Transcriptome analysis of the genes regulating phytohormone and cellular patterning in Lagerstroemia plant architecture

Genome-Wide Identification, Characterization, and Expression Analysis of the HD-Zip Gene Family in Lagerstroemia for Regulating Plant Height

The Homeodomain leucine zipper (HD-Zip) family of transcription factors is crucial in helping plants adapt to environmental changes and promoting their growth and development. Despite research on the HD-Zip family in various plants, studies in Lagerstroemia (crape myrtle) have not been reported. This study aimed to address this gap by comprehensively analyzing the HD-Zip gene family in crape myrtle. This study identified 52 HD-Zip genes in the genome of Lagerstroemia indica, designated as LinHDZ1-LinHDZ52. These genes were distributed across 22 chromosomes and grouped into 4 clusters (HD-Zip I-IV) based on their phylogenetic relationships. Most gene structures and motifs within each cluster were conserved. Analysis of protein properties, gene structure, conserved motifs, and cis-acting regulatory elements revealed diverse roles of LinHDZs in various biological contexts. Examining the expression patterns of these 52 genes in 6 tissues (shoot apical meristem, tender shoot, and mature shoot) of non-dwarf and dwarf crape myrtles revealed that 2 LinHDZs (LinHDZ24 and LinHDZ14) and 2 LinHDZs (LinHDZ9 and LinHDZ35) were respectively upregulated in tender shoot of non-dwarf crape myrtles and tender and mature shoots of dwarf crape myrtles, which suggested the important roles of these genes in regulate the shoot development of Lagerstroemia. In addition, the expression levels of 2 LinHDZs (LinHDZ23 and LinHDZ34) were significantly upregulated in the shoot apical meristem of non-dwarf crape myrtle. These genes were identified as key candidates for regulating Lagerstroemia plant height. This study enhanced the understanding of the functions of HD-Zip family members in the growth and development processes of woody plants and provided a theoretical basis for further studies on the molecular mechanisms underlying Lagerstroemia plant height.

Genome assembly and resequencing analyses provide new insights into the evolution, domestication and ornamental traits of crape myrtle

Crape myrtle (Lagerstroemia indica) is a globally used ornamental woody plant and is the representative species of Lagerstroemia. However, studies on the evolution and genomic breeding of L. indica have been hindered by the lack of a reference genome. Here we assembled the first high-quality genome of L. indica using PacBio combined with Hi-C scaffolding to anchor the 329.14-Mb genome assembly into 24 pseudochromosomes. We detected a previously undescribed independent whole-genome triplication event occurring 35.5 million years ago in L. indica following its divergence from Punica granatum. After resequencing 73 accessions of Lagerstroemia, the main parents of modern crape myrtle cultivars were found to be L. indica and L. fauriei. During the process of domestication, genetic diversity tended to decrease in many plants, but this was not observed in L. indica. We constructed a high-density genetic linkage map with an average map distance of 0.33 cM. Furthermore, we integrated the results of quantitative trait locus (QTL) using genetic mapping and bulk segregant analysis (BSA), revealing that the major-effect interval controlling internode length (IL) is located on chr1, which contains CDL15, CRG98, and GID1b1 associated with the phytohormone pathways. Analysis of gene expression of the red, purple, and white flower-colour flavonoid pathways revealed that differential expression of multiple genes determined the flower colour of L. indica, with white flowers having the lowest gene expression. In addition, BSA of purple- and green-leaved individuals of populations of L. indica was performed, and the leaf colour loci were mapped to chr12 and chr17. Within these intervals, we identified MYB35, NCED, and KAS1. Our genome assembly provided a foundation for investigating the evolution, population structure, and differentiation of Myrtaceae species and accelerating the molecular breeding of L. indica.

