced its tolerance to drought stress in Arabidopsis thaliana (A. thaliana). However, overexpression of PheNAC23ES exhibited the opposite functions. PheNAC23 was localized in the nucleus and had transactivation activity, while PheNAC23ES had a similar localization to the control green fluorescent protein and no transactivation activity. Further functional analysis revealed that PheNAC23ES could interact with PheNAC23, suggesting that PheNAC23ES might serve as a small interfering peptide that affects the function of PheNAC23 by binding to it.

Regulation of endogenous hormone and miRNA in leaves of alfalfa (Medicago sativa L.) seedlings under drought stress by endogenous nitric oxide

BACKGROUND

Alfalfa (Medicago sativa. L) is one of the best leguminous herbage in China and even in the world, with high nutritional and ecological value. However, one of the drawbacks of alfalfa is its sensitivity to dry conditions, which is a global agricultural problem. The objective of this study was to investigate the regulatory effects of endogenous nitric oxide (NO) on endogenous hormones and related miRNAs in alfalfa seedling leaves under drought stress. The effects of endogenous NO on endogenous hormones such as ABA, GA3, SA, and IAA in alfalfa leaves under drought stress were studied. In addition, high-throughput sequencing technology was used to identify drought-related miRNAs and endogenous NO-responsive miRNAs in alfalfa seedling leaves under drought stress.

RESULT

By measuring the contents of four endogenous hormones in alfalfa leaves, it was found that endogenous NO could regulate plant growth and stress resistance by inducing the metabolism levels of IAA, ABA, GA3, and SA in alfalfa, especially ABA and SA in alfalfa. In addition, small RNA sequencing technology and bioinformatics methods were used to analyze endogenous NO-responsive miRNAs under drought stress. It was found that most miRNAs were enriched in biological pathways and molecular functions related to hormones (ABA, ETH, and JA), phenylpropane metabolism, and plant stress tolerance.

CONCLUSION

In this study, the analysis of endogenous hormone signals and miRNAs in alfalfa leaves under PEG and PEG + cPTIO conditions provided an important basis for endogenous NO to improve the drought resistance of alfalfa at the physiological and molecular levels. It has important scientific value and practical significance for endogenous NO to improve plant drought resistance.

Multi-omics of Circular RNAs and Their Responses to Hormones in Moso Bamboo (Phyllostachys edulis)

Circular RNAs (circRNAs) are endogenous non-coding RNAs with covalently closed structures, which have important functions in plants. However, their biogenesis, degradation, and function upon treatment with gibberellins (GAs) and auxins (1-naphthaleneacetic acid, NAA) remain unknown. Here, we systematically identified and characterized the expression patterns, evolutionary conservation, genomic features, and internal structures of circRNAs using RNase R-treated libraries from moso bamboo (Phyllostachys edulis) seedlings. Moreover, we investigated the biogenesis of circRNAs dependent on both cis- and trans-regulation. We explored the function of circRNAs, including their roles in regulating microRNA (miRNA)-related genes and modulating the alternative splicing of their linear counterparts. Importantly, we developed a customized degradome sequencing approach to detect miRNA-mediated cleavage of circRNAs. Finally, we presented a comprehensive view of the participation of circRNAs in the regulation of hormone metabolism upon treatment of bamboo seedlings with GA and NAA. Collectively, our study provides insights into the biogenesis, function, and miRNA-mediated degradation of circRNAs in moso bamboo.

Identification and differential analysis of noncoding RNAs in response to drought in Phyllostachys aureosulcata f. spectabilis

The role of noncoding RNAs (ncRNAs) in plant resistance to abiotic stresses is increasingly being discovered. Drought stress is one of the most common stresses that affecting plant growth, and high intensity drought has a significant impact on the normal growth of plants. In this study, a high-throughput sequencing was performed on plant tissue samples of Phyllostachys aureosulcata f. spectabilis C. D. Chu et C. S. Chao by drought treatment for 0, 2, 4 and 6 days. The sequencing results were analysed bioinformatically. We detected 336,946 RNAs among all 12 samples, including 192,098 message RNAs (mRNAs), 142,761 long noncoding RNAs (lncRNAs), 1,670 circular RNAs (circRNAs), and 417 microRNAs (miRNAs). We detected 2,419 differentially expressed (DE) ncRNAs, including 213 DE circRNAs, 2,088 DE lncRNAs and 118 DE miRNAs. Then, we used Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) to functionally predict DE ncRNAs. The results showed that most DE ncRNAs are involved in the response to drought stress, mainly in biochemical reactions involved in some metabolites, as well as in organelle activities. In addition, we validated two random circRNAs and demonstrated their circularity. We also found a stable internal reference gene available for Phyllostachys aureosulcata f. spectabilis and validated the accuracy of this experiment by quantitative real-time polymerase chain reaction (qRT-PCR).

