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

Beyond skin-deep: targeting the plant surface for crop improvement

The above-ground plant surface is a well-adapted tissue layer that acts as an interface between the plant and its surrounding environment. As such, its primary role is to protect against desiccation and maintain the gaseous exchange required for photosynthesis. Further, this surface layer provides a barrier against pathogens and herbivory, while attracting pollinators and agents of seed dispersal. In the context of agriculture, the plant surface is strongly linked to post-harvest crop quality and yield. The epidermal layer contains several unique cell types adapted for these functions, while the non-lignified above-ground plant organs are covered by a hydrophobic cuticular membrane. This review aims to provide an overview of the latest understanding of the molecular mechanisms underlying crop cuticle and epidermal cell formation, with focus placed on genetic elements contributing towards quality, yield, drought tolerance, herbivory defence, pathogen resistance, pollinator attraction, and sterility, while highlighting the inter-relatedness of plant surface development and traits. Potential crop improvement strategies utilizing this knowledge are outlined in the context of the recent development of new breeding techniques.

SDG26 Is Involved in Trichome Control in Arabidopsis thaliana: Affecting Phytohormones and Adjusting Accumulation of H3K27me3 on Genes Related to Trichome Growth and Development

Plant trichomes formed by specialized epidermal cells play a role in protecting plants from biotic and abiotic stresses and can also influence the economic and ornamental value of plant products. Therefore, further studies on the molecular mechanisms of plant trichome growth and development are important for understanding trichome formation and agricultural production. SET Domain Group 26 (SDG26) is a histone lysine methyltransferase. Currently, the molecular mechanism by which SDG26 regulates the growth and development of Arabidopsis leaf trichomes is still unclear. We found that the mutant of Arabidopsis (sdg26) possessed more trichomes on its rosette leaves compared to the wild type (Col-0), and the trichome density per unit area of sdg26 is significantly higher than that of Col-0. The content of cytokinins and jasmonic acid was higher in sdg26 than in Col-0, while the content of salicylic acid was lower in sdg26 than in Col-0, which is conducive to trichome growth. By measuring the expression levels of trichome-related genes, we found that the expression of genes that positively regulate trichome growth and development were up-regulated, while the negatively regulated genes were down-regulated in sdg26. Through chromatin immunoprecipitation sequencing (ChIP-seq) analysis, we found that SDG26 can directly regulate the expression of genes related to trichome growth and development such as ZFP1, ZFP5, ZFP6, GL3, MYB23, MYC1, TT8, GL1, GIS2, IPT1, IPT3, and IPT5 by increasing the accumulation of H3K27me3 on these genes, which further affects the growth and development of trichomes. This study reveals the mechanism by which SDG26 affects the growth and development of trichomes through histone methylation. The current study provides a theoretical basis for studying the molecular mechanism of histone methylation in regulating leaf trichome growth and development and perhaps guiding the development of new crop varieties.

Lint percentage and boll weight QTLs in three excellent upland cotton (Gossypium hirsutum): ZR014121, CCRI60, and EZ60

BACKGROUND

Upland cotton (Gossypium hirsutum L.) is the most economically important species in the cotton genus (Gossypium spp.). Enhancing the cotton yield is a major goal in cotton breeding programs. Lint percentage (LP) and boll weight (BW) are the two most important components of cotton lint yield. The identification of stable and effective quantitative trait loci (QTLs) will aid the molecular breeding of cotton cultivars with high yield.

RESULTS

Genotyping by target sequencing (GBTS) and genome-wide association study (GWAS) with 3VmrMLM were used to identify LP and BW related QTLs from two recombinant inbred line (RIL) populations derived from high lint yield and fiber quality lines (ZR014121, CCRI60 and EZ60). The average call rate of a single locus was 94.35%, and the average call rate of an individual was 92.10% in GBTS. A total of 100 QTLs were identified; 22 of them were overlapping with the reported QTLs, and 78 were novel QTLs. Of the 100 QTLs, 51 QTLs were for LP, and they explained 0.29-9.96% of the phenotypic variation; 49 QTLs were for BW, and they explained 0.41-6.31% of the phenotypic variation. One QTL (qBW-E-A10-1, qBW-C-A10-1) was identified in both populations. Six key QTLs were identified in multiple-environments; three were for LP, and three were for BW. A total of 108 candidate genes were identified in the regions of the six key QTLs. Several candidate genes were positively related to the developments of LP and BW, such as genes involved in gene transcription, protein synthesis, calcium signaling, carbon metabolism, and biosynthesis of secondary metabolites. Seven major candidate genes were predicted to form a co-expression network. Six significantly highly expressed candidate genes of the six QTLs after anthesis were the key genes regulating LP and BW and affecting cotton yield formation.

