Updates on molecular mechanisms in the development of branched trichome in Arabidopsis and nonbranched in cotton

PaNAC089 is a membrane-tethered transcription factor (MTTF) that modulates flowering, chlorophyll breakdown and trichome initiation

Flowering and senescence are essential developmental stages of green plants, which are governed by complex molecular regulatory networks. However, the connection between flowering regulation and senescence regulation in London plane tree (Platanus acerifolia ) remains unknown. In this study, we identified a gene PaNAC089 from London plane tree, which encodes a membrane-tethered transcription factor (MTTF) belonging to the NAC (NAM, ATAF1/2, CUC2) transcription factor family. We investigated the functions of PaNAC089 in the regulation of flowering and senescence through the analysis of expression profiles and transgenic phenotypes. Heterologous overexpression of ΔPaNAC089 delayed flowering and inhibited chlorophyll breakdown to produce dark green rosette leaves in Arabidopsis . In addition, the trichome density of rosette leaves was decreased in transgenic lines. In ΔPaNAC089 overexpression plants, a series of functional genes with inhibited expression were identified by quantitative real-time polymerase chain reaction (qRT-PCR), including genes that regulate flowering, chlorophyll decomposition, and trichome initiation. Furthermore, Δ PaNAC089 directly binds to the promoter of CONSTANS (CO ) and NON-YELLOWING2 (NYE2 ) in the yeast one-hybrid assay. Consistent with this, luciferase (LUC) transient expression assays also showed that Δ PaNAC089 could inhibit the activity of NYE2 . To summarise, our data suggests that PaNAC089 is an MTTF that modulates flowering, chlorophyll breakdown and trichome initiation.

NbJAZ3 is required for jasmonate-meditated glandular trichome development in Nicotiana benthamiana

Exogenous methyl jasmonate (MeJA) treatment induces glandular trichome development in Nicotiana benthamiana, but the function of JAZ proteins, acting as core repressors, and their downstream genes have not been clearly shown in plants. Here, a bioinformatics analysis of 71 JAZ genes from tobacco, Arabidopsis thaliana, and tomato was carried out and shown to share highly conserved domains. Then, the expression profile of 17 NbJAZs in different tissues was analyzed, and NbJAZ3 was highly expressed in trichome. Through transgenic technology, we demonstrated that the glandular trichome density of NbJAZ3-overexpression lines significantly decreased with lower expression levels of NbWo, NbCycB2, and NbMIXTA. In contrast, the trichome density of NbJAZ3 RNAi lines slightly increased with higher expression level of NbWo. Given the negative protein feedback regulation relationship between NbCycB2 and NbWo, we verified that MeJA induced NbWo expression. NbWo was a direct target gene of NbJAZ3 and further demonstrated that NbJAZ3 inhibited the transcriptional activation of NbCycB2 by NbWo. Together, our findings outline a novel JA-meditated glandular trichome development model consisting of the NbJAZ3-NbWo-NbCycB2 axis.

RNA-Seq with a novel glabrous-ZM24fl reveals some key lncRNAs and the associated targets in fiber initiation of cotton

BACKGROUND

Cotton fiber is an important natural resource for textile industry and an excellent model for cell biology study. Application of glabrous mutant cotton and high-throughput sequencing facilitates the identification of key genes and pathways for fiber development and cell differentiation and elongation. LncRNA is a type of ncRNA with more than 200 nt in length and functions in the ways of chromatin modification, transcriptional and post-transcriptional modification, and so on. However, the detailed lncRNA and associated mechanisms for fiber initiation are still unclear in cotton.

RESULTS

In this study, we used a novel glabrous mutant ZM24fl, which is endowed with higher somatic embryogenesis, and functions as an ideal receptor for cotton genetic transformation. Combined with the high-throughput sequencing, fatty acid pathway and some transcription factors such as MYB, ERF and bHLH families were identified the important roles in fiber initiation; furthermore, 3,288 lncRNAs were identified, and some differentially expressed lncRNAs were also analyzed. From the comparisons of ZM24_0 DPA vs ZM24_-2 DPA and fl_0 DPA vs ZM24_0 DPA, one common lncRNA MSTRG 2723.1 was found that function upstream of fatty acid metabolism, MBY25-mediating pathway, and pectin metabolism to regulate fiber initiation. In addition, other lncRNAs MSTRG 3390.1, MSTRG 48719.1, and MSTRG 31176.1 were also showed potential important roles in fiber development; and the co-expression analysis between lncRNAs and targets showed the distinct models of different lncRNAs and complicated interaction between lncRNAs in fiber development of cotton.

CONCLUSIONS

From the above results, a key lncRNA MSTRG 2723.1 was identified that might mediate some key genes transcription of fatty acid metabolism, MYB25-mediating pathway, and pectin metabolism to regulate fiber initiation of ZM24 cultivar. Co-expression analysis implied that some other important lncRNAs (e.g., MSTRG 3390.1, MSTRG 48719.1, and MSTRG 31176.1) were also showed the different regulatory model and interaction between them, which proposes some valuable clues for the lncRNAs associated mechanisms in fiber development.

Rose without prickle: genomic insights linked to moisture adaptation

Prickles act against herbivores, pathogens or mechanical injury, while also preventing water loss. However, whether prickles have new function and the molecular genetics of prickle patterning remain poorly explored. Here, we generated a high-quality reference genome assembly for ‘Basye's Thornless’ (BT), a prickle-free cultivar of Rosa wichuraiana, to identify genetic elements related to stem prickle development. The BT genome harbors a high level of sequence diversity in itself and with cultivar ‘Old Blush’ (R. chinensis), a founder genotype in rose domestication. Inheritance of stem prickle density was determined and two QTL were identified. Differentially expressed genes in QTL were involved in water-related functions, suggesting that prickle density may hitchhike with adaptations to moist environments. While the prickle-related gene-regulatory-network (GRN) was highly conserved, the expression variation of key candidate genes was associated with prickle density. Our study provides fundamental resources and insights for genome evolution in the Rosaceae. Ongoing efforts on identification of the molecular bases for key rose traits may lead to improvements for horticultural markets.