Regulatory short RNAs: A decade's tale for manipulating salt tolerance in plants

Salt stress is a globally increasing environmental detriment to crop growth and productivity. Exposure to salt stress evokes a complex medley of cellular signals, which rapidly reprogram transcriptional and metabolic networks to shape plant phenotype. To date, genetic engineering approaches were used with success to enhance salt tolerance; however, their performance is yet to be evaluated under realistic field conditions. Regulatory short non-coding RNAs (rsRNAs) are emerging as next-generation candidates for engineering salt tolerance in crops. In view of this, the present review provides a comprehensive analysis of a decade's worth of functional studies on non-coding RNAs involved in salt tolerance. Further, we have integrated this knowledge of rsRNA-mediated regulation with the current paradigm of salt tolerance to highlight two regulatory complexes (RCs) for regulating salt tolerance in plants. Finally, a knowledge-driven roadmap is proposed to judiciously utilize RC component(s) for enhancing salt tolerance in crops.

A regulatory network driving shoot lignification in rapidly growing bamboo

Woody bamboo is environmentally friendly, abundant, and an alternative to conventional timber. Degree of lignification and lignin content and deposition affect timber properties. However, the lignification regulatory network in monocots is poorly understood. To elucidate the regulatory mechanism of lignification in moso bamboo (Phyllostachys edulis), we conducted integrated analyses using transcriptome, small RNA, and degradome sequencing followed by experimental verification. The lignification degree and lignin content increased with increased bamboo shoot height, whereas phenylalanine ammonia-lyase and Laccase activities first increased and then decreased with shoot growth. Moreover, we identified 11,504 differentially expressed genes (DEGs) in different portions of the 13th internodes of different height shoots; most DEGs associated with cell wall and lignin biosynthesis were upregulated, whereas some DEGs related to cell growth were downregulated. We identified a total of 1,502 miRNAs, of which 687 were differentially expressed. Additionally, in silico and degradome analyses indicated that 5,756 genes were targeted by 691 miRNAs. We constructed a regulatory network of lignification, including 11 miRNAs, 22 transcription factors, and 36 enzyme genes, in moso bamboo. Furthermore, PeLAC20 overexpression increased lignin content in transgenic Arabidopsis (Arabidopsis thaliana) plants. Finally, we proposed a reliable miRNA-mediated "MYB-PeLAC20" module for lignin monomer polymerization. Our findings provide definite insights into the genetic regulation of bamboo lignification. In addition to providing a platform for understanding related mechanisms in other monocots, these insights could be used to develop strategies to improve bamboo timber properties.

A comprehensive overview of miRNA targeting drought stress resistance in plants

MicroRNAs (miRNAs) are essential nonprotein-coding genes. In a range of organisms, miRNAs has been reported to play an essential role in regulating gene expressions at post-transcriptional level. They participate in most of the stress responsive processes in plants. Drought is an ultimate abiotic stress that affects the crop production. Therefore understanding drought stress responses are essential to improve the production of agricultural crops. Throughout evolution, plants have developed their own defense systems to cope with the adversities of environmental stresses. Among defensive mechanisms include the regulations of gene expression by miRNAs. Drought stress regulates the expression of some of the functionally conserved miRNAs in different plants. The given properties of miRNAs provide an insight to genetic alterations and enhancing drought resistance in cereal crops. The current review gives a summary to regulatory mechanisms in plants as well as miRNAs response to drought stresses in cereal crops. Some possible approaches and guidelines for the exploitation of drought stress miRNA responses to improve cereal crops are also described.

