CINCINNATA in Antirrhinum majus directly modulates genes involved in cytokinin and auxin signaling

Physiological Control and Genetic Basis of Leaf Curvature and Heading in Brassica rapa L

BACKGROUND

Heading is an important agronomic feature for Chinese cabbage, cabbage, and lettuce. The heading leaves function as nutrition storage organs, which contribute to the high quality and economic worth of leafy heads. Leaf development is crucial during the heading stage, most genes previously predicted to be involved in the heading process are based on Arabidopsis leaf development studies.

AIM OF REVIEW

Till date, there is no published review article that demonstrated a complete layout of all the identified regulators of leaf curvature and heading. In this review, we have summarized all the identified physiological and genetic regulators that are directly or indirectly involved in leaf curvature and heading in Brassica crops. By integrating all identified regulators that provide a coherent logic of leaf incurvature and heading, we proposed a molecular mechanism in Brassica crops with graphical illustrations. This review adds value to future breeding of distinct heading kinds of cabbage and Chinese cabbage by providing unique insights into leaf development.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Leaf curvature and heading are established by synergistic interactions among genes, transcription factors, microRNAs, phytohormones, and environmental stimuli that regulate primary and secondary morphogenesis. Various genes have been identified using transformation and genome editing that are responsible for the formation of leaf curvature and heading in Brassica crops. A range of leaf morphologies have been observed in Brassica, which are established because of the mutated determinants that are responsible for cell division and leaf polarity.

Genome-Wide Identification and Expression Analysis of the TCP Gene Family Related to Developmental and Abiotic Stress in Ginger

Ginger (Zingiber officinale Roscoe), a widely consumed edible and medicinal plant, possesses significant nutritional and economic value. Abiotic stresses such as drought and low temperatures can impact the growth and development of ginger. The plant-specific transcription factor Teosinte branched1/cycloidea/proliferating cell factor (TCP) has progressively been identified in various plants for its role in regulating plant growth and development as well as conferring resistance to abiotic stresses. However, limited information on the TCP family is available in ginger. In this study, we identified 20 TCP members in the ginger genome, which were randomly distributed across 9 chromosomes. Based on phylogenetic analysis, these ginger TCP were classified into two subfamilies: Class I (PCF) and Class II (CIN, CYC/TB). The classification of the identified ginger TCPs was supported by a multi-species phylogenetic tree and motif structure analysis, suggesting that the amplification of the ginger TCP gene family occurred prior to the differentiation of angiosperms. The promoter region of ginger TCP genes was found to contain numerous cis-acting elements associated with plant growth, development, and abiotic stress response. Among these elements, the stress response element, anaerobic induction, and MYB binding site play a dominant role in drought responsiveness. Additionally, expression pattern analysis revealed variations in the expression of ginger TCP gene among different tissues and in response to diverse abiotic stresses (drought, low temperature, heat, and salt). Our research offers a thorough examination of TCP members within the ginger plant. This analysis greatly contributes to the understanding of how TCP genes regulate tissue development and response to stress, opening up new avenues for further exploration in this field.

Genome-wide analysis of the TCP gene family and their expression pattern in Cymbidium goeringii

TCP gene family are specific transcription factors for plant, and considered to play an important role in development and growth. However, few related studies investigated the TCP gene trait and how it plays a role in growth and development of Orchidaceae. In this study, we obtained 14 TCP genes (CgTCPs) from the Spring Orchid Cymbidium goeringii genome. The classification results showed that 14 CgTCPs were mainly divided into two clades as follows: four PCF genes (Class I), nine CIN genes and one CYC gene (Class II). The sequence analysis showed that the TCP proteins of C. goeringii contain four conserved regions (basic Helix-Loop-Helix) in the TCP domain. The exon-intron structure varied in the clade according to a comparative investigation of the gene structure, and some genes had no introns. There are fewer CgTCP homologous gene pairs compared with Dendrobium catenatum and Phalaenopsis equestris, suggesting that the TCP genes in C. goeringii suffered more loss events. The majority of the cis-elements revealed to be enriched in the function of light responsiveness, followed by MeJA and ABA responsiveness, demonstrating their functions in regulating by light and phytohormones. The collinearity study revealed that the TCPs in D. catenatum, P. equestris and C. goeringii almost 1:1. The transcriptomic data and real-time reverse transcription-quantitative PCR (RT-qPCR) expression profiles showed that the flower-specific expression of the TCP class II genes (CgCIN2, CgCIN5 and CgCIN6) may be related to the regulation of florescence. Altogether, this study provides a comprehensive analysis uncovering the underlying function of TCP genes in Orchidaceae.

