Arabidopsis auxin response factor6 and 8 regulate jasmonic acid biosynthesis and floral organ development via repression of class 1 KNOX genes

Identification of male sterility-related genes in Saccharum officinarum and Saccharum spontaneum

KEY MESSAGE

Candidate male sterility genes were identified in sugarcane, which interacts with kinase-related proteins, transcription factors, and plant hormone signaling pathways to regulate stamen and anther development. Saccharum officinarum is a cultivated sugarcane species that its predominant feature is high sucrose content in stems. Flowering is necessary for breeding new cultivars but will terminate plant growth and reduce sugar yield. The wild sugarcane species Saccharum spontaneum has robust and viable pollen, whereas most S. officinarum accessions are male sterile, which is a desirable trait of a maternal parent in sugarcane breeding. To study male sterility and related regulatory pathways in sugarcane, we carried out RNAseq using flowers in different developmental stages between male-sterile S. officinarum accession 'LA Purple' and fertile S. spontaneum accession 'SES208'. Gene expression profiles were used to detect how genes are differentially expressed between male sterile and fertile flowers and to identify candidate genes for male sterility. Weighted gene correlation networks analysis (WGCNA) was conducted to investigate the regulatory networks. Transcriptomic analyses showed that 988 genes and 2888 alleles were differentially expressed in S. officinarum compared to S. spontaneum. Ten differentially expressed genes and thirty alleles were identified as candidate genes and alleles for male sterility in sugarcane. The gene Sspon.03G0007630 and two alleles of the gene Sspon.08G0002270, Sspon.08G0002270-2B and Sspon.08G0014700-1A, were involved in the early stamen or carpel development stages, while the remaining genes were classified into the post-meiosis stage. Gibberellin, auxin, and jasmonic acid signaling pathways are involved in the stamen development in sugarcane. The results expanded our knowledge of male sterility-related genes in sugarcane and generated genomic resources to facilitate the selection of ideal maternal parents to improve breeding efficiency.

The Arabidopsis ARID-HMG DNA-BINDING PROTEIN 15 modulates jasmonic acid signaling by regulating MYC2 during pollen development

The intricate process of male gametophyte development in flowering plants is regulated by jasmonic acid (JA) signaling. JA signaling initiates with the activation of the basic helix-loop-helix transcription factor (TF), MYC2, leading to the expression of numerous JA-responsive genes during stamen development and pollen maturation. However, the regulation of JA signaling during different stages of male gametophyte development remains less understood. This study focuses on the characterization of the plant ARID-HMG DNA-BINDING PROTEIN 15 (AtHMGB15) and its role in pollen development in Arabidopsis (Arabidopsis thaliana). Phenotypic characterization of a T-DNA insertion line (athmgb15-4) revealed delayed bolting, shorter siliques, and reduced seed set in mutant plants compared to the wild type. Additionally, AtHMGB15 deletion resulted in defective pollen morphology, delayed pollen germination, aberrant pollen tube growth, and a higher percentage of nonviable pollen grains. Molecular analysis indicated the downregulation of JA biosynthesis and signaling genes in the athmgb15-4 mutant. Quantitative analysis demonstrated that JA and its derivatives were ∼10-fold lower in athmgb15-4 flowers. Exogenous application of methyl jasmonate could restore pollen morphology and germination, suggesting that the low JA content in athmgb15-4 impaired JA signaling during pollen development. Furthermore, our study revealed that AtHMGB15 physically interacts with MYC2 to form a transcription activation complex. This complex promotes the transcription of key JA signaling genes, the R2R3-MYB TFs MYB21 and MYB24, during stamen and pollen development. Collectively, our findings highlight the role of AtHMGB15 as a positive regulator of the JA pathway, controlling the spatiotemporal expression of key regulators involved in Arabidopsis stamen and pollen development.

Enigmatic role of auxin response factors in plant growth and stress tolerance

Abiotic and biotic stresses globally constrain plant growth and impede the optimization of crop productivity. The phytohormone auxin is involved in nearly every aspect of plant development. Auxin acts as a chemical messenger that influences gene expression through a short nuclear pathway, mediated by a family of specific DNA-binding transcription factors known as Auxin Response Factors (ARFs). ARFs thus act as effectors of auxin response and translate chemical signals into the regulation of auxin responsive genes. Since the initial discovery of the first ARF in Arabidopsis, advancements in genetics, biochemistry, genomics, and structural biology have facilitated the development of models elucidating ARF action and their contributions to generating specific auxin responses. Yet, significant gaps persist in our understanding of ARF transcription factors despite these endeavors. Unraveling the functional roles of ARFs in regulating stress response, alongside elucidating their genetic and molecular mechanisms, is still in its nascent phase. Here, we review recent research outcomes on ARFs, detailing their involvement in regulating leaf, flower, and root organogenesis and development, as well as stress responses and their corresponding regulatory mechanisms: including gene expression patterns, functional characterization, transcriptional, post-transcriptional and post- translational regulation across diverse stress conditions. Furthermore, we delineate unresolved questions and forthcoming challenges in ARF research.

Rhytidome- and cork-type barks of holm oak, cork oak and their hybrids highlight processes leading to cork formation

BACKGROUND

The periderm is basic for land plants due to its protective role during radial growth, which is achieved by the polymers deposited in the cell walls. In most trees, like holm oak, the first periderm is frequently replaced by subsequent internal periderms yielding a heterogeneous outer bark made of a mixture of periderms and phloem tissues, known as rhytidome. Exceptionally, cork oak forms a persistent or long-lived periderm which results in a homogeneous outer bark of thick phellem cell layers known as cork. Cork oak and holm oak distribution ranges overlap to a great extent, and they often share stands, where they can hybridize and produce offspring showing a rhytidome-type bark.

RESULTS

Here we use the outer bark of cork oak, holm oak, and their natural hybrids to analyse the chemical composition, the anatomy and the transcriptome, and further understand the mechanisms underlying periderm development. We also include a unique natural hybrid individual corresponding to a backcross with cork oak that, interestingly, shows a cork-type bark. The inclusion of hybrid samples showing rhytidome-type and cork-type barks is valuable to approach cork and rhytidome development, allowing an accurate identification of candidate genes and processes. The present study underscores that abiotic stress and cell death are enhanced in rhytidome-type barks whereas lipid metabolism and cell cycle are enriched in cork-type barks. Development-related DEGs showing the highest expression, highlight cell division, cell expansion, and cell differentiation as key processes leading to cork or rhytidome-type barks.

CONCLUSION

Transcriptome results, in agreement with anatomical and chemical analyses, show that rhytidome and cork-type barks are active in periderm development, and suberin and lignin deposition. Development and cell wall-related DEGs suggest that cell division and expansion are upregulated in cork-type barks whereas cell differentiation is enhanced in rhytidome-type barks.

The auxin response factor (ARF) gene family in Cyclocarya paliurus: genome-wide identification and their expression profiling under heat and drought stresses

Auxin response factors (ARFs), as the main components of auxin signaling, play a crucial role in various processes of plant growth and development, as well as in stress response. So far, there have been no reports on the genome-wide identification of the ARF transcription factor family in Cyclocarya paliurus, a deciduous tree plant in the family Juglaceae. In this study, a total of 34 CpARF genes were identified based on whole genome sequence, and they were unevenly distributed on 16 chromosomes, with the highest distribution on chromosome 6. Domain analysis of CpARF proteins displayed that 31 out of 34 CpARF proteins contain a typical B3 domain (DBD domain), except CpARF12/ CpARF14/CpARF31, which all belong to Class VI. And 20 CpARFs (58.8%) contain an auxin_IAA binding domain, and are mainly distributed in classes I, and VI. Phylogenetic analysis showed that CpARF was divided into six classes (I-VI), each containing 4, 4, 1, 8, 4, and 13 members, respectively. Gene duplication analysis showed that there are 14 segmental duplications and zero tandem repeats were identified in the CpARF gene family of the C. paliurus genome. The Ka/Ks ratio of duplicate gene pairs indicates that CpARF genes are subjected to strong purification selection pressure. Synteny analysis showed that C. paliurus shared the highest homology in 74 ARF gene pairs with Juglans regia, followed by 73, 51, 25, and 11 homologous gene pairs with Populus trichocarpa, Juglans cathayensis, Arabidopsis, and rice, respectively. Promoter analysis revealed that 34 CpARF genes had cis-elements related to hormones, stress, light, and growth and development except for CpARF12. The expression profile analysis showed that almost all CpARF genes were differentially expressed in at least one tissue, and several CpARF genes displayed tissue-specific expression. Furthermore, 24 out of the 34 CpARF genes have significantly response to drought stress (P < 0.05), and most of them (16) being significantly down-regulated under moderate drought treatment. Meanwhile, the majority of CpARF genes (28) have significantly response to drought stress (P < 0.05), and most of them (26) are significantly down-regulated under severe drought treatment. Furthermore, 32 out of the 34 CpARF genes have significantly response to high, middle, and low salt stress under salt treatment (P < 0.05). Additionally, subcellular localization analysis confirmed that CpARF16 and CpARF32 were all localized to nucleus. Thus, our findings expand the understanding of the function of CpARF genes and provide a basis for further functional studies on CpARF genes in C. paliurus.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01474-1.