A natural antisense transcript acts as a negative regulator for the maize drought stress response gene ZmNAC48

Although plant-specific NAC transcription factors play crucial roles in response to abiotic stress, few reports describe the regulation of NAC genes in maize (Zea mays) by the cis-natural antisense transcripts (cis-NATs). In this study, 521 NAC genes from Gramineae were classified, of which 51 NAC genes contained cis-NATs. ZmNAC48 and cis-NATZmNAC48 co-localized to the same cell nucleus, and both transcripts responded to drought stress. Arabidopsis plants overexpressing ZmNAC48 had improved drought tolerance, lower rate of water loss, enhanced stomatal closure, and higher rates of survival. Transient expression in both maize protoplasts and tobacco leaves indicated that cis-NATZmNAC48 reduced ZmNAC48 expression. Western blotting and ribosome profiling analyses confirmed that cis-NATZmNAC48 lacked protein coding potential. Furthermore, the cis-NAT-derived small-interfering RNAs (nat-siRNAs) generated from the overlapping regions of ZmNAC48 and cis-NATZmNAC48 were detected in maize and transgenic Arabidopsis. Cis-NATZmNAC48 overexpressing maize showed higher water loss rate, increased stomatal opening, and had more dead leaves. Expression of ZmNAC48 and nat-siRNA was decreased in these plants. Taken together, our study indicates that both ZmNAC48 and cis-NATZmNAC48 are involved in plant drought stress responses, and that the double-stranded RNA-dependent mechanism is involved in the interaction between cis-NATZmNAC48 and ZmNAC48. Additionally, cis-NATZmNAC48 may negatively regulate ZmNAC48 to affect stomatal closure of maize.

PeSNAC-1 a NAC transcription factor from moso bamboo (Phyllostachys edulis) confers tolerance to salinity and drought stress in transgenic rice

NAC (NAM, AFAT and CUC) proteins play necessary roles in plant response to environmental stresses. However, the functional roles of NAC genes in moso bamboo (Phyllostachys edulis), an essential economic perennial woody bamboo species, are not well documented. In this study, we retrieved 152 PeNAC genes from the moso bamboo V2 genome, and PeSNAC-1 was isolated and functionally characterized. PeSNAC-1 was localized in the nucleus and had no transactivation activity in yeast. PeSNAC-1 extremely expressed in rhizome and young roots (0.1 and 0.5 cm) and was significantly induced by drought and salt treatments but repressed by abscisic acid (ABA), methyl jasmonate and high temperature (42 °C) in moso bamboo. Under water shortage and salinity conditions, survival ratios, Fv/Fm values, physiological indexes such as activities of superoxide dismutase, peroxidase and catalase and contents of malondialdehyde, H2O2 and proline were significantly higher in transgenic rice than the wild type, which suggests enhanced tolerance to drought and salt stress in PeSANC-1 overexpressed plants. Transcript levels of Na+/H+ antiporter and Na+ transporter genes (OsSOS1, OsNHX1 and OsHKT1;5), ABA signaling and biosynthesis genes (OsABI2, OsRAB16, OsPP2C68, OsLEA3-1, OsLEA3, OsNCED3, OsNCED4 and OsNCED5) and ABA-independent genes (OsDREB1A, OsDREB1B and OsDREB2A) were substantially higher in transgenic as compared with the wild type. Moreover, protein interaction analysis revealed that PeSNAC-1 could interact with stress responsive PeSNAC-2/4 and PeNAP-1/4/5 in both yeast and plant cells, which indicates a synergistic effect of those proteins in regulating the moso bamboo stress response. Our data demonstrate that PeSNAC-1 likely improved salt and drought stress tolerance via modulating gene regulation in both ABA-dependent and independent signaling pathways in transgenic rice. In addition, PeSNAC-1 functions as an important positive stress regulator in moso bamboo, participating in PeSNAC-1 and PeSNAC-2/4 or PeSNAC-1 and PeNAP-1/4/5 interaction networks.

Stress induced dynamic adjustment of conserved miR164:NAC module

Aims including the rationale

Salinity and drought are the two major stresses limiting the productivity of economically important crops such as Glycine max (soybean). The incidence of these stresses during the pod development stages affects the quality and quantity of seeds, which compromise the yield of soybean. The miR164:NAC module has been shown to play a critical role in regulating the response to salt and drought stress in several plant species. However, biological role of miR164:NAC module in salt stress in soybean is not fully understood.