Evolutionary analyses and expression patterns of TCP genes in Ranunculales

TCP transcription factors play a role in a large number of developmental processes and are at the crossroads of numerous hormonal biosynthetic and signaling pathways. The complete repertoire of TCP genes has already been characterized in several plant species, but not in any species of early diverging eudicots. We focused on the order Ranunculales because of its phylogenetic position as sister group to all other eudicots and its important morphological diversity. Results show that all the TCP genes expressed in the floral transcriptome of Nigella damascena (Ranunculaceae) are the orthologs of the TCP genes previously identified from the fully sequenced genome of Aquilegia coerulea. Phylogenetic analyses combined with the identification of conserved amino acid motifs suggest that six paralogous genes of class I TCP transcription factors were present in the common ancestor of angiosperms. We highlight independent duplications in core eudicots and Ranunculales within the class I and class II subfamilies, resulting in different numbers of paralogs within the main subclasses of TCP genes. This has most probably major consequences on the functional diversification of these genes in different plant clades. The expression patterns of TCP genes in Nigella damascena were consistent with the general suggestion that CIN and class I TCP genes may have redundant roles or take part in same pathways, while CYC/TB1 genes have more specific actions. Our findings open the way for future studies at the tissue level, and for investigating redundancy and subfunctionalisation in TCP genes and their role in the evolution of morphological novelties.

Genomic survey of TCP transcription factors in plants: Phylogenomics, evolution and their biology

The TEOSINTE BRANCHED1 (TBI1), CYCLOIDEA (CYC), and PROLIFERATING CELL NUCLEAR ANTIGEN FACTORS (PCF1 and PCF2) proteins truncated as TCP transcription factors carry conserved basic-helix-loop-helix (bHLH) structure, related to DNA binding functions. Evolutionary history of the TCP genes has shown their presence in early land plants. In this paper, we performed a comparative discussion on the current knowledge of the TCP Transcription Factors in lower and higher plants: their evolutionary history based on the phylogenetics of 849 TCP proteins from 37 plant species, duplication events, and biochemical roles in some of the plants species. Phylogenetics investigations confirmed the classification of TCP TFs into Class I (the PCF1/2), and Class II (the C- clade) factors; the Class II factors were further divided into the CIN- and CYC/TB1- subclade. A trace in the evolution of the TCP Factors revealed an absence of the CYC/TB1subclade in lower plants, and an independent evolution of the CYC/TB1subclade in both eudicot and monocot species. 54% of the total duplication events analyzed were biased towards the dispersed duplication, and we concluded that dispersed duplication events contributed to the expansion of the TCP gene family. Analysis in the TCP factors functional roles confirmed their involvement in various biochemical processes which mainly included promoting cell proliferation in leaves in Class I TCPs, and cell division during plant development in Class II TCP Factors. Apart from growth and development, the TCP Factors were also shown to regulate hormonal and stress response pathways. Although this paper does not exhaust the present knowledge of the TCP Transcription Factors, it provides a base for further exploration of the gene family.

Characterization of Cry2 genes (CRY2a and CRY2b) of B. napus and comparative analysis of BnCRY1 and BnCRY2a in regulating seedling photomorphogenesis

Cryptochrome 2 (CRY2) perceives blue/UV-A light and regulates photomorphogenesis in plants. However, besides Arabidopsis, CRY2 has been functionally characterized only in native species of japonica rice and tomato. In the present study, the BnCRY2a, generating a relatively longer cDNA and harboring an intron in its 5'UTR, has been characterized in detail. Western blot analysis revealed that BnCRY2a is light labile and degraded rapidly by 26S proteasome when seedlings are irradiated with blue light. For functional analysis, BnCRY2a was over-expressed in Brassica juncea, a related species more amenable to transformation. The BnCRY2a over-expression (BnCRY2a^OE) transgenics developed short hypocotyl and expanded cotyledons, accumulated more anthocyanin in light-grown seedlings, and displayed early flowering on maturity. Early flowering in BnCRY2a^OE transgenics was coupled with the up-regulation of many flowering-related genes such as FT. The present study also highlights the differential light sensitivity of cry1 and cry2 in controlling hypocotyl elongation growth in Brassica. BnCRY2a^OE seedlings developed much shorter hypocotyl under the low-intensity of blue light, while BnCRY1^OE seedling hypocotyls were shorter under the high-intensity blue light, compared to untransformed seedlings.