Quantitative Trait Loci and Transcriptome Analysis Reveal Genetic Basis of Fiber Quality Traits in CCRI70 RIL Population of Gossypium hirsutum

Upland cotton (Gossypium hirsutum) is widely planted around the world for its natural fiber, and producing high-quality fiber is essential for the textile industry. CCRI70 is a hybrid cotton plant harboring superior yield and fiber quality, whose recombinant inbred line (RIL) population was developed from two upland cotton varieties (sGK156 and 901-001) and were used here to investigate the source of high-quality related alleles. Based on the material of the whole population, a high-density genetic map was constructed using specific locus-amplified fragment sequencing (SLAF-seq). It contained 24,425 single nucleotide polymorphism (SNP) markers, spanning a distance of 4,850.47 centimorgans (cM) over 26 chromosomes with an average marker interval of 0.20 cM. In evaluating three fiber quality traits in nine environments to detect multiple environments stable quantitative trait loci (QTLs), we found 289 QTLs, of which 36 of them were stable QTLs and 18 were novel. Based on the transcriptome analysis for two parents and two RILs, 24,941 unique differentially expressed genes (DEGs) were identified, 473 of which were promising genes. For the fiber strength (FS) QTLs, 320 DEGs were identified, suggesting that pectin synthesis, phenylpropanoid biosynthesis, and plant hormone signaling pathways could influence FS, and several transcription factors may regulate fiber development, such as GAE6, C4H, OMT1, AFR18, EIN3, bZIP44, and GAI. Notably, the marker D13_56413025 in qFS-chr18-4 provides a potential basis for enhancing fiber quality of upland cotton via marker-assisted breeding and gene cloning of important fiber quality traits.

SlHair2 Regulates the Initiation and Elongation of Type I Trichomes on Tomato Leaves and Stems

Trichomes are hair-like structures that are essential for abiotic and biotic stress responses. Tomato Hair (H), encoding a C2H2 zinc finger protein, was found to regulate the multicellular trichomes on stems. Here, we characterized Solyc10g078990 (hereafter Hair2, H2), its closest homolog, to examine whether it was involved in trichome development. The H2 gene was highly expressed in the leaves, and its protein contained a single C2H2 domain and was localized to the nucleus. The number and length of type I trichomes on the leaves and stems of knock-out h2 plants were reduced when compared to the wild-type, while overexpression increased their number and length. An auto-activation test with various truncated forms of H2 using yeast two-hybrid (Y2H) suggested that H2 acts as a transcriptional regulator or co-activator and that its N-terminal region is important for auto-activation. Y2H and pull-down analyses showed that H2 interacts with Woolly (Wo), which regulates the development of type I trichomes in tomato. Luciferase complementation imaging assays confirmed that they had direct interactions, implying that H2 and Wo function together to regulate the development of trichomes. These results suggest that H2 has a role in the initiation and elongation of type I trichomes in tomato.

Genome-Wide Analysis of AAT Genes and Their Expression Profiling during Fiber Development in Cotton

Amino acid transporters (AATs) are a kind of membrane proteins that mediate the transport of amino acids across cell membranes in higher plants. The AAT proteins are involved in regulating plant cell growth and various developmental processes. However, the biological function of this gene family in cotton fiber development is not clear. In this study, 190, 190, 101, and 94 full-length AAT genes were identified from Gossypiumhirsutum, G. barbadense, G. arboreum, and G. raimondii. A total of 575 AAT genes from the four cotton species were divided into two subfamilies and 12 clades based on phylogenetic analysis. The AAT genes in the four cotton species were distributed on all the chromosomes. All GhAAT genes contain multiple exons, and each GhAAT protein has multiple conserved motifs. Transcriptional profiling and RT qPCR analysis showed that four GhATT genes tend to express specifically at the fiber initiation stage. Eight genes tend to express specifically at the fiber elongation and maturity stage, and four genes tend to express specifically at the fiber initiation and elongation stages. Our results provide a solid basis for further elucidating the biological function of AAT genes related to cotton fiber development and offer valuable genetic resources for crop improvement in the future.

Nonglandular prickle formation is associated with development and secondary metabolism-related genes in Rosa multiflora

Roses are among the most economically important ornamental plants worldwide. But prickles on the stem and leaves cause difficulties for cultivation or inconveniences during harvest and transportation, thus are an undesirable horticultural character. However, little is known about the molecular mechanisms of prickle development. In this study, we sought to develop Rosa multiflora (in the family Rosaceae) as a model plant to study prickle formation. The morphology, structure, and ontogeny of prickles were characterized, and transcriptome analysis of prickly and prickleless R. multiflora genotypes was performed. Morphological observation and microscopic analyses revealed that prickles of R. multiflora were non-glandular prickles (NGPs) and their maturation went through five developmental stages, which was accompanied by the accumulation of secondary metabolites such as lignin and anthocyanins. Comparative transcriptome analysis identified key pathways and hub genes potentially involved in prickle formation. Interestingly, among the differentially expressed genes (DEGs), several notable development and secondary metabolism-related transcription factors (TFs) including NAC, TCP, MYB, homeobox, and WRKY were up-regulated in prickly internodes. KEGG enrichment analysis indicated that DEGs were enriched in the pathways related to biosynthesis of secondary metabolites, flavonoids, and phenylpropanoids in the prickly R. multiflora. Our study provides novel insights into the molecular network underlying the regulation of prickle morphogenesis in R. multiflora, and the identified candidates might be applied to the genetic improvement of roses.