Methods

In this study, we identified 215 salt responsive miRNAs, using miScript miRNA array with a sensitive and a tolerant soybean genotype, William82 and INCASoy36, respectively. The targets of these salt regulated miRNAs were searched in the degradome datasets.

Key results

It was found that four salt stress deregulated miRNAs targeted the NAC transcription factor and among these miR164k and miR408d showed antagonistic expression in the two soybean genotypes. The expression of miR164k was higher in salt tolerant INCASoy36 as compared to salt sensitive William82, under unstressed conditions. However under salt stress, miR164k was downregulated in INCASoy36 (-2.65 fold), whereas it was upregulated in William82 (4.68 fold). A transient co-expression assay validated that gma-miR164k directs the cleavage of GmNAC1 transcript. Bioinformatics analysis revealed that the regulation of NAC transcription factor family by members of miR164 family is conserved across many species. The dynamic expression profiles of miR164 and NAC-TFs were captured in different tissues of rice, tobacco, and two soybean genotypes under drought and salt stress conditions.

Main conclusion

Collectively, our results suggest that genetically determined dynamic modulation of the conserved miR164:NAC-TF module may play an important role in determining the adaptive response of plants to stress.

Genome-wide analysis of laccase genes in moso bamboo highlights PeLAC10 involved in lignin biosynthesis and in response to abiotic stresses

Twenty-three PeLACs have been identified in moso bamboo, overexpression of PeLAC10 increases the lignin content and confers drought and phenolic acid tolerance in transgenic Arabidopsis. Laccases (LACs) have multifunction involved in the processes of cell elongation, lignification and stress response in plants. However, the function of laccases in bamboo remain unclear. Here, a total of 23 laccase genes (PeLAC1-PeLAC23) were identified in moso bamboo (Phyllostachys edulis). The diverse gene structure and expression pattern of PeLACs suggested that their function should be spatiotemporal and complicated, which was supported by the expression profiles in different tissues of moso bamboo. Eighteen PeLACs were identified as the targets of ped-miR397. The putative ped-miR397-binding site in the coding region of PeLAC10 was further confirmed by RLM-5' RACE, indicating that PeLAC10 was regulated by ped-miR397 after transcription. With the increasing shoot height, the expression abundance of PeLAC10 was up-regulated and reached the maximum in 15 cm shoots, while that of ped-miR397 was relative lower and showed the minimum in 15 cm shoots. PeLAC10 was up-regulated obviously under both ABA (100 μmol L^-1) and NaCl (400 mmol L^-1) treatments, and it was down-regulated under the GA₃ (100 μmol L^-1) treatment. The transgenic Arabidopsis plants over-expressing PeLAC10 became slightly smaller and their petioles were shorter than those of Col-0. However, they had a stronger capacity in resistance to phenolic acids and drought besides higher lignin content in stems. These results indicated that overexpression of PeLAC10 was helpful to increase the content of lignin in transgenic Arabidopsis and improve the adaptability to phenolic acid and drought stresses.

Overexpression of PheNAC3 from moso bamboo promotes leaf senescence and enhances abiotic stress tolerance in Arabidopsis

The NAC family is one of the largest transcription factor families unique to plants, which regulates the growth and development, biotic and abiotic stress responses, and maturation and senescence in plants. In this study, PheNAC3, a NAC gene, was isolated and characterized from moso bamboo (Phyllostachys edulis). PheNAC3 belong to the NAC1 subgroup and has a conserved NAC domain on the N-terminus, which with 88.74% similarity to ONAC011 protein. PheNAC3 localized in the nucleus and exhibited transactivation activity. PheNAC3 was upregulated during the process of senescence of leaves and detected shoots. PheNAC3 was also induced by ABA, MeJA, NaCl and darkness, but it had no remarkable response to PEG and SA treatments. Overexpression of PheNAC3 could cause precocious senescence in Arabidopsis. Transgenic Arabidopsis displayed faster seed germination, better seedling growth, and a higher survival rate than the wild-type under salt or drought stress conditions. Moreover, AtSAG12 associated with senescence and AtRD29A and AtRD29b related to ABA were upregulated by PheNAC3 overexpression, but AtCAB was inhibited. These findings show that PheNAC3 may participate in leaf senescence and play critical roles in the salt and drought stress response.