Integrating transcriptome and phytohormones analysis provided insights into plant height development in sesame

Plant height is a key agronomic trait influencing crops yield. The height of sesame plants is important for yield performance, lodging resistance and plant architecture. Although plant height is significantly distinct among sesame varieties, the genetic basis of plant height remains largely unknown. In this study, in order to tackle genetic insights into the sesame plant height development, a comprehensive transcriptome analysis was conducted using the stem tips from two sesame varieties with distinct plant height, Zhongzhi13 and ZZM2748, at five time points by BGI MGIseq2000 sequencing platform. A total of 16,952 genes were differentially expressed between Zhongzhi13 and ZZM2748 at five time points. KEGG and MapMan enrichment analyses and quantitative analysis of phytohormones indicated that hormones biosynthesis and signaling pathways were associated with sesame plant height development. Plenty of candidate genes involved in biosynthesis and signaling of brassinosteroid (BR), cytokinin (CK) and gibberellin (GA) which were major differential hormones between two varieties were identified, suggesting their critical roles in plant height regulation. WGCNA revealed a module which was significantly positively associated with the plant height trait and founded SiSCL9 was the hub gene involved in plant height development in our network. Further overexpression in transgenic Arabidopsis validated the function of SiSCL9 in the increase of plant height by 26.86%. Collectively, these results increase our understanding of the regulatory network controlling the development of plant height and provide a valuable genetic resource for improvement of plant architecture in sesame.

Gibberellin-related genes regulate dwarfing mechanism in wintersweet

Chimonanthus praecox (wintersweet) is an important cut flower and pot plant with a high ornamental and economic value in China. The development of dwarf wintersweet varieties has become an important research topic for the wintersweet industry. The lack of natural dwarf germplasm has hindered research into the molecular mechanisms of developing dwarf wintersweet, limiting its cultivation. After a long-term investigation and collection of germplasm resources of C. praecox, we obtained the germplasm of a dwarf C. praecox (dw). Here, the dwarf and normal C. praecox (NH) were used to identify the types of hormones regulating dw formation using phenotypic identification and endogenous hormone determination. Differentially expressed genes in the dw and NH groups were screened using transcriptome analysis. The functions of key genes in the dwarf trait were verified by heterologous expression. It was found that the internode length and cell number were significantly reduced in dw than in NH, and the thickness of the xylem and pith was significantly decreased. The dwarfness of dw could be recovered by exogenous gibberellic acid (GA) application, and endogenous GA levels showed that the GA4 content of dw was substantially lower than that of NH. Transcriptome differential gene analysis showed that the elevated expression of the CpGA2ox gene in the GA synthesis pathway and that of CpGAI gene in the signal transduction pathway might be the key mechanisms leading to dwarfing. Combined with the results of weighted gene co-expression network analysis, we selected the CpGAI gene for analysis and functional verification. These results showed that CpGAI is a nuclear transcriptional activator. Overexpression of CpGAI in Populus tomentosa Carr. showed that CpGAI could lead to the dwarfing in poplar. We analyzed the dwarfing mechanism of C. praecox, and the results provided a reference for dwarf breeding of wintersweet.

Identification and Expression Analysis of NAC Gene Family in Weeping Trait of Lagerstroemia indica

Lagerstroemia indica is a widely used ornamental plant in summer gardens because of its desirable plant shape. The weeping traits of plants are related to secondary cell wall thickness and hormone signaling. NAC (NAM-ATAF1/2-CUC2), as one of the plant-specific transcription factors, is a switch for the secondary cell wall and also involved in leaf senescence, phytohormone signaling, and other growth processes. We identified a total of 21 LiNAC genes from the transcriptome data, which we divided into 14 subgroups and 2 groups. The physicochemical characteristics of amino acids, subcellular localization, transmembrane structure, GO and KEGG enrichment, and expression patterns were also examined. The qRT-PCR analysis showed that the expressions of LiNAC8 and LiNAC13 in upright L. indica 'Shaoguifei' and weeping L. indica 'Xiariwuniang' were significantly higher from the beginning to the end of growth stage (S1-S3), and the expressions of 'Shaoguifei' were always higher than those of 'Xiariwuniang'. However, LiNAC2 showed a downward trend in S1-S3 and the relative expression level of 'Shaoguifei' was lower than that of 'Xiariwuniang'. It is hypothesized that these LiNAC genes may be involved in the regulation of weeping traits in L. indica. The results of this study provide a basis for analyzing the functions of LiNAC genes and help to explore the molecular regulatory mechanisms related to the weeping traits in L. indica.

