Arabidopsis JAGGED LATERAL ORGANS is expressed in boundaries and coordinates KNOX and PIN activity

The LOB domain protein, a novel transcription factor with multiple functions: A review

The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) protein, named for its LATERAL ORGAN BOUNDARIES (LOB) domain, is a member of a class of specific transcription factors commonly found in plants and is absent from all other groups of organisms. LBD TFs have been systematically identified in about 35 plant species and are involved in regulating various aspects of plant growth and development. However, research on the signaling network and regulatory functions of LBD TFs is insufficient, and only a few members have been studied. Moreover, a comprehensive review of these existing studies is lacking. In this review, the structure, regulatory mechanism and function of LBD TFs in recent years were reviewed in order to better understand the role of LBD TFs in plant growth and development, and to provide a new perspective for the follow-up study of LBD TFs.

Ectopic Expression of PtrLBD39 Retarded Primary and Secondary Growth in Populus trichocarpa

Primary and secondary growth of trees are needed for increments in plant height and stem diameter, respectively, affecting the production of woody biomass for applications in timber, pulp/paper, and related biomaterials. These two types of growth are believed to be both regulated by distinct transcription factor (TF)-mediated regulatory pathways. Notably, we identified PtrLBD39, a highly stem phloem-specific TF in Populus trichocarpa and found that the ectopic expression of PtrLBD39 in P. trichocarpa markedly retarded both primary and secondary growth. In these overexpressing plants, the RNA-seq, ChIP-seq, and weighted gene co-expression network analysis (WGCNA) revealed that PtrLBD39 directly or indirectly regulates TFs governing vascular tissue development, wood formation, hormonal signaling pathways, and enzymes responsible for wood components. This regulation led to growth inhibition, decreased fibrocyte secondary cell wall thickness, and reduced wood production. Therefore, our study indicates that, following ectopic expression in P. trichocarpa, PtrLBD39 functions as a repressor influencing both primary and secondary growth.

SlTCP24 and SlTCP29 synergistically regulate compound leaf development through interacting with SlAS2 and activating transcription of SlCKX2 in tomato

The complexity of compound leaves results primarily from the leaflet initiation and arrangement during leaf development. However, the molecular mechanism underlying compound leaf development remains a central research question. SlTCP24 and SlTCP29, two plant-specific transcription factors with the conserved TCP motif, are shown here to synergistically regulate compound leaf development in tomato. When both of them were knocked out simultaneously, the number of leaflets significantly increased, and the shape of the leaves became more complex. SlTCP24 and SlTCP29 could form both homodimers and heterodimers, and such dimerization was impeded by the leaf polarity regulator SlAS2, which interacted with SlTCP24 and SlTCP29. SlTCP24 and SlTCP29 could bind to the TCP-binding cis-element of the SlCKX2 promoter and activate its transcription. Transgenic plants with SlTCP24 and SlTCP29 double-gene knockout had a lowered transcript level of SlCKX2 and an elevated level of cytokinin. This work led to the identification of two key regulators of tomato compound leaf development and their targeted genes involved in cytokinin metabolic pathway. A model of regulation of compound leaf development was proposed based on observations of this study.

Genome-Wide Identification and Expression Analysis of AS2 Genes in Brassica rapa Reveal Their Potential Roles in Abiotic Stress

The ASYMMETRIC LEAVES2/LATERAL ORGAN BOUNDARIES (AS2/LOB) gene family plays a pivotal role in plant growth, induction of phytohormones, and the abiotic stress response. However, the AS2 gene family in Brassica rapa has yet to be investigated. In this study, we identified 62 AS2 genes in the B. rapa genome, which were classified into six subfamilies and distributed across 10 chromosomes. Sequence analysis of BrAS2 promotors showed that there are several typical cis-elements involved in abiotic stress tolerance and stress-related hormone response. Tissue-specific expression analysis showed that BrAS2-47 exhibited ubiquitous expression in all tissues, indicating it may be involved in many biological processes. Gene expression analysis showed that the expressions of BrAS2-47 and BrAS2-10 were significantly downregulated under cold stress, heat stress, drought stress, and salt stress, while BrAS2-58 expression was significantly upregulated under heat stress. RT-qPCR also confirmed that the expression of BrAS2-47 and BrAS2-10 was significantly downregulated under cold stress, drought stress, and salt stress, and in addition BrAS2-56 and BrAS2-4 also changed significantly under the three stresses. In addition, protein-protein interaction (PPI) network analysis revealed that the Arabidopsis thaliana genes AT5G67420 (homologous gene of BrAS2-47 and BrAS2-10) and AT3G49940 (homologous gene of BrAS2-58) can interact with NIN-like protein 7 (NLP7), which has been previously reported to play a role in resistance to adverse environments. In summary, our findings suggest that among the BrAS2 gene family, BrAS2-47 and BrAS2-10 have the most potential for the regulation of abiotic stress tolerance. These results will facilitate future functional investigations of BrAS2 genes in B. rapa.

Overexpression of Liriodendron tulipifera JAG Gene (LtuJAG) Changes Leaf Shapes in Transgenic Arabidopsis thaliana

In Arabidopsis thaliana, JAGGED (JAG) is a transcription inhibitor that controls the development of leaf polarity and regulates the expression of genes controlling lateral organ formation. Liriodendron tulipifera is an ornamental tree with extraordinary tulip-shaped flowers and goose web-like leaves, this is one of the suitable plants for morphological development research. To investigate the potential functions of the LtuJAG gene, we isolated the full-length LtuJAG from L. tulipifera, transferred it into A. thaliana via agrobacterium-mediated transformation, and monitored its expression pattern. Subcellular localization showed that LtuJAG was located in the nucleus. RT-qPCR assays indicated that LtuJAG was expressed mainly in leaf buds and flowers, but not in mature leaves and stems. GUS staining results showed that LtuJAG was expressed in the shoot apical meristem (SAM). Overexpressing LtuJAG changed A. thaliana leaf shapes, causing a moderate serration and a slight asymmetric distribution in the medio-lateral and proximal-distal axes. Ectopic expression of LtuJAG induced the expression of lateral organ boundary suppressors JAGGED LATERAL ORGANS (JLO) and ARABIDOPSIS THALIANA HOMEOBOX1 (ATH1). It also repressed the expression of the apical meristem suppressor class-1 KNOX gene (KNOX I) and altered endogenous hormone levels. Our results suggest that LtuJAG plays a role in negatively regulating leaf polarity formation in L. tulipifera.