A pair of DUF538 domain-containing proteins modulates plant growth and trichome development through the transcriptional regulation of GLABRA1 in Arabidopsis thaliana

SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) is an emerging plant growth regulator in trichome development, endoplasmic reticulum stress response, and phosphoinositide signaling, and belongs to the land plant-specific DUF538 domain-containing protein family. Despite its multifaceted roles, the functions of this protein family are poorly understood in plant growth and development. Here, we report that SVB-like (SVBL), the closest homolog of SVB, modulates plant growth and trichome development with SVB in Arabidopsis thaliana. Although none of the single mutants showed an obvious growth defect, the double mutants of svb svbl exhibited dwarfed plant growth. In trichome development, the defects in svb mutant were greatly enhanced by the additional mutation in SVBL, despite the single knockout of SVBL showing the mild defects. The double mutation reduced the transcript level of one of the central hub genes for trichome development, GLABRA1 (GL1), which in turn affects the other downstream genes, GLABRA2 (GL2), TRANSPARENT TESTA GLABRA2 (TTG2), TRIPTYCHON (TRY), CAPRICE (CPC), and ENHANCER OF TRY AND CPC1 (ETC1). In situ translational reporter assays showed that SVB and SVBL share highly similar localization patterns both at tissue and subcellular levels. The present study suggests that SVB and SVBL play a pivotal role in plant growth and trichome development by affecting a specific subset of known trichome developmental regulators, highlighting the importance of the DUF538 protein family in higher plants.

Function analysis of a cotton R2R3 MYB transcription factor GhMYB3 in regulating plant trichome development

Cotton is an important fibre-producing crop. Cotton fibres consist of highly elongated trichomes derived from the ovule. To improve the quality of cotton, it is necessary to identify the genes regulating fibre development. GhMYB3 was identified through bioinfomatic analysis and introduced to Arabidopsis and cotton to observe the phenotype. Protein inteaction and promoter bingding assays were conducted to explore the role of GhMYB3 in trichome fibre growth. Cotton fibre development might share a similar regulatory mechanism to Arabidopsis leaf trichomes, which is determined by the essential regulatory complex, MYB-bHLH-WD40. The GL1-like R2R3 MYB transcription factor GhMYB3 interacts with the AtGL3 protein involved in Arabidopsis trichome development. Ectopic expression of GhMYB3 could rescue the glabrous phenotype of the Arabidopsis gl1 mutant and produced more ectopic trichomes on inflorescence stems and floral organs, confirming its orthologous function in plant trichome development. The expression of GhMYB3 increased in response to exogenous gibberellin (GA₃ ), auxin (IAA) and methyl jasmonate (MeJA). Overexpression of this gene in cotton leads to a slight increase in fibre length and lint percentage, possibly by activating the transcription of its downstream gene GhRDL1 or other fibre-related genes. The results increase our understanding of the key role of GhMYB3 in positively controlling plant trichome development, and this gene could be a potential target for molecular breeding in cotton.

Transcriptome profiling reveals key genes in regulation of the tepal trichome development in Lilium pumilum D.C

A number of potential genes and pathways involved in tepal trichome development were identified in a natural lily mutant by transcriptome analysis and were confirmed with trichome and trichomeless species. Trichome is a specialized structure found on the surface of the plant with an important function in survival against abiotic and biotic stress. It is also an important economic trait in crop breeding. Extensive research has investigated the foliar trichome in model plants (Arabidopsis and tomato). However, the developmental mechanism of tepal trichome remains elusive. Lilium pumilum is an edible ornamental bulb and a good breeding parent possessing cold and salt-alkali resistance. Here, we found a natural mutant of Lilium pumilum grown on a highland whose tepals are covered by trichomes. Our data indicate that trichomes of the mutant are multicellular and branchless. Notably, stomata are also developed on the tepal of the mutant as well, suggesting there may be a correlation between trichome and stomata regulation. Furthermore, we isolated 27 differentially expressed genes (DEGs) by comparing the transcriptome profiling between the natural mutant and the wild type. These 27 genes belong to 4 groups: epidermal cell cycle and division, trichome morphogenesis, stress response, and transcription factors. Quantitative real-time PCR in Lilium pumilum (natural mutant and the wild type) and other lily species (Lilium leichtlinii var. maximowiczii/trichome; Lilium davidii var. willmottiae/, trichomeless) confirmed the validation of RNA-seq data and identified several trichome-related genes.

Advances in the Regulation of Epidermal Cell Development by C2H2 Zinc Finger Proteins in Plants

Plant epidermal cells, such as trichomes, root hairs, salt glands, and stomata, play pivotal roles in the growth, development, and environmental adaptation of terrestrial plants. Cell fate determination, differentiation, and the formation of epidermal structures represent basic developmental processes in multicellular organisms. Increasing evidence indicates that C2H2 zinc finger proteins play important roles in regulating the development of epidermal structures in plants and plant adaptation to unfavorable environments. Here, we systematically summarize the molecular mechanism underlying the roles of C2H2 zinc finger proteins in controlling epidermal cell formation in plants, with an emphasis on trichomes, root hairs, and salt glands and their roles in plant adaptation to environmental stress. In addition, we discuss the possible roles of homologous C2H2 zinc finger proteins in trichome development in non-halophytes and salt gland development in halophytes based on bioinformatic analysis. This review provides a foundation for further study of epidermal cell development and abiotic stress responses in plants.