CONCLUSIONS

A total of 100 stable QTLs for LP and BW in upland cotton were identified in this study; these QTLs could be used in cotton molecular breeding programs. Putative candidate genes of the six key QTLs were identified; this result provided clues for future studies on the mechanisms of LP and BW developments.

An R2R3 MYB gene GhMYB3 functions in drought stress by negatively regulating stomata movement and ROS accumulation

MYB transcription factors are one of the largest TF families involved in plant growth and development as well as biotic and abiotic stresses. In this study, we report the identification and functional characterization of a stress-responsive MYB gene (GhMYB3) from drought stress related transcriptome of upland cotton. GhMYB3, belonging to the R2R3-type, has high sequence similarity with AtMYB3 and was localized in the nucleus. Silence of GhMYB3 enhanced the drought tolerance of cotton seedlings and plants, reduced the water loss rate, and enhanced stomatal closure. In addition, GhMYB3i lines exhibited less ROS accumulation, as well as higher antioxidant enzyme activity and increased content of anthocyanins and proanthocyanidins than WT plants after drought stress. The expression level of flavonoid biosynthesis- and stress-related genes were up-regulated in GhMYB3i lines under drought stress condition. These results demonstrated that GhMYB3 acted as a negative regulator in upland cotton response to drought stress by regulating stomatal closure and ROS accumulation.

The R2R3-MYB-SG7 transcription factor CaMYB39 orchestrates surface phenylpropanoid metabolism and pathogen resistance in chickpea

Flavonoids are important plant pigments and defense compounds; understanding the transcriptional regulation of flavonoid biosynthesis may enable engineering crops with improved nutrition and stress tolerance. Here, we characterize R2R3-MYB domain subgroup 7 transcription factor CaMYB39, which regulates flavonol biosynthesis primarily in chickpea trichomes. CaMYB39 overexpression in chickpea was accompanied by a change in flux availability for the phenylpropanoid pathway, particularly flavonol biosynthesis. Lines overexpressing CaMYB39 showed higher isoflavonoid levels, suggesting its role in regulating isoflavonoid pathway. CaMYB39 transactivates the transcription of early flavonoid biosynthetic genes (EBG). FLAVONOL SYNTHASE2, an EBG, encodes an enzyme with higher substrate specificity for dihydrokaempferol than other dihydroflavonols explaining the preferential accumulation of kaempferol derivatives as prominent flavonols in chickpea. Interestingly, CaMYB39 overexpression increased trichome density and enhanced the accumulation of diverse flavonol derivatives in trichome-rich tissues. Moreover, CaMYB39 overexpression reduced reactive oxygen species levels and induced defense gene expression which aids in partially blocking the penetration efficiency of the fungal pathogen, Ascochyta rabiei, resulting in lesser symptoms, thus establishing its role against deadly Ascochyta blight (AB) disease. Overall, our study reports an instance where R2R3-MYB-SG7 member, CaMYB39, besides regulating flavonol biosynthesis, modulates diverse pathways like general phenylpropanoid, isoflavonoid, trichome density, and defense against necrotrophic fungal infection in chickpea.

Comparative analysis of the MYB gene family in seven Ipomoea species

The MYB transcription factors regulate plant growth, development, and defense responses. However, information about the MYB gene family in Ipomoea species is rare. Herein, we performed a comprehensive genome-wide comparative analysis of this gene family among seven Ipomoea species, sweet potato (I. batatas), I. trifida, I. triloba, I. nil, I. purpurea, I. cairica, and I. aquatic, and identified 296, 430, 411, 291, 226, 281, and 277 MYB genes, respectively. The identified MYB genes were classified into five types: 1R-MYB (MYB-related), 2R-MYB (R2R3-MYB), 3R-MYB (R1R2R3-MYB), 4R-MYB, and 5R-MYB, and the MYB-related or R2R3-MYB type was the most abundant MYB genes in the seven species. The Ipomoea MYB genes were classed into distinct subgroups based on the phylogenetic topology and the classification of the MYB superfamily in Arabidopsis. Analysis of gene structure and protein motifs revealed that members within the same phylogenetic group presented similar exon/intron and motif organization. The identified MYB genes were unevenly mapped on the chromosomes of each Ipomoea species. Duplication analysis indicated that segmental and tandem duplications contribute to expanding the Ipomoea MYB genes. Non-synonymous substitution (Ka) to synonymous substitution (Ks) [Ka/Ks] analysis showed that the duplicated Ipomoea MYB genes are mainly under purifying selection. Numerous cis-regulatory elements related to stress responses were detected in the MYB promoters. Six sweet potato transcriptome datasets referring to abiotic and biotic stresses were analyzed, and MYB different expression genes' (DEGs') responses to stress treatments were detected. Moreover, 10 sweet potato MYB DEGs were selected for qRT-PCR analysis. The results revealed that four responded to biotic stress (stem nematodes and Ceratocystis fimbriata pathogen infection) and six responded to the biotic stress (cold, drought, and salt). The results may provide new insights into the evolution of MYB genes in the Ipomoea genome and contribute to the future molecular breeding of sweet potatoes.