Rice transcriptional repressor OsTIE1 controls anther dehiscence and male sterility by regulating JA biosynthesis

Proper anther dehiscence is essential for successful pollination and reproduction in angiosperms, and jasmonic acid (JA) is crucial for the process. However, the mechanisms underlying the tight regulation of JA biosynthesis during anther development remain largely unknown. Here, we demonstrate that the rice (Oryza sativa L.) ethylene-response factor-associated amphiphilic repression (EAR) motif-containing protein TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTORS (TCP) INTERACTOR CONTAINING EAR MOTIF PROTEIN1 (OsTIE1) tightly regulates JA biosynthesis by repressing TCP transcription factor OsTCP1/PCF5 during anther development. The loss of OsTIE1 function in Ostie1 mutants causes male sterility. The Ostie1 mutants display inviable pollen, early stamen filament elongation, and precocious anther dehiscence. In addition, JA biosynthesis is activated earlier and JA abundance is precociously increased in Ostie1 anthers. OsTIE1 is expressed during anther development, and OsTIE1 is localized in nuclei and has transcriptional repression activity. OsTIE1 directly interacts with OsTCP1, and overexpression of OsTCP1 caused early anther dehiscence resembling that of Ostie1. JA biosynthesis genes including rice LIPOXYGENASE are regulated by the OsTIE1-OsTCP1 complex. Our findings reveal that the OsTIE1-OsTCP1 module plays a critical role in anther development by finely tuning JA biosynthesis and provide a foundation for the generation of male sterile plants for hybrid seed production.

Identification of microRNAs and their target genes associated with chasmogamous and cleistogamous flower development in Viola prionantha

MAIN CONCLUSION

Differentially expressed microRNAs were found associated with the development of chasmogamous and cleistogamous flowers in Viola prionantha, revealing potential roles of microRNAs in the developmental evolution of dimorphic flowers. In Viola prionantha, chasmogamous (CH) flowers are induced by short daylight, while cleistogamous (CL) flowers are triggered by long daylight. How environmental factors and microRNAs (miRNAs) affect dimorphic flower formation remains unknown. In this study, small RNA sequencing was performed on CH and CL floral buds at different developmental stages in V. prionantha, differentially expressed miRNAs (DEmiRNAs) were identified, and their target genes were predicted. In CL flowers, Viola prionantha miR393 (vpr-miR393a/b) and vpr-miRN3366 were highly expressed, while in CH flowers, vpr-miRN2005, vpr-miR172e-2, vpr-miR166m-3, vpr-miR396f-2, and vpr-miR482d-2 were highly expressed. In the auxin-activated signaling pathway, vpr-miR393a/b and vpr-miRN2005 could target Vpr-TIR1/AFB and Vpr-ARF2, respectively, and other DEmiRNAs could target genes involved in the regulation of transcription, e.g., Vpr-AP2-7. Moreover, Vpr-UFO and Vpr-YAB5, the main regulators in petal and stamen development, were co-expressed with Vpr-TIR1/AFB and Vpr-ARF2 and showed lower expression in CL flowers than in CH flowers. Some V. prionantha genes relating to the stress/defense responses were co-expressed with Vpr-TIR1/AFB, Vpr-ARF2, and Vpr-AP2-7 and highly expressed in CL flowers. Therefore, in V. prionantha, CH-CL flower development may be regulated by the identified DEmiRNAs and their target genes, thus providing the first insight into the formation of dimorphic flowers in Viola.

The genetic control of herkogamy

Herkogamy is the spatial separation of anthers and stigmas within complete flowers, and is a key floral trait that promotes outcrossing in many angiosperms. The degree of separation between pollen-producing anthers and receptive stigmas has been shown to influence rates of self-pollination amongst plants, with a reduction in herkogamy increasing rates of successful selfing in self-compatible species. Self-pollination is becoming a critical issue in horticultural crops grown in environments where biotic pollinators are limited, absent, or difficult to utilise. In these cases, poor pollination results in reduced yield and misshapen fruit. Whilst there is a growing body of work elucidating the genetic basis of floral organ development, the genetic and environmental control points regulating herkogamy are poorly understood. A better understanding of the developmental and regulatory pathways involved in establishing varying degrees of herkogamy is needed to provide insights into the production of flowers more adept at selfing to produce consistent, high-quality fruit. This review presents our current understanding of herkogamy from a genetics and hormonal perspective.

Auxin and carbohydrate control flower bud development in Anthurium andraeanum during early stage of sexual reproduction

BACKGROUND

Flower buds of Anthurium andraeanum frequently cease to grow and abort during the early flowering stage, resulting in prolonged planting times and increased commercialization costs. Nevertheless, limited knowledge exists of the mechanism of flower development after initiation in A. andraeanum.

RESULTS

In this study, the measurement of carbohydrate flow and intensity between leaves and flowers during different growth stages showed that tender leaves are strong sinks and their concomitant flowers are weak ones. This suggested that the tender leaves compete with their concomitant flower buds for carbohydrates during the early growth stages, potentially causing the abortion of the flower buds. The analysis of transcriptomic differentially expressed genes suggested that genes related to sucrose metabolism and auxin response play an important role during flower bud development. Particularly, co-expression network analysis found that AaSPL12 is a hub gene engaged in flower development by collaborating carbohydrate and auxin signals. Yeast Two Hybrid assays revealed that AaSPL12 can interact with AaARP, a protein that serves as an indicator of dormancy. Additionally, the application of exogenous IAA and sucrose can suppress the expression of AaARP, augment the transcriptional abundance of AaSPL12, and consequently expedite flower development in Anthurium andraeanum.

CONCLUSIONS

Collectively, our findings indicated that the combination of auxin and sugar signals could potentially suppress the repression of AaARP protein to AaSPL12, thus advancing the development of flower buds in Anthurium andraeanum.

The transcription factors and pathways underpinning male reproductive development in Arabidopsis

As Arabidopsis flowers mature, specialized cells within the anthers undergo meiosis, leading to the production of haploid microspores that differentiate into mature pollen grains, each containing two sperm cells for double fertilization. During pollination, the pollen grains are dispersed from the anthers to the stigma for subsequent fertilization. Transcriptomic studies have identified a large number of genes expressed over the course of male reproductive development and subsequent functional characterization of some have revealed their involvement in floral meristem establishment, floral organ growth, sporogenesis, meiosis, microsporogenesis, and pollen maturation. These genes encode a plethora of proteins, ranging from transcriptional regulators to enzymes. This review will focus on the regulatory networks that control male reproductive development, starting from flower development and ending with anther dehiscence, with a focus on transcription factors and some of their notable target genes.

KNOTTED1-like homeobox (KNOX) transcription factors - Hubs in a plethora of networks: A review

KNOX (KNOTTED1-like HOMEOBOX) belongs to a class of important homeobox genes, which encode the homeodomain proteins binding to the specific element of target genes, and widely participate in plant development. Advancements in genetics and molecular biology research generate a large amount of information about KNOX genes in model and non-model plants, and their functions in different developmental backgrounds are gradually becoming clear. In this review, we summarize the known and presumed functions of the KNOX gene in plants, focusing on horticultural plants and crops. The classification and structural characteristics, expression characteristics and regulation, interacting protein factors, functions, and mechanisms of KNOX genes are systematically described. Further, the current research gaps and perspectives were discussed. These comprehensive data can provide a reference for the directional improvement of agronomic traits through KNOX gene regulation.

