Cotton (Gossypium hirsutum) 14-3-3 proteins participate in regulation of fibre initiation and elongation by modulating brassinosteroid signalling

Transcriptional and translational landscape fine-tune genome annotation and explores translation control in cotton

INTRODUCTION

The unavailability of intergenic region annotation in whole genome sequencing and pan-genomics hinders efforts to enhance crop improvement.

OBJECTIVES

Despite advances in research, the impact of post-transcriptional regulation on fiber development and translatome profiling at different stages of fiber growth in cotton (G. hirsutum) remains unexplored.

METHODS

We utilized a combination of reference-guided de novo transcriptome assembly and ribosome profiling techniques to uncover the hidden mechanisms of translational control in eight distinct tissues of upland cotton.

RESULTS

Our study identified P-site distribution at three-nucleotide periodicity and dominant ribosome footprint at 27 nucleotides. Specifically, we have detected 1,589 small open reading frames (sORFs), including 1,376 upstream ORFs (uORFs) and 213 downstream ORFs (dORFs), as well as 552 long non-coding RNAs (lncRNAs) with potential coding functions, which fine-tune the annotation of the cotton genome. Further, we have identified novel genes and lncRNAs with strong translation efficiency (TE), while sORFs were found to affect mRNA transcription levels during fiber elongation. The reliability of these findings was confirmed by the high consistency in correlation and synergetic fold change between RNA-sequencing (RNA-seq) and Ribosome-sequencing (Ribo-seq) analyses. Additionally, integrated omics analysis of the normal fiber ZM24 and short fiber pag1 cotton mutant revealed several differentially expressed genes (DEGs), and fiber-specific expressed (high/low) genes associated with sORFs (uORFs and dORFs). These findings were further supported by the overexpression and knockdown of GhKCS6, a gene associated with sORFs in cotton, and demonstrated the potential regulation of the mechanism governing fiber elongation on both the transcriptional and post-transcriptional levels.

CONCLUSION

Reference-guided transcriptome assembly and the identification of novel transcripts fine-tune the annotation of the cotton genome and predicted the landscape of fiber development. Our approach provided a high-throughput method, based on multi-omics, for discovering unannotated ORFs, hidden translational control, and complex regulatory mechanisms in crop plants.

Systematically and Comprehensively Understanding the Regulation of Cotton Fiber Initiation: A Review

Cotton fibers provide an important source of raw materials for the textile industry worldwide. Cotton fiber is a kind of single cell that differentiates from the epidermis of the ovule and provides a perfect research model for the differentiation and elongation of plant cells. Cotton fiber initiation is the first stage throughout the entire developmental process. The number of fiber cell initials on the seed ovule epidermis decides the final fiber yield. Thus, it is of great significance to clarify the mechanism underlying cotton fiber initiation. Fiber cell initiation is controlled by complex and interrelated regulatory networks. Plant phytohormones, transcription factors, sugar signals, small signal molecules, functional genes, non-coding RNAs, and histone modification play important roles during this process. Here, we not only summarize the different kinds of factors involved in fiber cell initiation but also discuss the mechanisms of these factors that act together to regulate cotton fiber initiation. Our aim is to synthesize a systematic and comprehensive review of different factors during fiber initiation that will provide the basics for further illustrating these mechanisms and offer theoretical guidance for improving fiber yield in future molecular breeding work.

The transcription factor ERF108 interacts with AUXIN RESPONSE FACTORs to mediate cotton fiber secondary cell wall biosynthesis

Phytohormones play indispensable roles in plant growth and development. However, the molecular mechanisms underlying phytohormone-mediated regulation of fiber secondary cell wall (SCW) formation in cotton (Gossypium hirsutum) remain largely underexplored. Here, we provide mechanistic evidence for functional interplay between the APETALA2/ethylene response factor (AP2/ERF) transcription factor GhERF108 and auxin response factors GhARF7-1 and GhARF7-2 in dictating the ethylene-auxin signaling crosstalk that regulates fiber SCW biosynthesis. Specifically, in vitro cotton ovule culture revealed that ethylene and auxin promote fiber SCW deposition. GhERF108 RNA interference (RNAi) cotton displayed remarkably reduced cell wall thickness compared with controls. GhERF108 interacted with GhARF7-1 and GhARF7-2 to enhance the activation of the MYB transcription factor gene GhMYBL1 (MYB domain-like protein 1) in fibers. GhARF7-1 and GhARF7-2 respond to auxin signals that promote fiber SCW thickening. GhMYBL1 RNAi and GhARF7-1 and GhARF7-2 virus-induced gene silencing (VIGS) cotton displayed similar defects in fiber SCW formation as GhERF108 RNAi cotton. Moreover, the ethylene and auxin responses were reduced in GhMYBL1 RNAi plants. GhMYBL1 directly binds to the promoters of GhCesA4-1, GhCesA4-2, and GhCesA8-1 and activates their expression to promote cellulose biosynthesis, thereby boosting fiber SCW formation. Collectively, our findings demonstrate that the collaboration between GhERF108 and GhARF7-1 or GhARF7-2 establishes ethylene-auxin signaling crosstalk to activate GhMYBL1, ultimately leading to the activation of fiber SCW biosynthesis.

Using association mapping and local interval haplotype association analysis to improve the cotton drought stress response

Drought stress has a serious impact on the growth and development of cotton. To explore the relevant molecular mechanism of the drought stress response in cotton, gene mapping based on the QTL interval mapped by simplified genome BSA-seq of the drought-resistance-related RIL population was performed. A QTL region spanning 2.02 Mb on chromosome D07 was selected, and 201 resource materials were genotyped using 9 KASP markers in the interval. After local interval haplotype association analysis, the overlap of the 110 kb peak region confirmed the reliability of this region, and at the same time, the role of GhGF14-30, the only gene in the overlapping region, was modeled in the response of cotton to drought stress. qRTPCR analysis of the materials and population parents proved that this gene plays a role in the drought stress response in cotton. Virus-induced gene silencing proved the importance of this gene in drought-sensitive materials, and drought-resistance-related marker genes also proved that the GhGF14-30 gene may play an important role in the ABA and SOS signaling pathways. This study provides a basis for mining drought stress response functional genes in cotton and lays the foundation for the molecular mechanism of the GhGF14-30 gene in response to drought stress in cotton.

A comprehensive overview of cotton genomics, biotechnology and molecular biological studies

Cotton is an irreplaceable economic crop currently domesticated in the human world for its extremely elongated fiber cells specialized in seed epidermis, which makes it of high research and application value. To date, numerous research on cotton has navigated various aspects, from multi-genome assembly, genome editing, mechanism of fiber development, metabolite biosynthesis, and analysis to genetic breeding. Genomic and 3D genomic studies reveal the origin of cotton species and the spatiotemporal asymmetric chromatin structure in fibers. Mature multiple genome editing systems, such as CRISPR/Cas9, Cas12 (Cpf1) and cytidine base editing (CBE), have been widely used in the study of candidate genes affecting fiber development. Based on this, the cotton fiber cell development network has been preliminarily drawn. Among them, the MYB-bHLH-WDR (MBW) transcription factor complex and IAA and BR signaling pathway regulate the initiation; various plant hormones, including ethylene, mediated regulatory network and membrane protein overlap fine-regulate elongation. Multistage transcription factors targeting CesA 4, 7, and 8 specifically dominate the whole process of secondary cell wall thickening. And fluorescently labeled cytoskeletal proteins can observe real-time dynamic changes in fiber development. Furthermore, research on the synthesis of cotton secondary metabolite gossypol, resistance to diseases and insect pests, plant architecture regulation, and seed oil utilization are all conducive to finding more high-quality breeding-related genes and subsequently facilitating the cultivation of better cotton varieties. This review summarizes the paramount research achievements in cotton molecular biology over the last few decades from the above aspects, thereby enabling us to conduct a status review on the current studies of cotton and provide strong theoretical support for the future direction.