CINCINNATA-Like TCP Transcription Factors in Cell Growth - An Expanding Portfolio

Post-mitotic cell growth is a key process in plant growth and development. Cell expansion drives major growth during morphogenesis and is influenced by both endogenous factors and environmental stimuli. Though both isotropic and anisotropic cell growth can contribute to organ size and shape at different degrees, anisotropic cell growth is more likely to contribute to shape change. While much is known about the mechanisms that increase cellular turgor and cell-wall biomass during expansion, the genetic factors that regulate these processes are less studied. In the past quarter of a century, the role of the CINCINNATA-like TCP (CIN-TCP) transcription factors has been well documented in regulating diverse aspects of plant growth and development including flower asymmetry, plant architecture, leaf morphogenesis, and plant maturation. The molecular activity of the CIN-TCP proteins common to these biological processes has been identified as their ability to suppress cell proliferation. However, reports on their role regulating post-mitotic cell growth have been scanty, partly because of functional redundancy among them. In addition, it is difficult to tease out the effect of gene activity on cell division and expansion since these two processes are linked by compensation, a phenomenon where perturbation in proliferation is compensated by an opposite effect on cell growth to keep the final organ size relatively unaltered. Despite these technical limitations, recent genetic and growth kinematic studies have shown a distinct role of CIN-TCPs in promoting cellular growth in cotyledons and hypocotyls, the embryonic organs that grow solely by cell expansion. In this review, we highlight these recent advances in our understanding of how CIN-TCPs promote cell growth.

Meta-analysis of transcriptomic studies of cytokinin-treated rice roots defines a core set of cytokinin response genes

Cytokinins regulate diverse aspects of plant growth and development, primarily through modulation of gene expression. The cytokinin-responsive transcriptome has been thoroughly described in dicots, especially Arabidopsis, but much less so in monocots. Here, we present a meta-analysis of five different transcriptomic analyses of rice (Oryza sativa) roots treated with cytokinin, including three previously unpublished experiments. We developed a treatment method in which hormone is added to the media of rice seedlings grown in sterile hydroponic culture under a continuous airflow, which resulted in minimal perturbation of the seedlings, thus greatly reducing changes in gene expression in the absence of exogenous hormone. We defined a core set of 205 upregulated and 86 downregulated genes that were differentially expressed in at least three of the transcriptomic datasets. This core set includes genes encoding the type-A response regulators (RRs) and cytokinin oxidases/dehydrogenases, which have been shown to be primary cytokinin response genes. GO analysis revealed that the upregulated genes were enriched for terms related to cytokinin/hormone signaling and metabolism, while the downregulated genes were significantly enriched for genes encoding transporters. Variations of type-B RR binding motifs were significantly enriched in the promoters of the upregulated genes, as were binding sites for other potential partner transcription factors. The promoters of the downregulated genes were generally enriched for distinct cis-acting motifs and did not include the type-B RR binding motif. This analysis provides insight into the molecular mechanisms underlying cytokinin action in a monocot and provides a useful foundation for future studies of this hormone in rice and other cereals.

Auxin and Cytokinin Interplay during Leaf Morphogenesis and Phyllotaxy

Auxins (IAA) and cytokinins (CKs) are the most influential phytohormones, having multifaceted roles in plants. They are key regulators of plant growth and developmental processes. Additionally, their interplay exerts tight control on plant development and differentiation. Although several reviews have been published detailing the auxin-cytokinin interplay in controlling root growth and differentiation, their roles in the shoot, particularly in leaf morphogenesis are largely unexplored. Recent reports have provided new insights on the roles of these two hormones and their interplay on leaf growth and development. In this review, we focus on the effect of auxins, CKs, and their interactions in regulating leaf morphogenesis. Additionally, the regulatory effects of the auxins and CKs interplay on the phyllotaxy of plants are discussed.