Heat-responsive microRNAs participate in regulating the pollen fertility stability of CMS-D2 restorer line under high-temperature stress

Anther development and pollen fertility of cytoplasmic male sterility (CMS) conditioned by Gossypium harknessii cytoplasm (CMS-D2) restorer lines are susceptible to continuous high-temperature (HT) stress in summer, which seriously hinders the large-scale application of "three-line" hybrids in production. Here, integrated small RNA, transcriptome, degradome, and hormone profiling was performed to explore the roles of microRNAs (miRNAs) in regulating fertility stability in mature pollens of isonuclear alloplasmic near-isogenic restorer lines NH and SH under HT stress at two environments. A total of 211 known and 248 novel miRNAs were identified, of which 159 were differentially expressed miRNAs (DEMs). Additionally, 45 DEMs in 39 miRNA clusters (PmCs) were also identified, and most highly expressed miRNAs were significantly induced in SH under extreme HT, especially four MIR482 and six MIR6300 family miRNAs. PmC28 was located in the fine-mapped interval of the Rf1 gene and contained two DEMs, gra-miR482_L-2R + 2 and gma-miR2118a-3p_R + 1_1ss18TG. Transcriptome sequencing identified 6281 differentially expressed genes, of which heat shock protein (HSP)-related genes, such as HSP70, HSP22, HSP18.5-C, HSP18.2 and HSP17.3-B, presented significantly reduced expression levels in SH under HT stress. Through integrating multi-omics data, we constructed a comprehensive molecular network of miRNA-mRNA-gene-KEGG containing 35 pairs of miRNA/target genes involved in regulating the pollen development in response to HT, among which the mtr-miR167a_R + 1, tcc-miR167c and ghr-miR390a, tcc-miR396c_L-1 and ghr-MIR169b-p3_1ss6AG regulated the pollen fertility by influencing ARF8 responsible for the auxin signal transduction, ascorbate and aldarate metabolism, and the sugar and lipid metabolism and transport pathways, respectively. Further combination with hormone analysis revealed that HT-induced jasmonic acid signaling could activate the expression of downstream auxin synthesis-related genes and cause excessive auxin accumulation, followed by a cascade of auxin signal transduction, ultimately resulting in pollen abortion. The results provide a new understanding of how heat-responsive miRNAs regulate the stability of fertility restoration for CMS-D2 cotton under heat stress.

AUXIN RESPONSE FACTOR protein accumulation and function

Auxin is a key regulator of plant developmental processes. Its effects on transcription are mediated by the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. ARFs tightly control specific auxin responses necessary for proper plant growth and development. Recent research has revealed that regulated ARF protein accumulation and ARF nucleo-cytoplasmic partitioning can determine auxin transcriptional outputs. In this review, we explore these recent findings and consider the potential for regulated ARF accumulation in driving auxin responses in plants.

MYB transcription factors and their roles in the male reproductive development of flowering plants

As one of the largest transcription factor families with complex functional differentiation in plants, the MYB transcription factors (MYB TFs) play important roles in the physiological and biochemical processes of plant growth and development. Male reproductive development, an essential part of sexual reproduction in flowering plants, is undoubtedly regulated by MYB TFs. In this review, we summarize the roles of the MYB TFs involved in the three stages of male reproductive development: pollen grains formation and maturation, filament elongation and anther dehiscence, and fertilization. Also, the potential downstream target genes and upstream regulators of these MYB TFs are discussed. Furthermore, we propose the underlying regulatory mechanisms of these MYB TFs: (1) A complex network of MYB TFs regulates various aspects of male reproductive development; (2) MYB homologous genes in different species may be functionally conserved or differentiated; (3) MYB TFs often form regulatory complexes with bHLH TFs.

Loss of susceptibility, an underexplored approach for durable resistance to plant-parasitic nematodes

This article comments on:

Noureddine Y, da Rocha M, An J, Médina C, Mejias J, Mulet K, Quentin M, Abad P, Zouine M, Favery B, Jaubert-Possamai S. 2023. AUXIN RESPONSIVE FACTOR8 regulates development of the feeding site induced by root-knot nematodes in tomato. Journal of Experimental Botany 74, 5752–5766.

Humic Substances Isolated from Recycled Biomass Trigger Jasmonic Acid Biosynthesis and Signalling

Intensive agriculture maintains high crop yields through chemical inputs, which are well known for their adverse effects on environmental quality and human health. Innovative technologies are required to reduce the risk generated by the extensive and harmful use of pesticides. The plant biostimulants made from humic substances isolated from recyclable biomass offer an alternative approach to address the need for replacing conventional agrochemicals without compromising the crop yield. The stimulatory effects of humic substances are commonly associated with plant hormones, particularly auxins. However, jasmonic acid (JA) is crucial metabolite in mediating the defence responses and governing plant growth and development. This work aimed to evaluate the changes in the biosynthesis and signalling pathway of JA in tomato seedlings treated with humic acids (HA) isolated from vermicompost. We use the tomato model system cultivar Micro-Tom (MT) harbouring a reporter gene fused to a synthetic promoter that responds to jasmonic acid (JERE::GUS). The transcript levels of genes involved in JA generation and activity were also determined using qRT-PCR. The application of HA promoted plant growth and altered the JA status, as revealed by both GUS and qRT-PCR assays. Both JA enzymatic synthesis (LOX, OPR3) and JA signalling genes (JAZ and JAR) were found in higher transcription levels in plants treated with HA. In addition, ethylene (ETR4) and auxin (ARF6) signalling components were positively modulated by HA, revealing a hormonal cross-talk. Our results prove that the plant defence system linked to JA can be emulated by HA application without growth inhibition.

Comparative transcriptome analysis reveals the involvement of an MYB transcriptional activator, SmMYB108, in anther dehiscence in eggplant

Male sterility is a highly attractive agronomic trait as it effectively prevents self-fertilization and facilitates the production of high-quality hybrid seeds in plants. Timely release of mature pollen following anther dehiscence is essential for stamen development in flowering plants. Although several theories have been proposed regarding this, the specific mechanism of anther development in eggplant remains elusive. In this study, we selected an R2R3-MYB transcription factor gene, SmMYB108, that encodes a protein localized primarily in the nucleus by comparing the transcriptomics of different floral bud developmental stages of the eggplant fertile line, F142. Quantitative reverse transcription polymerase chain reaction revealed that SmMYB108 was preferentially expressed in flowers, and its expression increased significantly on the day of flowering. Overexpression of SmMYB108 in tobacco caused anther dehiscence. In addition, we found that SmMYB108 primarily functions as a transcriptional activator via C-terminal activation (amino acid 262-317). Yeast one-hybrid and dual-luciferase reporter assays revealed that genes (SmMYB21, SmARF6, and SmARF8) related to anther development targeted the SmMYB108 promoter. Overall, our results provide insights into the molecular mechanisms involved in the regulation of anther development by SmMYB108.

Modulating auxin response stabilizes tomato fruit set

Fruit formation depends on successful fertilization and is highly sensitive to weather fluctuations that affect pollination. Auxin promotes fruit initiation and growth following fertilization. Class A auxin response factors (Class A ARFs) repress transcription in the absence of auxin and activate transcription in its presence. Here, we explore how multiple members of the ARF family regulate fruit set and fruit growth in tomato (Solanum lycopersicum) and Arabidopsis thaliana, and test whether reduction of SlARF activity improves yield stability in fluctuating temperatures. We found that several tomato Slarf mutant combinations produced seedless parthenocarpic fruits, most notably mutants deficient in SlARF8A and SlARF8B genes. Arabidopsis Atarf8 mutants deficient in the orthologous gene had less complete parthenocarpy than did tomato Slarf8a Slarf8b mutants. Conversely, Atarf6 Atarf8 double mutants had reduced fruit growth after fertilization. AtARF6 and AtARF8 likely switch from repression to activation of fruit growth in response to a fertilization-induced auxin increase in gynoecia. Tomato plants with reduced SlARF8A and SlARF8B gene dosage had substantially higher yield than the wild type under controlled or ambient hot and cold growth conditions. In field trials, partial reduction in the SlARF8 dose increased yield under extreme temperature with minimal pleiotropic effects. The stable yield of the mutant plants resulted from a combination of early onset of fruit set, more fruit-bearing branches and more flowers setting fruits. Thus, ARF8 proteins mediate the control of fruit set, and relieving this control with Slarf8 mutations may be utilized in breeding to increase yield stability in tomato and other crops.