Single-cell RNA landscape of the special fiber initiation process in Bombax ceiba

As a new source of natural fibers, the Bombax ceiba tree can provide thin, light, extremely soft and warm fiber material for the textile industry. Natural fibers are an ideal model system for studying cell growth and differentiation, but the molecular mechanisms that regulate fiber initiation are not fully understood. In B. ceiba, we found that fiber cells differentiate from the epidermis of the inner ovary wall. Each initiated cell then divides into a cluster of fiber cells that eventually develop into mature fibers, a process very different from the classical fiber initiation process of cotton. We used high-throughput single-cell RNA sequencing (scRNA-seq) to examine the special characteristics of fiber initiation in B. ceiba. A total of 15 567 high-quality cells were identified from the inner wall of the B. ceiba ovary, and 347 potential marker genes for fiber initiation cell types were identified. Two major cell types, initiated fiber cells and epidermal cells, were identified and verified by RNA in situ hybridization. A developmental trajectory analysis was used to reconstruct the process of fiber cell differentiation in B. ceiba. Comparative analysis of scRNA-seq data from B. ceiba and cotton (Gossypium hirsutum) confirmed that the additional cell division process in B. ceiba is a novel species-specific mechanism for fiber cell development. Candidate genes and key regulators that may contribute to fiber cell differentiation and division in B. ceiba were identified. This work reveals gene expression signatures during B. ceiba fiber initiation at a single-cell resolution, providing a new strategy and viewpoint for investigation of natural fiber cell differentiation and development.

Advances in understanding epigenetic regulation of plant trichome development: a comprehensive review

Plant growth and development are controlled by a complex gene regulatory network, which is currently a focal point of research. It has been established that epigenetic factors play a crucial role in plant growth. Trichomes, specialized appendages that arise from epidermal cells, are of great significance in plant growth and development. As a model system for studying plant development, trichomes possess both commercial and research value. Epigenetic regulation has only recently been implicated in the development of trichomes in a limited number of studies, and microRNA-mediated post-transcriptional regulation appears to dominate in this context. In light of this, we have conducted a review that explores the interplay between epigenetic regulations and the formation of plant trichomes, building upon existing knowledge of hormones and transcription factors in trichome development. Through this review, we aim to deepen our understanding of the regulatory mechanisms underlying trichome formation and shed light on future avenues of research in the field of epigenetics as it pertains to epidermal hair growth.

Genome-wide chromatin accessibility landscape and dynamics of transcription factor networks during ovule and fiber development in cotton

BACKGROUND

The development of cotton fiber is regulated by the orchestrated binding of regulatory proteins to cis-regulatory elements associated with developmental genes. The cis-trans regulatory dynamics occurred throughout the course of cotton fiber development are elusive. Here we generated genome-wide high-resolution DNase I hypersensitive sites (DHSs) maps to understand the regulatory mechanisms of cotton ovule and fiber development.

RESULTS

We generated DNase I hypersensitive site (DHS) profiles from cotton ovules at 0 and 3 days post anthesis (DPA) and fibers at 8, 12, 15, and 18 DPA. We obtained a total of 1185 million reads and identified a total of 199,351 DHSs through ~ 30% unique mapping reads. It should be noted that more than half of DNase-seq reads mapped multiple genome locations and were not analyzed in order to achieve a high specificity of peak profile and to avoid bias from repetitive genomic regions. Distinct chromatin accessibilities were observed in the ovules (0 and 3 DPA) compared to the fiber elongation stages (8, 12, 15, and 18 DPA). Besides, the chromatin accessibility during ovules was particularly elevated in genomic regions enriched with transposable elements (TEs) and genes in TE-enriched regions were involved in ovule cell division. We analyzed cis-regulatory modules and revealed the influence of hormones on fiber development from the regulatory divergence of transcription factor (TF) motifs. Finally, we constructed a reliable regulatory network of TFs related to ovule and fiber development based on chromatin accessibility and gene co-expression network. From this network, we discovered a novel TF, WRKY46, which may shape fiber development by regulating the lignin content.

CONCLUSIONS

Our results not only reveal the contribution of TEs in fiber development, but also predict and validate the TFs related to fiber development, which will benefit the research of cotton fiber molecular breeding.

Brassinosteroids regulate cotton fiber elongation by modulating very-long-chain fatty acid biosynthesis

Brassinosteroid (BR), a growth-promoting phytohormone, regulates many plant growth processes including cell development. However, the mechanism by which BR regulates fiber growth is poorly understood. Cotton (Gossypium hirsutum) fibers are an ideal single-cell model in which to study cell elongation due to their length. Here we report that BR controls cotton fiber elongation by modulating very-long-chain fatty acid (VLCFA) biosynthesis. BR deficiency reduces the expression of 3-ketoacyl-CoA synthases (GhKCSs), the rate-limiting enzymes involved in VLCFA biosynthesis, leading to lower saturated VLCFA contents in pagoda1 (pag1) mutant fibers. In vitro ovule culture experiments show that BR acts upstream of VLCFAs. Silencing of BRI1-EMS-SUPPRESOR 1.4 (GhBES1.4), encoding a master transcription factor of the BR signaling pathway, significantly reduces fiber length, whereas GhBES1.4 overexpression produces longer fibers. GhBES1.4 regulates endogenous VLCFA contents and directly binds to BR RESPONSE ELEMENTS (BRREs) in the GhKCS10_At promoter region, which in turn regulates GhKCS10_At expression to increase endogenous VLCFA contents. GhKCS10_At overexpression promotes cotton fiber elongation, whereas GhKCS10_At silencing inhibits cotton fiber growth, supporting a positive regulatory role for GhKCS10_At in fiber elongation. Overall, these results uncover a mechanism of fiber elongation through crosstalk between BR and VLCFAs at the single-cell level.

Gossypium mustelinum genome and an introgression population enrich interspecific genetics and breeding in cotton

KEY MESSAGE

Genomic and genetic resources of G. mustelinum were effective for identifying genes for qualitative and quantitative traits. Gossypium mustelinum represents the earliest diverging evolutionary lineage of polyploid Gossypium, representing a rich gene pool for numerous desirable traits lost in cotton cultivars. Accurate information of the genomic features and the genetic architecture of objective traits are essential for the discovery and utilization of G. mustelinum genes. Here, we presented a chromosome-level genome assembly of G. mustelinum and developed an introgression population of the G. mustelinum in the background of G. hirsutum that contained 264 lines. We precisely delimited the boundaries of the 1,662 introgression segments with the help of G. mustelinum genome assembly, and 87% of crossover regions (COs) were less than 5 Kb. Genes for fuzzless and green fuzz were discovered, and a total of 14 stable QTLs were identified with 12 novel QTLs across four independent environments. A new fiber length QTL, qUHML/SFC-A11, was confined to a 177-Kb region, and GmOPB4 and GmGUAT11 were considered as the putative candidate genes as potential negative regulator for fiber length. We presented a genomic and genetic resource of G. mustelinum, which we demonstrated that it was efficient for identifying genes for qualitative and quantitative traits. Our study built a valuable foundation for cotton genetics and breeding.

Improved cotton yield: Can we achieve this goal by regulating the coordination of source and sink?

Cotton is one of the major cash crops globally. It is characterized by determinate growth and multiple fruiting, which makes the source-sink contradiction more obvious. Coordination between source and sink is crucial for normal growth, yield, and quality of cotton. Numerous studies reported how the assimilate transport and distribution under varying environmental cues affected crop yields. However, less is known about the functional mechanism underlying the assimilate transport between source and sink, and how their distribution impacts cotton growth. Here, we provided an overview of the assimilate transport and distribution mechanisms , and discussed the regulatory mechanisms involved in source-sink balance in relation to cotton yield. Therefore, this review enriched our knowledge of the regulatory mechanism involved in source-sink relationship for improved cotton yield.

Recent progression and future perspectives in cotton genomic breeding

Upland cotton is an important global cash crop for its long seed fibers and high edible oil and protein content. Progress in cotton genomics promotes the advancement of cotton genetics, evolutionary studies, functional genetics, and breeding, and has ushered cotton research and breeding into a new era. Here, we summarize high-impact genomics studies for cotton from the last 10 years. The diploid Gossypium arboreum and allotetraploid Gossypium hirsutum are the main focus of most genetic and genomic studies. We next review recent progress in cotton molecular biology and genetics, which builds on cotton genome sequencing efforts, population studies, and functional genomics, to provide insights into the mechanisms shaping abiotic and biotic stress tolerance, plant architecture, seed oil content, and fiber development. We also suggest the application of novel technologies and strategies to facilitate genome-based crop breeding. Explosive growth in the amount of novel genomic data, identified genes, gene modules, and pathways is now enabling researchers to utilize multidisciplinary genomics-enabled breeding strategies to cultivate "super cotton", synergistically improving multiple traits. These strategies must rise to meet urgent demands for a sustainable cotton industry.