Comparative Transcriptomic, Anatomical and Phytohormone Analyses Provide New Insights Into Hormone-Mediated Tetraploid Dwarfing in Hybrid Sweetgum (Liquidambar styraciflua × L. formosana)

Polyploid breeding is an effective approach to improve plant biomass and quality. Both fast growth and dwarf types of in vitro or ex vitro plants are produced after polyploidization. However, little is known regarding the dwarf type mechanism in polyploids grown in vitro. In this study, the morphological and cytological characteristics were measured in tetraploid and diploid hybrid sweetgum (Liquidambar styraciflua × L. formosana) with the same genetic background. RNA sequencing (RNA-Seq) was used to analyse shoot and root variations between tetraploid and diploid plants; important metabolites were validated. The results showed that the shoot and root lengths were significantly shorter in tetraploids than in diploids after 25 d of culture. Most tetraploid root cells were wider and more irregular, and the length of the meristematic zone was shorter, while tetraploid cells were significantly larger than diploid cells. Differentially expressed genes (DEGs) were significantly enriched in the plant growth and organ elongation pathways, such as plant hormone biosynthesis and signal transduction, sugar and starch metabolism, and cell cycles. Hormone biosynthesis and signal transduction genes, such as YUCCA, TAA1, GH3, SAUR, CPS, KO, KAO, GA20ox, GA3ox, BAS1 and CYCD3, which help to regulate organ elongation, were generally downregulated. The auxin, gibberellin, and brassinolide (BL) contents in roots and stems were significantly lower in tetraploids than in diploids, which may greatly contribute to slow growth in the roots and stems of tetraploid regenerated plants. Exogenous gibberellic acid (GA₃) and indole-3-acetic acid (IAA), which induced plant cell elongation, could significantly promote growth in the stems and roots of tetraploids. In summary, comparative transcriptomics and metabolite analysis showed that the slow growth of regenerated tetraploid hybrid sweetgum was strongly related to auxin and gibberellin deficiency. Our findings provide insights into the molecular mechanisms that underlie dwarfism in allopolyploid hybrid sweetgum.

Transcriptome-Wide Characterization of Seed Aging in Rice: Identification of Specific Long-Lived mRNAs for Seed Longevity

Various long-lived mRNAs are stored in seeds, some of which are required for the initial phase of germination and are critical to seed longevity. However, the seed-specific long-lived mRNAs involved in seed longevity remain poorly understood in rice. To identify these mRNAs in seeds, we first performed aging experiment with 14 rice varieties, and categorized them as higher longevity (HL) and lower longevity (LL) rice varieties in conventional rice and hybrid rice, respectively. Second, RNA-seq analysis showed that most genes showed similar tendency of expression changes during natural and artificial aging, suggesting that the effects of these two aging methods on transcription are comparable. In addition, some differentially expressed genes (DEGs) in the HL and LL varieties differed after natural aging. Furthermore, several specific long-lived mRNAs were identified through a comparative analysis of HL and LL varieties after natural aging, and similar sequence features were also identified in the promoter of some specific long-lived mRNAs. Overall, we identified several specific long-lived mRNAs in rice, including gibberellin receptor gene GID1, which may be associated with seed longevity.

Morphological, transcriptomic and metabolomic analyses of Sophora davidii mutants for plant height