Stem Cell Basis for Fractal Patterns: Axillary Meristem Initiation

Whereas stem cell lineages are of enormous importance in animal development, their roles in plant development have only been appreciated in recent years. Several specialized lineages of stem cells have been identified in plants, such as meristemoid mother cells and vascular cambium, as well as those located in the apical meristems. The initiation of axillary meristems (AMs) has recently gained intensive attention. AMs derive from existing stem cell lineages that exit from SAMs and define new growth axes. AMs are in fact additional rounds of SAMs, and display the same expression patterns and functions as the embryonic SAM, creating a fractal branching pattern. Their formation takes place in leaf-meristem boundaries and mainly comprises two key stages. The first stage is the maintenance of the meristematic cell lineage in an undifferentiated state. The second stage is the activation, proliferation, and re-specification to form new stem cell niches in AMs, which become the new postembryonic "fountain of youth" for organogenesis. Both stages are tightly regulated by spatially and temporally interwound signaling networks. In this mini-review, I will summarize the most up-to-date understanding of AM establishment and mainly focus on how the leaf axil meristematic cell lineage is actively maintained and further activated to become CLV3-expressed stem cells, which involves phytohormonal cascades, transcriptional regulations, epigenetic modifications, as well as mechanical signals.

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

Genome-wide identification and expression analysis of LBD transcription factor genes in Moso bamboo (Phyllostachys edulis)

BACKGROUND

Moso bamboo, the fastest growing plant on earth, is an important source for income in large areas of Asia, mainly cultivated in China. Lateral organ boundaries domain (LBD) proteins, a family of transcription factors unique to plants, are involved in multiple transcriptional regulatory pathways and play important roles in lateral organ development, pathogen response, secondary growth, and hormone response. The LBD gene family has not previously been characterized in moso bamboo (Phyllostachys edulis).

RESULTS

In this study, we identified 55 members of the LBD gene family from moso bamboo and found that they were distributed non-uniformly across its 18 chromosomes. Phylogenetic analysis showed that the moso bamboo LBD genes could be divided into two classes. LBDs from the same class share relatively conserved gene structures and sequences encoding similar amino acids. A large number of hormone response-associated cis-regulatory elements were identified in the LBD upstream promoter sequences. Synteny analysis indicated that LBDs in the moso bamboo genome showed greater collinearity with those of O. sativa (rice) and Zea mays (maize) than with those of Arabidopsis and Capsicum annuum (pepper). Numerous segmental duplicates were found in the moso bamboo LBD gene family. Gene expression profiles in four tissues showed that the LBD genes had different spatial expression patterns. qRT-PCR assays with the Short Time-series Expression Miner (STEM) temporal expression analysis demonstrated that six genes (PeLBD20, PeLBD29, PeLBD46, PeLBD10, PeLBD38, and PeLBD06) were consistently up-regulated during the rapid growth and development of bamboo shoots. In addition, 248 candidate target genes that function in a variety of pathways were identified based on consensus LBD binding motifs.

CONCLUSIONS

In the current study, we identified 55 members of the moso bamboo transcription factor LBD and characterized for the first time. Based on the short-time sequence expression software and RNA-seq data, the PeLBD gene expression was analyzed. We also investigated the functional annotation of all PeLBDs, including PPI network, GO, and KEGG enrichment based on String database. These results provide a theoretical basis and candidate genes for studying the molecular breeding mechanism of rapid growth of moso bamboo.

Analysis of Shoot Architecture Traits in Edamame Reveals Potential Strategies to Improve Harvest Efficiency

Edamame is a type of green, vegetable soybean and improving shoot architecture traits for edamame is important for breeding of high-yield varieties by decreasing potential loss due to harvesting. In this study, we use digital imaging technology and computer vision algorithms to characterize major traits of shoot architecture for edamame. Using a population of edamame PIs, we seek to identify underlying genetic control of different shoot architecture traits. We found significant variations in the shoot architecture of the edamame lines including long-skinny and candle stick-like structures. To quantify the similarity and differences of branching patterns between these edamame varieties, we applied a topological measurement called persistent homology. Persistent homology uses algebraic geometry algorithms to measure the structural similarities between complex shapes. We found intriguing relationships between the topological features of branching networks and pod numbers in our plant population, suggesting combination of multiple topological features contribute to the overall pod numbers on a plant. We also identified potential candidate genes including a lateral organ boundary gene family protein and a MADS-box gene that are associated with the pod numbers. This research provides insight into the genetic regulation of shoot architecture traits and can be used to further develop edamame varieties that are better adapted to mechanical harvesting.

Heterologous expression of chrysanthemum TOPLESS corepressor CmTPL1-1 alters meristem maintenance and organ development in Arabidopsis thaliana

TOPLESS (TPL)/TOPLESS-related (TPR) corepressors are important regulators of plant growth and development, but their functions in chrysanthemum (Chrysanthemum morifolium) are currently unclear. In this study, a chrysanthemum TPL/TPR family gene, designated CmTPL1-1, was characterized. This gene encodes an 1135-amino-acid polypeptide harboring a conserved N-terminal domain and two C-terminal WD40 domains. CmTPL1-1 showed no transcriptional activity in yeast, and a localization experiment indicated that it localized to the nuclei in onion epidermal cells. Transcript profiling established that the gene was most highly expressed in the stem apex. The heterologous expression of CmTPL1-1 in Arabidopsis thaliana produced a pleiotropic phenotype, including smaller leaves, shorter siliques, increased meristem number, asymmetrical petal distribution and reduced stamen number. In transgenic plants, four AtARFs were downregulated, while six AtIAAs and two AtGH3s were upregulated at the transcript level; moreover, the expression of three key class I KNOTTED-like homeobox (KNOX) genes was upregulated. In addition, by yeast two-hybrid screening of a chrysanthemum cDNA library, we found that CmTPL1-1 could interact with CmWOX4, CmLBD38 and CmLBD36, and these interactions were confirmed by bimolecular fluorescence complementation (BiFC) assays. Overall, we speculated that heterologous expression of CmTPL1-1 regulates plant growth and development by interacting with auxin signaling in Arabidopsis.

flasher, a novel mutation in a glucosinolate modifying enzyme, conditions changes in plant architecture and hormone homeostasis

Meristem function is underpinned by numerous genes that affect hormone levels, ultimately controlling phyllotaxy, the transition to flowering and general growth properties. Class I KNOX genes are major contributors to this process, promoting cytokinin biosynthesis but repressing gibberellin production to condition a replication competent state. We identified a suppressor mutant of the KNOX1 mutant brevipedicellus (bp) that we termed flasher (fsh), which promotes stem and pedicel elongation, suppresses early senescence, and negatively affects reproductive development. Map-based cloning and complementation tests revealed that fsh is due to an E40K change in the flavin monooxygenase GS-OX5, a gene encoding a glucosinolate (GSL) modifying enzyme. In vitro enzymatic assays revealed that fsh poorly converts substrate to product, yet the levels of several GSLs are higher in the suppressor line, implicating FSH in feedback control of GSL flux. FSH is expressed predominantly in the vasculature in patterns that do not significantly overlap those of BP, implying a non-cell autonomous mode of meristem control via one or more GSL metabolites. Hormone analyses revealed that cytokinin levels are low in bp, but fsh restores cytokinin levels to near normal by activating cytokinin biosynthesis genes. In addition, jasmonate levels in the fsh suppressor are significantly lower than in bp, which is likely due to elevated expression of JA inactivating genes. These observations suggest the involvement of the GSL pathway in generating one or more negative effectors of growth that influence inflorescence architecture and fecundity by altering the balance of hormonal regulators.