Genome-Wide Identification of BES1 Gene Family in Six Cucurbitaceae Species and Its Expression Analysis in Cucurbita moschata

The BES1 (BRI1-EMSSUPPRESSOR1) gene family play a vital role in the BR (brassinosteroid) signaling pathway, which is involved in the growth and development, biotic, abiotic, and hormone stress response in many plants. However, there are few reports of BES1 in Cucurbita moschata. In this study, 50 BES1 genes were identified in six Cucurbitaceae species by genome-wide analysis, which could be classified into 3 groups according to their gene structural features and motif compositions, and 13 CmoBES1 genes in Cucurbita moschata were mapped on 10 chromosomes. Quantitative real-time PCR analysis showed that the CmoBES1 genes displayed differential expression under different abiotic stress and hormone treatments. Subcellular localization showed that the most of CmoBES1 proteins localized in nucleus and cytoplasm, and transactivation assay indicated 9 CmoBES1 proteins played roles as transcription factors. Our analysis of BES1s diversity, localization, and expression in Curcubitaceae contributes to the better understanding of the essential roles of these transcription factors in plants.

A deletion/duplication in the Ligon lintless-2 locus induces siRNAs that inhibit cotton fiber cell elongation

Most cultivated cotton (Gossypium hirsutum L.) varieties have two types of seed fibers: short fuzz fiber strongly adhered to the seed coat, and long lint fiber used in the textile industry. The Ligon lintless-2 (Li2) cotton mutant has a normal vegetative phenotype but produces very short lint fiber on the seeds. The Li2 mutation is controlled by a single dominant gene. We discovered a large structural rearrangement at the end of chromosome D13 in the Li2 mutant based on whole-genome sequencing and genetic mapping of segregating populations. The rearrangement contains a 177-kb deletion and a 221-kb duplication positioned as a tandem inverted repeat. The gene Gh_D13G2437 is located at the junction of the inverted repeat in the duplicated region. During transcription such structure spontaneously forms self-complementary hairpin RNA of Gh_D13G2437 followed by production of small interfering RNA (siRNA). Gh_D13G2437 encodes a Ran-Binding Protein 1 (RanBP1) that preferentially expresses during cotton fiber elongation. The abundance of siRNA produced from Gh_D13G2437 reciprocally corresponds with the abundance of highly homologous (68%-98% amino acid sequence identity) RanBP1 family transcripts during fiber elongation, resulting in a shorter fiber phenotype in the Li2. Overexpression of Gh_D13G2437 in the Li2 mutant recovered the long lint fiber phenotype. Taken together, our findings revealed that siRNA-induced silencing of a family of RanBP1s inhibit elongation of cotton fiber cells in the Li2 mutant.

Evolutionary divergence of duplicated genomes in newly described allotetraploid cottons

Wild relatives of domesticated plants provide a rich resource for crop improvement and a valuable comparative perspective for understanding genomic, physiological, and agricultural traits. Here, we provide high-quality reference genomes of one early domesticated form of the economically most important cotton species, Gossypium hirsutum, and two other wild species, to clarify evolutionary relationships and understand the genomic changes that characterize these species and their close relatives. We document abundant gene resources involved in adaptation to environmental challenges, highlighting the potential for introgression of favorable genes into domesticated cotton and for increasing resilience to climate variability. Our study complements other recent genomic analyses in the cotton genus and provides a valuable foundation for breeding improved cotton varieties.

Allotetraploid cotton (Gossypium) species represents a model system for the study of plant polyploidy, molecular evolution, and domestication. Here, chromosome-scale genome sequences were obtained and assembled for two recently described wild species of tetraploid cotton, Gossypium ekmanianum [(AD)₆, Ge] and Gossypium stephensii [(AD)₇, Gs], and one early form of domesticated Gossypium hirsutum, race punctatum [(AD)₁, Ghp]. Based on phylogenomic analysis, we provide a dated whole-genome level perspective for the evolution of the tetraploid Gossypium clade and resolved the evolutionary relationships of Gs, Ge, and domesticated G. hirsutum. We describe genomic structural variation that arose during Gossypium evolution and describe its correlates—including phenotypic differentiation, genetic isolation, and genetic convergence—that contributed to cotton biodiversity and cotton domestication. Presence/absence variation is prominent in causing cotton genomic structural variations. A presence/absence variation-derived gene encoding a phosphopeptide-binding protein is implicated in increasing fiber length during cotton domestication. The relatively unimproved Ghp offers the potential for gene discovery related to adaptation to environmental challenges. Expanded gene families enoyl-CoA δ isomerase 3 and RAP2-7 may have contributed to abiotic stress tolerance, possibly by targeting plant hormone-associated biochemical pathways. Our results generate a genomic context for a better understanding of cotton evolution and for agriculture.

GhBZR3 suppresses cotton fiber elongation by inhibiting very-long-chain fatty acid biosynthesis

The BRASSINAZOLE-RESISTANT (BZR) transcription factor is a core component of brassinosteroid (BR) signaling and is involved in the development of many plant species. BR is essential for the initiation and elongation of cotton fibers. However, the mechanism of BR-regulating fiber development and the function of BZR is poorly understood in Gossypium hirsutum L. (cotton). Here, we identified a BZR family transcription factor protein referred to as GhBZR3 in cotton. Overexpression of GhBZR3 in Arabidopsis caused shorter root hair length, hypocotyl length, and hypocotyl cell length, indicating that GhBZR3 negatively regulates cell elongation. Pathway enrichment analysis from VIGS-GhBZR3 cotton plants found that fatty acid metabolism and degradation might be the regulatory pathway that is primarily controlled by GhBZR3. Silencing GhBZR3 expression in cotton resulted in taller plant height as well as longer fibers. The very-long-chain fatty acid (VLCFA) content was also significantly increased in silenced GhBZR3 plants compared with the wild type. The GhKCS13 promoter, a key gene for VLCFA biosynthesis, contains two GhBZR3 binding sites. The results of yeast one-hybrid, electrophoretic mobility shift, and luciferase assays revealed that GhBZR3 directly interacted with the GhKCS13 promoter to suppress gene expression. Taken together, these results indicate that GhBZR3 negatively regulates cotton fiber development by reducing VLCFA biosynthesis. This study not only deepens our understanding of GhBZR3 function in cotton fiber development, but also highlights the potential of improving cotton fiber length and plant growth using GhBZR3 and its related genes in future cotton breeding programs.

Expansion and Diversification of the 14-3-3 Gene Family in Camellia sinensis

14-3-3 proteins are signal moderators in sensing various stresses and play essential functions in plant growth and development. Although, 14-3-3 gene families have been identified and characterized in many plant species, its evolution has not been studied systematically. In this study, the plant 14-3-3 family was comprehensively analyzed from green algae to angiosperm. Our result indicated that plant 14-3-3 originated during the early evolutionary history of green algae and expanded in terricolous plants. Twenty-six 14-3-3 genes were identified in the tea genome. RNA-seq analysis showed that tea 14-3-3 genes display different expression patterns in different organs. Moreover, the expression of most tea 14-3-3 genes displayed variable expression patterns under different abiotic and biotic stresses. In conclusion, our results elucidate the evolutionary origin of plant 14-3-3 genes, and beneficial for understanding their biological functions and improving tea agricultural traits in the future.

The bHLH/HLH transcription factors GhFP2 and GhACE1 antagonistically regulate fiber elongation in cotton

Basic helix-loop-helix/helix-loop-helix (bHLH/HLH) transcription factors play important roles in cell elongation in plants. However, little is known about how bHLH/HLH transcription factors antagonistically regulate fiber elongation in cotton (Gossypium hirsutum). In this study, we report that two bHLH/HLH transcription factors, fiber-related protein 2 (GhFP2) and ACTIVATOR FOR CELL ELONGATION 1 (GhACE1), function in fiber development of cotton. GhFP2 is an atypical bHLH protein without the basic region, and its expression is regulated by brassinosteroid (BR)-related BRASSINAZOLE RESISTANT 1 (GhBZR1). Overexpression of GhFP2 in cotton hindered fiber elongation, resulting in shortened fiber length. In contrast, suppression of GhFP2 expression in cotton promoted fiber development, leading to longer fibers compared with the wild-type. GhFP2 neither contains a DNA-binding domain nor has transcriptional activation activity. Furthermore, we identified GhACE1, a bHLH protein that interacts with GhFP2 and positively regulates fiber elongation. GhACE1 could bind to promoters of plasma membrane intrinsic protein 2;7 (GhPIP2;7) and expansions 8 (GhEXP8) for directly activating their expression, but the interaction between GhFP2 and GhACE1 suppressed transcriptional activation of these target genes by GhACE1. Taken together, our results indicate that GhACE1 promotes fiber elongation by activating expressions of GhPIP2;7 and GhEXP8, but its transcription activation on downstream genes may be obstructed by BR-modulated GhFP2. Thus, our data reveal a key mechanism for fiber cell elongation through a pair of antagonizing HLH/bHLH transcription factors in cotton.

