GLABRA2, A Common Regulator for Epidermal Cell Fate Determination and Anthocyanin Biosynthesis in Arabidopsis

An inquiry into the radial patterning of root hair cell distribution in eudicots

The root epidermis of tracheophytes consists of hair-forming cells (HCs) and nonhair cells (NCs). The HC distribution pattern is classified into three types: random (Type I), vertically alternating (Type II), and radial (Type III). Type III is found only in core eudicots and is known to be position-dependent in superrosids with HCs positioned between two underlying cortical cells. However, the evolution of Type III and the universality of its position dependency in eudicots remain unclear. We surveyed the HC distribution in basal and Type III-exhibiting core eudicots and conducted genomic analyses to get insight into whether eudicots share the same genetic network to establish Type III. Our survey revealed no canonical Type III in basal eudicots but a reverse Type III, with NCs between two cortical cells and HCs on a single cortical cell, in Papaveraceae of basal eudicots. Type III-exhibiting species from both superrosids and superasterids showed the canonical position dependency of HCs. However, some key components for Type III determination were absent in the genomes of Papaveraceae and Type III-exhibiting superasterids. Our findings identify a novel position-dependent type of HC patterning, reverse Type III, and suggest that Type III emerged independently or diversified during eudicot evolution.

The HD-ZIP IV transcription factor GLABRA2 acts as an activator for proanthocyanidin biosynthesis in Medicago truncatula seed coat

Proanthocyanidins (PAs), a group of flavonoids, are found in leaves, flowers, fruits, and seed coats of many plant species. PAs are primarily composed of epicatechin units in the seed coats of the model legume species, Medicago truncatula. It can be synthesized from two separate pathways, the leucoanthocyanidin reductase (MtLAR) pathway and the anthocyanidin synthase (MtANS) pathway, which produce epicatechin through anthocyanidin reductase (MtANR). These pathways are mainly controlled by the MYB-bHLH-WD40 (MBW) ternary complex. Here, we characterize a class IV homeodomain-leucine zipper (HD-ZIP IV) transcription factor, GLABRA2 (MtGL2), which contributes to PA biosynthesis in the seed coat of M. truncatula. Null mutation of MtGL2 results in dark brown seed coat, which is accompanied by reduced PAs accumulation and increased anthocyanins content. The MtGL2 gene is predominantly expressed in the seed coat during the early stages of seed development. Genetic and molecular analyses indicate that MtGL2 positively regulates PA biosynthesis by directly activating the expression of MtANR. Additionally, our results show that MtGL2 is strongly induced by the MBW activator complexes that are involved in PA biosynthesis. Taken together, our results suggest that MtGL2 acts as a novel positive regulator in PA biosynthesis, expanding the regulatory network and providing insights for genetic engineering of PA production.

Advances in functional studies of plant MYC transcription factors

Myelocytomatosis (MYC) transcription factors (TFs) belong to the basic helix-loop-helix (bHLH) family in plants and play a central role in governing a wide range of physiological processes. These processes encompass plant growth, development, adaptation to biotic and abiotic stresses, as well as secondary metabolism. In recent decades, significant strides have been made in comprehending the multifaceted regulatory functions of MYCs. This advancement has been achieved through the cloning of MYCs and the characterization of plants with MYC deficiencies or overexpression, employing comprehensive genome-wide 'omics' and protein-protein interaction technologies. MYCs act as pivotal components in integrating signals from various phytohormones' transcriptional regulators to orchestrate genome-wide transcriptional reprogramming. In this review, we have compiled current research on the role of MYCs as molecular switches that modulate signal transduction pathways mediated by phytohormones and phytochromes. This comprehensive overview allows us to address lingering questions regarding the interplay of signals in response to environmental cues and developmental shift. It also sheds light on the potential implications for enhancing plant resistance to diverse biotic and abiotic stresses through genetic improvements achieved by plant breeding and synthetic biology efforts.