An integrated analysis of cell-type specific gene expression reveals genes regulated by REVOLUTA and KANADI1 in the Arabidopsis shoot apical meristem

In the Arabidopsis thaliana shoot apical meristem (SAM) the expression domains of Class III Homeodomain Leucine Zipper (HD-ZIPIII) and KANADI (KAN) genes are separated by a narrow boundary region from which new organs are initiated. Disruption of this boundary through either loss of function or ectopic expression of HD-ZIPIII and KAN causes ectopic or suppression of organ formation respectively, raising the question of how these transcription factors regulate organogenesis at a molecular level. In this study we develop a multi-channel FACS/RNA-seq approach to characterize global patterns of gene expression across the HD-ZIPIII-KAN1 SAM boundary. We then combine FACS, RNA-seq and perturbations of HD-ZIPIII and KAN expression to identify genes that are both responsive to REV and KAN1 and normally expressed in patterns that correlate with REV and KAN1. Our data reveal that a significant number of genes responsive to REV are regulated in opposite ways depending on time after induction, with genes associated with auxin response and synthesis upregulated initially, but later repressed. We also characterize the cell type specific expression patterns of auxin responsive genes and identify a set of genes involved in organogenesis repressed by both REV and KAN1.

The plant hormone auxin promotes the formation of lateral organs such as leaves and flowers in a specific region of the shoot called the peripheral zone. Although the restriction of organogenesis to the peripheral zone is known to depend on the Class III Homeodomain Leucine Zipper (HD-ZIPIII) and KANADI1 (KAN1) genes, the transcriptional pathways downstream of these genes have not been studied in the shoot. In this study we investigate regulatory interactions between REVOLUTA (REV), KAN1 and auxin by developing a cell-type specific transcriptomics approach to analyse gene expression patterns and responses to perturbations. Using this approach, we identify cell-type specific genes that respond to changes in REV and KAN1 expression in the shoot. Our data reveal that while REV promotes auxin-related gene expression over the short term, both REV and KAN1 repress auxin induced genes over the long-term, consistent with their influence on organogenesis. We also identify a common set of genes repressed by REV and KAN1 that promote organogenesis.

Phylogeny and Functions of LOB Domain Proteins in Plants

Lateral organ boundaries (LOB) domain (LBD) genes, a gene family encoding plant-specific transcription factors, play important roles in plant growth and development. At present, though there have been a number of genome-wide analyses on LBD gene families and functional studies on individual LBD proteins, the diverse functions of LBD family members still confuse researchers and an effective strategy is required to summarize their functional diversity. To further integrate and improve our understanding of the phylogenetic classification, functional characteristics and regulatory mechanisms of LBD proteins, we review and discuss the functional characteristics of LBD proteins according to their classifications under a phylogenetic framework. It is proved that this strategy is effective in the anatomy of diverse functions of LBD family members. Additionally, by phylogenetic analysis, one monocot-specific and one eudicot-specific subclade of LBD proteins were found and their biological significance in monocot and eudicot development were also discussed separately. The review will help us better understand the functional diversity of LBD proteins and facilitate further studies on this plant-specific transcription factor family.

CRISPR/Cas9-Mediated Multiplex Genome Editing of JAGGED Gene in Brassica napus L

Pod shattering resistance is an essential component to achieving a high yield, which is a substantial objective in polyploid rapeseed cultivation. Previous studies have suggested that the Arabidopsis JAGGED (JAG) gene is a key factor implicated in the regulatory web of dehiscence fruit. However, its role in controlling pod shattering resistance in oilseed rape is still unknown. In this study, multiplex genome editing was carried out by the CRISPR/Cas9 system on five homoeologs (BnJAG.A02, BnJAG.C02, BnJAG.C06, BnJAG.A07, and BnJAG.A08) of the JAG gene. Knockout mutagenesis of all homoeologs drastically affected the development of the lateral organs in organizing pod shape and size. The cylindrical body of the pod comprised a number of undifferentiated cells like a callus, without distinctive valves, replum, septum, and valve margins. Pseudoseeds were produced, which were divided into two halves with an incomplete layer of cells (probably septum) that separated the undifferentiated cells. These mutants were not capable of generating any productive seeds for further generations. However, one mutant line was identified in which only a BnJAG.A08-NUB-Like paralog of the JAG gene was mutated. Knockout mutagenesis in BnJAG.A08-NUB gene caused significant changes in the pod dehiscence zone. The replum region of the mutant was increased to a great extent, resulting in enlarged cell size, bumpy fruit, and reduced length compared with the wild type. A higher replum-valve joint area may have increased the resistance to pod shattering by ~2-fold in JAG mutants compared with wild type. Our results offer a basis for understanding variations in Brassica napus fruit by mutating JAG genes and providing a way forward for other Brassicaceae species.

TWI1 regulates cell-to-cell movement of OSH15 to control leaf cell fate

Cell pattern formation in plant leaves has attracted much attention from both plant biologists and breeders. However, in rice, the molecular mechanism remains unclear. Here, we describe the isolation and functional characterization of TWISTED-LEAF1 (TWI1), a critical gene involved in the development of the mestome sheath, vascular bundle sheath, interveinal mesophyll and sclerenchyma in rice leaves. Mutant twi1 plants have twisted leaves which might be caused by the compromised development and disordered patterning of bundle sheath, sclerenchyma and interveinal mesophyll cells. Expression of TWI1 can functionally rescue these mutant phenotypes. TWI1 encodes a transcription factor binding protein that interacts with OSH15, a class I KNOTTED1-like homeobox (KNOX) transcription factor. The cell-to-cell trafficking of OSH15 is restricted through its interaction with TWI1. Knockout or knockdown of OSH15 in twi1 rescues the twisted leaf phenotype. These studies reveal a key factor controlling cell pattern formation in rice leaves.