Genome-Wide Analysis of SRNF Genes in Gossypium hirsutum Reveals the Role of GhSRNF18 in Primary Root Growth

Root systems are instrumental for water and nutrient uptake and the anchorage of plants in the soil. Root regulating GL2-interacting repressors (GIRs) contain a Short RING-like Zinc-Finger (SRNF) domain, but there has been no comprehensive characterization about this gene family in any plant species. Here, we renamed the GIR-like proteins as SRNF proteins due to their conserved domain and identified 140 SRNF genes from 16 plant species including 24 GhSRNF genes in Gossypium hirsutum. Phylogenetic analysis of the SRNFs revealed both similarities and divergences between five subfamilies. Notably, synteny analysis revealed that polyploidization and whole-genome duplication contribute to the expansion of the GhSRNF gene family. Various cis-acting regulatory elements were shown to be pertinent to light, phytohormone, defense responsive, and meristem regulation. Furthermore, GhSRNF2/15 were predominantly expressed in root, whereas the expression of GhSRNF18 is positively correlated with the primary root (PR) length in G. hirsutum, quantified by quantitative real-time PCR (qRT-PCR). Over-expression of GhSRNF18 in Arabidopsis and virus-induced gene silencing (VIGS) of GhSRNF18 in G. hirsutum has revealed the role of GhSRNF18 in PR growth. The over-expression of GhSRNF18 in Arabidopsis resulted in an increase of meristematic activities and auxin accumulations in PRs, which were consistent with the transcriptomic data. Our results suggested that GhSRNF18 positively regulates PR growth. This study increased our understanding of the SRNF gene family in plants and provided a novel rationale for the further investigation of cotton root morphogenesis regulated by the GhSRNFs.

Sphingolipid Profile during Cotton Fiber Growth Revealed That a Phytoceramide Containing Hydroxylated and Saturated VLCFA Is Important for Fiber Cell Elongation

Cotton fiber is a single-celled seed trichrome that arises from the epidermis of the ovule’s outer integument. The fiber cell displays high polar expansion and thickens but not is disrupted by cell division. Therefore, it is an ideal model for studying the growth and development of plant cells. Sphingolipids are important components of membranes and are also active molecules in cells. However, the sphingolipid profile during fiber growth and the differences in sphingolipid metabolism at different developmental stages are still unclear. In this study, we detected that there were 6 classes and 95 molecular species of sphingolipids in cotton fibers by ultrahigh performance liquid chromatography-MS/MS (UHPLC-MS/MS). Among these, the phytoceramides (PhytoCer) contained the most molecular species, and the PhytoCer content was highest, while that of sphingosine-1-phosphate (S1P) was the lowest. The content of PhytoCer, phytoceramides with hydroxylated fatty acyls (PhytoCer-OHFA), phyto-glucosylceramides (Phyto-GluCer), and glycosyl-inositol-phospho-ceramides (GIPC) was higher than that of other classes in fiber cells. With the development of fiber cells, phytosphingosine-1-phosphate (t-S1P) and PhytoCer changed greatly. The sphingolipid molecular species Ceramide (Cer) d18:1/26:1, PhytoCer t18:1/26:0, PhytoCer t18:0/26:0, PhytoCer t18:1/h20:0, PhytoCer t18:1/h26:0, PhytoCer t18:0/h26:0, and GIPC t18:0/h16:0 were significantly enriched in 10-DPA fiber cells while Cer d18:1/20:0, Cer d18:1/22:0, and GIPC t18:0/h18:0 were significantly enriched in 20-DPA fiber cells, indicating that unsaturated PhytoCer containing hydroxylated and saturated very long chain fatty acids (VLCFA) play some role in fiber cell elongation. Consistent with the content analysis results, the related genes involved in long chain base (LCB) hydroxylation and unsaturation as well as VLCFA synthesis and hydroxylation were highly expressed in rapidly elongating fiber cells. Furthermore, the exogenous application of a potent inhibitor of serine palmitoyltransferase, myriocin, severely blocked fiber cell elongation, and the exogenous application of sphingosine antagonized the inhibition of myriocin for fiber elongation. Taking these points together, we concluded that sphingolipids play crucial roles in fiber cell elongation and SCW deposition. This provides a new perspective for further studies on the regulatory mechanism of the growth and development of cotton fiber cells.

Improved genome assembly and pan-genome provide key insights into eggplant domestication and breeding

Eggplant (Solanum melongena L.) is an important horticultural crop and one of the most widely grown vegetables from the Solanaceae family. It was domesticated from a wild, prickly progenitor carrying small, round, non-anthocyanic fruits. We obtained a novel, highly contiguous genome assembly of the eggplant '67/3' reference line, by Hi-C retrofitting of a previously released short read- and optical mapping-based assembly. The sizes of the 12 chromosomes and the fraction of anchored genes in the improved assembly were comparable to those of a chromosome-level assembly. We resequenced 23 accessions of S. melongena representative of the worldwide phenotypic, geographic, and genetic diversity of the species, and one each from the closely related species Solanum insanum and Solanum incanum. The eggplant pan-genome contained approximately 51.5 additional megabases and 816 additional genes compared with the reference genome, while the pan-plastome showed little genetic variation. We identified 53 selective sweeps related to fruit color, prickliness, and fruit shape in the nuclear genome, highlighting selection leading to the emergence of present-day S. melongena cultivars from its wild ancestors. Candidate genes underlying the selective sweeps included a MYBL1 repressor and CHALCONE ISOMERASE (for fruit color), homologs of Arabidopsis GLABRA1 and GLABROUS INFLORESCENCE STEMS2 (for prickliness), and orthologs of tomato FW2.2, OVATE, LOCULE NUMBER/WUSCHEL, SUPPRESSOR OF OVATE, and CELL SIZE REGULATOR (for fruit size/shape), further suggesting that selection for the latter trait relied on a common set of orthologous genes in tomato and eggplant.