The AtERF19 gene regulates meristem activity and flower organ size in plants

Ethylene-responsive factors (ERFs) have diverse functions in the regulation of various plant developmental processes. Here, we demonstrate the dual role of an Arabidopsis ERF gene, AtERF19, in regulating reproductive meristem activity and flower organ size through the regulation of genes involved in CLAVATA-WUSCHEL (CLV-WUS) and auxin signaling, respectively. We found that AtERF19 stimulated the formation of flower primordia and controlled the number of flowers produced by activating WUS and was negatively regulated by CLV3. 35S::AtERF19 expression resulted in significantly more flowers, whereas 35S::AtERF19 + SRDX dominant-negative mutants produced fewer flowers. In addition, AtERF19 also functioned to control flower organ size by promoting the division/expansion of the cells through activating Small Auxin Up RNA Gene 32 (SAUR32), which positively regulated MYB21/24 in the auxin signaling pathway. 35S::AtERF19 and 35S::SAUR32 resulted in similarly larger flowers, whereas 35S::AtERF19 + SRDX and 35S::SAUR32-RNAi mutants produced smaller flowers than the wild type. The functions of AtERF19 were confirmed by the production of similarly more and larger flowers in 35S::AtERF19 transgenic tobacco (Nicotiana benthamiana) and in transgenic Arabidopsis which ectopically expressed the orchid gene (Nicotiana benthamiana) PaERF19 than in wild-type plants. The finding that AtERF19 regulates genes involved in both CLV-WUS and auxin signaling during flower development significantly expands the current knowledge of the multifunctional evolution of ERF genes in plants. The results presented in this work indicate a dual role for the transcription factor AtERF19 in controlling the number of flowers produced and flower organ size through the regulation of genes involved in CLV-WUS and auxin signaling, respectively. Our findings expand the knowledge of the roles of ERF genes in the regulation of reproductive development.

Brassinosteroid signals cooperate with katanin-mediated microtubule severing to control stamen filament elongation

Proper stamen filament elongation is essential for pollination and plant reproduction. Plant hormones are extensively involved in every stage of stamen development; however, the cellular mechanisms by which phytohormone signals couple with microtubule dynamics to control filament elongation remain unclear. Here, we screened a series of Arabidopsis thaliana mutants showing different microtubule defects and revealed that only those unable to sever microtubules, lue1 and ktn80.1234, displayed differential floral organ elongation with less elongated stamen filaments. Prompted by short stamen filaments and severe decrease in KTN1 and KTN80s expression in qui-2 lacking five BZR1-family transcription factors (BFTFs), we investigated the crosstalk between microtubule severing and brassinosteroid (BR) signaling. The BFTFs transcriptionally activate katanin-encoding genes, and the microtubule-severing frequency was severely reduced in qui-2. Taken together, our findings reveal how BRs can regulate cytoskeletal dynamics to coordinate the proper development of reproductive organs.

Transcriptomic analysis reveals candidate genes associated with anther development in Lilium Oriental Hybrid 'Siberia'

Lily (Lilium spp. and hybrids) is an important cut flower crop worldwide. Lily flowers have large anthers, which release a large amount of pollen that stains the tepals or clothing and thus can affect the commercial value of cut flowers. In this study, lily Oriental 'Siberia' was used to investigate the regulatory mechanism of lily anther development, which may provide information to prevent pollen pollution in the future. Based on the flower bud length, anther length and color, and anatomical observations, lily anther development was categorized into five stages: green (G), green-to-yellow 1 (GY1), green-to-yellow 2 (GY2), yellow (Y), and purple (P). Total RNA was extracted from the anthers at each stage for transcriptomic analysis. A total of 268.92-Gb clean reads were generated, and 81,287 unigenes were assembled and annotated. The number of differentially expressed genes (DEGs) and unique genes were largest for the pairwise comparison between the G and GY1 stages. The G and P samples were clustered separately, whereas the GY1, GY2, and Y samples were clustered together in scatter plots from a principal component analysis. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses of DEGs detected in the GY1, GY2, and Y stages revealed that the pectin catabolic process, hormone levels, and phenylpropanoid biosynthesis were enriched. The DEGs associated with jasmonic acid biosynthesis and signaling were highly expressed at the early stages (G and GY1), whereas the DEGs associated with phenylpropanoid biosynthesis were mainly expressed in the intermediate stages (GY1, GY2, and Y). The DEGs involved in the pectin catabolic process were expressed at advanced stages (Y and P). Cucumber mosaic virus-induced gene silencing of LoMYB21 and LoAMS caused a strongly inhibited anther dehiscence phenotype, but without affecting the development of other floral organs. These results provide novel insights for understanding the regulatory mechanism of anther development in lily and other plants.

Engineered Cleistogamy in Camelina sativa for bioconfinement

Camelina sativa is a self-pollinating and facultative outcrossing oilseed crop. Genetic engineering has been used to improve camelina yield potential for altered fatty acid composition, modified protein profiles, improved seed and oil yield, and enhanced drought resistance. The deployment of transgenic camelina in the field posits high risks related to the introgression of transgenes into non-transgenic camelina and wild relatives. Thus, effective bioconfinement strategies need to be developed to prevent pollen-mediated gene flow (PMGF) from transgenic camelina. In the present study, we overexpressed the cleistogamy (i.e. floral petal non-openness)-inducing PpJAZ1 gene from peach in transgenic camelina. Transgenic camelina overexpressing PpJAZ1 showed three levels of cleistogamy, affected pollen germination rates after anthesis but not during anthesis, and caused a minor silicle abortion only on the main branches. We also conducted field trials to examine the effects of the overexpressed PpJAZ1 on PMGF in the field, and found that the overexpressed PpJAZ1 dramatically inhibited PMGF from transgenic camelina to non-transgenic camelina under the field conditions. Thus, the engineered cleistogamy using the overexpressed PpJAZ1 is a highly effective bioconfinement strategy to limit PMGF from transgenic camelina, and could be used for bioconfinement in other dicot species.

Genome-wide screening and characterization of long noncoding RNAs involved in flowering/bolting of Lactuca sativa

BACKGROUND

Lettuce (Lactuca sativa L.) is considered the most important vegetable in the leafy vegetable group. However, bolting affects quality, gives it a bitter taste, and as a result makes it inedible. Bolting is an event induced by the coordinated effects of various environmental factors and endogenous genetic components. Although bolting/flowering responsive genes have been identified in most sensitive and non-sensitive species, non-coding RNA molecules like long non-coding RNAs (lncRNAs) have not been investigated in lettuce. Hence, in this study, potential long non-coding RNAs that regulate flowering /bolting were investigated in two lettuce strains S24 (resistant strain) and S39 (susceptible strain) in different flowering times to better understand the regulation of lettuce bolting mechanism. For this purpose, we used two RNA-seq datasets to discover the lncRNA transcriptome profile during the transition from vegetative to reproductive phase.

RESULTS

For identifying unannotated transcripts in these datasets, a 7-step pipeline was employed to filter out these transcripts and terminate with 293 novel lncRNAs predicted by PLncPRO and CREMA. These transcripts were then utilized to predict cis and trans flowering-associated targets and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Computational predictions of target gene function showed the involvement of putative flowering-related genes and enrichment of the floral regulators FLC, CO, FT, and SOC1 in both datasets. Finally, 17 and 18 lncRNAs were proposed as competing endogenous target mimics (eTMs) for novel and known lncRNA miRNAs, respectively.

CONCLUSION

Overall, this study provides new insights into lncRNAs that control the flowering time of plants known for bolting, such as lettuce, and opens new windows for further study.