Drawing a Line: Grasses and Boundaries

Delineation between distinct populations of cells is essential for organ development. Boundary formation is necessary for the maintenance of pluripotent meristematic cells in the shoot apical meristem (SAM) and differentiation of developing organs. Boundaries form between the meristem and organs, as well as between organs and within organs. Much of the research into the boundary gene regulatory network (GRN) has been carried out in the eudicot model Arabidopsis thaliana. This work has identified a dynamic network of hormone and gene interactions. Comparisons with other eudicot models, like tomato and pea, have shown key conserved nodes in the GRN and species-specific alterations, including the recruitment of the boundary GRN in leaf margin development. How boundaries are defined in monocots, and in particular the grass family which contains many of the world’s staple food crops, is not clear. In this study, we review knowledge of the grass boundary GRN during vegetative development. We particularly focus on the development of a grass-specific within-organ boundary, the ligule, which directly impacts leaf architecture. We also consider how genome engineering and the use of natural diversity could be leveraged to influence key agronomic traits relative to leaf and plant architecture in the future, which is guided by knowledge of boundary GRNs.

Role of LBD14 during ABA-mediated control of root system architecture in Arabidopsis

The LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) gene family encode plant-specific transcription factors that regulate various aspects of plant growth and development. Arabidopsis genome has 42 LBD genes. Several LBD genes, such as LBD16, -18, -29, and -33, have been shown to function in lateral root (LR) development via auxin signaling. Although abscisic acid (ABA) is a well-known antistress plant hormone regulating various plant developmental processes, it also plays a role in LR growth regulation. Our recent study showed that LBD14 expression is downregulated by ABA during the entire steps of LR development. The RNAi-induced downregulation and overexpression of LBD14 indicated that LBD14 promotes LR formation. LBD14RNAi enhanced the ABA-induced suppression of LR density compared with the wild type, suggesting that LBD14 is involved in the ABA-mediated control of LR formation. Our study provides an insight into the signaling mechanism of developmental plasticity whereby ABA controls LR branching via LBD14 downregulation under abiotic stress conditions.

Genome-Wide Identification of Arabidopsis LBD29 Target Genes Reveals the Molecular Events behind Auxin-Induced Cell Reprogramming during Callus Formation

Auxin-induced callus formation represents an important cell reprogramming process during in vitro regeneration of plants, in which the pericycle or pericycle-like cells within plant organs are reprogrammed into the pluripotent cell mass termed callus that is generally required for subsequent regeneration of root or shoot. However, the molecular events behind cell reprogramming during auxin-induced callus formation are largely elusive. We previously identified that auxin-induced LATERAL ORGAN BOUNDARIES DOMAIN (LBD) transcription factors act as the master regulators to trigger auxin-induced callus formation. Here, by ChIP-seq (chromatin immunoprecipitation-based sequencing) and RNA sequencing approaches, we identified the potential LBD29 target genes at the genome-wide level and outlined the molecular events of LBD-triggered cell reprogramming during callus formation. We showed that LBD29 preferentially bound to the G-box (CACGTG) and TGGGC[C/T] motifs and potentially targeted >350 genes, among which the genes related to methylation, reactive oxygen species (ROS) metabolism, cell wall hydrolysis and lipid metabolism were rapidly activated, while most of the light-responsive genes were suppressed by LBD29. Further examination of a few representative genes validated that they were targeted by LBD29 and participated in the regulation of cell reprogramming during callus formation. Our data not only outline a framework of the early molecular events behind auxin-induced cell reprogramming of callus formation, but also provide a valuable resource for identification of genes that regulate cell fate switch during in vitro regeneration of plants.

Coiled-coil motif in LBD16 and LBD18 transcription factors are critical for dimerization and biological function in arabidopsis

The LATERAL ORGAN BOUNDARIES (LOB) DOMAIN (LBD) gene family members encode a class of plant-specific transcription factors that play important roles in many different aspects of plant growth and development. The LBD proteins contain a conserved LOB domain harboring a Leu zipper-like coiled-coil motif, which has been predicted to mediate protein-protein interactions among the LBD family members. Dimerization of transcription factors is crucial for the modulation of their DNA-binding affinity, specificity, and diversity, contributing to the transcriptional regulation of distinct cellular and biological responses. Our various molecular and biochemical experiments with genetic approaches on LBD16 and LBD18, which are known to control lateral root development in Arabidopsis, demonstrated that the conserved Leu or Val residues in the coiled-coil motifs of these transcription factors are critical for their dimerization as well as the transcriptional regulation to display their biological functions during lateral root formation. We further showed that beside the coiled-coil motif, the carboxyl-terminal region in LBD18 acts as an additional dimerization domain. These findings provide a molecular framework for the homo- and hetero-dimerization of the LBD family proteins for displaying their distinct and diverse biological functions in plants.

Genomewide analysis of the lateral organ boundaries domain gene family in Eucalyptus grandis reveals members that differentially impact secondary growth

Lateral Organ Boundaries Domain (LBD) proteins are plant-specific transcription factors playing crucial roles in growth and development. However, the function of LBD proteins in Eucalyptus grandis remains largely unexplored. In this study, LBD genes in E. grandis were identified and characterized using bioinformatics approaches. Gene expression patterns in various tissues and the transcriptional responses of EgLBDs to exogenous hormones were determined by qRT-PCR. Functions of the selected EgLBDs were studied by ectopically overexpressing in a hybrid poplar (Populus alba × Populus glandulosa). Expression levels of genes in the transgenic plants were investigated by RNA-seq. Our results showed that there were forty-six EgLBD members in the E. grandis genome and three EgLBDs displayed xylem- (EgLBD29) or phloem-preferential expression (EgLBD22 and EgLBD37). Confocal microscopy indicated that EgLBD22, EgLBD29 and EgLBD37 were localized to the nucleus. Furthermore, we found that EgLBD22, EgLBD29 and EgLBD37 were responsive to the treatments of indol-3-acetic acid and gibberellic acid. More importantly, we demonstrated EgLBDs exerted different influences on secondary growth. Namely, 35S::EgLBD37 led to significantly increased secondary xylem, 35S::EgLBD29 led to greatly increased phloem fibre production, and 35S::EgLBD22 showed no obvious effects. We revealed that key genes related to gibberellin, ethylene and auxin signalling pathway as well as cell expansion were significantly up- or down-regulated in transgenic plants. Our new findings suggest that LBD genes in E. grandis play important roles in secondary growth. This provides new mechanisms to increase wood or fibre production.