Functional Modules in the Meristems: "Tinkering" in Action

BACKGROUND

A feature of higher plants is the modular principle of body organisation. One of these conservative morphological modules that regulate plant growth, histogenesis and organogenesis is meristems-structures that contain pools of stem cells and are generally organised according to a common principle. Basic content: The development of meristems is under the regulation of molecular modules that contain conservative interacting components and modulate the expression of target genes depending on the developmental context. In this review, we focus on two molecular modules that act in different types of meristems. The WOX-CLAVATA module, which includes the peptide ligand, its receptor and the target transcription factor, is responsible for the formation and control of the activity of all meristem types studied, but it has its own peculiarities in different meristems. Another regulatory module is the so-called florigen-activated complex, which is responsible for the phase transition in the shoot vegetative meristem (e.g., from the vegetative shoot apical meristem to the inflorescence meristem).

CONCLUSIONS

The review considers the composition and functions of these two functional modules in different developmental programmes, as well as their appearance, evolution and use in plant breeding.

Genome-wide identification of HD-ZIP gene family and screening of genes related to prickle development in Zanthoxylum armatum

Zanthoxylum armatum is an important cash crop for medicinal and food purposes in Asia. However, its stems and leaves are covered with a large number of prickles, which cause many problems in the production process. The homeodomain leucine zipper (HD-ZIP) gene family is a class of transcription factors unique to plants that play an important role in biological processes such as morphogenesis, signal transduction, and secondary metabolite synthesis. However, little is known about HD-ZIP gene information that may be involved in prickle development of Z. armatum. Here, we identified 76 ZaHDZ genes from the Z. armatum genome and classified them into four subfamilies (I-IV) based on phylogenetic analysis, a classification further supported by gene structure and conserved motif analysis. Seventy-six ZaHDZ genes were unevenly distributed on chromosomes. Evolutionary analysis revealed that the expansion of ZaHDZ genes mainly were due to whole-genome duplication (WGD) or segmental duplication, and they experienced strong purifying selection pressure in the process of evolution. A total of 47 cis-elements were identified in the promoter region of ZaHDZ genes. Quantitative real-time polymerase chain reaction analysis was performed on subfamily IV ZaHDZ gene expression levels in five tissues and under four hormone treatments. Finally, ZaHDZ16 was predicted to be the candidate gene most likely to be involved in prickle development of Z. armatum. These results contribute to a better understanding of the characteristics of HD-ZIP gene family and lay a foundation for further study on the function of genes related to prickle development of Z. armatum.

Novel Roles of SPATULA in the Control of Stomata and Trichome Number, and Anthocyanin Biosynthesis

The bHLH transcription factor SPATULA (SPT) has been identified as a regulator during different stages of Arabidopsis development, including the control of leaf size. However, the mechanism via which it performs this function has not been elucidated. To better understand the role of SPT during leaf development, we used a transcriptomic approach to identify putative target genes. We found putative SPT target genes related to leaf development, and to stomata and trichome formation. Furthermore, genes related to anthocyanin biosynthesis. In this work, we demonstrate that SPT is a negative regulator of stomata number and a positive regulator of trichome number. In addition, SPT is required for sucrose-mediated anthocyanin biosynthesis.

RNAseq-Based Working Model for Transcriptional Regulation of Crosstalk between Simultaneous Abiotic UV-B and Biotic Stresses in Plants

Plants adjust their secondary metabolism by altering the expression of corresponding genes to cope with both abiotic and biotic stresses. In the case of UV-B radiation, plants produce protective flavonoids; however, this reaction is impeded during pattern-triggered immunity (PTI) induced by pathogens. Pathogen attack can be mimicked by the application of microbial associated molecular patterns (e.g., flg22) to study crosstalk between PTI and UV-B-induced signaling pathways. Switching from Arabidopsis cell cultures to in planta studies, we analyzed whole transcriptome changes to gain a deeper insight into crosstalk regulation. We performed a comparative transcriptomic analysis by RNAseq with four distinct mRNA libraries and identified 10778, 13620, and 11294 genes, which were differentially expressed after flg22, UV-B, and stress co-treatment, respectively. Focusing on genes being either co-regulated with the UV-B inducible marker gene chalcone synthase CHS or the flg22 inducible marker gene FRK1 identified a large set of transcription factors from diverse families, such as MYB, WRKY, or NAC. These data provide a global view of transcriptomic reprogramming during this crosstalk and constitute a valuable dataset for further deciphering the underlying regulatory mechanism(s), which appear to be much more complex than previously anticipated. The possible involvement of MBW complexes in this context is discussed.