BREVIPEDICELLUS/KNAT1 targets TCP15 to modulate filament elongation during Arabidopsis late stamen development

The Arabidopsis homeodomain protein BREVIPEDICELLUS/KNAT1 represses the expression of the gene encoding the transcription factor TCP15 to limit filament growth at late stages of stamen development.

Hormones and Flower Development in Arabidopsis

Sexual reproduction requires the participation of two gametes, female and male. In angiosperms, gametes develop in specialized organs, pollen (containing the male gametes) develops in the stamens, and the ovule (containing the female gamete) develops in the gynoecium. In Arabidopsis thaliana, the female and male sexual organs are found within the same structure called flower, surrounded by the perianth, which is composed of petals and sepals. During flower development, different organs emerge in an established order and throughout their development distinct tissues within each organ are differentiated. All this requires the coordination and synchronization of several biological processes. To achieve this, hormones and genes work together. These components can interact at different levels generating hormonal interplay and both positive and negative feedback loops, which in turn, gives robustness, stability, and flexibility to flower development. Here, we summarize the progress made on elucidating the role of different hormonal pathways during flower development in Arabidopsis thaliana.

Transcriptomic analysis reveals the formation mechanism of anemone-type flower in chrysanthemum

BACKGROUND

The ray and disc florets on the chrysanthemum capitulum are morphologically diverse and have remarkably abundant variant types, resulting in a rich variety of flower types. An anemone shape with pigmented and elongated disk florets is an important trait in flower shape breeding of chrysanthemums. The regulatory mechanism of their anemone-type disc floret formation was not clear, thus limiting the directional breeding of chrysanthemum flower types. In this study, we used morphological observation, transcriptomic analysis, and gene expression to investigate the morphogenetic processes and regulatory mechanisms of anemone-type chrysanthemum.

RESULT

Scanning electron microscopy (SEM) observation showed that morphological differences between non-anemone-type disc florets and anemone-type disc florets occurred mainly during the petal elongation period. The anemone-type disc florets elongated rapidly in the later stages of development. Longitudinal paraffin section analysis revealed that the anemone-type disc florets were formed by a great number of cells in the middle layer of the petals with vigorous division. We investigated the differentially expressed genes (DEGs) using ray and disc florets of two chrysanthemum cultivars, 082 and 068, for RNA-Seq and their expression patterns of non-anemone-type and anemone-type disc florets. The result suggested that the CYCLOIDEA2 (CYC2s), MADS-box genes, and phytohormone signal-related genes appeared significantly different in both types of disc florets and might have important effects on the formation of anemone-type disc florets. In addition, it is noteworthy that the auxin and jasmonate signaling pathways might play a vital role in developing anemone-type disc florets.

CONCLUSIONS

Based on our findings, we propose a regulatory network for forming non-anemone-type and anemone-type disc florets. The results of this study lead the way to further clarify the mechanism of the anemone-type chrysanthemum formation and lay the foundation for the directive breeding of chrysanthemum petal types.

Ustilaginoidea virens Nuclear Effector SCRE4 Suppresses Rice Immunity via Inhibiting Expression of a Positive Immune Regulator OsARF17

Rice false smut caused by the biotrophic fungal pathogen Ustilaginoidea virens has become one of the most important diseases in rice. The large effector repertory in U. virens plays a crucial role in virulence. However, current knowledge of molecular mechanisms how U. virens effectors target rice immune signaling to promote infection is very limited. In this study, we identified and characterized an essential virulence effector, SCRE4 (Secreted Cysteine-Rich Effector 4), in U. virens. SCRE4 was confirmed as a secreted nuclear effector through yeast secretion, translocation assays and protein subcellular localization, as well as up-regulation during infection. The SCRE4 gene deletion attenuated the virulence of U. virens to rice. Consistently, ectopic expression of SCRE4 in rice inhibited chitin-triggered immunity and enhanced susceptibility to false smut, substantiating that SCRE4 is an essential virulence factor. Furthermore, SCRE4 transcriptionally suppressed the expression of OsARF17, an auxin response factor in rice, which positively regulates rice immune responses and resistance against U. virens. Additionally, the immunosuppressive capacity of SCRE4 depended on its nuclear localization. Therefore, we uncovered a virulence strategy in U. virens that transcriptionally suppresses the expression of the immune positive modulator OsARF17 through nucleus-localized effector SCRE4 to facilitate infection.

Systematic Identification and Expression Analysis of the Auxin Response Factor (ARF) Gene Family in Ginkgo biloba L

Auxin participates in various physiological and molecular response-related developmental processes and is a pivotal hormone that regulates phenotypic formation in plants. Auxin response factors (ARFs) are vital transcription factors that mediate downstream auxin signaling by explicitly binding to auxin-responsive genes' promoters. Here, to investigate the possible developmental regulatory functions of ARFs in Ginkgo biloba, through employing comprehensive bioinformatics, we recognized 15 putative GbARF members. Conserved domains and motifs, gene and protein structure, gene duplication, GO enrichment, transcriptome expression profiles, and qRT-PCR all showed that Group I and III members were highly conserved. Among them, GbARF10b and GbARF10a were revealed as transcriptional activators in the auxin response for the development of Ginkgo male flowers through sequences alignment, cis-elements analysis and GO annotation; the results were corroborated for the treatment of exogenous SA. Moreover, the GbARFs expansion occurred predominantly by segmental duplication, and most GbARFs have undergone purifying selection. The Ka/Ks ratio test identified the functional consistence of GbARF2a and GbARF2c, GbARF10b, and GbARF10a in tissue expression profiles and male flower development. In summary, our study established a new research basis for exploring Ginkgo GbARF members' roles in floral organ development and hormone response.

Maize miR167-ARF3/30-polyamine oxidase 1 module-regulated H2O2 production confers resistance to maize chlorotic mottle virus

Maize chlorotic mottle virus (MCMV) is the key pathogen causing maize lethal necrosis (MLN). Due to the sharply increased incidence of MLN in many countries, there is an urgent need to identify resistant lines and uncover the underlying resistance mechanism. Here, we showed that the abundance of maize (Zea mays) microR167 (Zma-miR167) positively modulates the degree of resistance to MCMV. Zma-miR167 directly targets Auxin Response Factor3 (ZmARF3) and ZmARF30, both of which negatively regulate resistance to MCMV. RNA-sequencing coupled with gene expression assays revealed that both ZmARF3 and ZmARF30 directly bind the promoter of Polyamine Oxidase 1 (ZmPAO1) and activate its expression. Knockdown or inhibition of enzymatic activity of ZmPAO1 suppressed MCMV infection. Nevertheless, MCMV-encoded p31 protein directly targets ZmPAO1 and enhances the enzyme activity to counteract Zma-miR167-mediated defense to some degree. We uncovered a role of the Zma-miR167-ZmARF3/30 module for restricting MCMV infection by regulating ZmPAO1 expression, while MCMV employs p31 to counteract this defense.

Genome-Wide Identification of Auxin Response Factors in Peanut (Arachis hypogaea L.) and Functional Analysis in Root Morphology

Auxin response factors (ARFs) play important roles in plant growth and development; however, research in peanut (Arachis hypogaea L.) is still lacking. Here, 63, 30, and 30 AhARF genes were identified from an allotetraploid peanut cultivar and two diploid ancestors (A. duranensis and A. ipaensis). Phylogenetic tree and gene structure analysis showed that most AhARFs were highly similar to those in the ancestors. By scanning the whole-genome for ARF-recognized cis-elements, we obtained a potential target gene pool of AhARFs, and the further cluster analysis and comparative analysis showed that numerous members were closely related to root development. Furthermore, we comprehensively analyzed the relationship between the root morphology and the expression levels of AhARFs in 11 peanut varieties. The results showed that the expression levels of AhARF14/26/45 were positively correlated with root length, root surface area, and root tip number, suggesting an important regulatory role of these genes in root architecture and potential application values in peanut breeding.