LBD14/ASL17 Positively Regulates Lateral Root Formation and is Involved in ABA Response for Root Architecture in Arabidopsis

The LATERAL ORGAN BOUNDARIES (LOB) DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) gene family members play key roles in diverse aspects of plant development. Previous studies have shown that LBD16, 18, 29 and 33 are critical for integrating the plant hormone auxin to control lateral root development in Arabidopsis thaliana. In the present study, we show that LBD14 is expressed exclusively in the root where it promotes lateral root (LR) emergence. Repression of LBD14 expression by ABA correlates with the inhibitory effects of ABA on LR emergence. Transient gene expression assays with Arabidopsis protoplasts demonstrated that LBD14 is a nuclear-localized transcriptional activator. The knock-down of LBD14 expression by RNA interference (RNAi) resulted in reduced LR formation by delaying both LR primordium development and LR emergence, whereas overexpression of LBD14 in Arabidopsis enhances LR formation. We show that ABA (but not other plant hormones such as auxin, brassinosteroids and cytokinin) specifically down-regulated β-glucuronidase (GUS) expression under the control of the LBD14 promoter in transgenic Arabidopsis during LR development from initiation to emergence and endogenous LBD14 transcript levels in the root. Moreover, RNAi of LBD14 enhanced the LR suppression in response to ABA, whereas LBD14 overexpression did not alter the ABA-mediated suppression of LR formation. Taken together, these results suggest that LBD14 promoting LR formation is one of the critical factors regulated by ABA to inhibit LR growth, contributing to the regulation of the Arabidopsis root system architecture in response to ABA.

The LATERAL ORGAN BOUNDARIES Domain gene family in grapevine: genome-wide characterization and expression analyses during developmental processes and stress responses

LATERAL ORGAN BOUNDARIES (LOB) DOMAIN (LBD) constitute a family of plant-specific transcription factors with key roles in the regulation of plant organ development, pollen development, plant regeneration, pathogen response, and anthocyanin and nitrogen metabolisms. However, the role of LBDs in fruit ripening and in grapevine (Vitis vinifera L.) development and stress responses is poorly documented. By performing a model curation of LBDs in the latest genome annotation 50 genes were identified. Phylogenetic analysis showed that LBD genes can be grouped into two classes mapping on 16 out of the 19 V. vinifera chromosomes. New gene subclasses were identified that have not been characterized in other species. Segmental and tandem duplications contributed significantly to the expansion and evolution of the LBD gene family in grapevine as noticed for other species. The analysis of cis-regulatory elements and transcription factor binding sites in the VviLBD promoter regions suggests the involvement of several hormones in the regulation of LBDs expression. Expression profiling suggest the involvement of LBD transcription factors in grapevine development, berry ripening and stress responses. Altogether this study provides valuable information and robust candidate genes for future functional analysis aiming to clarify mechanisms responsible for the onset of fruit ripening and fruit defense strategies.

The Arabidopsis JAGGED LATERAL ORGANS (JLO) gene sensitizes plants to auxin

Plant growth and development of new organs depend on the continuous activity of the meristems. In the shoot, patterns of organ initiation are determined by PINFORMED (PIN)-dependent auxin distribution, while the undifferentiated state of meristem cells requires activity of KNOTTED LIKE HOMEOBOX (KNOX) transcription factors. Cell proliferation and differentiation of the root meristem are regulated by the largely antagonistic functions of auxin and cytokinins. It has previously been shown that the transcription factor JAGGED LATERAL ORGANS (JLO), a member of the LATERAL ORGAN BOUNDARY DOMAIN (LBD) family, coordinates KNOX and PIN expression in the shoot and promotes root meristem growth. Here we show that JLO is required for the establishment of the root stem cell niche, where it interacts with the auxin/PLETHORA pathway. Auxin signaling involves the AUX/IAA co-repressor proteins, ARF transcription factors and F-box receptors of the TIR1/AFB1-5 family. Because jlo mutants fail to degrade the AUX/IAA protein BODENLOS, root meristem development is inhibited. We also demonstrate that the expression levels of two auxin receptors, TIR1 and AFB1, are controlled by JLO dosage, and that the shoot and root defects of jlo mutants are alleviated in jlo plants expressing TIR1 and AFB1 from a transgene. The finding that the auxin sensitivity of a plant can be differentially regulated through control of auxin receptor expression can explain how different developmental processes can be integrated by the activity of a key transcription factor.

Dimerization in LBD16 and LBD18 Transcription Factors Is Critical for Lateral Root Formation

LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKEs (hereafter referred to as LBD) are plant-specific transcription factors that play important roles in a plethora of plant growth and development. The leucine (Leu) zipper-like coiled-coil motif in the lateral organ boundaries domain of the class I LBD proteins has been proposed to mediate protein dimerization, but it has not been experimentally assessed yet. LBD16 and LBD18 have been well characterized to play important roles in lateral root development in Arabidopsis (Arabidopsis thaliana). Here, we investigated the role of the coiled-coil motif in the dimerization of LBD16 and LBD18 and in transcriptional regulation and biological function. We built the molecular models of the coiled coil of LBD16 and LBD18, providing the probable Leu zipper models of the helix dimer. Using a variety of molecular techniques, such as bimolecular fluorescence complementation, luciferase complementation imaging, GST pull down, and coimmunoprecipitation assays, we showed that the conserved Leu or valine residues in the coiled-coil motif are critical for the dimerization of LBD16 or LBD18. Using transgenic Arabidopsis plants that overexpress HA:LBD16 or HA:LBD16Q in lbd16 or HA:LBD18 or HA:LBD18Q in lbd18, we demonstrated that the homodimerization of LBD18 mediated by the coiled-coil motif is crucial for transcriptional regulation via promoter binding and for lateral root formation. In addition, we found that the carboxyl-terminal region beyond the coiled-coil motif in LBD18 acts as an additional dimerization domain. These results provide a molecular basis for homodimerization and heterodimerization among the 42 Arabidopsis LBD family members for displaying their biological functions.

A Maize Gene Regulatory Network for Phenolic Metabolism

The translation of the genotype into phenotype, represented for example by the expression of genes encoding enzymes required for the biosynthesis of phytochemicals that are important for interaction of plants with the environment, is largely carried out by transcription factors (TFs) that recognize specific cis-regulatory elements in the genes that they control. TFs and their target genes are organized in gene regulatory networks (GRNs), and thus uncovering GRN architecture presents an important biological challenge necessary to explain gene regulation. Linking TFs to the genes they control, central to understanding GRNs, can be carried out using gene- or TF-centered approaches. In this study, we employed a gene-centered approach utilizing the yeast one-hybrid assay to generate a network of protein-DNA interactions that participate in the transcriptional control of genes involved in the biosynthesis of maize phenolic compounds including general phenylpropanoids, lignins, and flavonoids. We identified 1100 protein-DNA interactions involving 54 phenolic gene promoters and 568 TFs. A set of 11 TFs recognized 10 or more promoters, suggesting a role in coordinating pathway gene expression. The integration of the gene-centered network with information derived from TF-centered approaches provides a foundation for a phenolics GRN characterized by interlaced feed-forward loops that link developmental regulators with biosynthetic genes.