GrTCP11, a Cotton TCP Transcription Factor, Inhibits Root Hair Elongation by Down-Regulating Jasmonic Acid Pathway in Arabidopsis thaliana

TCP transcription factors play important roles in diverse aspects of plant development as transcriptional activators or repressors. However, the functional mechanisms of TCPs are not well understood, especially in cotton fibers. Here, we identified a total of 37 non-redundant TCP proteins from the diploid cotton (Gossypium raimondii), which showed great diversity in the expression profile. GrTCP11, an ortholog of AtTCP11, was preferentially expressed in cotton anthers and during fiber initiation and secondary cell wall synthesis stages. Overexpression of GrTCP11 in Arabidopsis thaliana reduced root hair length and delayed flowering. It was found that GrTCP11 negatively regulated genes involved in jasmonic acid (JA) biosynthesis and response, such as AtLOX4, AtAOS, AtAOC1, AtAOC3, AtJAZ1, AtJAZ2, AtMYC2, and AtERF1, which resulted in a decrease in JA concentration in the overexpressed transgenic lines. As with the JA-deficient mutant dde2-2, the transgenic line 4-1 was insensitive to 50 μM methyl jasmonate, compared with the wild-type plants. The results suggest that GrTCP11 may be an important transcription factor for cotton fiber development, by negatively regulating JA biosynthesis and response.

The Regulation of CIN-like TCP Transcription Factors

TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR 1 and 2 (TCP) family proteins are the plant-specific transcription factors extensively participating in diverse developmental processes by integrating external cues with internal signals. The roles of CINCINNATA (CIN)-like TCPs are conserved in control of the morphology and size of leaves, petal development, trichome formation and plant flowering. The tight regulation of CIN-like TCP activity at transcriptional and post-transcriptional levels are central for plant developmental plasticity in response to the ever-changing environmental conditions. In this review, we summarize recent progresses with regard to the function and regulation of CIN-like TCPs. CIN-like TCPs are regulated by abiotic and biotic cues including light, temperature and pathogens. They are also finely controlled by microRNA319 (miRNA319), chromatin remodeling complexes and auxin homeostasis. The protein degradation plays critical roles in tightly controlling the activity of CIN-like TCPs as well.

MicroRNA160 regulates leaf curvature in potato (Solanum tuberosum L. cv. Désirée)

Leaf development is a complex process and factors such as size, shape, curvature, compounding, and texture determine the final leaf morphology. MicroRNA160 is one of the crucial players that has been shown to regulate lamina formation and compounding in tomato. In this study, we show that miR160 also regulates leaf curvature in potato. miR160 targets a group of Auxin Response Factors - StARF10, StARF16, and StARF17 - that are proposed to function majorly as repressors of auxin signaling. We observed that overexpression of miR160 (miR160-OE) results in decrease in the levels of these ARFs along with hypersensitivity to exogenous auxin treatment, whereas knockdown of miR160 (miR160-KD) causes increased ARF levels and auxin hyposensitivity. The leaves of miR160-OE plants have a high positive curvature, but of miR160-KD plants are flattened compared to wildtype. A prolonged activation of cell cycle - as indicated by increased levels of StCYCLIND3;2 - in the center region of miR160-OE leaves appears to have caused this positive curvature. However, a comparable StTCP4 activity at both center and margin regions of miR160-KD leaves could be the cause for its flattened leaf phenotype. In summary, we show that miR160 plays an important role in regulating leaf curvature in potato plants.

Phenotypic and genetic characterization of tomato mutants provides new insights into leaf development and its relationship to agronomic traits

BACKGROUND

Tomato mutants altered in leaf morphology are usually identified in the greenhouse, which demands considerable time and space and can only be performed in adequate periods. For a faster but equally reliable scrutiny method we addressed the screening in vitro of 971 T-DNA lines. Leaf development was evaluated in vitro in seedlings and shoot-derived axenic plants. New mutants were characterized in the greenhouse to establish the relationship between in vitro and in vivo leaf morphology, and to shed light on possible links between leaf development and agronomic traits, a promising field in which much remains to be discovered.