Molecular Mechanisms of Plant Trichome Development

Plant trichomes, protrusions formed from specialized aboveground epidermal cells, provide protection against various biotic and abiotic stresses. Trichomes can be unicellular, bicellular or multicellular, with multiple branches or no branches at all. Unicellular trichomes are generally not secretory, whereas multicellular trichomes include both secretory and non-secretory hairs. The secretory trichomes release secondary metabolites such as artemisinin, which is valuable as an antimalarial agent. Cotton trichomes, also known as cotton fibers, are an important natural product for the textile industry. In recent years, much progress has been made in unraveling the molecular mechanisms of trichome formation in Arabidopsis thaliana, Gossypium hirsutum, Oryza sativa, Cucumis sativus, Solanum lycopersicum, Nicotiana tabacum, and Artemisia annua. Here, we review current knowledge of the molecular mechanisms underlying fate determination and initiation, elongation, and maturation of unicellular, bicellular and multicellular trichomes in several representative plants. We emphasize the regulatory roles of plant hormones, transcription factors, the cell cycle and epigenetic modifications in different stages of trichome development. Finally, we identify the obstacles and key points for future research on plant trichome development, and speculated the development relationship between the salt glands of halophytes and the trichomes of non-halophytes, which provides a reference for future studying the development of plant epidermal cells.

Revealing Genetic Differences in Fiber Elongation between the Offspring of Sea Island Cotton and Upland Cotton Backcross Populations Based on Transcriptome and Weighted Gene Coexpression Networks

Fiber length is an important indicator of cotton fiber quality, and the time and rate of cotton fiber cell elongation are key factors in determining the fiber length of mature cotton. To gain insight into the differences in fiber elongation mechanisms in the offspring of backcross populations of Sea Island cotton Xinhai 16 and land cotton Line 9, we selected two groups with significant differences in fiber length (long-fiber group L and short-fiber group S) at different fiber development stages 0, 5, 10 and 15 days post-anthesis (DPA) for transcriptome comparison. A total of 171.74 Gb of clean data was obtained by RNA-seq, and eight genes were randomly selected for qPCR validation. Data analysis identified 6055 differentially expressed genes (DEGs) between two groups of fibers, L and S, in four developmental periods, and gene ontology (GO) term analysis revealed that these DEGs were associated mainly with microtubule driving, reactive oxygen species, plant cell wall biosynthesis, and glycosyl compound hydrolase activity. Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis indicated that plant hormone signaling, mitogen-activated protein kinase (MAPK) signaling, and starch and sucrose metabolism pathways were associated with fiber elongation. Subsequently, a sustained upregulation expression pattern, profile 19, was identified and analyzed using short time-series expression miner (STEM). An analysis of the weighted gene coexpression network module uncovered 21 genes closely related to fiber development, mainly involved in functions such as cell wall relaxation, microtubule formation, and cytoskeletal structure of the cell wall. This study helps to enhance the understanding of the Sea Island–Upland backcross population and identifies key genes for cotton fiber development, and these findings will provide a basis for future research on the molecular mechanisms of fiber length formation in cotton populations.

Molecular Traits and Functional Exploration of BES1 Gene Family in Plants

The BES1 (BRI1-EMSSUPPRESSOR1) gene family is a unique class of transcription factors that play dynamic roles in the Brassinosteroids (BRs) signaling pathway. The published genome sequences of a large number of plants provide an opportunity to identify and perform a comprehensive functional study on the BES1 gene family for their potential roles in developmental processes and stress responses. A total of 135 BES1 genes in 27 plant species were recognized and characterized, which were divided into five well-conserved subfamilies. BES1 was not found in lower plants, such as Cyanophora paradoxa and Galdieria sulphuraria. The spatial expression profiles of BES1s in Arabidopsis, rice, and cotton, as well as their response to abiotic stresses, were analyzed. The overexpression of two rice BES1 genes, i.e., OsBES1-3 and OsBES1-5, promotes root growth under drought stress. The overexpression of GhBES1-4 from cotton enhanced the salt tolerance in Arabidopsis. Five protein interaction networks were constructed and numerous genes co-expressed with GhBES1-4 were characterized in transgenic Arabidopsis. BES1 may have evolved in the ancestors of the first land plants following its divergence from algae. Our results lay the foundation for understanding the complex mechanisms of BES1-mediated developmental processes and abiotic stress tolerance.

The Pivotal Role of Major Chromosomes of Sub-Genomes A and D in Fiber Quality Traits of Cotton

Lack of precise information about the candidate genes involved in a complex quantitative trait is a major obstacle in the cotton fiber quality improvement, and thus, overall genetic gain in conventional phenotypic selection is low. Recent molecular interventions and advancements in genome sequencing have led to the development of high-throughput molecular markers, quantitative trait locus (QTL) fine mapping, and single nucleotide polymorphisms (SNPs). These advanced tools have resolved the existing bottlenecks in trait-specific breeding. This review demonstrates the significance of chromosomes 3, 7, 9, 11, and 12 of sub-genomes A and D carrying candidate genes for fiber quality. However, chromosome 7 carrying SNPs for stable and potent QTLs related to fiber quality provides great insights for fiber quality-targeted research. This information can be validated by marker-assisted selection (MAS) and transgene in Arabidopsis and subsequently in cotton.

Glucose regulates cotton fiber elongation by interacting with brassinosteroid

In plants, glucose (Glc) plays important roles, as a nutrient and signal molecule, in the regulation of growth and development. However, the function of Glc in fiber development of upland cotton (Gossypium hirsutum) is unclear. Here, using gas chromatography-mass spectrometry (GC-MS), we found that the Glc content in fibers was higher than that in ovules during the fiber elongation stage. In vitro ovule culture revealed that lower Glc concentrations promoted cotton fiber elongation, while higher concentrations had inhibitory effects. The hexokinase inhibitor N-acetylglucosamine (NAG) inhibited cotton fiber elongation in the cultured ovules, indicating that Glc-mediated fiber elongation depends on the Glc signal transduced by hexokinase. RNA sequencing (RNA-seq) analysis and hormone content detection showed that 150mM Glc significantly activated brassinosteroid (BR) biosynthesis, and the expression of signaling-related genes was also increased, which promoted fiber elongation. In vitro ovule culture clarified that BR induced cotton fiber elongation in a dose-dependent manner. In hormone recovery experiments, only BR compensated for the inhibitory effects of NAG on fiber elongation in a Glc-containing medium. However, the ovules cultured with the BR biosynthetic inhibitor brassinazole and from the BR-deficient cotton mutant pag1 had greatly reduced fiber elongation at all the Glc concentrations tested. This demonstrates that Glc does not compensate for the inhibition of fiber elongation caused by BR biosynthetic defects, suggesting that the BR signaling pathway works downstream of Glc during cotton fiber elongation. Altogether, our study showed that Glc plays an important role in cotton fibre elongation, and crosstalk occurs between Glc and BR signaling during modulation of fiber elongation.

bHLH transcription factors LP1 and LP2 regulate longitudinal cell elongation

Basic helix-loop-helix/helix-loop-helix (bHLH/HLH) transcription factors play substantial roles in plant cell elongation. In this study, two bHLH/HLH homologous proteins leaf related protein 1 and leaf-related protein 2 (AtLP1 and AtLP2) were identified in Arabidopsis thaliana. LP1 and LP2 play similar positive roles in longitudinal cell elongation. Both LP1 and LP2 overexpression plants exhibited long hypocotyls, elongated cotyledons, and particularly long leaf blades. The elongated leaves resulted from increased longitudinal cell elongation. lp1 and lp2 loss-of-function single mutants did not display distinct phenotypes, but the lp1lp2 double mutant showed decreased leaf length associated with less longitudinal polar cell elongation. Furthermore, the phenotype of lp1lp2 could be rescued by the expression of LP1 or LP2. Expression of genes related to cell elongation was upregulated in LP1 and LP2 overexpression plants but downregulated in lp1lp2 double mutant plants compared with that of wild type. LP1 and LP2 proteins could directly bind to the promoters of Longifolia1 (LNG1) and LNG2 to activate the expression of these cell elongation related genes. Both LP1 and LP2 could interact with two other bHLH/HLH proteins, IBH1 (ILI1 binding BHLH Protein1) and IBL1 (IBH1-like1), thereby suppressing the transcriptional activation of LP1 and LP2 to the target genes LNG1 and LNG2. Thus, our data suggested that LP1 and LP2 act as positive regulators to promote longitudinal cell elongation by activating the expression of LNG1 and LNG2 genes in Arabidopsis. Moreover, homodimerization of LP1 and LP2 may be essential for their function, and interaction between LP1/LP2 and other bHLH/HLH proteins may obstruct transcriptional regulation of target genes by LP1 and LP2.