Genome-Wide Analysis of Soybean LATERAL ORGAN BOUNDARIES Domain-Containing Genes: A Functional Investigation of GmLBD12

Plant-specific () genes play critical roles in various plant growth and development processes. However, the number and characteristics of genes in soybean [ (L.) Merr.] remain unknown. Here, we identified 90 homologous genes in the soybean genome that phylogenetically clustered into two classes (I and II). The majority of the genes were evenly distributed across all 20 soybean chromosomes, and 77 (81.11%) of them were detected in segmental duplicated regions. Furthermore, the exon-intron organization and motif composition for each were analyzed. A close phylogenetic relationship was identified between the soybean genes and 41 previously reported genes of different plants in the same group, providing insights into their putative functions. Expression analysis indicated that more than half of the genes were expressed, with the two gene classes showing differential tissue expression characteristics; in addition, they were differentially induced by biotic and abiotic stresses. To further explore the functions of genes in soybean, was selected for functional characterization. GmLBD12 was mainly localized to the nucleus and showed high expression in root and seed tissues. Overexpressing in (L.) Heynh resulted in increases in lateral root (LR) number and plant height. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis demonstrated that was induced by drought, salt, cold, indole acetic acid (IAA), abscisic acid (ABA), and salicylic acid SA treatments. This study provides the first comprehensive analysis of the soybean gene family and a valuable foundation for future functional studies of genes.

Characterization of the LBD gene family in Brachypodium: a phylogenetic and transcriptional study

An unambiguous nomenclature is proposed for the twenty-eight-member LOB domain transcription factor family in Brachypodium . Expression analysis provides unique transcript patterns that are characteristic of a wide range of organs and plant parts. LOB (lateral organ boundaries)-domain proteins define a family of plant-specific transcription factors involved in developmental processes from embryogenesis to seed production. They play a crucial role in shaping the plant architecture through coordinating cell fate at meristem to organ boundaries. Despite their high potential importance, our knowledge of them is limited, especially in the case of monocots. In this study, we characterized LOB domain protein coding genes (LBDs) of Brachypodium distachyon, a model plant for grasses, and present their phylogenetic relationships and an overall spatial expression study. In the Brachypodium genome database, 28 LBDs were found and then classified based on the presence of highly conserved LOB domain motif. Their transcript amounts were measured via quantitative real-time RT-PCR in 37 different plant parts from root tip to generative organs. Comprehensive phylogenetic analysis suggests that there are neither Brachypodium- nor monocot-specific lineages among LBDs, but there are differences in terms of complexity of subclasses between monocots and dicots. Although LBDs in Brachypodium have wide variation of tissue-specific expression and relative transcript levels, overall expression patterns show similarity to their counterparts in other species. The varying transcript profiles we observed support the hypothesis that Brachypodium LBDs have diverse but conserved functions in plant organogenesis.

The LBD12-1 Transcription Factor Suppresses Apical Meristem Size by Repressing Argonaute 10 Expression

The shoot apical meristem (SAM) consists of a population of multipotent cells that generates all aerial structures and regenerates itself. SAM maintenance and lateral organ development are regulated by several complex signaling pathways, in which the Argonaute gene-mediated pathway plays a key role. One Argonaute gene, AGO10, functions as a microRNA locker that attenuates miR165/166 activity and positively regulates shoot apical meristem development, but little is known about when and how AGO10 is regulated at the transcriptional level. In this work, we showed that transgenic rice plants overexpressing LBD12-1, an LBD family transcription factor, exhibited stunted growth, twisted leaves, abnormal anthers, and reduced SAM size. Further research revealed that LBD12-1 directly binds to the promoter region and represses the expression of AGO10. Overexpression of AGO10 in an LBD12-1 overexpression background rescued the growth defect phenotype of LBD12-1-overexpressing plants. The expression of LBD12-1 and its binding ability to the AGO10 promoter is induced by stress. lbd12-1 loss-of-function mutants showed similar phenotypes and SAM size to the wild type under normal conditions, but lbd12-1 had a larger SAM under salt stress. Our findings provide novel insights into the regulatory mechanism of AGO10 by which SAM size is controlled under stress conditions.

The founder-cell transcriptome in the Arabidopsis apetala1 cauliflower inflorescence meristem

BACKGROUND

Although the pattern of lateral organ formation from apical meristems establishes species-specific plant architecture, the positional information that confers cell fate to cells as they transit to the meristem flanks where they differentiate, remains largely unknown. We have combined fluorescence-activated cell sorting and RNA-seq to characterise the cell-type-specific transcriptome at the earliest developmental time-point of lateral organ formation using DORNRÖSCHEN-LIKE::GFP to mark founder-cell populations at the periphery of the inflorescence meristem (IM) in apetala1 cauliflower double mutants, which overproliferate IMs.

RESULTS

Within the lateral organ founder-cell population at the inflorescence meristem, floral primordium identity genes are upregulated and stem-cell identity markers are downregulated. Additional differentially expressed transcripts are involved in polarity generation and boundary formation, and in epigenetic and post-translational changes. However, only subtle transcriptional reprogramming within the global auxin network was observed.

CONCLUSIONS

The transcriptional network of differentially expressed genes supports the hypothesis that lateral organ founder-cell specification involves the creation of polarity from the centre to the periphery of the IM and the establishment of a boundary from surrounding cells, consistent with bract initiation. However, contrary to the established paradigm that sites of auxin response maxima pre-pattern lateral organ initiation in the IM, auxin response might play a minor role in the earliest stages of lateral floral initiation.

A genome-wide analysis of the ASYMMETRIC LEAVES2/LATERAL ORGAN BOUNDARIES (AS2/LOB) gene family in barley (Hordeum vulgare L.)

ASYMMETRIC LEAVES2/LATERAL ORGAN BOUNDARIES (AS2/LOB) genes are a family of plant specific transcription factors, which play an important role in the regulation of plant lateral organ development and metabolism. However, a genome-wide analysis of the AS2/LOB gene family is still not available for barley. In the present study, 24 AS2-like (ASL)/LOB domain (LBD) genes were identified based on the barley (Hordeum vulgare L.) genome sequence. A phylogenetic tree of ASL/LBD proteins from barley, Arabidopsis, maize, and rice was constructed. The ASL/LBD genes were classified into two classes, class I and class II, which were divided into five and two subgroups, respectively. Genes homologous in barley and Arabidopsis were analyzed. In addition, the structure and chromosomal locations of the genes were analyzed. Expression profiles indicated that barley HvASL/LBD genes exhibit a variety of expression patterns, suggesting that they are involved in various aspects of physiological and developmental processes. This genome-wide analysis of the barley AS2/LOB gene family contributes to our understanding of the functions of the AS2/LOB gene family.