The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis

The production of Arabidopsis seed mucilage involves complex polysaccharide biosynthetic pathways and developmental processes in seed epidermal cells. Although the polysaccharide components of Arabidopsis seed mucilage have been identified, their regulatory mechanism requires further investigation. Here, we show that Class II KNOX gene family members KNAT3 and KNAT7 play an essential role in regulating mucilage production in the early developmental stages of Arabidopsis seeds. Double mutant knat3knat7 resulted in defective seed mucilage production and columellae formation, whereas knat3 showed a normal phenotype compared with wild type, and the mucilage thickness in knat7 was slightly disturbed. Rhamnogalacturonan I (RG-I) and its biosynthetic substrates galacturonic acid and rhamnose were reduced in both the adherent and soluble mucilage of knat3knat7. Comparative transcriptome analysis on whole seeds suggested that polysaccharide, glucosinolate and anthocyanin biosynthetic pathways were specifically repressed in knat3knat7. Transient co-expression of KNAT3 and KNAT7 with promoter regions of candidate genes in Arabidopsis protoplasts revealed that both KNAT3 and KNAT7 act as positive regulators of the RG-I biosynthetic gene MUCILAGE-MODIFIED 4 (MUM4, AT1G53500). Collectively, our results demonstrate that KNAT3 and KNAT7 are multifunctional transcription factors in secondary cell wall development and redundantly modulate mucilage biosynthesis in Arabidopsis seeds.

Genome-Wide Identification and Expression Analysis of Homeodomain Leucine Zipper Subfamily IV (HD-ZIP IV) Gene Family in Cannabis sativa L

The plant-specific homeodomain zipper family (HD-ZIP) of transcription factors plays central roles in regulating plant development and environmental resistance. HD-ZIP transcription factors IV (HDZ IV) have been involved primarily in the regulation of epidermal structure development, such as stomata and trichomes. In our study, we identified nine HDZ IV-encoding genes in Cannabis sativa L. by conducting a computational analysis of cannabis genome resources. Our analysis suggests that these genes putatively encode proteins that have all the conserved domains of HDZ IV transcription factors. The phylogenetic analysis of HDZ IV gene family members of cannabis, rice (Oryza sativa), and Arabidopsis further implies that they might have followed distinct evolutionary paths after divergence from a common ancestor. All the identified cannabis HDZ IV gene promoter sequences have multiple regulation motifs, such as light- and hormone-responsive elements. Furthermore, experimental evidence shows that different HDZ IV genes have different expression patterns in root, stem, leaf, and flower tissues. Four genes were primarily expressed in flowers, and the expression of CsHDG5 (XP_030501222.1) was also correlated with flower maturity. Fifty-nine genes were predicted as targets of HDZ IV transcription factors. Some of these genes play central roles in pathogen response, flower development, and brassinosteroid signaling. A subcellular localization assay indicated that one gene of this family is localized in the Arabidopsis protoplast nucleus. Taken together, our work lays fundamental groundwork to illuminate the function of cannabis HDZ IV genes and their possible future uses in increasing cannabis trichome morphogenesis and secondary metabolite production.