The peu-miR160a-PeARF17.1/PeARF17.2 module participates in the adventitious root development of poplar

Deep roots give rise to flourishing leaves, and the two complement each other. However, the genetic mechanisms underlying adventitious rooting for forest trees have remained elusive. In this study, we verified that peu-miR160a targets six poplar genes AUXIN RESPONSE FACTORS (ARFs), PeARF10.1, PeARF16.1, PeARF16.2, PeARF16.3, PeARF17.1 and PeARF17.2, using 5'RLM-RACE. Interaction experiments with peu-miR160a and PeARFs in poplar protoplasts further confirmed that peu-miR160a targets and negatively regulates the six PeARFs. Peu-miR160a and its target genes exhibited obvious temporal expression in different stages of adventitious root development, and they could also be induced by IAA and abscisic acid. Peu-miR160a-overexpressing lines exhibited a significant shortening of adventitious root length, an increase in the number of lateral roots, severe dwarfing and shortened internodes. In addition, the overexpression of PeARF17.1 or mPeARF17.2 (peu-miR160a-resistant version of PeARF17.2) significantly increased the number of adventitious roots. Furthermore, PeARF17.1-overexpressing lines had multiple branches with no visible trunk, although the adventitious root length of the PeARF17.1-overexpressing lines was significantly increased. Our findings reveal that the peu-miR160a - PeARF17.1/PeARF17.2 module is an important regulator involved in the development of the adventitious roots of poplar.

Genome-Wide Investigation of the NAC Gene Family and Its Potential Association with the Secondary Cell Wall in Moso Bamboo

NAC (NAM, ATAF, and CUC) transcription factors (TFs) are implicated in the transcriptional regulation of diverse processes and have been characterized in a number of plant species. However, NAC TFs are still not well understood in bamboo, especially their potential association with the secondary cell wall (SCW). Here, 94 PeNACs were identified and characterized in moso bamboo (Phyllostachys edulis). Based on their gene structures and conserved motifs, the PeNACs were divided into 11 groups according to their homologs in Arabidopsis. PeNACs were expressed variously in different tissues of moso bamboo, suggesting their functional diversity. Fifteen PeNACs associated with the SCW were selected for co-expression analysis and validation. It was predicted that 396 genes were co-expressed with the 15 PeNACs, in which 16 and 55 genes were involved in the lignin catabolic process and cellulose biosynthetic process respectively. As the degree of lignification in the growing bamboo shoots increased, all 15 PeNACs were upregulated with a trend of rising first and then decreasing except PeNAC37, which increased continuously. These results indicated that these PeNACs might play important roles in SCW biosynthesis and lignification in bamboo shoots. Seven of 15 PeNACs had been found positively co-expressed with seven PeMYBs, and they had similar expression patterns with those of the PeMYBs in bamboo shoots. The targeted sites of miR164 were found in 16 PeNACs, of which three PeNACs associated with SCW were validated to have an opposite expression trend to that of miR164 in growing bamboo shoots. In addition, three PeNACs were selected and verified to have self-activation activities. These results provide comprehensive information of the NAC gene family in moso bamboo, which will be helpful for further functional studies of PeNACs to reveal the molecular regulatory mechanisms of bamboo wood property.

Genome-wide identification of growth-regulating factors in moso bamboo (Phyllostachys edulis): in silico and experimental analyses

Growth-regulating factor (GRF), a small plant-specific transcription factor (TF) family, is extensively involved in the regulation of growth and developmental processes. However, the GRF family has not been comprehensively studied in moso bamboo (Phyllostachys edulis), a typical non-timber forest member. Here, 18 GRF genes were identified and characterized from the moso bamboo genome, and they clustered into three subfamilies (A, B and C). PeGRF genes were analyzed to determine their gene structures, conserved motifs and promoter. The non-synonymous/synonymous substitution ratios of paralogous and orthologous were less than 1, indicating that the GRF family mainly experienced purifying selection during evolution. According to the analysis of tissue-specific expression patterns, the participation of moso bamboo GRFs might be required during the formation and development of these five tissues. Moreover, PeGRF proteins might be involved in the regulation of plant development in biological processes. The qRT-PCR analysis demonstrated that PeGRF genes played essential roles in combating hormonal stresses and they might be involved in hormone regulation. PeGRF11, a nuclear localized protein as assessed by a subcellular localization assay, could interact with PeGIF3 in yeast and in planta according to yeast two-hybridization and bimolecular fluorescence complementation assays (BiFC) assays. But PeGRF11, as a TF, had no transcriptional activity in yeast. These results provide useful information for future functional research on the GRF genes in moso bamboo.