Comparative Genomics and Functional Studies of Putative m6A Methyltransferase (METTL) Genes in Cotton

N6-methyladenosine (m6A) RNA modification plays important regulatory roles in plant development and adapting to the environment, which requires methyltransferases to achieve the methylation process. However, there has been no research regarding m6A RNA methyltransferases in cotton. Here, a systematic analysis of the m6A methyltransferase (METTL) gene family was performed on twelve cotton species, resulting in six METTLs identified in five allotetraploid cottons, respectively, and three to four METTLs in the seven diploid species. Phylogenetic analysis of protein-coding sequences revealed that METTL genes from cottons, Arabidopsis thaliana, and Homo sapiens could be classified into three clades (METTL3, METTL14, and METTL-like clades). Cis-element analysis predicated the possible functions of METTL genes in G. hirsutum. RNA-seq data revealed that GhMETTL14 (GH_A07G0817/GH_D07G0819) and GhMETTL3 (GH_A12G2586/GH_D12G2605) had high expressions in root, stem, leaf, torus, petal, stamen, pistil, and calycle tissues. GhMETTL14 also had the highest expression in 20 and 25 dpa fiber cells, implying a potential role at the cell wall thickening stage. Suppressing GhMETTL3 and GhMETTL14 by VIGS caused growth arrest and even death in G. hirsutum, along with decreased m6A abundance from the leaf tissues of VIGS plants. Overexpression of GhMETTL3 and GhMETTL14 produced distinct differentially expressed genes (DEGs) in A. thaliana, indicating their possible divergent functions after gene duplication. Overall, GhMETTLs play indispensable but divergent roles during the growth of cotton plants, which provides the basis for the systematic investigation of m6A in subsequent studies to improve the agronomic traits in cotton.

TRY intron2 determined its expression in inflorescence activated by SPL9 and MADS-box genes in Arabidopsis

Plant trichomes are specialized epidermal cells that protect plants from insects and pathogens. In Arabidopsis, epidermal hairs decrease as internodes increase in height, with only few epidermal hairs produced on the sepals abaxial surface of the early flowers. TRIPTYCHON (TRY) is known to be a negative regulator of epidermal hair development in Arabidopsis, suppressing the formation of ectopic epidermal hairs in the inflorescence. Here, we reported that the second intron of TRY gene plays a critical role in trichome spatial distribution in Arabidopsis. The expression of TRY rises with the increasing stem nodes and reaches the peak in the inflorescence, while the trichomes distribution decrease. The transgenic plants showed that TRY promoter could only drive the genomic instead of coding sequences combined with GUS reporter gene, which indicates that the regulatory elements of TRY expression in inflorescence could be located in the intron regions. Multiple SPLs and MADS-box binding sites were found in the TRY intron2 sequence. Further genetic and biochemistry assays revealed that the flowering-related genes such as SPL9 could bind to these cis-elements directly, contributing to the TRY spatial expression. Since cotton fiber and Arabidopsis trichomes share similar regulatory mechanism, extended analysis showed that the intron2 of cotton TRY genes also contain the cis-elements. Thus, the introns harboring the transcription element may be the general way to regulate the gene expression in different plants and provides molecular clues for the related crops' traits design.

Systematical characterization of GRF gene family in sorghum, and their potential functions in aphid resistance

Sorghum (Sorghum bicolor) is the fifth important cereal and an industrial energy crop in the world. Growth Regulation Factors (GRFs) play an important role in response to environmental stress, however, the knowledge of GRFs relating to the pest resistance is lacking. Here, we identified 8 GRF genes harboring the typical QLQ (glutamine, leucine, glutamine) and WRC (tryptophan, arginine, cysteine) domains in Sorghum, which could be classified into 4 clades through phylogenetic analysis. The SbGRF genes express in most tissues, while more than half of them express at the highest level in inflorescence. To further investigate their possible role in stress response, we analyzed the transcriptomics data. The results showed that SbGRFs could respond to the abiotic stresses including heat, salt and drought stress. Furthermore, combined the data with qRT-PCR, SbGRF1, 2, 4 and 7 were identified as dominant genes response to the aphid-induced stress. SSR markers close to these genes were also searched. Above all, we summarized the SbGRFs and provided their potential roles in aphid response.