SlARF2a plays a negative role in mediating axillary shoot formation

SlARF2a is expressed in most plant organs, including roots, leaves, flowers and fruits. A detailed expression study revealed that SlARF2a is mainly expressed in the leaf nodes and cross-sections of the nodes indicated that SlARF2a expression is restricted to vascular organs. Decapitation or the application of 6-benzylaminopurine (BAP) can initially promote axillary shoots, during which SlARF2a expression is significantly reduced. Down-regulation of SlARF2a expression results in an increased frequency of dicotyledons and significantly increased lateral organ development. Stem anatomy studies have revealed significantly altered cambia and phloem in tomato plants expressing down-regulated levels of ARF2a, which is associated with obvious alterations in auxin distribution. Further analysis has revealed that altered auxin transport may occur via altered pin expression. To identify the interactions of AUX/IAA and TPL with ARF2a, four axillary shoot development repressors that are down-regulated during axillary shoot development, IAA3, IAA9, SlTPL1 and SlTPL6, were tested for their direct interactions with ARF2a. Although none of these repressors are directly involved in ARF2a activity, similar expression patterns of IAA3, IAA9 and ARF2a implied they might work tightly in axillary shoot formation and other developmental processes.

Genome-Wide Identification, Evolutionary Analysis and Expression Profiles of LATERAL ORGAN BOUNDARIES DOMAIN Gene Family in Lotus japonicus and Medicago truncatula

The LATERAL ORGANBOUNDARIESDOMAIN (LBD) gene family has been well-studied in Arabidopsis and play crucial roles in the diverse growth and development processes including establishment and maintenance of boundary of developmental lateral organs. In this study we identified and characterized 38 LBD genes in Lotus japonicus (LjLBD) and 57 LBD genes in Medicago truncatula (MtLBD), both of which are model legume plants that have some specific development features absent in Arabidopsis. The phylogenetic relationships, their locations in the genome, genes structure and conserved motifs were examined. The results revealed that all LjLBD and MtLBD genes could be distinctly divided into two classes: Class I and II. The evolutionary analysis showed that Type I functional divergence with some significantly site-specific shifts may be the main force for the divergence between Class I and Class II. In addition, the expression patterns of LjLBD genes uncovered the diverse functions in plant development. Interestingly, we found that two LjLBD proteins that were highly expressed during compound leaf and pulvinus development, can interact via yeast two-hybrid assays. Taken together, our findings provide an evolutionary and genetic foundation in further understanding the molecular basis of LBD gene family in general, specifically in L. japonicus and M. truncatula.

LOB Domain Proteins: Beyond Lateral Organ Boundaries

LATERAL ORGAN BOUNDARIES DOMAIN (LBD) proteins defined by a conserved LATERAL ORGAN BOUNDARIES (LOB) domain are key regulators of plant organ development. Recent studies have expanded their functional diversity beyond the definition of lateral organ boundaries to pollen development, plant regeneration, photomorphogenesis, pathogen response, and specific developmental functions in non-model plants, such as poplar and legumes. The identification of a range of upstream regulators, protein partners, and downstream targets of LBD family members has unraveled the molecular networks of LBD-dependent processes. Moreover, it has been demonstrated that LBD proteins have essential roles in integrating developmental changes in response to phytohormone signaling or environmental cues. As we discuss here, these novel discoveries of LBD functions and their molecular contexts promote a better understanding of this plant-specific transcription factor family.

Divide et impera: boundaries shape the plant body and initiate new meristems

485 I. 485 II. 486 III. 491 IV. 491 V. 495 495 References 495 SUMMARY: Boundaries, established and maintained in different regions of the plant body, have diverse functions in development. One role is to separate different cell groups, for example the differentiating cells of a leaf primordium from the pluripotent cells of the apical meristem. Boundary zones are also established during compound leaf development, to separate young leaflets from each other, and in many other positions of the plant body. Recent studies have demonstrated that different boundary zones share similar properties. They are characterized by a low rate of cell divisions and specific patterns of gene expression. In addition, the levels of the plant hormones auxin and brassinosteroids are down-regulated in boundary zones, resulting in a low differentiation level of boundary cells. This feature seems to be crucial for a second important role of boundary zones, the formation of new meristems. The primary shoot meristem, as well as secondary and ectopic shoot meristems, initiate from boundary cells that exhibit competence for meristem formation.

Mechanical stress contributes to the expression of the STM homeobox gene in Arabidopsis shoot meristems

The role of mechanical signals in cell identity determination remains poorly explored in tissues. Furthermore, because mechanical stress is widespread, mechanical signals are difficult to uncouple from biochemical-based transduction pathways. Here we focus on the homeobox gene SHOOT MERISTEMLESS (STM), a master regulator and marker of meristematic identity in Arabidopsis. We found that STM expression is quantitatively correlated to curvature in the saddle-shaped boundary domain of the shoot apical meristem. As tissue folding reflects the presence of mechanical stress, we test and demonstrate that STM expression is induced after micromechanical perturbations. We also show that STM expression in the boundary domain is required for organ separation. While STM expression correlates with auxin depletion in this domain, auxin distribution and STM expression can also be uncoupled. STM expression and boundary identity are thus strengthened through a synergy between auxin depletion and an auxin-independent mechanotransduction pathway at the shoot apical meristem.

HANABA TARANU (HAN) Bridges Meristem and Organ Primordia Boundaries through PINHEAD, JAGGED, BLADE-ON-PETIOLE2 and CYTOKININ OXIDASE 3 during Flower Development in Arabidopsis

Shoot organ primordia are initiated from the shoot apical meristem and develop into leaves during the vegetative stage, and into flowers during the reproductive phase. Between the meristem and the newly formed organ primordia, a boundary with specialized cells is formed that separates meristematic activity from determinate organ growth. Despite interactions that have been found between boundary regulators with genes controlling meristem maintenance or primordial development, most boundary studies were performed during embryogenesis or vegetative growth, hence little is known about whether and how boundaries communicate with meristem and organ primordia during the reproductive stage. We combined genetic, molecular and biochemical tools to explore interactions between the boundary gene HANABA TARANU (HAN) and two meristem regulators BREVIPEDICELLUS (BP) and PINHEAD (PNH), and three primordia-specific genes PETAL LOSS (PTL), JAGGED (JAG) and BLADE-ON-PETIOLE (BOP) during flower development. We demonstrated the key role of HAN in determining petal number, as part of a set of complex genetic interactions. HAN and PNH transcriptionally promote each other, and biochemically interact to regulate meristem organization. HAN physically interacts with JAG, and directly stimulates the expression of JAG and BOP2 to regulate floral organ development. Further, HAN directly binds to the promoter and intron of CYTOKININ OXIDASE 3 (CKX3) to modulate cytokinin homeostasis in the boundary. Our data suggest that boundary-expressing HAN communicates with the meristem through the PNH, regulates floral organ development via JAG and BOP2, and maintains boundary morphology through CKX3 during flower development in Arabidopsis.