Transcription factors DkBZR1/2 regulate cell wall degradation genes and ethylene biosynthesis genes during persimmon fruit ripening

BRASSINAZOLE RESISTANT (BZR) transcription factors are critical components of the brassinosteroid signalling pathway, but their possible roles in fruit ripening have rarely been reported. In this study, four BZR sequences were isolated from persimmon fruit. Among the four BZR genes, DkBZR1/2 were expressed in persimmon fruit; DkBZR1 protein amount decreased and dephosphorylated DkBZR2 gradually accumulated during the storage period. DkBZR1/2 proteins were localized in both the nucleus and cytoplasm and accumulated in the nucleus after 24-epibrassinolide treatment. DkBZR1 suppressed the transcription of Diospyros kaki endo-1,4-betaglucanase 1 (DkEGase1) and 1-aminocyclopropane-1-carboxylate synthase 1 (DkACS1) by binding to the BR response element (BRRE) in their promoters, and DkBZR2 activated the transcription of pectate lyase 1 (DkPL1) and 1-aminocyclopropane-1-carboxylate oxidase 2 (DkACO2) by binding to the E-box motif in their promoters. Transient overexpression of DkBZR2 promoted the conversion of acid-soluble pectin to water-soluble pectin and increased ethylene production in persimmon fruit. Our findings indicate that DkBZR1 and DkBZR2 serve as repressors and activators of persimmon fruit ripening, respectively.

Phosphorylation of WRKY16 by MPK3-1 is essential for its transcriptional activity during fiber initiation and elongation in cotton (Gossypium hirsutum)

Cotton, one of the most important crops in the world, produces natural fiber materials for the textile industry. WRKY transcription factors play important roles in plant development and stress responses. However, little is known about whether and how WRKY transcription factors regulate fiber development of cotton so far. In this study, we show that a fiber-preferential WRKY transcription factor, GhWRKY16, positively regulates fiber initiation and elongation. GhWRKY16-silenced transgenic cotton displayed a remarkably reduced number of fiber protrusions on the ovule and shorter fibers compared to the wild-type. During early fiber development, GhWRKY16 directly binds to the promoters of GhHOX3, GhMYB109, GhCesA6D-D11, and GhMYB25 to induce their expression, thereby promoting fiber initiation and elongation. Moreover, GhWRKY16 is phosphorylated by the mitogen-activated protein kinase GhMPK3-1 at residues T-130 and S-260. Phosphorylated GhWRKY16 directly activates the transcription of GhMYB25, GhHOX3, GhMYB109, and GhCesA6D-D11 for early fiber development. Thus, our data demonstrate that GhWRKY16 plays a crucial role in fiber initiation and elongation, and that GhWRKY16 phosphorylation by GhMPK3-1 is essential for the transcriptional activation on downstream genes during the fiber development of cotton.

Orchestration of plant development and defense by indirect crosstalk of salicylic acid and brassinosteorid signaling via transcription factor GhTINY2

Salicylic acid (SA) and brassinosteroids (BRs) are well known to regulate diverse processes of plant development and stress responses, but the mechanisms by which these phytohormones mediate the growth and defense trade-off are largely unclear. In addition, little is known about the roles of DEHYDRATION RESPONSIVE ELEMENT BINDING transcription factors, especially in biotic stress and plant growth. Here, we identified a cotton (Gossypium hirsutum) APETALA2/ETHYLENE RESPONSIVE FACTOR gene GhTINY2 that is strongly induced by Verticillium dahliae. Overexpression of GhTINY2 in cotton and Arabidopsis enhanced tolerance to V. dahliae, while knockdown of expression increased the susceptibility of cotton to the pathogen. GhTINY2 was found to promote SA accumulation and SA signaling transduction by directly activating expression of WRKY51. Moreover, GhTINY2-overexpressing cotton and Arabidopsis showed retardation of growth, increased sensitivity to inhibitors of BR biosynthesis, down-regulation of several BR-induced genes, and up-regulation of BR-repressed genes, while GhTINY2-RNAi cotton showed the opposite effects. We further determined that GhTINY2 negatively regulates BR signaling by interacting with BRASSINAZOLE-RESISTANT 1 (BZR1) and restraining its transcriptional activation of the expression of INDOLE-3-ACETIC ACID INDUCIBLE 19 (IAA19). These findings indicate that GhTINY2 fine-tunes the trade-off between immunity and growth via indirect crosstalk between WRKY51-mediated SA biosynthesis and BZR1-IAA19-regulated BR signaling.

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.

LCM and RNA-seq analyses revealed roles of cell cycle and translational regulation and homoeolog expression bias in cotton fiber cell initiation

Background

Cotton fibers provide a powerful model for studying cell differentiation and elongation. Each cotton fiber is a singular and elongated cell derived from epidermal-layer cells of a cotton seed. Efforts to understand this dramatic developmental shift have been impeded by the difficulty of separation between fiber and epidermal cells.

Results

Here we employed laser-capture microdissection (LCM) to separate these cell types. RNA-seq analysis revealed transitional differences between fiber and epidermal-layer cells at 0 or 2 days post anthesis. Specifically, down-regulation of putative cell cycle genes was coupled with upregulation of ribosome biosynthesis and translation-related genes, which may suggest their respective roles in fiber cell initiation. Indeed, the amount of fibers in cultured ovules was increased by cell cycle progression inhibitor, Roscovitine, and decreased by ribosome biosynthesis inhibitor, Rbin-1. Moreover, subfunctionalization of homoeologs was pervasive in fiber and epidermal cells, with expression bias towards 10% more D than A homoeologs of cell cycle related genes and 40–50% more D than A homoeologs of ribosomal protein subunit genes. Key cell cycle regulators were predicted to be epialleles in allotetraploid cotton. MYB-transcription factor genes displayed expression divergence between fibers and ovules. Notably, many phytohormone-related genes were upregulated in ovules and down-regulated in fibers, suggesting spatial-temporal effects on fiber cell development.

Conclusions

Fiber cell initiation is accompanied by cell cycle arrest coupled with active ribosome biosynthesis, spatial-temporal regulation of phytohormones and MYB transcription factors, and homoeolog expression bias of cell cycle and ribosome biosynthesis genes. These valuable genomic resources and molecular insights will help develop breeding and biotechnological tools to improve cotton fiber production.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07579-1.

Roles of the 14-3-3 gene family in cotton flowering

BACKGROUND

In plants, 14-3-3 proteins, also called GENERAL REGULATORY FACTORs (GRFs), encoded by a large multigene family, are involved in protein-protein interactions and play crucial roles in various physiological processes. No genome-wide analysis of the GRF gene family has been performed in cotton, and their functions in flowering are largely unknown.

RESULTS

In this study, 17, 17, 31, and 17 GRF genes were identified in Gossypium herbaceum, G. arboreum, G. hirsutum, and G. raimondii, respectively, by genome-wide analyses and were designated as GheGRFs, GaGRFs, GhGRFs, and GrGRFs, respectively. A phylogenetic analysis revealed that these proteins were divided into ε and non-ε groups. Gene structural, motif composition, synteny, and duplicated gene analyses of the identified GRF genes provided insights into the evolution of this family in cotton. GhGRF genes exhibited diverse expression patterns in different tissues. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that the GhGRFs interacted with the cotton FLOWERING LOCUS T homologue GhFT in the cytoplasm and nucleus, while they interacted with the basic leucine zipper transcription factor GhFD only in the nucleus. Virus-induced gene silencing in G. hirsutum and transgenic studies in Arabidopsis demonstrated that GhGRF3/6/9/15 repressed flowering and that GhGRF14 promoted flowering.