RESULTS

Following the screening in vitro of tomato T-DNA lines, putative mutants altered in leaf morphology were evaluated in the greenhouse. The comparison of results in both conditions indicated a general phenotypic correspondence, showing that in vitro culture is a reliable system for finding mutants altered in leaf development. Apart from providing homogeneous conditions, the main advantage of screening in vitro lies in the enormous time and space saving. Studies on the association between phenotype and nptII gene expression showed co-segregation in two lines (P > 99%). The use of an enhancer trap also allowed identifying gain-of-function mutants through reporter expression analysis. These studies suggested that genes altered in three other mutants were T-DNA tagged. New mutants putatively altered in brassinosteroid synthesis or perception, mutations determining multiple pleiotropic effects, lines affected in organ curvature, and the first tomato mutant with helical growth were discovered. Results also revealed new possible links between leaf development and agronomic traits, such as axillary branching, flower abscission, fruit development and fruit cracking. Furthermore, we found that the gene tagged in mutant 2635-MM encodes a Sterol 3-beta-glucosyltransferase. Expression analysis suggested that abnormal leaf development might be due to the lack-off-function of this gene.

CONCLUSION

In vitro culture is a quick, efficient and reliable tool for identifying tomato mutants altered in leaf morphology. The characterization of new mutants in vivo revealed new links between leaf development and some agronomic traits. Moreover, the possible implication of a gene encoding a Sterol 3-beta-glucosyltransferase in tomato leaf development is reported.

The CIN-TCP transcription factors promote commitment to differentiation in Arabidopsis leaf pavement cells via both auxin-dependent and independent pathways

Cells in organ primordia undergo active proliferation at an early stage to generate sufficient number, before exiting proliferation and entering differentiation. However, how the actively proliferating cells are developmentally reprogrammed to acquire differentiation potential during organ maturation is unclear. Here, we induced a microRNA-resistant form of TCP4 at various developmental stages of Arabidopsis leaf primordium that lacked the activity of TCP4 and its homologues and followed its effect on growth kinematics. By combining this with spatio-temporal gene expression analysis, we show that TCP4 commits leaf cells within the transition zone to exit proliferation and enter differentiation. A 24-hour pulse of TCP4 activity was sufficient to impart irreversible differentiation competence to the actively dividing cells. A combination of biochemical and genetic analyses revealed that TCP4 imparts differentiation competence by promoting auxin response as well as by directly activating HAT2, a HD-ZIP II transcription factor-encoding gene that also acts downstream to auxin response. Our study offers a molecular link between the two major organ maturation factors, CIN-like TCPs and HD-ZIP II transcription factors and explains how TCP activity restricts the cell number and final size in a leaf.

Cells in a young organ primordium proliferate to generate sufficient number, before they exit division cycle and enter differentiation programme at later stages. While factors that drive cell cycle progression have been identified and studied in detail in diverse eukaryotic species, developmental factors that promote exit from division and entry into differentiation are less known, especially in the plant kingdom. Here, we show that the class II TCP proteins, notably TCP4, irreversibly reprogram the mitotic cells to exit division and acquire differentiation competence by auxin response as well as direct activation of HAT2 transcription. Our work offers a molecular link between class II TCP and HD-ZIP II genes during the cell differentiation and leaf maturation.

Molecular cartography of leaf development - role of transcription factors

Organ elaboration in plants occurs almost exclusively by an increase in cell number and size. Leaves, the planar lateral appendages of plants, are no exception. Forward and reverse genetic approaches have identified several genes whose role in leaf morphogenesis has been inferred from their primary effect on cell number and size, thereby distinguishing them as either promoters or inhibitors of cell proliferation and expansion. While such classification is useful in studying size control, a similar link between genes and shape generation is poorly understood. Computational modelling can provide a conceptual framework to re-evaluate the known genetic information and assign specific morphogenetic roles to the transcription factor-encoding genes. Here we discuss recent advances in our understanding of the roles of transcription factors in the planar growth of leaf lamina in two orthogonal dimensions.

Leaf development and evolution

Plant leaves are differentiated organs that arise sequentially from a population of pluripotent stem cells at the shoot apical meristem (SAM). There is substantial diversity in leaf shape, much of which depends on the size and arrangement of outgrowths at the leaf margin. These outgrowths are generated by a patterning mechanism similar to the phyllotactic processes producing organs at the SAM, which involves the transcription factors CUP-SHAPED COTYLEDON and the phytohormone auxin. In the leaf, this patterning mechanism creates sequential protrusions and indentations along the margin. The size, shape, and distribution of these protrusions also depend on the overall growth of the leaf lamina. Globally, growth is regulated by a complex genetic network controlling the distribution of cell proliferation and the timing of differentiation. Evolutionary changes in margin form arise from changes in two different classes of homeobox genes that modify the outcome of marginal patterning in diverse ways, and are under intense investigation.