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.

Inheritance, QTLs, and Candidate Genes of Lint Percentage in Upland Cotton

Cotton (Gossypium spp.) is an important natural fiber plant. Lint percentage (LP) is one of the most important determinants of cotton yield and is a typical quantitative trait with high variation and heritability. Many cotton LP genetic linkages and association maps have been reported. This work summarizes the inheritance, quantitative trait loci (QTLs), and candidate genes of LP to facilitate LP genetic study and molecular breeding. More than 1439 QTLs controlling LP have been reported. Excluding replicate QTLs, 417 unique QTLs have been identified on 26 chromosomes, including 243 QTLs identified at LOD >3. More than 60 are stable, major effective QTLs that can be used in marker-assisted selection (MAS). More than 90 candidate genes for LP have been reported. These genes encode MYB, HOX, NET, and other proteins, and most are preferentially expressed during fiber initiation and elongation. A putative molecular regulatory model of LP was constructed and provides the foundation for the genetic study and molecular breeding of LP.

Comprehensive identification and expression analysis of CRY gene family in Gossypium

BACKGROUND

The cryptochromes (CRY) are specific blue light receptors of plants and animals, which play crucial roles in physiological processes of plant growth, development, and stress tolerance.

RESULTS

In the present work, a systematic analysis of the CRY gene family was performed on twelve cotton species, resulting in 18, 17, 17, 17, and 17 CRYs identified in five alloteraploid cottons (Gossypium hirsutum, G. barbadense, G. tomentosum, G. mustelinum and G. darwinii), respectively, and five to nine CRY genes in the seven diploid species. Phylogenetic analysis of protein-coding sequences revealed that CRY genes from cottons and Arabidopsis thaliana could be classified into seven clades. Synteny analysis suggested that the homoeolog of G. hirsutum Gh_A02G0384 has undergone an evolutionary loss event in the other four allotetraploid cotton species. Cis-element analysis predicated the possible functions of CRY genes in G. hirsutum. RNA-seq data revealed that Gh_D09G2225, Gh_A09G2012 and Gh_A11G1040 had high expressions in fiber cells of different developmental states. In addition, the expression levels of one (Gh_A03G0120), 15 and nine GhCRY genes were down-regulated following the PEG, NaCl and high-temperature treatments, respectively. For the low-temperature treatment, five GhCRY genes were induced, and five were repressed. These results indicated that most GhCRY genes negatively regulate the abiotic stress treatments.

CONCLUSION

We report the structures, domains, divergence, synteny, and cis-elements analyses systematically of G. hirsutum CRY genes. Possible biological functions of GhCRY genes in differential tissues as well as in response to abiotic stress during the cotton plant life cycle were predicted.

Genetic Diversity and Genome-Wide Association Study of Morphological and Quality Traits in Peach Using Two Spanish Peach Germplasm Collections

Peach [Prunus persica (L.) Batsch] is one of the most important stone fruits species in world production. Spanish peach production is currently the second largest in the world and the available cultivars in Spain includes a great source of genetic diversity with variability in fruit quality traits and postharvest disorders tolerance. In order to explore the genetic diversity and single nucleotide polymorphism (SNP)-trait associations in the Spanish germplasm, the new peach 18K SNP v2 array was used to genotype 287 accessions belonging to the two National Peach Germplasm Collections placed at the Agrifood Research and Technology Centre of Aragon (CITA) and at the Experimental Station of Aula Dei (EEAD)-CSIC. The high density of the new SNP array allowed the identification of 30 groups of synonymies, which had not been identified before using low-density markers. In addition, a possible large-scale molecular event in 'Starcrest', a sport of 'Springcrest', was detected showing a possible chromosome replacement of a 13.5 Mb region. Previous suggestions about Spanish diversification regions agreed with our genetic diversity and linkage disequilibrium (LD) decay results using high-density markers. A genome-wide association study (GWAS) detected 34 significant SNP-trait association with the type of leaf glands (TLG), fruit hairiness (FH), and flesh texture (FT). The impact of the significant SNPs was studied with SnpEff. Candidate genes encode several important family proteins involved in trichome formation and powdery mildew resistance (linked to TLG in peach). The genetic distance among cultivars obtained, together with SNP-trait associations found, provide new knowledge for marker-assisted selection and crossing approaches in peach breeding programmes.