CONCLUSIONS

Here, 82 GRF genes were identified in cotton, and their gene and protein features, classification, evolution, and expression patterns were comprehensively and systematically investigated. The GhGRF3/6/9/15 interacted with GhFT and GhFD to form florigen activation complexs that inhibited flowering. However, GhGRF14 interacted with GhFT and GhFD to form florigen activation complex that promoted flowering. The results provide a foundation for further studies on the regulatory mechanisms of flowering.

MIXTAs and phytohormones orchestrate cotton fiber development

Cotton is the largest source of natural fiber for textile industry in the world. Cotton fibers are seed trichomes that make cotton unique among plants. Cotton fibers originate from ovule epidermal cells and serve as an excellent model to study the process of cell differentiation in plants. Characterization of factors contributing to fiber development will help to reveal general mechanisms of cell differentiation in plants. Transcription factors (TFs), especially MYB-MIXTA-like (MML) factors, appear to have evolved unique roles in fiber development. In addition, phytohormones including brassinosteroids, jasmonic acid, GA and auxin also play an important role in regulating fiber development. Here, we summarize the mechanisms of MIXTAs and phytohormones orchestrating cotton fiber development. The progress in understanding molecular basis of fiber development will facilitate future genetic engineering and breeding to improve cotton fiber quality and yield.

14-3-3 Proteins Are Involved in BR-Induced Ray Petal Elongation in Gerbera hybrida

14-3-3 proteins play a major role in the regulation of primary metabolism, protein transport, ion channel activity, signal transduction and biotic/abiotic stress responses. However, their involvement in petal growth and development is largely unknown. Here, we identified and characterized the expression patterns of seven genes of the 14-3-3 family in gerbera. While none of the genes showed any tissue or developmental specificity of spatiotemporal expression, all seven predicted proteins have the nine α-helices typical of 14-3-3 proteins. Following treatment with brassinolide, an endogenous brassinosteroid, the Gh14-3-3 genes displayed various response patterns; for example, Gh14-3-3b and Gh14-3-3f reached their highest expression level at early (2 h) and late (24 h) timepoints, respectively. Further study revealed that overexpression of Gh14-3-3b or Gh14-3-3f promoted cell elongation, leading to an increase in ray petal length. By contrast, silencing of Gh14-3-3b or Gh14-3-3f inhibited petal elongation, which was eliminated partly by brassinolide. Correspondingly, the expression of petal elongation-related and brassinosteroid signaling-related genes was modified in transgenic petals. Taken together, our research suggests that Gh14-3-3b and Gh14-3-3f are positive regulators of brassinosteroid-induced ray petal elongation and thus provides novel insights into the molecular mechanism of petal growth and development.

Phytohormone-Based Regulation of Trichome Development

Phytohormones affect plant growth and development. Many phytohormones are involved in the initiation of trichome development, which can help prevent damage from UV radiation and insect bites and produce fragrance, flavors, and compounds used as pharmaceuticals. Phytohormones promote the participation of transcription factors in the initiation of trichome development; for example, the transcription factors HDZIP, bHLH and MYB interact and form transcriptional complexes to regulate trichome development. Jasmonic acid (JA) mediates the progression of the endoreduplication cycle to increase the number of multicellular trichomes or trichome size. Moreover, there is crosstalk between phytohormones, and some phytohormones interact with each other to affect trichome development. Several new techniques, such as the CRISPR-Cas9 system and single-cell transcriptomics, are available for investigating gene function, determining the trajectory of individual trichome cells and elucidating the regulatory network underlying trichome cell lineages. This review discusses recent advances in the modulation of trichome development by phytohormones, emphasizes the differences and similarities between phytohormones initially present in trichomes and provides suggestions for future research.

GhN/AINV13 positively regulates cotton stress tolerance by interacting with the 14-3-3 protein

Neutral/alkaline invertases (N/AINVs) are sucrose hydrolases with important roles in plants. In this study, 15, 15, 15, 29, and 30 N/AINVs were identified in the Gossypium species, G. raimondii, G. herbaceum, G. arboreum, G. hirsutum, and G. barbadense, respectively. Along with two previously discovered branches, α and β, a new clade γ was first discovered in our study. Investigation of gene collinearity showed that whole-genome duplication (WGD) and polyploidization were responsible for the expansion of the N/AINV gene family in allopolyploid Gossypium. Moreover, expression patterns revealed that GhN/AINV3/13/17/23/24/28 from the β clade is highly expressed during the period of fiber initiation. The invertase activity of GhN/AINV13 and GhN/AINV23 were confirmed by restoring defects of invertase-deficient yeast mutant SEY2102. Treatments of abiotic stress showed that most GhN/AINVs were induced in response to polyethylene glycol (PEG) or salt stress. A virus-induced gene-silencing (VIGS) experiment and yeast two-hybrid assay demonstrated that GhN/AINV13 may interact with their positive regulators Gh14-3-3 proteins and participate in the fiber initiation or stress tolerance of cotton. Our results provided fundamental information regarding N/AINVs and highlight their potential functions in cotton stress tolerance.

From Sequencing to Genome Editing for Cotton Improvement

Traditional breeding techniques are proven, but additional knowledge learned from genome sequencing provides vast new data that might help identify gene targets for improving cotton sustainability. CRISPR/Cas9 provides a powerful tool for precision cotton breeding. Here, we discuss the opportunities and challenges of genome sequencing and editing for cotton improvement.

Map-based cloning of qBWT-c12 discovered brassinosteroid-mediated control of organ size in cotton

Assuring fiber yield stability is the primary objective for cotton breeders since the world population is on the rise, and the demand for cotton fiber is increasing every year. Thus, enhancing average cotton boll weight (BWT) could improve seed cotton production, and ultimately to increase cotton fiber yield. This study accomplished the map-based cloning of a novel boll weight regulating locus, qBWT-c12, in cotton. Bulk segregation analysis detected linked markers, aided in the detection of a stable BWT regulating locus, qBWT-c12, on Chr12 in a novel boll size mutant, BS41. Progeny evaluation confined the qBWT-c12 to a 0.89 cM interval between the AD-A12_07 and AD-FM_44 markers in recombinant derived F₃ and F₄ populations. Homology mapping detected a 40 bp insertion-deletion (InDel) site in the AD-FM_44 clone sequence situated +341 downstream of GhBRH1_A12, which showed complete linkage to the BWT phenotype. The suppressed expression of GhBRH1_A12 suggested its putative involvement during early boll development events in BS41. Although brassinosteroid (BR) biosynthesis and signaling pathway genes were up regulated in different tissues, but the organ growth was suppressed leading to dwarf plants, smaller leaves, and de-morphed smaller bolls in BS41. Thus, a disruption in the BR signal cascade is anticipated and could be related to lower GhBRH1_A12 expression in BS41.This study firstly reported the genetic dissection of boll size regulation of G. barbadense in G. hirsutum background using map-based cloning of a BWT regulating locus, qBWT-c12. Moreover, it also emphasized the putative role GhBRH1_A12 in regulating BR homeostasis and its potential to modulate plant growth and boll development in cotton.

High-density linkage map construction and QTL analyses for fiber quality, yield and morphological traits using CottonSNP63K array in upland cotton (Gossypium hirsutum L.)

Background

Improving fiber quality and yield are the primary research objectives in cotton breeding for enhancing the economic viability and sustainability of Upland cotton production. Identifying the quantitative trait loci (QTL) for fiber quality and yield traits using the high-density SNP-based genetic maps allows for bridging genomics with cotton breeding through marker assisted and genomic selection. In this study, a recombinant inbred line (RIL) population, derived from cross between two parental accessions, which represent broad allele diversity in Upland cotton, was used to construct high-density SNP-based linkage maps and to map the QTLs controlling important cotton traits.

Results

Molecular genetic mapping using RIL population produced a genetic map of 3129 SNPs, mapped at a density of 1.41 cM. Genetic maps of the individual chromosomes showed good collinearity with the sequence based physical map. A total of 106 QTLs were identified which included 59 QTLs for six fiber quality traits, 38 QTLs for four yield traits and 9 QTLs for two morphological traits. Sub-genome wide, 57 QTLs were mapped in A sub-genome and 49 were mapped in D sub-genome. More than 75% of the QTLs with favorable alleles were contributed by the parental accession NC05AZ06. Forty-six mapped QTLs each explained more than 10% of the phenotypic variation. Further, we identified 21 QTL clusters where 12 QTL clusters were mapped in the A sub-genome and 9 were mapped in the D sub-genome. Candidate gene analyses of the 11 stable QTL harboring genomic regions identified 19 putative genes which had functional role in cotton fiber development.