Genome-Wide Identification and Expression Pattern Analysis of KNOX Gene Family in Orchidaceae

The establishment of lateral organs and subsequent plant architecture involves factors intrinsic to the stem apical meristem (SAM) from which they are derived. KNOTTED1-LIKE HOMEOBOX (KNOX) genes are a family of plant-specific homeobox transcription factors that especially act in determining stem cell fate in SAM. Although KNOXs have been studied in many land plants for decades, there is a dearth of knowledge on KNOX's role in Orchidaceae, the largest and most diverse lineage of flowering plants. In this study, a total of 32 putative KNOX genes were identified in the genomes of five orchid species and further designated into two classes (Class I and Class II) based on phylogenetic relationships. Sequence analysis showed that most orchid KNOX proteins retain four conserved domains (KNOX1, KNOX2, ELK, and Homeobox_KN). Comparative analysis of gene structure showed that the exon-intron structure is conserved in the same clade but most orchids exhibited longer intron, which may be a unique feature of Orchidaceae. Cis-elements identified in the promoter region of orchid KNOXs were found mostly enriched in a function of light responsiveness, followed by MeJA and ABA responsiveness, indicative of their roles in modulating light and phytohormones. Collinear analysis unraveled a one-to-one correspondence among KNOXs in orchids, and all KNOX genes experienced strong purifying selection, indicating the conservation of this gene family has been reinforced across the Orchidaceae lineage. Expression profiles based on transcriptomic data and real-time reverse transcription-quantitative PCR (RT-qPCR) revealed a stem-specific expression of KNOX Class I genes and a broader expression pattern of Class II genes. Taken together, our results provided a comprehensive analysis to uncover the underlying function of KNOX genes in Orchidaceae.

Genome-wide identification and expression analysis of auxin response factors in peanut (Arachis hypogaea L.)

Auxin response factors (ARFs) are transcription factors that regulate the expression of auxin response genes, and have important functions in plant growth and development. In this study, available genome data for peanut (Arachis hypogaea L.) were used to identify AhARF genes. In total, 61 AhARFs and 23 AtARFs were divided into six groups (I-VI). Molecular structural analysis revealed that the protein members of AhARF contain at least two domains, the B3 domain and the Auxin-resp domain, and that some have a C-terminal dimerisation domain. Screening of the transcriptome data of 22 tissues of A. hypogaea cv. Tifrunner in a public database showed high expression levels of AhARF2 and AhARF6. AhARF6 was expressed more highly in the stem and branch than in the root and leaf of the wild species Arachis monticola (A. mon) and cultivated species H103. After treatment with exogenous auxin (NAA), the expression of AhARF6 was inhibited, and this inhibition was greater in A. mon than in H103. The transcriptome map revealed that the expression of AhARF6 was higher in the larger pods of H8107 and ZP06 than in the medium pods of H103 and small pods of A. mon. Moreover, AhARF6-5 was proven to be localised in the nucleus, consistent with the location of AtARF6. These results suggest that AhARF6 may play an important role in pod development in peanut.

Auxin and its role in plant development: structure, signalling, regulation and response mechanisms

Auxins are plant hormones that play a central role in controlling plant growth and development across different environmental conditions. Even at low concentrations, auxins can regulate gene expression through specific transcription factors and proteins that are modulated to environmental responses in the signalling cascade. Auxins are synthesized in tissues with high cell division activity and distributed by specific transmembrane proteins that regulate efflux and influx. This review presents recent advances in understanding the biosynthetic pathways, both dependent and independent of tryptophan, highlighting the intermediate indole compounds (indole-3-acetamide, indole-3-acetaldoxime, indole-3-pyruvic acid and tryptamine) and the key enzymes for auxin biosynthesis, such as YUCs and TAAs. In relation to the signalling cascade, it has been shown that auxins influence gene expression regulation by the connection between synthesis and distribution. Moreover, the molecular action of the auxin response factors and auxin/indole-3-acetic acid transcription factors with the F-box TIR1/AFB auxin receptors regulates gene expression. In addition, the importance of microRNAs in the auxin signalling pathway and their influence on plant plasticity to environmental fluctuations is also demonstrated. Finally, this review describes the chemical and biological processes involving auxins in plants.

Auxin Interactions with Other Hormones in Plant Development

Auxin is a crucial growth regulator that governs plant development and responses to environmental perturbations. It functions at the heart of many developmental processes, from embryogenesis to organ senescence, and is key to plant interactions with the environment, including responses to biotic and abiotic stimuli. As remarkable as auxin is, it does not act alone, but rather solicits the help of, or is solicited by, other endogenous signals, including the plant hormones abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellic acid, jasmonates, salicylic acid, and strigolactones. The interactions between auxin and other hormones occur at multiple levels: hormones regulate one another's synthesis, transport, and/or response; hormone-specific transcriptional regulators for different pathways physically interact and/or converge on common target genes; etc. However, our understanding of this crosstalk is still fragmentary, with only a few pieces of the gigantic puzzle firmly established. In this review, we provide a glimpse into the complexity of hormone interactions that involve auxin, underscoring how patchy our current understanding is.

Transcriptomic analysis provides insights into the AUXIN RESPONSE FACTOR 6-mediated repression of nicotine biosynthesis in tobacco (Nicotiana tabacum L.)

KEY MESSAGE

NtARF6 overexpression represses nicotine biosynthesis in tobacco. Transcriptome analysis suggests that NtARF6 acts as a regulatory hub that connect different phytohormone signaling pathways to antagonize the jasmonic acid-induced nicotine biosynthesis. Plant specialized metabolic pathways are regulated by a plethora of molecular regulators that form complex networks. In Nicotiana tabacum, nicotine biosynthesis is regulated by transcriptional activators, such as NtMYC2 and the NIC2-locus ERFs. However, the underlying molecular mechanism of the regulatory feedback is largely unknown. Previous research has shown that NbARF1, a nicotine synthesis repressor, reduces nicotine accumulation in N. benthamiana. In this study, we demonstrated that overexpression of NtARF6, an ortholog of NbARF1, was able to reduce pyridine alkaloid accumulation in tobacco. We found that NtARF6 could not directly repress the transcriptional activities of the key nicotine pathway structural gene promoters. Transcriptomic analysis suggested that this NtARF6-induced deactivation of alkaloid biosynthesis might be achieved by the antagonistic effect between jasmonic acid (JA) and other plant hormone signaling pathways, such as ethylene (ETH), salicylic acid (SA), abscisic acid (ABA). The repression of JA biosynthesis is accompanied by the induction of ETH, ABA, and SA signaling and pathogenic infection defensive responses, resulting in counteracting JA-induced metabolic reprogramming and decreasing the expression of nicotine biosynthetic genes in vivo. This study provides transcriptomic evidence for the regulatory mechanism of the NtARF6-mediated repression of alkaloid biosynthesis and indicates that this ARF transcription factor might act as a regulatory hub to connect different hormone signaling pathways in tobacco.

Links between Regulatory Systems of ROS and Carbohydrates in Reproductive Development

To thrive on the earth, highly sophisticated systems to finely control reproductive development have been evolved in plants. In addition, deciphering the mechanisms underlying the reproductive development has been considered as a main research avenue because it leads to the improvement of the crop yields to fulfill the huge demand of foods for the growing world population. Numerous studies revealed the significance of ROS regulatory systems and carbohydrate transports and metabolisms in the regulation of various processes of reproductive development. However, it is poorly understood how these mechanisms function together in reproductive tissues. In this review, we discuss mode of coordination and integration between ROS regulatory systems and carbohydrate transports and metabolisms underlying reproductive development based on the hitherto findings. We then propose three mechanisms as key players that integrate ROS and carbohydrate regulatory systems. These include ROS-dependent programmed cell death (PCD), mitochondrial and respiratory metabolisms as sources of ROS and energy, and functions of arabinogalactan proteins (AGPs). It is likely that these key mechanisms govern the various signals involved in the sequential events required for proper seed production.