The Analysis of Genes and Phytohormone Metabolic Pathways Associated with Leaf Shape Development in Liriodendron chinense via De Novo Transcriptome Sequencing

The leaf, a photosynthetic organ that plays an indispensable role in plant development and growth, has a certain ability to adapt to the environment and exhibits tremendous diversity among angiosperms. Liriodendron chinense, an ancestral angiosperm species, is very popular in landscaping. The leaf of this species has two lobes and resembles a Qing Dynasty Chinese robe; thus, leaf shape is the most valuable ornamental trait of the tree. In this work, to determine the candidate genes associated with leaf development in L. chinense, scanning electron microscopy (SEM) was employed to distinguish the developmental stages of tender leaves. Four stages were clearly separated, and transcriptome sequencing was performed for two special leaf stages. Altogether, there were 48.23 G clean reads in the libraries of the two leaf developmental stages, and 48,107 assembled unigenes were annotated with five databases. Among four libraries, 3118 differentially expressed genes (DEGs) were enriched in expression profiles. We selected ten DEGs associated with leaf development and validated their expression patterns via quantitative real-time PCR (qRT-PCR) assays. Most validation results were closely correlated with the RNA-sequencing data. Taken together, we examined the dynamic process of leaf development and indicated that several transcription factors and phytohormone metabolism genes may participate in leaf shape development. The transcriptome data analysis presented in this work aims to provide basic insights into the mechanisms mediating leaf development, and the results serve as a reference for the genetic breeding of ornamental traits in L. chinense.

CIN-TCP transcription factors: Transiting cell proliferation in plants

Leaves are the most conspicuous planar organs in plants, designed for efficient capture of sunlight and its conversion to energy that is channeled into sustaining the entire biosphere. How a few founder cells derived from the shoot apical meristem give rise to diverse leaf forms has interested naturalists and developmental biologists alike. At the heart of leaf morphogenesis lie two simple cellular processes, division and expansion, that are spatially and temporally segregated in a developing leaf. In leaves of dicot model species, cell division occurs predominantly at the base, concomitant with the expansion and differentiation of cells at the tip of the lamina that drives increase in leaf surface area. The timing of the transition from one cell fate (division) to the other (expansion) within a growing leaf lamina is a critical determinant of final leaf shape, size, complexity and flatness. The TCP proteins, unique to plant kingdom, are sequence-specific DNA-binding transcription factors that control several developmental and physiological traits. A sub-group of class II TCPs, called CINCINNATA-like TCPs (CIN-TCPs henceforth), are key regulators of the timing of the transition from division to expansion in dicot leaves. The current review highlights recent advances in our understanding of how the pattern of CIN-TCP activity is translated to the dynamic spatio-temporal control of cell-fate transition through the transactivation of cell-cycle regulators, growth-repressing microRNAs, and interactions with the chromatin remodeling machinery to modulate phytohormone responses. Unravelling how environmental inputs influence CIN-TCP-mediated growth control is a challenge for future studies. © 2018 IUBMB Life, 70(8):718-731, 2018.

Genome-Wide Identification of TCP Family Transcription Factors in Medicago truncatula Reveals Significant Roles of miR319-Targeted TCPs in Nodule Development

TCP proteins, the plant-specific transcription factors, are involved in the regulation of multiple aspects of plant development among different species, such as leaf development, branching, and flower symmetry. However, thus far, the roles of TCPs in legume, especially in nodulation are still not clear. In this study, a genome-wide analysis of TCP genes was carried out to discover their evolution and function in Medicago truncatula. In total, 21 MtTCPs were identified and classified into class I and class II, and the class II MtTCPs were further divided into two subclasses, CIN and CYC/TB1. The expression profiles of MtTCPs are dramatically different. The universal expression of class I MtTCPs was detected in all organs. However, the MtTCPs in CIN subclass were highly expressed in leaf and most of the members in CYC/TB1 subclass were highly expressed in flower. Such organ-specific expression patterns of MtTCPs suggest their different roles in plant development. In addition, most MtTCPs were down-regulated during the nodule development, except for the putative MtmiR319 targets, MtTCP3, MtTCP4, and MtTCP10A. Overexpression of MtmiR319A significantly reduced the expression level of MtTCP3/4/10A/10B and resulted in the decreased nodule number, indicating the important roles of MtmiR319-targeted MtTCPs in nodulation. Taken together, this study systematically analyzes the MtTCP gene family at a genome-wide level and their possible functions in nodulation, which lay the basis for further explorations of MtmiR319/MtTCPs module in association with nodule development in M. truncatula.