Conclusion

We constructed a high-density genetic map of SNPs in Upland cotton. Collinearity between genetic and physical maps indicated no major structural changes in the genetic mapping populations. Most traits showed high broad-sense heritability. One hundred and six QTLs were identified for the fiber quality, yield and morphological traits. Majority of the QTLs with favorable alleles were contributed by improved parental accession. More than 70% of the mapped QTLs shared the similar map position with previously reported QTLs which suggest the genetic relatedness of Upland cotton germplasm. Identification of QTL clusters could explain the correlation among some fiber quality traits in cotton. Stable and major QTLs and QTL clusters of traits identified in the current study could be the targets for map-based cloning and marker assisted selection (MAS) in cotton breeding. The genomic region on D12 containing the major stable QTLs for micronaire, fiber strength and lint percentage could be potential targets for MAS and gene cloning of fiber quality traits in cotton.

Proteomic profiling of cotton fiber developmental transition from cell elongation to secondary wall deposition

Cotton fiber developmental transition from elongation to secondary cell wall biosynthesis is a critical growth shifting phase that affects fiber final length, strength, and other properties. Morphological dynamic analysis indicated an asynchronous fiber developmental pattern between two most important commercial cotton species, Gossypium hirsutum (Gh) and G. barbadense (Gb). Using isobaric tags for relative and absolute quantitation techniques, we examined the temporal changes of protein expression at three representative development periods (15-19, 19-23, and 23-27 dpa) in both species. Strikingly, a large proportion of differentially expressed proteins (DEPs) were identified at 19-23 dpa in Gh and at 23-27 dpa in Gb, corresponding to their fiber developmental transition timing. To better understand fiber transitional development, we comparatively analyzed those DEPs in 19-23 dpa of Gh vs. in 23-27 dpa of Gb, and noted that these cotton species indeed share fundamentally similar fiber developmental features under the biological processes. We also showed that there are limited overlaps in both specific upregulated and downregulated proteins between the two species, suggesting species-specific protein regulations in the development process. Proteomic profiling results revealed dynamic changes of several key proteins and biological processes that are potentially correlated with fiber developmental transition. During the transition, upregulated proteins are mainly involved in carbohydrate/energy metabolism, oxidation-reduction, cytoskeleton, protein turnover, Ca2+ signaling, etc., whereas important downregulated proteins are mostly involved in phenylpropanoid and flavonoid secondary metabolism pathways. The gene expressions of several changed proteins in this key stage were also examined by quantitative reverse transcription polymerase chain reaction. Overall, the present study provides accurate pictures of the regulatory networks of functional proteins during the fiber developmental transition, therefore highlighting candidate genes/proteins and related pathways for the cotton fiber improvement.

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

Trichomes are specialized epidermal cells and a vital plant organ that protect plants from various harms and provide valuable resources for plant development and use. Some key genes related to trichomes have been identified in the model plant Arabidopsis thaliana through glabrous mutants and gene cloning, and the hub MYB-bHLH-WD40, consisting of several factors including GLABRA1 (GL1), GL3, TRANSPARENT TESTA GLABRA1 (TTG1), and ENHANCER OF GLABRA3 (EGL3), has been established. Subsequently, some upstream transcription factors, phytohormones and epigenetic modification factors have also been studied in depth. In cotton, a very important fibre and oil crop globally, in addition to the key MYB-like factors, more important regulators and potential molecular mechanisms (e.g. epigenetic modifiers, distinct metabolic pathways) are being exploited during different fibre developmental stages. This occurs due to increased cotton research, resulting in the discovery of more complex regulation mechanisms from the allotetraploid genome of cotton. In addition, some conservative as well as specific mediators are involved in trichome development in other species. This study summarizes molecular mechanisms in trichome development and provides a detailed comparison of the similarities and differences between Arabidopsis and cotton, analyses the possible reasons for the discrepancy in identification of regulators, and raises future questions and foci for understanding trichome development in more detail.

Optimization of polar distribution of GhPIN3a in the ovule epidermis improves cotton fiber development

This article comments on: Zeng J, Zhang M, Hou L, Bai W, Yan X, Hou N, Wang H, Huang J, Zhao J, and Pei Y. 2019. Cytokinin inhibits cotton fiber initiation via disrupting PIN3a-mediated IAA asymmetric accumulation in ovule epidermis. Journal of Experimental Botany 70, 3139–3151.

Cytokinin inhibits cotton fiber initiation by disrupting PIN3a-mediated asymmetric accumulation of auxin in the ovule epidermis

Auxin-dependent cell expansion is crucial for initiation of fiber cells in cotton (Gossypium hirsutum), which ultimately determines fiber yield and quality. However, the regulation of this process is far from being well understood. In this study, we demonstrate an antagonistic effect between cytokinin (CK) and auxin on cotton fiber initiation. In vitro and in planta experiments indicate that enhanced CK levels can reduce auxin accumulation in the ovule integument, which may account for the defects in the fiberless mutant xu142fl. In turn, supplementation with auxin can recover fiber growth of CK-treated ovules and mutant ovules. We further found that GhPIN3a is a key auxin transporter for fiber-cell initiation and is polarly localized to the plasma membranes of non-fiber cells, but not to those of fiber cells. This polar localization allows auxin to be transported within the ovule integument while specifically accumulating in fiber cells. We show that CKs antagonize the promotive effect of auxin on fiber cell initiation by undermining asymmetric accumulation of auxin in the ovule epidermis through down-regulation of GhPIN3a and disturbance of the polar localization of the protein.

MaBZR1/2 act as transcriptional repressors of ethylene biosynthetic genes in banana fruit

Banana fruit (Musa acuminate L.) ripening is a complex genetical process affected by multiple phytohormones and expression of various genes. However, whether plant hormone brassinosteroid (BR) is involved in this process remains obscure. In this work, three genes that encode BR core signaling components brassinazole resistant (BZR) proteins, namely MaBZR1 to MaBZR3, were characterized from banana fruit. MaBZR1-MaBZR3 exhibited both nuclear and cytoplasmic localization and behaved as transcription inhibitors. Expression analysis showed that MaBZR1/2/3 were continuously decreased as fruit ripening proceeded, indicating their negative roles in banana ripening. Moreover, gel shift and transient expression assays demonstrated that MaBZR1/2 could suppress the transcription of ethylene biosynthetic genes, including MaACS1, MaACO13 and MaACO14, which increased gradually during the banana ripening, via specifically binding to CGTGT/CG sequence in their promoters. Importantly, exogenous application of BRs promotes banana ripening, which is presumably due to the accelerated expression of MaACS1 and MaACO13/14, and consequently the ethylene production. Our study indicates that MaBZR1/2 act as transcriptional repressors of ethylene biosynthetic genes during banana fruit ripening.

Fiber Quality Improvement in Upland Cotton (Gossypium hirsutum L.): Quantitative Trait Loci Mapping and Marker Assisted Selection Application

Genetic improvement in fiber quality is one of the main challenges for cotton breeders. Fiber quality traits are controlled by multiple genes and are classified as complex quantitative traits, with a negative relationship with yield potential, so the genetic gain is low in traditional genetic improvement by phenotypic selection. The availability of Gossypium genomic sequences facilitates the development of high-throughput molecular markers, quantitative trait loci (QTL) fine mapping and gene identification, which helps us to validate candidate genes and to use marker assisted selection (MAS) on fiber quality in breeding programs. Based on developments of high density linkage maps, QTLs fine mapping, marker selection and omics, we have performed trait dissection on fiber quality traits in diverse populations of upland cotton. QTL mapping combined with multi-omics approaches such as, RNA sequencing datasets to identify differentially expressed genes have benefited the improvement of fiber quality. In this review, we discuss the application of molecular markers, QTL mapping and MAS for fiber quality improvement in upland cotton.