Basic Helix-Loop-Helix (bHLH) Transcription Factors Regulate a Wide Range of Functions in Arabidopsis

The basic helix-loop-helix (bHLH) transcription factor family is one of the largest transcription factor gene families in Arabidopsis thaliana, and contains a bHLH motif that is highly conserved throughout eukaryotic organisms. Members of this family have two conserved motifs, a basic DNA binding region and a helix-loop-helix (HLH) region. These proteins containing bHLH domain usually act as homo- or heterodimers to regulate the expression of their target genes, which are involved in many physiological processes and have a broad range of functions in biosynthesis, metabolism and transduction of plant hormones. Although there are a number of articles on different aspects to provide detailed information on this family in plants, an overall summary is not available. In this review, we summarize various aspects of related studies that provide an overview of insights into the pleiotropic regulatory roles of these transcription factors in plant growth and development, stress response, biochemical functions and the web of signaling networks. We then provide an overview of the functional profile of the bHLH family and the regulatory mechanisms of other proteins.

Identification of the Group III WRKY Subfamily and the Functional Analysis of GhWRKY53 in Gossypium hirsutum L

WRKY transcription factors had multiple functions in plant secondary metabolism, leaf senescence, fruit ripening, adaptation to biotic and abiotic stress, and plant growth and development. However, knowledge of the group III WRKY subfamily in fiber development in upland cotton (Gossypium hirsutum L.) is largely absent. Previous studies have shown that there were 21 putative group III WRKY members in G. hirsutum L. These putative amino acid sequences from the III WRKY group were phylogenetically clustered into three clades. Multiple alignment, conservative motif analysis, and gene structure analysis showed that the members clustered together in the phylogenetic tree had similar motifs and gene structures. Expression pattern analysis revealed that variation in the expression levels of these genes in different tissues and fiber development stages. To better understand the functions of putative group III WRKY genes in G. hirsutum L., we selected the cotton fiber initiation-related gene GhWRKY53 for cloning and functional identification. The subcellular localization experiment of GhWRKY53 in Nicotiana tabacum leaves showed that it was located in the nucleus. The heterologous expression of GhWRKY53 in Arabidopsis thaliana could significantly increase the density of trichomes. Twelve proteins that interacted with GhWRKY53 were screened from the cotton fiber cDNA library by yeast two-hybrid experiment. This study findings lay a foundation for further research on the role of the GhWRKY53 during cotton fiber development and provide a new insight for further studying putative group III WRKY genes in G. hirsutum L. Our research results also provide vital information for the genetic mechanism of high-quality cotton fiber formation and essential genetic resources for cotton fiber quality improvement.

Recent Advances and Future Perspectives in Cotton Research

Cotton is not only the world's most important natural fiber crop, but it is also an ideal system in which to study genome evolution, polyploidization, and cell elongation. With the assembly of five different cotton genomes, a cotton-specific whole-genome duplication with an allopolyploidization process that combined the A- and D-genomes became evident. All existing A-genomes seemed to originate from the A₀-genome as a common ancestor, and several transposable element bursts contributed to A-genome size expansion and speciation. The ethylene production pathway is shown to regulate fiber elongation. A tip-biased diffuse growth mode and several regulatory mechanisms, including plant hormones, transcription factors, and epigenetic modifications, are involved in fiber development. Finally, we describe the involvement of the gossypol biosynthetic pathway in the manipulation of herbivorous insects, the role of GoPGF in gland formation, and host-induced gene silencing for pest and disease control. These new genes, modules, and pathways will accelerate the genetic improvement of cotton.

Ubiquitylome analysis reveals the involvement of ubiquitination in the bast fiber growth of ramie

A total of 249 sites from 197 proteins showed a differential ubiquitination level in the fiber development of ramie barks. The function of two differentially ubiquitinated proteins for fiber growth was demonstrated. Ubiquitination is one of the most common post-translational modifications of proteins, and it plays essential roles in plant growth and development. However, the involvement of ubiquitination in the growth of plant fibers remains largely unknown. We compared the ubiquitylome of the top and middle stems of ramie bark, with different fiber growth stages. We identified 249 differentially ubiquitinated sites in 197 proteins in fiber-developing barks in the stems and found that seven were homologs of Arabidopsis proteins associated with fiber growth. Overexpression of the differentially ubiquitinated proteins, RWA3 homolog whole_GLEAN_10024150 and MYB protein whole_GLEAN_10015497, significantly promoted fiber growth in transgenic Arabidopsis, indicating their involvement in this process. We also found that the abundance of these proteins decreased when their ubiquitination levels increased and vice versa in the fiber-developing bark. These results indicated that the abundance of these two proteins was adjusted through ubiquitin-dependent degradation. Collectively, our findings provide important insights into the involvement of ubiquitination in the growth of ramie fibers.

Conserved and non-conserved functions of the rice homologs of the Arabidopsis trichome initiation-regulating MBW complex proteins

BACKGROUND

Trichome initiation in Arabidopsis is regulated by a MYB-bHLH-WD40 (MBW) transcriptional activator complex formed by the R2R3 MYB transcription factor GLABRA1 (GL1), MYB23 or MYB82, the bHLH transcription factor GLABRA3 (GL3), ENHANCER OF GLABRA3 (EGL3) or TRANSPARENT TESTA8 (TT8), and the WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1). However, the functions of the rice homologs of the MBW complex proteins remained uncharacterized.