Deep Sequencing of Small RNA Reveals the Molecular Regulatory Network of AtENO2 Regulating Seed Germination

Seed germination is a key step in the new life cycle of plants. In agriculture, we regard the rapid and consistent process of seed germination as one of the necessary conditions to measure the high quality and yield of crops. ENO2 is a key enzyme in glycolysis, which also plays an important role in plant growth and abiotic stress responses. In our study, we found that the time of seed germination in AtENO2 mutation (eno2^-) was earlier than that of wild type (WT) in Arabidopsis thaliana. Previous studies have shown that microRNAs (miRNAs) were vital in seed germination. After deep sequencing of small RNA, we found 590 differentially expressed miRNAs in total, of which 87 were significantly differentially expressed miRNAs. By predicting the target genes of miRNAs and analyzing the GO annotation, we have counted 18 genes related to seed germination, including ARF family, TIR1, INVC, RR19, TUDOR2, GA3OX2, PXMT1, and TGA1. MiR9736-z, miR5059-z, ath-miR167a-5p, ath-miR167b, ath-miR5665, ath-miR866-3p, miR10186-z, miR8165-z, ath-miR857, ath-miR399b, ath-miR399c-3p, miR399-y, miR163-z, ath-miR393a-5p, and ath-miR393b-5p are the key miRNAs regulating seed germination-related genes. Through KEGG enrichment analysis, we found that phytohormone signal transduction pathways were significantly enriched, and these miRNAs mentioned above also participate in the regulation of the genes in plant hormone signal transduction pathways, thus affecting the synthesis of plant hormones and further affecting the process of seed germination. This study laid the foundation for further exploration of the AtENO2 regulation for seed germination.

Auxin and gibberellin signaling cross-talk promotes hypocotyl xylem expansion and cambium homeostasis

During secondary growth, the thickening of plant organs, wood (xylem) and bast (phloem) is continuously produced by the vascular cambium. In Arabidopsis hypocotyl and root, we can distinguish two phases of secondary growth based on cell morphology and production rate. The first phase, in which xylem and phloem are equally produced, precedes the xylem expansion phase in which xylem formation is enhanced and xylem fibers differentiate. It is known that gibberellins (GA) trigger this developmental transition via degradation of DELLA proteins and that the cambium master regulator BREVIPEDICELLUS/KNAT1 (BP/KNAT1) and receptor like kinases ERECTA and ERL1 regulate this process downstream of GA. However, our understanding of the regulatory network underlying GA-mediated secondary growth is still limited. Here, we demonstrate that DELLA-mediated xylem expansion in Arabidopsis hypocotyl is mainly achieved through DELLA family members RGA and GAI, which promote cambium senescence. We further show that AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8, which physically interact with DELLAs, specifically repress phloem proliferation and induce cambium senescence during the xylem expansion phase. Moreover, the inactivation of BP in arf6 arf8 background revealed an essential role for ARF6 and ARF8 in cambium establishment and maintenance. Overall, our results shed light on a pivotal hormone cross-talk between GA and auxin in the context of plant secondary growth.

Investigating the presence of compensatory evolution in dicamba resistant IAA16 mutated kochia (Bassia scoparia)†

BACKGROUND

Lack of fitness costs has been reported for multiple herbicide resistance traits, but the underlying evolutionary mechanisms are not well understood. Compensatory evolution that ameliorates resistance costs, has been documented in bacteria and insects but rarely studied in weeds. Dicamba resistant IAA16 (G73N) mutated kochia was previously found to have high fecundity in the absence of competition, regardless of significant vegetative growth defects. To understand if costs of dicamba resistance can be compensated through traits promoting reproductive success in kochia, we thoroughly characterized the reproductive growth and development of different G73N kochia biotypes. Flowering phenology, seed production and reproductive allocation were quantified through greenhouse studies, floral (stigma-anthers distance) and seed morphology, as well as resulting mating and seed dispersal systems were studied through time-course microcopy images.

RESULTS

G73N covaried with multiple phenological, morphological and ecological traits that improve reproductive fitness: (i) 16-60% higher reproductive allocation; (ii) longer reproduction phase through early flowering (2-7 days); (iii) smaller stigma-anthers separation (up to 60% reduction of herkogamy and dichogamy) that can potentially promote selfing and reproductive assurance; (iv) 'winged' seeds with 30-70% longer sepals that facilitate long-distance seed dispersal.

CONCLUSION

The current study demonstrates that costs of herbicide resistance can be ameliorated through coevolution of other fitness penalty alleviating traits. As illustrated in a hypothetical model, the evolution of herbicide resistance is an ongoing fitness maximization process, which poses challenges to contain the spread of resistance. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Tracking the time-dependent and tissue-specific processes of arsenic accumulation and stress responses in rice (Oryza sativa L.)

The present study analysed time (0.5 h to 24 h) and tissue [roots, old leaves (OL) and young leaves (YL)] dependent nature of arsenic (As) accumulation and ensuing responses in two contrasting varieties of rice (Oryza sativa L.); Pooja (tolerant) and CO-50 (moderately sensitive). Arsenic accumulation was 5.4-, 4.7- and 7.3-fold higher at 24 h in roots, OL and YL, respectively of var. CO-50 than that in var. Pooja. Arsenic accumulation in YL depicted a delayed accumulation; at 2 h onwards in var. Pooja (0.23 µg g^-1 dw) while at 1 h onwards in var. CO50 (0.26 µg g^-1 dw). The responses of oxidative stress parameters, antioxidant enzymes, metabolites and ions were also found to be tissue- and time-dependent and depicted differential pattern in the two varieties. Among hormone, salicylic acid and abscisic acid showed variable response in var. Pooja and var. CO-50. Metabolite analysis depicted an involvement of various metabolites in As stress responses of two varieties. In conclusion, an early sensing of the As stress, proper coordination of hormones, biochemical responses, ionic and metabolic profiles allowed var. Pooja to resist As stress and reduce As accumulation more effectively as compared to that of var. CO-50.

Auxin and Flower Development: A Blossoming Field

The establishment of the species-specific floral organ body plan involves many coordinated spatiotemporal processes, which include the perception of positional information that specifies floral meristem and floral organ founder cells, coordinated organ outgrowth coupled with the generation and maintenance of inter-organ and inter-whorl boundaries, and the termination of meristem activity. Auxin is integrated within the gene regulatory networks that control these processes and plays instructive roles at the level of tissue-specific biosynthesis and polar transport to generate local maxima, perception, and signaling. Key features of auxin function in several floral contexts include cell nonautonomy, interaction with cytokinin gradients, and the central role of MONOPTEROS and ETTIN to regulate canonical and noncanonical auxin response pathways, respectively. Arabidopsis flowers are not representative of the enormous angiosperm floral diversity; therefore, comparative studies are required to understand how auxin underlies these developmental differences. It will be of great interest to compare the conservation of auxin pathways among flowering plants and to discuss the evolutionary role of auxin in floral development.

Protein Levels of Several Arabidopsis Auxin Response Factors Are Regulated by Multiple Factors and ABA Promotes ARF6 Protein Ubiquitination

The auxin response factor (ARF) transcription factors are a key component in auxin signaling and play diverse functions in plant growth, development, and stress response. ARFs are regulated at the transcript level and posttranslationally by protein modifications. However, relatively little is known regarding the control of ARF protein levels. We expressed five different ARFs with an HA (hemagglutinin) tag and observed that their protein levels under the same promoter varied considerably. Interestingly, their protein levels were affected by several hormonal and environmental conditions, but not by the auxin treatment. ABA (abscisic acid) as well as 4 °C and salt treatments decreased the levels of HA-ARF5, HA-ARF6, and HA-ARF10, but not that of HA-ARF19, while 37 °C treatment increased the levels of the four HA-ARFs, suggesting that the ARF protein levels are regulated by multiple factors. Furthermore, MG132 inhibited the reduction of HA-ARF6 level by ABA and 4 °C treatments, suggesting that these treatments decrease HA-ARF6 level through 26S proteasome-mediated protein degradation. It was also found that ABA treatment drastically increased HA-ARF6 ubiquitination, without strongly affecting the ubiquitination profile of the total proteins. Together, these results reveal another layer of control on ARFs, which could serve to integrate multiple hormonal and environmental signals into the ARF-regulated gene expression.