A Survey of Enhanced Cold Tolerance and Low-Temperature-Induced Anthocyanin Accumulation in a Novel Zoysia japonica Biotype

Zoysia japonica is a warm-season turfgrass that is extensively used in landscaping, sports fields, and golf courses worldwide. Uncovering the low-temperature response mechanism of Z. japonica can help to accelerate the development of new cold-tolerant cultivars, which could be used to prolong the ornamental and usage duration of turf. A novel Z. japonica biotype, YueNong-9 (YN-9), was collected from northeastern China for this study. Phenotypic measurements, cold-tolerance investigation, and whole-transcriptome surveys were performed on YN-9 and LanYin-3 (LY-3), the most popular Z. japonica cultivar in Southern China. The results indicated the following: YN-9 has longer second and third leaves than LY-3; when exposed to the natural low temperature during winter in Guangzhou, YN-9 accumulated 4.74 times more anthocyanin than LY-3; after cold acclimation and freezing treatment, 83.25 ± 9.55% of YN-9 survived while all LY-3 leaves died, and the dark green color index (DGCI) value of YN-9 was 1.78 times that of LY-3; in YN-9, there was a unique up-regulation of Phenylalanine ammonia-lyase (PAL), Homeobox-leucine Zipper IV (HD-ZIP), and ATP-Binding Cassette transporter B8 (ABCB8) expressions, as well as a unique down-regulation of zinc-regulated transporters and iron-regulated transporter-like proteins (ZIPs) expression, which may promote anthocyanin biosynthesis, transport, and accumulation. In conclusion, YN-9 exhibited enhanced cold tolerance and is thus an excellent candidate for breeding cold-tolerant Z. japonica variety, and its unique low-temperature-induced anthocyanin accumulation and gene responses provide ideas and candidate genes for the study of low-temperature tolerance mechanisms and genetic engineering breeding.

EIN3 and RSL4 interfere with an MYB-bHLH-WD40 complex to mediate ethylene-induced ectopic root hair formation in Arabidopsis

The alternating cell specifications of root epidermis to form hair cells or nonhair cells in Arabidopsis are determined by the expression level of GL2, which is activated by an MYB-bHLH-WD40 (WER-GL3-TTG1) transcriptional complex. The phytohormone ethylene (ET) has a unique effect of inducing N-position epidermal cells to form root hairs. However, the molecular mechanisms underlying ET-induced ectopic root hair development remain enigmatic. Here, we show that ET promotes ectopic root hair formation through down-regulation of GL2 expression. ET-activated transcription factors EIN3 and its homolog EIL1 mediate this regulation. Molecular and biochemical analyses further revealed that EIN3 physically interacts with TTG1 and interferes with the interaction between TTG1 and GL3, resulting in reduced activation of GL2 by the WER-GL3-TTG1 complex. Furthermore, we found through genetic analysis that the master regulator of root hair elongation, RSL4, which is directly activated by EIN3, also participates in ET-induced ectopic root hair development. RSL4 negatively regulates the expression of GL2, likely through a mechanism similar to that of EIN3. Therefore, our work reveals that EIN3 may inhibit gene expression by affecting the formation of transcription-activating protein complexes and suggests an unexpected mutual inhibition between the hair elongation factor, RSL4, and the hair specification factor, GL2. Overall, this study provides a molecular framework for the integration of ET signaling and intrinsic root hair development pathway in modulating root epidermal cell specification.

The Carboxyl-Terminus of TRANSPARENT TESTA GLABRA1 Is Critical for Its Functions in Arabidopsis

The Arabidopsis WD40 repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) regulates cell fate determination, including trichome initiation and root hair formation, as well as secondary metabolism such as flavonoid biosynthesis and seed coat mucilage production. TTG1 regulates different processes via regulating the expression of its downstream target genes by forming MYB-bHLH-WD40 (MBW) activator complexes with different R2R3 MYB and bHLH transcription factors. Here, we report the identification of the carboxyl (C)-terminus as a critical domain for TTG1′s functions in Arabidopsis. We found that the ttg1Δ15aa mutant shows pleiotropic phenotypes identical to a TTG1 loss-of-function mutant. Gene sequencing indicates that a single nucleotide substitution in TTG1 led to a premature stop at the W327 residue, leading to the production of a truncated TTG1 protein with a deletion of the last 15 C-terminal amino acids. The expression of TTG1 under the control of its native promoter fully restored the ttg1Δ15aa mutant phenotypes. Consistent with these observations, the expression levels of TTG1 downstream genes such as GLABRA2 (GL2) and CAPRICE (CPC) were reduced in the ttg1Δ15aa mutant. Assays in Arabidopsis protoplast show that TTG1Δ15aa failed to interact with the bHLH transcription factor GL3, and the deletion of the last 3 C-terminal amino acids or the 339L amino acid alone fully abolished the interaction of TTG1 with GL3. Furthermore, the expression of TTG1Δ3aa under the control of TTG1 native promoter failed to restore the ttg1Δ15aa mutant phenotypes. Taken together, our results suggest that the C-terminal domain of TTG1 is required for its proper function in Arabidopsis.