RESULTS

Based on amino acid sequence identity and similarity, and protein interaction prediction, we identified OsGL1s, OsGL3s and OsTTG1s as rice homologs of the MBW complex proteins. By using protoplast transfection, we show that OsGL1D, OsGL1E, OsGL3B and OsTTG1A were predominantly localized in the nucleus, OsGL3B functions as a transcriptional activator and is able to interact with GL1 and TTG1. By using yeast two-hybrid and protoplast transfection assays, we show that OsGL3B is able to interact with OsGL1E and OsTTG1A, and OsGL1E and OsTTG1A are also able to interact with GL3. On the other hand, we found that OsGL1D functions as a transcription activator, and it can interact with GL3 but not OsGL3B. Furthermore, our results show that expression of OsTTG1A in the ttg1 mutant restored the phenotypes including alternations in trichome and root hair formation, seed color, mucilage production and anthocyanin biosynthesis, indicating that OsTTG1A and TTG1 may have similar functions.

CONCLUSION

These results suggest that the rice homologs of the Arabidopsis MBW complex proteins are able to form MBW complexes, but may have conserved and non-conserved functions.

Assessing and integrating the transcriptome analysis with plant development, trichomes, and secondary metabolites yield potential in Mentha arvensisL

Mentha arvensisL. (corn mint) is well known for the production of menthol, a used commodity in flavouring industries, and provides natural fragrances. Glandular trichomes are responsible for producing specific secondary metabolites in vascular plants having species chemistry. Ten cultivars/varieties of M. arvensis, namely, Saksham, Kosi, Himalaya, Gomti, Sambhav, Kalka, Damroo, Kushal, and Shivalik, were used to study the developmental regulation of trichomes, essential oil yield, chemical constituents of essential oil and morphological parameters were estimated with gene expression using a randomized block design. Simultaneously, RNA sequence-based transcriptome analysis was done to reveal the transcription factors and differential gene analysis, which are responsible for the biosynthesis of essential oil as well as trichome development. Plant growth showed the maximum transition between 35 and 50 days stage, while essential oil and its metabolite bioconversion was observed in between 70 and 100 days stage. Glandular trichomes were maximally increased between 50, 70 days, and 100 days stage in var. Kosi followed by var. Saryu which has rapid growth in oil content. Menthol reductase activity was found to be a regulatory element during development, as it follows the inverse trend of menthol content and leads to menthol accumulation in subcuticular spaces. Transcriptional factors, cog, and nonredundant novel genes were identified. The composition of mintessential oils is regulated at multiple levels, including transcript abundance, catalytic properties of enzyme catalysts, and cell type-specific epigenetic processes.

Comparative Transcriptome Analysis Reveals Key Genes and Pathways Involved in Prickle Development in Eggplant

Eggplant is one of the most important vegetables worldwide. Prickles on the leaves, stems and fruit calyxes of eggplant may cause difficulties during cultivation, harvesting and transportation, and therefore is an undesirable agronomic trait. However, limited knowledge about molecular mechanisms of prickle morphogenesis has hindered the genetic improvement of eggplant. In this study, we performed the phenotypic characterization and transcriptome analysis on prickly and prickleless eggplant genotypes to understand prickle development at the morphological and molecular levels. Morphological analysis revealed that eggplant prickles were multicellular, lignified and layered organs. Comparative transcriptome analysis identified key pathways and hub genes involved in the cell cycle as well as flavonoid biosynthetic, photosynthetic, and hormone metabolic processes during prickle development. Interestingly, genes associated with flavonoid biosynthesis were up-regulated in developing prickles, and genes associated with photosynthesis were down-regulated in developing and matured prickles. It was also noteworthy that several development-related transcription factors such as bHLH, C2H2, MYB, TCP and WRKY were specifically down- or up-regulated in developing prickles. Furthermore, four genes were found to be differentially expressed within the Pl locus interval. This study provides new insights into the regulatory molecular mechanisms underlying prickle morphogenesis in eggplant, and the genes identified might be exploited in breeding programs to develop prickleless eggplant cultivars.

Differentiation in the genetic basis of stem trichome development between cultivated tetraploid cotton species

BACKGROUND

Cotton stem trichomes and seed fibers are each single celled structures formed by protrusions of epidermal cells, and were found sharing the overlapping molecular mechanism. Compared with fibers, cotton stem trichomes are more easily observed, but the molecular mechanisms underlying their development are still poorly understood.

RESULTS

In this study, Gossypium hirsutum (Gh) and G. barbadense (Gb) were found to differ greatly in percentages of varieties/accessions with glabrous stems and in trichome density, length, and number per trichopore. Gh varieties normally had long singular and clustered trichomes, while Gb varieties had short clustered trichomes. Genetic mapping using five F₂ populations from crosses between glabrous varieties and those with different types of stem trichomes revealed that much variation among stem trichome phenotypes could be accounted for by different combinations of genes/alleles on Chr. 06 and Chr. 24. The twenty- six F₁ generations from crosses between varieties with different types of trichomes had varied phenotypes, further suggesting that the trichomes of tetraploid cotton were controlled by different genes/alleles. Compared to modern varieties, a greater proportion of Gh wild accessions were glabrous or had shorter and denser trichomes; whereas a smaller proportion of Gb primitive accessions had glabrous stems. A close correlation between fuzz fiber number and stem trichome density was observed in both Gh and Gb primitive accessions and modern varieties.

CONCLUSION

Based on these findings, we hypothesize that stem trichomes evolved in parallel with seed fibers during the domestication of cultivated tetraploid cotton. In addition, the current results illustrated that stem trichome can be used as a morphological index of fiber quality in cotton conventional breeding.