Transcription factors NF-YA2 and NF-YA10 regulate leaf growth via auxin signaling in Arabidopsis

In plants, leaf is crucial for photosynthesis and respiration. Leaf area and quantity are important for leaf vegetables to increase biomass. The process of leaf development involves coordinated regulation among small RNAs, transcription factors and hormones. Here, we found leaf size were regulated by transcription factors NF-YA2 and NF-YA10 in Arabidopsis. NF-YA2 and NF-YA10 overexpression increased biomass accumulation through promoting leaf growth and cell expansion. NF-YA2 and NF-YA10 were expressed in SAM and leaf vasculature. Endogenous IAA content reduced by 20% and 24% in transgenic Arabidopsis plants overexpressing NF-YA2 and NF-YA10 compared to wild-type plants. Chromatin immunoprecipitation assays revealed that NF-YA2 and NF-YA10 bound directly to the cis-element CCAAT in the promoter of the YUC2, and decreased the expression of YUC2, a YUCCA family gene. The auxin transporter gene PIN1 and auxin response factor1 and 2 (ARF1 and ARF2) genes, transcriptional repressors, were downregulated. These findings showed leaf development was regulated by NF-YA2 and NF-YA10 through the auxin-signaling pathway and may provide a new insight into the genetic engineering of vegetables biomass and crop productivity.

Evolution and Expression Patterns of TCP Genes in Asparagales

CYCLOIDEA-like genes are involved in the symmetry gene network, limiting cell proliferation in the dorsal regions of bilateral flowers in core eudicots. CYC-like and closely related TCP genes (acronym for TEOSINTE BRANCHED1, CYCLOIDEA, and PROLIFERATION CELL FACTOR) have been poorly studied in Asparagales, the largest order of monocots that includes both bilateral flowers in Orchidaceae (ca. 25.000 spp) and radially symmetrical flowers in Hypoxidaceae (ca. 200 spp). With the aim of assessing TCP gene evolution in the Asparagales, we isolated TCP-like genes from publicly available databases and our own transcriptomes of Cattleya trianae (Orchidaceae) and Hypoxis decumbens (Hypoxidaceae). Our matrix contains 452 sequences representing the three major clades of TCP genes. Besides the previously identified CYC specific core eudicot duplications, our ML phylogenetic analyses recovered an early CIN-like duplication predating all angiosperms, two CIN-like Asparagales-specific duplications and a duplication prior to the diversification of Orchidoideae and Epidendroideae. In addition, we provide evidence of at least three duplications of PCF-like genes in Asparagales. While CIN-like and PCF-like genes have multiplied in Asparagales, likely enhancing the genetic network for cell proliferation, CYC-like genes remain as single, shorter copies with low expression. Homogeneous expression of CYC-like genes in the labellum as well as the lateral petals suggests little contribution to the bilateral perianth in C. trianae. CIN-like and PCF-like gene expression suggests conserved roles in cell proliferation in leaves, sepals and petals, carpels, ovules and fruits in Asparagales by comparison with previously reported functions in core eudicots and monocots. This is the first large scale analysis of TCP-like genes in Asparagales that will serve as a platform for in-depth functional studies in emerging model monocots.

TCP factors: new kids on the signaling block

The TCP transcription factors govern key plant developmental processes and have profound effects on the growth patterns of meristems and organs, partly explained by direct transcriptional control of cell cycle genes. This view is nevertheless incomplete, as accumulated evidence indicates that TCPs also act through other mechanisms, such as the regulation of hormone activity. Several TCP factors not only act as mediators of hormone-induced changes in cell proliferation, but also as modulators, or even key players, of hormone synthesis, transport and signal transduction. This adds another layer of complexity to the role of TCPs in plant development.