Involvement of ABA Responsive SVB Genes in the Regulation of Trichome Formation in Arabidopsis

Trichome formation in Arabidopsis is regulated by several key regulators, and plants hormones such as gibberellin, salicylic acid, jasmonic acid and cytokinins have been shown to regulate trichome formation by affecting the transcription or activities of the key regulators. We report here the identification of two abscisic acid (ABA) responsive genes, SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) and SVB2 as trichome formation regulator genes in Arabidopsis. The expression levels of SVB and SVB2 were increased in response to ABA treatment, their expression levels were reduced in the ABA biosynthesis mutant aba1-5, and they have similar expression pattern. In addition to the trichome defects reported previously for the svb single mutant, we found that even though the trichome numbers were largely unaffected in both the svb and svb2 single mutants generate by using CRISPR/Cas9 gene editing, the trichome numbers were greatly reduced in the svb svb2 double mutants. On the other hand, trichome numbers were increased in SVB or SVB2 overexpression plants. RT-PCR results show that the expression of the trichome formation key regulator gene ENHANCER OF GLABRA3 (EGL3) was affected in the svb svb2 double mutants. Our results suggest that SVB and SVB2 are ABA responsive genes, and SVB and SVB2 function redundantly to regulate trichome formation in Arabidopsis.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Integrated Analysis of Small RNA, Transcriptome, and Degradome Sequencing Reveals the MiR156, MiR5488 and MiR399 are Involved in the Regulation of Male Sterility in PTGMS Rice

A photoperiod- and thermo-sensitive genic male sterile (PTGMS) line is the basic material for two-hybrid rice and is an important genetic breeding resource. Peiai64S (PA64S) is an important germplasm resource of PTGMS rice, and it has been applied to two-line hybrid rice systems in China. Pollen fertility in PA64S is regulated by the temperature and photoperiod, but the mechanism of the fertility transition is unclear. In this study, we obtained the male fertile plant PA64S(F) and the male sterile plant PA64S(S) by controlling different temperatures under long light conditions and used the male fertile and sterile plants to investigate the role of microRNAs (miRNAs) in regulating male fertility in rice. We performed the small RNA library sequencing of anthers from PA64S(S) and PA64S(F). A total of 196 miRNAs were identified-166 known miRNAs among 27 miRNA families and 30 novel miRNAs. In the transcriptome analysis, the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of differentially expressed genes revealed significant enrichment in the synthesis and metabolism of fatty acids and some secondary metabolism pathways such as fatty acid metabolism and phenylalanine metabolism. With a comprehensive analysis of miRNA, transcriptome, and degradome sequencing, we identified that 13 pairs of miRNA/target genes regulated male fertility in rice by responding to temperature change, among which the miR156, miR5488, and miR399 affect the male fertility of PA64S by influencing SPLs, the lignin synthesis of anther walls, and the flavonoid metabolism pathway. The results provide a new understanding of PTGMS rice, which will help us better understand the potential regulatory mechanisms of male sterility in the future.