Knockdown of the DUF647 family member RUS4 impairs stamen development and pollen maturation in Arabidopsis

ROOT UV-B SENSITIVE4 (RUS4) encodes a Domain of Unknown Function647 (DUF647) protein, whose function is poorly understood. We have previously shown the artificial microRNA knockdown Arabidopsis RUS4 plants, referred to as amiR-RUS4, have severely reduced male fertility with a defect in anther dehiscence. Here, we show that amiR-RUS4 plants are also defective in pollen maturation and germination. Promoter-reporter analysis shows that RUS4 is highly expressed in tapetal layer, developing microspores, mature and germinating pollen, strongly suggesting its role in the process of pollen maturation. As the translational RUS4-GFP fusion protein has been localized to the chloroplasts where the first step of jasmonic acid (JA) biosynthesis takes place, leading to the hypothesis that RUS4 may be involved in JA-mediated stamen development. We show that expression of several JA metabolic genes increased markedly in flower buds of the amiR-RUS4 plants compared to that of the wild-type. We further show that transcript abundance of a clade of the JA-responsive MYB transcript factor genes, especially MYB108, reduced significantly in stamens of amiR-RUS4 plants relative to the wild-type; these MYB transcript factors have been shown to be required for JA-mediated stamen and pollen maturation. Our data suggest that RUS4 may play a role in coordinating anther dehiscence and pollen maturation by affecting the expression of JA-related genes.

Molecular Mechanism of Slow Vegetative Growth in Populus Tetraploid

Tetraploid plants often have altered rates of vegetative growth relative to their diploid progenitors. However, the molecular basis for altered growth rates remains a mystery. This study reports microRNA (miRNA) and gene expression differences in Populus tetraploids and counterpart diploids using RNA and miRNA sequencing. The results showed that there was no significant difference between young leaves in the expression of vegetative growth-related miRNAs. However, as leaves aged, the expression of auxin- and gibberellin-related miRNAs was significantly upregulated, while the expression of senescence-related miRNAs was significantly downregulated. The dose effect enhanced the negative regulation of the target genes with ARFs, GA20ox, GA3ox, and GAMYB being downregulated, and TCP and NAC being upregulated. As a result, the chloroplast degradation of tetraploid leaves was accelerated, the photosynthetic rate was decreased, and the synthesis and decomposition ability of carbohydrate was decreased.

Transcriptome sequencing and metabolite profiling analyses provide comprehensive insight into molecular mechanisms of flower development in Dendrobium officinale (Orchidaceae)

This research provides comprehensive insight into the molecular networks and molecular mechanisms underlying D. officinale flower development. Flowers are complex reproductive organs and play a crucial role in plant propagation, while also providing sustenance for insects and natural bioactive metabolites for humans. However, knowledge about gene regulation and floral metabolomes in flowers is limited. In this study, we used an important orchid species (Dendrobium officinale), whose flowers can be used to make herbal tea, to perform transcriptome sequencing and metabolic profiling of early- and medium-stage flower buds, as well as opened flowers, to provide comprehensive insight into the molecular mechanisms underlying flower development. A total of 8019 differentially expressed genes (DEGs) and 239 differentiated metabolites were found. The transcription factors that were identified and analyzed belong exclusively to the MIKC-type MADS-box proteins and auxin responsive factors that are known to be involved in flower development. The expression of genes involved in chlorophyll and carotenoid biosynthesis strongly matched the metabolite accumulation patterns. The genes related to flavonoid and polysaccharide biosynthesis were active during flower development. Interestingly, indole-3-acetic acid and abscisic acid, whose trend of accumulation was inverse during flower development, may play an important role in this process. Collectively, the identification of DEGs and differentiated metabolites could help to illustrate the regulatory networks and molecular mechanisms important for flower development in this orchid.

Overexpression of Arabidopsis microRNA167 induces salicylic acid-dependent defense against Pseudomonas syringae through the regulation of its targets ARF6 and ARF8

microRNAs are powerful regulators of growth, development, and stress responses in plants. The Arabidopsis thaliana microRNA miR167 was previously found to regulate diverse processes including flower development, root development, and response to osmotic stress by controlling the patterns of expression of its target genes AUXIN RESPONSE FACTOR 6 (ARF6), ARF8, and IAA-Ala RESISTANT 3. Here, we report that miR167 also modulates defense against pathogens through ARF6 and ARF8. miR167 is differentially expressed in response to the bacterial pathogen Pseudomonas syringae, and overexpression of miR167 confers very high levels of resistance. This resistance appears to be due to suppression of auxin responses and is partially dependent upon salicylic acid signaling, and also depends upon altered stomatal behavior in these plants. Closure of stomata upon the detection of P. syringae is an important aspect of the basal defense response, as it prevents bacterial cells from entering the leaf interior and causing infection. Plants overexpressing miR167 constitutively maintain small stomatal apertures, resulting in very high resistance when the pathogen is inoculated onto the leaf surface. Additionally, the systemic acquired resistance (SAR) response is severely compromised in plants overexpressing miR167, in agreement with previous work showing that the activation of SAR requires intact auxin signaling responses. This work highlights a new role for miR167, and also emphasizes the importance of hormonal balance in short- and long-term defense and of stomata as an initial barrier to pathogen entry.

Uncloaking lncRNA-meditated gene expression as a potential regulator of CMS in cotton (Gossypium hirsutum L.)

Cytoplasmic male sterility is a well-proven mechanism for cotton hybrid production. Long non-coding RNAs belong to a class of transcriptional regulators that function in multiple biological processes. The cDNA libraries from the flower buds of the cotton CGMS, it's restorer (Rf) and maintainer lines were sequenced using high throughput NGS technique. A total of 1531 lncRNAs showed significant differential expression patterns between these three lines. Functional analysis of the co-expression network of lncRNA-mRNA using gene ontology vouchsafes that, lncRNAs play a crucial role in cytoplasmic male sterility and fertility restoration through pollen development, INO80 complex, development of anther wall tapetum, chromatin remodeling, and histone modification. Additionally, 94 lncRNAs were identified as putative precursors of 49 miRNAs. qRT-PCR affirms the concordance of expression pattern to RNA-seq data. These findings divulge the lncRNA driven miRNA-mediated regulation of gene expression profiling superintended for a better understanding of the CMS mechanisms of cotton.

A role for the Auxin Response Factors ARF6 and ARF8 homologs in petal spur elongation and nectary maturation in Aquilegia

The petal spur of the basal eudicot Aquilegia is a key innovation associated with the adaptive radiation of the genus. Previous studies have shown that diversification of Aquilegia spur length can be predominantly attributed to variation in cell elongation. However, the genetic pathways that control the development of petal spurs are still being investigated. Here, we focus on a pair of closely related homologs of the AUXIN RESPONSE FACTOR family, AqARF6 and AqARF8, to explore their roles in Aquileiga coerulea petal spur development. Expression analyses of the two genes show that they are broadly expressed in vegetative and floral organs, but have relatively higher expression in petal spurs, particularly at later stages. Knockdown of the two AqARF6 and AqARF8 transcripts using virus-induced gene silencing resulted in largely petal-specific defects, including a significant reduction in spur length due to a decrease in cell elongation. These spurs also exhibited an absence of nectar production, which was correlated with downregulation of STYLISH homologs that have previously been shown to control nectary development. This study provides the first evidence of ARF6/8 homolog-mediated petal development outside the core eudicots. The genes appear to be specifically required for cell elongation and nectary maturation in the Aquilegia petal spur.

ARF family identification in Tamarix chinensis reveals the salt responsive expression of TcARF6 targeted by miR167

Auxin response factors (ARFs) are important transcription factors (TFs) that are differentially expressed in response to various abiotic stresses. The important roles of ARFs and small RNA-ARF pathways in mediating plant growth and stress responses have emerged in several recent studies. However, no studies on the involvement of ARFs in tamarisk trees, which are resistant to salinity, have been conducted. In this study, systematic analysis revealed 12 TcARF genes belonging to five different groups in Tamarix chinensis. The microRNA response elements of miR160, which belongs to group I and miR167, which belongs to group III, were conserved in terms of their location and sequence. Moreover, digital gene expression profiles suggested that a potential miR167 target gene, TcARF6, was rapidly expressed in response to salt stress. Cloning of TcARF6 revealed that TcARF6 could be an activation TF with a glutamine-rich region and expression pattern analysis revealed that the expression of TcARF6 was significantly downregulated specifically in the roots. A significant negative correlation in the expression pattern of tch-miR167/TcARF6 indicated that this module may play a key role in the response to salt stress. Overall, these results provide basic information on the posttranscriptional regulation of TcARF6 for future investigations of the T. chinensis salt-stress response.