UDP-Glucose Dehydrogenases: Identification, Expression, and Function Analyses in Upland Cotton (Gossypium hirsutum)

UDP-glucose dehydrogenase (UGD; EC1.1.1.22) is a NAD⁺-dependent enzyme that catalyzes the two-fold oxidation of UDP-glucose (UDP-Glc) to produce UDP-glucuronic acid and plays an important role in plant cell wall synthesis. A total of 42 UGD genes from four Gossypium genomes including G. hirsutum, G. arboretum, G. barbadense, and G. raimondii were identified and found that the UGD gene family has conservative evolution patterns in gene structure and protein domain. The growth of fibers can be effectively promoted after adding the UDP-Glc to the medium, and the GhUGD gene expression enhanced. In addition, the transgenic Arabidopsis lines over-expressing GH_D12G1806 had longer root lengths and higher gene expression level than the wild-type plants of Columbia-0. These results indicated that UGD may play important roles in cotton fiber development and has a guiding significance for dissecting fiber development mechanism.

Transcriptome Analysis Reveals a Gene Expression Pattern Associated with Fuzz Fiber Initiation Induced by High Temperature in Gossypium barbadense

Gossypium barbadense is an important source of natural textile fibers, as is Gossypium hirsutum. Cotton fiber development is often affected by various environmental factors, such as abnormal temperature. However, little is known about the underlying mechanisms of temperature regulating the fuzz fiber initiation. In this study, we reveal that high temperatures (HT) accelerate fiber development, improve fiber quality, and induced fuzz initiation of a thermo-sensitive G. barbadense variety L7009. It was proved that fuzz initiation was inhibited by low temperature (LT), and 4 dpa was the stage most susceptible to temperature stress during the fuzz initiation period. A total of 43,826 differentially expressed genes (DEGs) were identified through comparative transcriptome analysis. Of these, 9667 were involved in fiber development and temperature response with 901 transcription factor genes and 189 genes related to plant hormone signal transduction. Further analysis of gene expression patterns revealed that 240 genes were potentially involved in fuzz initiation induced by high temperature. Functional annotation revealed that the candidate genes related to fuzz initiation were significantly involved in the asparagine biosynthetic process, cell wall biosynthesis, and stress response. The expression trends of sixteen genes randomly selected from the RNA-seq data were almost consistent with the results of qRT-PCR. Our study revealed several potential candidate genes and pathways related to fuzz initiation induced by high temperature. This provides a new view of temperature-induced tissue and organ development in Gossypium barbadense.

Genome-wide association study reveals the genetic basis of fiber quality traits in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Fiber quality is an important economic trait of cotton, and its improvement is a major goal of cotton breeding. To better understand the genetic mechanisms responsible for fiber quality traits, we conducted a genome-wide association study to identify and mine fiber-quality-related quantitative trait loci (QTLs) and genes.

RESULTS

In total, 42 single nucleotide polymorphisms (SNPs) and 31 QTLs were identified as being significantly associated with five fiber quality traits. Twenty-five QTLs were identified in previous studies, and six novel QTLs were firstly identified in this study. In the QTL regions, 822 genes were identified and divided into four clusters based on their expression profiles. We also identified two pleiotropic SNPs. The SNP locus i52359Gb was associated with fiber elongation, strength, length and uniformity, while i11316Gh was associated with fiber strength and length. Moreover, these two SNPs were nonsynonymous and located in genes Gh_D09G2376 and Gh_D06G1908, respectively. RT-qPCR analysis revealed that these two genes were preferentially expressed at one or more stages of cotton fiber development, which was consistent with the RNA-seq data. Thus, Gh_D09G2376 and Gh_D06G1908 may be involved in fiber developmental processes.

CONCLUSIONS

The findings of this study provide insights into the genetic bases of fiber quality traits, and the identified QTLs or genes may be applicable in cotton breeding to improve fiber quality.

Heterologous overexpression of the GbTCP5 gene increased root hair length, root hair and stem trichome density, and lignin content in transgenic Arabidopsis

Teosinte branched1/cycloidea/proliferating cell factor1 (TCP) is a plant-specific protein family member involved in plant growth and development. However, the functions of most members of the cotton TCP family are unknown. In this study, the GbTCP5 gene encodes a sea-island cotton class II TCP CIN subclass transcription factor. The GbTCP5 transcription factor is located in the nucleus, has transcriptional activation activity, and can bind to TCP II cis-acting elements. GbTCP5 was widely expressed in tissues with the highest transcript level in the calyx. GbTCP5 is expressed at different developmental stages of the fiber and has significantly high transcriptional level expression in the fibers at 20, 30 and 35 days post anthesis (DPA). Heterologous overexpression of the GbTCP5 gene increased root hair length, root hair and stem trichome density, and stem lignin content in transgenic Arabidopsis compared to the wild type (WT). GbTCP5 binds the promoters of the GL3, EGL3, CPC, MYB46, LBD30, CesA4, VND7, CCOMT1, and CAD5 genes to upregulate their expression. Moreover, the homologous genes of these genes are expressed in the fibers of different developmental stages of the sea-island cotton fiber. These results indicate that GbTCP5 regulates root hair development and secondary wall formation in Arabidopsis and may be a candidate gene for improving cotton fiber quality.