Whole-transcriptome analysis of differentially expressed genes in the mutant and normal capitula of Chrysanthemum morifolium

BACKGROUND

Chrysanthemum morifolium is one of the most economically important and popular floricultural crops in the family Asteraceae. Chrysanthemum flowers vary considerably in terms of colors and shapes. However, the molecular mechanism controlling the development of chrysanthemum floral colors and shapes remains an enigma. We analyzed a cut-flower chrysanthemum variety that produces normal capitula composed of ray florets with normally developed pistils and purple corollas and mutant capitula comprising ray florets with green corollas and vegetative buds instead of pistils.

RESULTS

We conducted a whole-transcriptome analysis of the differentially expressed genes (DEGs) in the mutant and normal capitula using third-generation and second-generation sequencing techniques. We identified the DEGs between the mutant and normal capitula to reveal important regulators underlying the differential development. Many transcription factors and genes related to the photoperiod and GA pathways, floral organ identity, and the anthocyanin biosynthesis pathway were differentially expressed between the normal and mutant capitula. A qualitative analysis of the pigments in the florets of normal and mutant capitula indicated anthocyanins were synthesized and accumulated in the florets of normal capitula, but not in the florets of mutant capitula. These results provide clues regarding the molecular basis of the replacement of Chrysanthemum morifolium ray florets with normally developed pistils and purple corollas with mutant ray florets with green corollas and vegetative buds. Additionally, the study findings will help to elucidate the molecular mechanisms underlying floral organ development and contribute to the development of techniques for studying the regulation of flower shape and color, which may enhance chrysanthemum breeding.

CONCLUSIONS

The whole-transcriptome analysis of DEGs in mutant and normal C. morifolium capitula described herein indicates the anthocyanin deficiency of the mutant capitula may be related to the mutation that replaces ray floret pistils with vegetative buds. Moreover, pistils may be required for the anthocyanin biosynthesis in the corollas of chrysanthemum ray florets.

Whole-transcriptome analysis of differentially expressed genes in the mutant and normal capitula of Chrysanthemum morifolium

BACKGROUND

Chrysanthemum morifolium is one of the most economically important and popular floricultural crops in the family Asteraceae. Chrysanthemum flowers vary considerably in terms of colors and shapes. However, the molecular mechanism controlling the development of chrysanthemum floral colors and shapes remains an enigma. We analyzed a cut-flower chrysanthemum variety that produces normal capitula composed of ray florets with normally developed pistils and purple corollas and mutant capitula comprising ray florets with green corollas and vegetative buds instead of pistils.

RESULTS

We conducted a whole-transcriptome analysis of the differentially expressed genes (DEGs) in the mutant and normal capitula using third-generation and second-generation sequencing techniques. We identified the DEGs between the mutant and normal capitula to reveal important regulators underlying the differential development. Many transcription factors and genes related to the photoperiod and GA pathways, floral organ identity, and the anthocyanin biosynthesis pathway were differentially expressed between the normal and mutant capitula. A qualitative analysis of the pigments in the florets of normal and mutant capitula indicated anthocyanins were synthesized and accumulated in the florets of normal capitula, but not in the florets of mutant capitula. These results provide clues regarding the molecular basis of the replacement of Chrysanthemum morifolium ray florets with normally developed pistils and purple corollas with mutant ray florets with green corollas and vegetative buds. Additionally, the study findings will help to elucidate the molecular mechanisms underlying floral organ development and contribute to the development of techniques for studying the regulation of flower shape and color, which may enhance chrysanthemum breeding.

CONCLUSIONS

The whole-transcriptome analysis of DEGs in mutant and normal C. morifolium capitula described herein indicates the anthocyanin deficiency of the mutant capitula may be related to the mutation that replaces ray floret pistils with vegetative buds. Moreover, pistils may be required for the anthocyanin biosynthesis in the corollas of chrysanthemum ray florets.