Identification of novel miRNAs and their target genes from Populus szechuanica infected with Melampsora larici-populina

Two novel miRNAs were selected from a pre-constructed RNA library of Populus szechuanica infected with the foliar rust fungus Melampsora larici-populina in order to detect the genes regulated as targets of the miRNAs novel_mir_11 and novel_mir_357. The novel miRNAs were identified from P. szechuanica using stem-loop methods and their precursors were able to fold into a complete stem loop structure. The predicted target genes of the novel miRNAs were verified with RNA ligase-mediated 5' rapid amplification of cDNA ends (RLM-5'RACE). The full-length sequences of target genes, RPM1 and RPS2/5, in P. szechuanica were obtained through rapid amplification of cDNA ends (RACE) and officially named PsRPM1 and PsRPS2/5. These genes contain nucleotide binding site-leucine-rich repeats (NBS-LRR) domains typical of resistance genes. The expression levels of miRNAs and their target genes in different periods post infection were analysed with quantitative real-time PCR (qRT-PCR). After infection with the foliar rust fungus, the expression levels of the novel miRNAs and their target genes were dynamic. Both novel_mir_11 and novel_mir_357 negatively regulated the expression of their target genes. In this study, the regulatory effects of two novel miRNAs through their target genes were characterized to provide further mechanistic information regarding the interaction between Populus and a foliar rust fungus. Results of this study improve our understanding of the defence response mechanisms of Populus and will stimulate future work to characterize strategies to prevent and control Populus diseases.

Engineering Drought Resistance in Forest Trees

Climatic stresses limit plant growth and productivity. In the past decade, tree improvement programs were mainly focused on yield but it is obvious that enhanced stress resistance is also required. In this review we highlight important drought avoidance and tolerance mechanisms in forest trees. Genomes of economically important trees species with divergent resistance mechanisms can now be exploited to uncover the mechanistic basis of long-term drought adaptation at the whole plant level. Molecular tree physiology indicates that osmotic adjustment, antioxidative defense and increased water use efficiency are important targets for enhanced drought tolerance at the cellular and tissue level. Recent biotechnological approaches focused on overexpression of genes involved in stress sensing and signaling, such as the abscisic acid core pathway, and down-stream transcription factors. By this strategy, a suite of defense systems was recruited, generally enhancing drought and salt stress tolerance under laboratory conditions. However, field studies are still scarce. Under field conditions trees are exposed to combinations of stresses that vary in duration and magnitude. Variable stresses may overrule the positive effect achieved by engineering an individual defense pathway. To assess the usability of distinct modifications, large-scale experimental field studies in different environments are necessary. To optimize the balance between growth and defense, the use of stress-inducible promoters may be useful. Future improvement programs for drought resistance will benefit from a better understanding of the intricate networks that ameliorate molecular and ecological traits of forest trees.

Engineering Drought Resistance in Forest Trees

Climatic stresses limit plant growth and productivity. In the past decade, tree improvement programs were mainly focused on yield but it is obvious that enhanced stress resistance is also required. In this review we highlight important drought avoidance and tolerance mechanisms in forest trees. Genomes of economically important trees species with divergent resistance mechanisms can now be exploited to uncover the mechanistic basis of long-term drought adaptation at the whole plant level. Molecular tree physiology indicates that osmotic adjustment, antioxidative defense and increased water use efficiency are important targets for enhanced drought tolerance at the cellular and tissue level. Recent biotechnological approaches focused on overexpression of genes involved in stress sensing and signaling, such as the abscisic acid core pathway, and down-stream transcription factors. By this strategy, a suite of defense systems was recruited, generally enhancing drought and salt stress tolerance under laboratory conditions. However, field studies are still scarce. Under field conditions trees are exposed to combinations of stresses that vary in duration and magnitude. Variable stresses may overrule the positive effect achieved by engineering an individual defense pathway. To assess the usability of distinct modifications, large-scale experimental field studies in different environments are necessary. To optimize the balance between growth and defense, the use of stress-inducible promoters may be useful. Future improvement programs for drought resistance will benefit from a better understanding of the intricate networks that ameliorate molecular and ecological traits of forest trees.