Sophora davidii is an important plant resource in the karst region of Southwest China, but S. davidii plant-height mutants are rarely reported. Therefore, we performed phenotypic, anatomic structural, transcriptomic and metabolomic analyses to study the mechanisms responsible for S. davidii plant-height mutants. Phenotypic and anatomical observations showed that compared to the wild type, the dwarf mutant displayed a significant decrease in plant height, while the tall mutant displayed a significant increase in plant height. The dwarf mutant cells were smaller and more densely arranged, while those of the wild type and the tall mutant were larger and loosely arranged. Transcriptomic analysis revealed that differentially expressed genes (DEGs) involved in cell wall biosynthesis, expansion, phytohormone biosynthesis, signal transduction pathways, flavonoid biosynthesis and phenylpropanoid biosynthesis were significantly enriched in the S. davidii plant-height mutants. Metabolomic analysis revealed 57 significantly differential metabolites screened from both the dwarf and tall mutants. A total of 8 significantly different flavonoid compounds were annotated to LIPID MAPS, and three metabolites (chlorogenic acid, kaempferol and scopoletin) were involved in phenylpropanoid biosynthesis and flavonoid biosynthesis. These results shed light on the molecular mechanisms of plant height in S. davidii mutants and provide insight for further molecular breeding programs.

A Genetic Linkage Map of BC2 Population Reveals QTL Associated with Plant Architecture Traits in Lagerstroemia

Plant architecture improvement is of great significance in influencing crop yield, harvesting efficiency and ornamental value, by changing the spatial structure of the canopy. However, the mechanism on plant architecture in woody plants is still unclear. In order to study the genetic control of plant architecture traits and promote marker-assisted selection (MAS), a genetic linkage map was constructed, and QTL mapping was performed. In this study, using 188 BC2 progenies as materials, a genetic map of Lagerstroemia was constructed using amplification fragment length polymorphisms (AFLP) and simple sequence repeats (SSR) markers, and the QTLs of four key plant architecture traits (plant height, crown width, primary lateral branch height and internode length) were analyzed. The genetic map contains 22 linkage groups, including 198 AFLP markers and 36 SSR markers. The total length of the genome covered by the map is 1272 cM, and the average distance between markers is 6.8 cM. Three QTLs related to plant height were located in LG1, LG4 and LG17 linkage groups, and the phenotypic variation rates were 32.36, 16.18 and 12.73%, respectively. A QTL related to crown width was located in LG1 linkage group, and the phenotypic variation rate was 18.07%. Two QTLs related to primary lateral branch height were located in the LG1 and LG7 linkage groups, and the phenotypic variation rates were 20.59 and 15.34%, respectively. Two QTLs related to internode length were located in the LG1 and LG20 linkage groups, and the phenotypic variation rates were 14.86 and 9.87%. The results provide a scientific basis for finely mapping genes of plant architecture traits and marker-assisted breeding in Lagerstroemia.

Comparative transcriptome analyses provide novel insights into etiolated shoot development of walnut (Juglans regia L.)

In general, genes promoting IAA, CTK GA and ethylene biosynthesis were upregulated, while genes participating in ABA, chlorophyll and starch biosynthesis pathways performed opposite tendency during etiolation. Etiolation as a method for rejuvenation plays an important role in the vegetative propagation of woody plants. However, the molecular mechanism of etiolated shoot development remains unclear. In this study, we investigated changes at different etiolation stages of Juglans regia. The histology and transcriptome of J. regia were analysed using etiolated stems, which were treated in darkness for 30, 60, 90 days. The results showed that the ratios of pith (Pi) diameter/stem diameter (D), cortex (Co) width/D, and phloem (Ph) width/D increased, while the ratio of xylem (Xy) width/D decreased after etiolation, and the difference in these ratios between etiolated stems and the control was more significant at 60 days than 90 days. Differentially expressed genes (DEGs) were significantly enriched in pathways such as plant hormone biosynthesis and signal transduction, chlorophyll biosynthesis and degradation, and starch and sucrose metabolism. The difference in the contents of indole-3-acetic acid (IAA), abscisic acid (ABA), sugar and chlorophyll between etiolated stems and the control increased with increasing treatment duration; in contrast, the concentrations of gibberellin (GA), zeatin (ZT), and starch, as well as the difference between the etiolated stems and control were lowest at 60 days among the three stages. On the whole, the positive effect of etiolation on the rejuvenation of walnut stems changed as the treatment period increased. The present investigation lays a foundation for future studies on the effect of etiolation on rejuvenation and for promoting the efficiency of vegetative propagation.