Understanding the role of phytohormones in cotton fiber development through omic approaches; recent advances and future directions

Cotton is among the most important fiber crops for the textile-based industry, thanks to its cellulose-rich mature fibers. The fiber initiation and elongation are one of the best models for deciphering mechanisms of single-cell differentiation and growth, that also target of fiber development programs. During the last couple of decades, high yielding omics approaches (genomics, transcriptomics, and proteomics), have helped in the identification of several genes and gene products involved in fiber development along with functional relationship to phytohormones. For example, MYB transcription factor family and Sus gene family have been evidenced by controlling cotton fiber initiation. Most importantly, the biosynthesis, responses, and transporting of phytohormones is documented to participate in the initiation of cotton fibers. Herein, in this review, the reliable genetic evidence by manipulating the above genes in cotton have been summarized to describe the relationships among key phytohormones, transcription factors, proteins, and downstream fiber growth-related genes such as Sus. The effect of other important factors such as ROS, fatty acid metabolism, and actin (globular multi-functional proteins) over fiber development has also been discussed. The challenges and deficiencies in the research of cotton fiber development have been mentioned along with a future perspective to discover new crucial genes using multiple omics analysis.

A cotton α1,3-/4-fucosyltransferase-encoding gene, FucT4, plays an important role in cell elongation and is significantly associated with fiber quality

Fucosylation, one of the key posttranslational modifications, plays an important role in plants. It is involved in the development, signal transduction, reproduction, and disease resistance. α1,3-/4-Fucosyltransferase is responsible for transferring L-fucose from GDP-L-fucose to the N-glycan to exert fucosylational functions. However, the roles of the fucosyltransferase gene in cotton remain unknown. This study provided a comprehensive investigation of its possible functions. A genome-wide analysis identified four, four, eight, and eight FucT genes presented in the four sequenced cotton species, diploid Gossypium raimondii, G. arboreum, tetraploid G. hirsutum acc. TM-1, and G. barbadense cv. H7124, respectively. These FucTs were classified into two groups, with FucT4 homologs alone as a group. We isolated FucT4 in TM-1 and H7124, and named it GhFucT4 and GbFucT4, respectively. Quantitative RT-PCR and transcriptome data demonstrated that GhFucT4 had the highest expression levels in fibers among all GhFucT genes. Association studies and QTL co-localization supported the possible involvement of GhFucT4 in cotton fiber development. GhFucT4 and GbFucT4 shared high sequence identities, and FucT4 had higher expression in H7124 fiber tissues compared with TM-1. Furthermore, ectopic expression of FucT4 in transgenic Arabidopsis promoted root cell elongation, upregulated expression of genes related to cell wall loosening, and led to longer primary root. These results collectively indicate that FucT4 plays an important role in promoting cell elongation and modulating fiber development, which could be utilized to improve fiber quality traits in cotton breeding.

Dissection of Dynamic Transcriptome Landscape of Leaf, Bract, and Lupulin Gland in Hop (Humulus lupulus L.)

The hop plant (Humulus lupulus L.) produces several valuable secondary metabolites, such as prenylflavonoid, bitter acids, and essential oils. These compounds are biosynthesized in glandular trichomes (lupulin glands) endowed with pharmacological properties and widely implicated in the beer brewing industry. The present study is an attempt to generate exhaustive information of transcriptome dynamics and gene regulatory mechanisms involved in biosynthesis and regulation of these compounds, developmental changes including trichome development at three development stages, namely leaf, bract, and mature lupulin glands. Using high-throughput RNA-Seq technology, a total of 61.13, 50.01, and 20.18 Mb clean reads in the leaf, bract, and lupulin gland libraries, respectively, were obtained and assembled into 43,550 unigenes. The putative functions were assigned to 30,996 transcripts (71.17%) based on basic local alignment search tool similarity searches against public sequence databases, including GO, KEGG, NR, and COG families, which indicated that genes are principally involved in fundamental cellular and molecular functions, and biosynthesis of secondary metabolites. The expression levels of all unigenes were analyzed in leaf, bract, and lupulin glands tissues of hop. The expression profile of transcript encoding enzymes of BCAA metabolism, MEP, and shikimate pathway was most up-regulated in lupulin glands compared with leaves and bracts. Similarly, the expression levels of the transcription factors and structural genes that directly encode enzymes involved in xanthohumol, bitter acids, and terpenoids biosynthesis pathway were found to be significantly enhanced in lupulin glands, suggesting that production of these metabolites increases after the leaf development. In addition, numerous genes involved in primary metabolism, lipid metabolism, photosynthesis, generation of precursor metabolites/energy, protein modification, transporter activity, and cell wall component biogenesis were differentially regulated in three developmental stages, suggesting their involvement in the dynamics of the lupulin gland development. The identification of differentially regulated trichome-related genes provided a new foundation for molecular research on trichome development and differentiation in hop. In conclusion, the reported results provide directions for future functional genomics studies for genetic engineering or molecular breeding for augmentation of secondary metabolite content in hop.

Development of Parasitic Organs of a Stem Holoparasitic Plant in Genus Cuscuta

Parasitic plants infect a broad range of plant species including economically important crops. They survive by absorbing water, minerals, and photosynthates from their hosts. To support their way of life, parasitic plants generally establish parasitic organs that allow them to attach to their hosts and to efficiently absorb substances from the vascular system of the host. Here, we summarize the recent progress in understanding the mechanisms underlying the formation of these parasitic organs, focusing on the process depicted in the stem holoparasitic genus, Cuscuta. An attachment structure called "holdfast" on the stem surface is induced by the light and contact stimuli. Concomitantly with holdfast formation, development of an intrusive structure called haustorium initiates in the inner cortex of the Cuscuta stem, and it elongates through apoplastic space of the host tissue. When haustoria reaches to host vascular tissues, they begin to form vascular conductive elements to connect vascular tissue of Cuscuta stem to those of host. Recent studies have shown parasite-host interaction in the interfacial cell wall, and regulation of development of these parasitic structures in molecular level. We also briefly summarize the role of host receptor in the control of compatibility between Cuscuta and hosts, on which occurrence of attachment structure depends, and the role of plant-to-plant transfer of long-distance signals after the establishment of conductive structure.