TCP Transcription Factors at the Interface between Environmental Challenges and the Plant's Growth Responses

Plants are sessile and as such their reactions to environmental challenges differ from those of mobile organisms. Many adaptions involve growth responses and hence, growth regulation is one of the most crucial biological processes for plant survival and fitness. The plant-specific TEOSINTE BRANCHED 1, CYCLOIDEA, PCF1 (TCP) transcription factor family is involved in plant development from cradle to grave, i.e., from seed germination throughout vegetative development until the formation of flowers and fruits. TCP transcription factors have an evolutionary conserved role as regulators in a variety of plant species, including orchids, tomatoes, peas, poplar, cotton, rice and the model plant Arabidopsis. Early TCP research focused on the regulatory functions of TCPs in the development of diverse organs via the cell cycle. Later research uncovered that TCP transcription factors are not static developmental regulators but crucial growth regulators that translate diverse endogenous and environmental signals into growth responses best fitted to ensure plant fitness and health. I will recapitulate the research on TCPs in this review focusing on two topics: the discovery of TCPs and the elucidation of their evolutionarily conserved roles across the plant kingdom, and the variety of signals, both endogenous (circadian clock, plant hormones) and environmental (pathogens, light, nutrients), TCPs respond to in the course of their developmental roles.

Distinct Regulatory Changes Underlying Differential Expression of TEOSINTE BRANCHED1-CYCLOIDEA-PROLIFERATING CELL FACTOR Genes Associated with Petal Variations in Zygomorphic Flowers of Petrocosmea spp. of the Family Gesneriaceae

CYCLOIDEA (CYC)-like genes, belonging to the plant-specific TCP transcription factor family that is named after TEOSINTE BRANCHED1 (TB1) from maize (Zea mays), CYC from Antirrhinum majus, and the PROLIFERATING CELL FACTORS (PCF) from rice (Oryza sativa), have conserved dorsal identity function in patterning floral zygomorphy mainly through specific expression in dorsal petals of a flower. Their expression changes are usually related to morphological diversity of zygomorphic flowers. However, it is still a challenge to elucidate the molecular mechanism underlying their expression differentiation. It is also unknown whether CINCINNATA (CIN)-like TCP genes, locally controlling cell growth and proliferation, are involved in the evolution of floral zygomorphy. To address these questions, we selected two closely related species, i.e. Petrocosmea glabristoma and Petrocosmea sinensis, with distinct petal morphology to conduct expression, hybridization, mutant, and allele-specific expression analyses. The results show that the size change of the dorsal petals between the two species is mainly mediated by the expression differentiation of CYC1C and CYC1D, while the shape variation of all petals is related to the expression change of CIN1. In reciprocal F1 hybrids, the expression of CYC1C, CYC1D, and CIN1 conforms to an additive inheritance mode, consistent with the petal phenotypes of hybrids. Through allele-specific expression analyses, we find that the expression differentiation of these TCP genes is underlain by distinctly different types of regulatory changes. We suggest that highly redundant paralogs with identical expression patterns and interspecific expression differentiation may be controlled by remarkably different regulatory pathways because natural selection may favor different regulatory modifications rather than coding sequence changes of key developmental genes in generating morphological diversity.

TCP15 modulates cytokinin and auxin responses during gynoecium development in Arabidopsis

We studied the role of Arabidopsis thaliana TCP15, a member of the TEOSINTE BRANCHED1-CYCLOIDEA-PCF (TCP) transcription factor family, in gynoecium development. Plants that express TCP15 from the 35S CaMV promoter (35S:TCP15) develop flowers with defects in carpel fusion and a reduced number of stigmatic papillae. In contrast, the expression of TCP15 fused to a repressor domain from its own promoter causes the development of outgrowths topped with stigmatic papillae from the replum. 35S:TCP15 plants show lower levels of the auxin indoleacetic acid and reduced expression of the auxin reporter DR5 and the auxin biosynthesis genes YUCCA1 and YUCCA4, suggesting that TCP15 is a repressor of auxin biosynthesis. Treatment of plants with cytokinin enhances the developmental effects of expressing TCP15 or its repressor form. In addition, treatment of a knock-out double mutant in TCP15 and the related gene TCP14 with cytokinin causes replum enlargement, increased development of outgrowths, and the induction of the auxin biosynthesis genes YUCCA1 and YUCCA4. A comparison of the phenotypes observed after cytokinin treatment of plants with altered expression levels of TCP15 and auxin biosynthesis genes suggests that TCP15 modulates gynoecium development by influencing auxin homeostasis. We propose that the correct development of the different tissues of the gynoecium requires a balance between auxin levels and cytokinin responses, and that TCP15 participates in a feedback loop that helps to adjust this balance.