Small RNA and Transcriptome Sequencing Reveal a Potential miRNA-Mediated Interaction Network That Functions during Somatic Embryogenesis in Lilium pumilum DC. Fisch

Plant somatic embryos are widely used in the fields of germplasm conservation, breeding for genetic engineering and artificial seed production. MicroRNAs (miRNAs) play pivotal roles in somatic embryogenesis (SE) regulation. However, their regulatory roles during various stages of SE remain unclear. In this study, six types of embryogenic samples of Lilium pumilum DC. Fisch., including organogenic callus, embryogenic callus induced for 4 weeks, embryogenic callus induced for 6 weeks, globular embryos, torpedo embryos and cotyledon embryos, were prepared for small RNA sequencing. The results revealed a total of 2,378,760 small RNA reads, among which the most common size was 24 nt. Four hundred and fifty-two known miRNAs, belonging to more than 86 families, 57 novel miRNAs and 40 miRNA*s were identified. The 86 known miRNA families were sorted according to an alignment with their homologs across 24 land plants into the following four categories: 23 highly conserved, 4 moderately conserved, 15 less conserved and 44 species-specific miRNAs. Differentially expressed known miRNAs were identified during various stages of SE. Subsequently, the expression levels of 12 differentially expressed miRNAs and 4 targets were validated using qRT-PCR. In addition, six samples were mixed in equal amounts for transcript sequencing, and the sequencing data were used as transcripts for miRNA target prediction. A total of 66,422 unigenes with an average length of 800 bp were assembled from 56,258,974 raw reads. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment indicated that 38,004 and 15,497 unigenes were successfully assigned to GO terms and KEGG pathways, respectively. Among the unigenes, 2,182 transcripts were predicted to be targets for 396 known miRNAs. The potential targets of the identified miRNAs were mostly classified into the following GO terms: cell, binding and metabolic process. Enriched KEGG analysis demonstrated that carbohydrate metabolism was the predominant pathway in Lilium SE. Thus, we performed systemic characterization, homology comparisons and profiling of miRNA expression, and we constructed an miRNA-target network during Lilium SE for the first time. Our findings establish a foundation for the further exploration of critical genes and elucidation of SE in Lilium.

A NAC Transcription Factor Represses Putrescine Biosynthesis and Affects Drought Tolerance

Arginine decarboxylase (ADC)-mediated putrescine biosynthesis plays an important role in plant stress responses, but the transcriptional regulation of ADC in response to abiotic stress is not well understood. We isolated a NAM, ATAF1/2, and CUC (NAC) domain-containing transcription factor, PtrNAC72, from trifoliate orange (Poncirus trifoliata) by yeast one-hybrid screening. PtrNAC72, localized to the nucleus, binds specifically to the promoter of PtADC and acts as a transcriptional repressor. PtrNAC72 expression was induced by cold, drought, and abscisic acid. ADC messenger RNA abundance and putrescine levels were decreased in transgenic tobacco (Nicotiana nudicaulis) plants overexpressing PtrNAC72 but increased, compared with the wild type, in an Arabidopsis (Arabidopsis thaliana) transfer DNA insertion mutant, nac72 While transgenic tobacco lines overexpressing PtrNAC72 were more sensitive to drought, plants of the Arabidopsis nac72 mutant exhibited enhanced drought tolerance, consistent with the accumulation of reactive oxygen species in the tested genotypes. In addition, exogenous application of putrescine to the overexpression lines restored drought tolerance, while treatment with d-arginine, an ADC inhibitor, compromised the drought tolerance of nac72 Taken together, these results demonstrate that PtrNAC72 is a repressor of putrescine biosynthesis and may negatively regulate the drought stress response, at least in part, via the modulation of putrescine-associated reactive oxygen species homeostasis.