Dissecting genetic diversity and genomic background of Petunia cultivars with contrasting growth habits

The cultivated petunia (Petunia ×hybrida) is derived from the progenitor species P. axillaris and P. integrifolia. The hybridization dates back only to the 1830s, though intensive breeding efforts have yielded cultivars exhibiting incredible diversity for many traits, including growth habit, flower color, and flower size. Until now, little is known about the genetic diversity and genomic background of modern cultivars. Here we selected a panel of 13 cultivars with contrasting growth habits and three wild species (the progenitors and P. exserta) to estimate the genomic contribution from the ancestral species and to study whether the variation of the genetic origin could be associated with different breeding programs or morphological variability. Transcriptome sequencing identified 1,164,566 SNPs representing 98.4% (32,451) of the transcripts that cover 99.2% (of 52,697,361 bp) of the P. axillaris transcriptome. Cultivars with an upright growth habit had more homozygous alleles and more P. axillaris-derived alleles than trailing cultivars, while mounded cultivars had intermediate heterozygosity. Unlike previous studies, we found the proportions of alleles derived from each progenitor species varied across cultivars but overall were not biased toward one progenitor species, suggesting diverse selection during cultivar development. For trailing cultivars, alleles potentially introgressed from other wild species ("out" alleles) were enriched. The "out" alleles were clustered in particular regions of chromosomes, suggesting that these regions may be hotspots of introgression. Transcripts in these regions were enriched with gene ontology terms associated with growth habit. This study provides novel insight into the contributions of progenitor species to the genomic background of modern petunia cultivars and identifies genome regions that may harbor genes conferring the trailing growth habit for further exploration.

Transcriptome profiles reveal that gibberellin-related genes regulate weeping traits in crape myrtle

Plant architecture includes vital traits that influence and benefit crops, and economically important trees. Different plant architectures provide natural beauty. Weeping ornamental plants are aesthetically appealing to people. The regulatory mechanism controlling the weeping trait is poorly understood in crape myrtle. To investigate the weeping trait mechanism, transcriptional profiling of different organs in weeping and upright crape myrtle was performed based on phenotype. Phenotypic and histological analyses demonstrated that endodermal cells were absent, and that new shoot phenotypes could be rescued by the GA3 treatment of weeping plants. The transcriptional analysis and coexpression network analysis (WGCNA) of differentially expressed genes indicated that GA synthesis and signal transduction pathways play a role in weeping traits. When the expression level of a negative element of GA signaling, LfiGRAS1, was reduced by virus-induced gene silencing (VIGS), new branches grew in infected plants in a negatively geotropic manner. An integrated analysis implied that GA had a strong influence on weeping crape myrtle by interacting with other factors. This study helps to elucidate the mechanism governing the weeping trait and can improve the efficiency of breeding in Lagerstroemia.

Screening of Applicable SSR Molecular Markers Linked to Creeping Trait in Crape Myrtle

Creeping plants have unique ornamental value because they have more branches and flowers and the creeping trait is rare in crape myrtle (Lagerstroemia indica L.). In this study, the first filial generation (F1) population was derived from Lagerstroemia fauriei Koehne (standard) and L. indica “Creole” (creeping) and the backcross1 (BC1) population was derived from the backcross of F1 individual S82 (creeping) and L. fauriei. The segregation of the creeping trait was analyzed for 174 seedlings of the BC1 population to examine the linkage relationship between simple sequence repeat (SSR) molecular markers and the creeping trait. Creeping genes were screened using bulked segregant analysis combined with 322 SSR primers, which were detected with good polymorphism. The results show that two SSR markers (S364 and LYS12) were detected, with genetic distances of 23.49 centimorgan (cM) and 25.86 cM from the loci controlling the plant opening angle trait and the branching angle trait, respectively. The accuracy rate for phenotypic verification using S364 and LYS12 was 76.51% and 74.14%, respectively. Our results provide basic information for the molecular marker-assisted selective breeding and cloning of the creeping gene to improve architecture diversity in the breeding of crape myrtle.