A Pivotal Role of Hormones in Regulating Cotton Fiber Development

Cotton is the main source of renewable fiber in the world and is primarily used for textile production. Cotton fibers are single cells differentiated from the ovule epidermis and are an excellent model system for studying cell elongation, polyploidization, and cell wall biosynthesis. Plant hormones, which are present in relatively low concentrations, play important roles in various developmental processes, and recently, multiple reports have revealed the pivotal roles of hormones in regulating cotton fiber development. For example, exogenous application of hormones has been shown to promote the initiation and growth of fiber cells. However, a comprehensive understanding about phytohormone regulating fiber development is still unknown. Here, we focus on recent advances in elucidating the roles of multiple phytohormones in the control of fiber development, namely auxin, gibberellin, brassinosteroid, ethylene, cytokinin, abscisic acid, and strigolactones. We not only review the identification of genes involved in hormone biosynthetic and signaling pathways but also discuss the mechanisms of these phytohormones in regulating the initiation and elongation of fiber cells in cotton. Auxin, gibberellin, brassinosteroid, ethylene, jasmonic acid, and strigolactones play positive roles in fiber development, whereas cytokinin and abscisic acid inhibit fiber growth. Our aim is to provide a comprehensive review of the role of phytohormones in cotton fiber development that will serve as the basis for further elucidation of the mechanisms by which plant hormones regulate fiber growth.

The cotton XLIM protein (GhXLIM6) is required for fiber development via maintaining dynamic F-actin cytoskeleton and modulating cellulose biosynthesis

LIM domain proteins are cysteine-rich proteins, and are often considered as actin bundlers and transcription factors in plants. However, the roles of XLIM proteins in plants (especially in cotton) remain unexplored in detail so far. In this study, we identified a cotton XLIM protein (GhXLIM6) that is preferentially expressed in cotton fiber during whole elongation stage and early secondary cell wall (SCW) synthesis stage. The GhXLIM6-silenced transgenic cotton produces shorter fibers with thinner cell walls, compared with wild-type (WT). GhXLIM6 protein could directly bind F-actin and promote actin polymerization both in vitro and in vivo. It also acts as a transcription factor to suppress GhKNL1 expression through binding the PAL-box element of GhKNL1 promoter, and subsequently regulate the expression of CesA genes related to cellulose biosynthesis and deposition in SCWs of cotton fibers. The cellulose content in fibers of GhXLIM6RNAi cotton is lower than that in WT. Taken together, these data reveal the dual roles of GhXLIM6 in fiber development. On one hand, GhXLIM6 functions in fiber elongation through binding to F-actin to maintain the dynamic F-actin cytoskeleton. On the other hand, GhXLIM6 fine-tunes fiber SCW formation, probably through directly suppressing transcription of GhKNL1 to promote cellulose biosynthesis.

Characterization of bHLH/HLH genes that are involved in brassinosteroid (BR) signaling in fiber development of cotton (Gossypium hirsutum)

BACKGROUND

Basic helix-loop-helix/helix-loop-helix (bHLH/HLH) transcription factors play important roles in plant development. Many reports have suggested that bHLH/HLH proteins participate in brassinosteroid (BR) hormone signaling pathways to promote cell elongation. Cotton fibers are single-cells and derived from seed surface. To explore the roles of bHLH/HLH proteins in cotton fiber development progress by modulating BR signaling pathway, we performed a systematic analysis of the bHLH/HLH gene family in upland cotton (Gossypium hirsutum) genome.

RESULTS

In this study, we identified 437 bHLH/HLH genes in upland cotton (G. hirsutum) genome. Phylogenetic analysis revealed that GhbHLH/HLH proteins were split into twenty six clades in the tree. These GhbHLH/HLH genes are distributed unevenly in different chromosomes of cotton genome. Segmental duplication is the predominant gene duplication event and the major contributor for amplification of GhbHLH/HLH gene family. The GhbHLH/HLHs within the same group have conserved exon/intron pattern and their encoding proteins show conserved motif composition. Based on transcriptome data, we identified 77 GhbHLH/HLH candidates that are expressed at relatively high levels in cotton fibers. As adding exogenous BR (brassinolide, BL) or brassinazole (Brz, a BR biosynthesis inhibitor), expressions of these GhbHLH/HLH genes were up-regulated or down-regulated in cotton fibers. Furthermore, overexpression of GhbHLH282 (one of the BR-response genes) in Arabidopsis not only promoted the plant growth, but also changed plant response to BR signaling.

CONCLUSION

Collectively, these data suggested that these GhbHLH/HLH genes may participate in BR signaling transduction during cotton fiber development. Thus, our results may provide a valuable reference data as the basis for further studying the roles of these bHLH/HLH genes in cotton fiber development.

Genome-wide identification of the GhARF gene family reveals that GhARF2 and GhARF18 are involved in cotton fibre cell initiation

Auxin signalling plays an essential role in regulating plant development. Auxin response factors (ARFs), which are critical components of auxin signalling, modulate the expression of early auxin-responsive genes by binding to auxin response factor elements (AuxREs). However, there has been no comprehensive characterization of this gene family in cotton. Here, we identified 56 GhARF genes in the assembled Gossypium hirsutum genome. This gene family was divided into 17 subfamilies, and 44 members of them were distributed across 21 chromosomes. GhARF6 and GhARF11 subfamily genes were predominantly expressed in vegetative tissues, whereas GhARF2 and GhARF18 subfamily genes were highly expressed during seed fibre cell initiation. GhARF2-1 and GhARF18-1 were exclusively expressed in trichomes, organs similar to cotton seed fibre cells, and overexpression of these genes in Arabidopsis enhances trichome initiation. Comparative transcriptome analysis combined with AuxRE prediction revealed 11 transcription factors as potential target genes of GhARF2 and GhARF18. Six of these genes were significantly expressed during seed fibre cell initiation and were bound by GhARF2-1 and GhARF18-1 in yeast one-hybrid assays. Our results suggest that GhARF2 and GhARF18 genes may be key regulators of cotton seed fibre initiation by regulating the expression of several transcription factor genes. This study deepens our understanding of auxin-mediated initiation of cotton seed fibre cells and helps us in breeding better cotton varieties in the future.

Inhibition of Heat Shock proteins HSP90 and HSP70 induce oxidative stress, suppressing cotton fiber development

Cotton fiber is a specialized unicellular structure useful for the study of cellular differentiation and development. Heat shock proteins (HSPs) have been shown to be involved in various developmental processes. Microarray data analysis of five Gossypium hirsutum genotypes revealed high transcript levels of GhHSP90 and GhHSP70 genes at different stages of fiber development, indicating their importance in the process. Further, we identified 26 and 55 members of HSP90 and HSP70 gene families in G. hirsutum. The treatment of specific inhibitors novobiocin (Nov; HSP90) and pifithrin/2-phenylethynesulfonamide (Pif; HSP70) in in-vitro cultured ovules resulted in a fewer number of fiber initials and retardation in fiber elongation. The molecular chaperone assay using bacterially expressed recombinant GhHSP90-7 and GhHSP70-8 proteins further confirmed the specificity of inhibitors. HSP inhibition disturbs the H₂O₂ balance that leads to the generation of oxidative stress, which consequently results in autophagy in the epidermal layer of the cotton ovule. Transmission electron microscopy (TEM) of inhibitor-treated ovule also corroborates autophagosome formation along with disrupted mitochondrial cristae. The perturbations in transcript profile of HSP inhibited ovules show differential regulation of different stress and fiber development-related genes and pathways. Altogether, our results indicate that HSP90 and HSP70 families play a crucial role in cotton fiber differentiation and development by maintaining cellular homeostasis.

Genome-Wide Analysis of the GRF Family Reveals Their Involvement in Abiotic Stress Response in Cassava

GENERAL REGULATORY FACTOR (GRF) proteins play vital roles in the regulation of plant growth, development, and response to abiotic stress. However, little information is known for this gene family in cassava (Manihot esculenta). In this study, 15 MeGRFs were identified from the cassava genome and were clustered into the ε and the non-ε groups according to phylogenetic, conserved motif, and gene structure analyses. Transcriptomic analyses showed eleven MeGRFs with constitutively high expression in stems, leaves, and storage roots of two cassava genotypes. Expression analyses revealed that the majority of GRFs showed transcriptional changes under cold, osmotic, salt, abscisic acid (ABA), and H₂O₂ treatments. Six MeGRFs were found to be commonly upregulated by abiotic stress, ABA, and H₂O₂ treatments, which may be the converging points of multiple signaling pathways. Interaction network analysis identified 18 possible interactors of MeGRFs. Taken together, this study elucidates the transcriptional control of MeGRFs in tissue development and the responses of abiotic stress and related signaling in cassava. Some constitutively expressed, tissue-specific, and abiotic stress-responsive candidate MeGRF genes were identified for the further genetic improvement of crops.