Seed mucilage evolution: Diverse molecular mechanisms generate versatile ecological functions for particular environments

Plant myxodiasporous species have the ability to release a polysaccharidic mucilage upon imbibition of the seed (myxospermy) or the fruit (myxocarpy). This is a widespread capacity in angiosperms providing multiple ecological functions including higher germination efficiency under environmental stresses. It is unclear whether myxodiaspory has one or multiple evolutionary origins and why it was supposedly lost in several species. Here, we summarize recent advances on three main aspects of myxodiaspory. (a) It represents a combination of highly diverse traits at different levels of observation, ranging from the dual tissular origin of mucilage secretory cells to diverse mucilage polysaccharidic composition and ultrastructural organization. (b) An asymmetrical selection pressure is exerted on myxospermy-related genes that were first identified in Arabidopsis thaliana. The A. thaliana and the flax intra-species mucilage variants show that myxospermy is a fast-evolving trait due to high polymorphism in a few genes directly acting on mucilage establishment. In A. thaliana, these actors are downstream of a master regulatory complex and an original phylogenetic overview provided here illustrates that this complex has sequentially evolved after the common ancestor of seed plants and was fully established in the common ancestor of the rosid clade. (c) Newly identified myxodiaspory ecological functions indicate new perspectives such as soil microorganism control and plant establishment support.

The HD-ZIP IV transcription factor GL2-LIKE regulates male flowering time and fertility in cucumber

Cucumber is dioecious by nature, having both male and female flowers, and is a model system for unisexual flower development. Knowledge related to male flowering is limited, but it is reported to be regulated by transcription factors and hormone signals. Here, we report functional characterization of the cucumber (Cucumis sativus) GL2-LIKE gene, which encodes a homeodomain leucine zipper (HD-ZIP) IV transcription factor that plays an important role in regulating male flower development. Spatial-temporal expression analyses revealed high-level expression of CsGL2-LIKE in the male flower buds and anthers. CsGL2-LIKE is closely related to AtGL2, which is known to play a key role in trichome development. However, ectopic expression of CsGL2-LIKE in Arabidopsis gl2-8 mutant was unable to rescue the gl2-8 phenotype. Interestingly, the silencing of CsGL2-LIKE delayed male flowering by inhibiting the expression of the florigen gene FT and reduced pollen vigor and seed viability. Protein-protein interaction assays showed that CsGL2-LIKE interacts with the jasmonate ZIM domain protein CsJAZ1 to form a HD-ZIP IV-CsJAZ1 complex. Collectively, our study indicates that CsGL2-LIKE regulates male flowering in cucumber, and reveals a novel function of a HD-ZIP IV transcription factor in regulating male flower development of cucumber.

Abnormal expression of bHLH3 disrupts a flavonoid homeostasis network, causing differences in pigment composition among mulberry fruits

Mulberry fruits with high concentrations of anthocyanins are favored by consumers because of their good taste, bright color, and high nutritional value. However, neither the regulatory mechanism controlling flavonoid biosynthesis in mulberry nor the molecular basis of different mulberry fruit colors is fully understood. Here, we report that a flavonoid homeostasis network comprising activation and feedback regulation mechanisms determines mulberry fruit color. In vitro and in vivo assays showed that MYBA-bHLH3-TTG1 regulates the biosynthesis of anthocyanins, while TT2L1 and TT2L2 work with bHLH3 or GL3 and form a MYB-bHLH-WD40 (MBW) complex with TTG1 to regulate proanthocyanidin (PA) synthesis. Functional and expression analyses showed that bHLH3 is a key regulator of the regulatory network controlling mulberry fruit coloration and that MYB4 is activated by MBW complexes and participates in negative feedback control of the regulatory network to balance the accumulation of anthocyanins and proanthocyanidins. Our research demonstrates that the interaction between bHLH3 and MYB4 in the homeostasis regulatory network ensures that the fruits accumulate desirable flavonoids and that this network is stable in pigment-rich mulberry fruits. However, the abnormal expression of bHLH3 disrupts the balance of the network and redirects flavonoid metabolic flux in pale-colored fruits, resulting in differences in the levels and proportions of anthocyanins, flavones, and flavonols among differently colored mulberry fruits (red, yellow, and white). The results of our study reveal the molecular basis of the diversity of mulberry fruit colors.