Cooperative action of the paralogous maize lateral organ boundaries (LOB) domain proteins RTCS and RTCL in shoot-borne root formation

The paralogous maize (Zea mays) LBD (Lateral Organ Boundaries Domain) genes rtcs (rootless concerning crown and seminal roots) and rtcl (rtcs-like) emerged from an ancient whole-genome duplication. RTCS is a key regulator of crown root initiation. The diversity of expression, molecular interaction and phenotype of rtcs and rtcl were investigated. The rtcs and rtcl genes display highly correlated spatio-temporal expression patterns in roots, despite the significantly higher expression of rtcs. Both RTCS and RTCL proteins bind to LBD downstream promoters and act as transcription factors. In line with its auxin inducibility and binding to auxin response elements of rtcs and rtcl promoters, ARF34 (AUXIN RESPONSE FACTOR 34) acts as transcriptional activator. Yeast two-hybrid screening combined with bimolecular fluorescence complementation (BiFC) experiments revealed conserved and unique interaction partners of RTCS and RTCL. The rtcl mutation leads to defective shoot-borne root elongation early in development. Cooperative action of RTCS and RTCL during shoot-borne root formation was demonstrated by rtcs-dependent repression of rtcl transcription in coleoptilar nodes. Although RTCS is instrumental in shoot-borne root initiation, RTCL controls shoot-borne root elongation early in development. Their conserved role in auxin signaling, but diverse function in shoot-borne root formation, is underscored by their conserved and unique interaction partners.

CLAUSA restricts tomato leaf morphogenesis and GOBLET expression

Leaf morphogenesis and differentiation are highly flexible processes. The development of compound leaves is characterized by an extended morphogenesis stage compared with that of simple leaves. The tomato mutant clausa (clau) possesses extremely elaborate compound leaves. Here we show that this elaboration is generated by further extension of the morphogenetic window, partly via the activity of ectopic meristems present on clau leaves. Further, we propose that CLAU might negatively affect expression of the NAM/CUC gene GOBLET (GOB), an important modulator of compound-leaf development, as GOB expression is elevated in clau mutants and reducing GOB expression suppresses the clau phenotype. Expression of GOB is also elevated in the compound leaf mutant lyrate (lyr), and the remarkable enhancement of the clau phenotype by lyr suggests that clau and lyr affect leaf development and GOB in different pathways.

Overexpression of the Arabidopsis thaliana signalling peptide TAXIMIN1 affects lateral organ development

Lateral organ boundary formation is highly regulated by transcription factors and hormones such as auxins and brassinosteroids. However, in contrast to many other developmental processes in plants, no role for signalling peptides in the regulation of this process has been reported yet. The first characterization of the secreted cysteine-rich TAXIMIN (TAX) signalling peptides in Arabidopsis is presented here. TAX1 overexpression resulted in minor alterations in the primary shoot and root metabolome, abnormal fruit morphology, and fusion of the base of cauline leaves to stems forming a decurrent leaf attachment. The phenotypes at the paraclade junction match TAX1 promoter activity in this region and are similar to loss of LATERAL ORGAN FUSION (LOF) transcription factor function. Nevertheless, TAX1 expression was unchanged in lof1lof2 paraclade junctions and, conversely, LOF gene expression was unchanged in TAX1 overexpressing plants, suggesting TAX1 may act independently. This study identifies TAX1 as the first plant signalling peptide influencing lateral organ separation and implicates the existence of a peptide signal cascade regulating this process in Arabidopsis.

Organogenesis in plants: initiation and elaboration of leaves

Plant organs initiate from meristems and grow into diverse forms. After initiation, organs enter a morphological phase where they develop their shape, followed by differentiation into mature tissue. Investigations into these processes have revealed numerous factors necessary for proper development, including transcription factors such as the KNOTTED-LIKE HOMEOBOX (KNOX) genes, the hormone auxin, and miRNAs. Importantly, these factors have been shown to play a role in organogenesis in various diverse model species, revealing both deep conservation of regulatory strategies and evolutionary novelties that led to new plant forms. We review here recent work in understanding the regulation of organogenesis and in particular leaf formation, highlighting how regulatory modules are often redeployed in different organ types and stages of development to achieve diverse forms through the balance of growth and differentiation.

LATERAL ORGAN BOUNDARIES DOMAIN (LBD)10 interacts with SIDECAR POLLEN/LBD27 to control pollen development in Arabidopsis

During male gametophyte development in Arabidopsis thaliana, the microspores undergo an asymmetric division to produce a vegetative cell and a generative cell, which undergoes a second division to give rise to two sperm cells. SIDECAR POLLEN/LATERAL ORGAN BOUNDARIES DOMAIN (LBD) 27 plays a key role in the asymmetric division of microspores. Here we provide molecular genetic evidence that a combinatorial role of LBD10 with LBD27 is crucial for male gametophyte development in Arabidopsis. Expression analysis, genetic transmission and pollen viability assays, and pollen development analysis demonstrated that LBD10 plays a role in the male gametophyte function primarily at germ cell mitosis. In the mature pollen of lbd10 and lbd10 expressing a dominant negative version of LBD10, LBD10:SRDX, aberrant microspores such as bicellular and smaller tricellular pollen appeared at a ratio of 10-15% with a correspondingly decreased ratio of normal tricellular pollen, whereas in lbd27 mutants, 70% of the pollen was aborted. All pollen in the lbd10 lbd27 double mutants was aborted and severely shrivelled compared with that of the single mutants, indicating that LBD10 and LBD27 are essential for pollen development. Gene expression and subcellular localization analyses of LBD10:GFP and LBD27:RFP during pollen development indicated that posttranscriptional and/or posttranslational controls are involved in differential accumulation and subcellular localization of LBD10 and LBD27 during pollen development, which may contribute in part to combinatorial and distinct roles of LBD10 with LBD27 in microspore development. In addition, we showed that LBD10 and LBD27 interact to form a heterodimer for nuclear localization.

Beyond the Divide: Boundaries for Patterning and Stem Cell Regulation in Plants

The initiation of plant lateral organs from the shoot apical meristem (SAM) is closely associated with the formation of specialized domains of restricted growth known as the boundaries. These zones are required in separating the meristem from the growing primordia or adjacent organs but play a much broader role in regulating stem cell activity and shoot patterning. Studies have revealed a network of genes and hormone pathways that establish and maintain boundaries between the SAM and leaves. Recruitment of these pathways is shown to underlie a variety of processes during the reproductive phase including axillary meristems production, flower patterning, fruit development, and organ abscission. This review summarizes the role of conserved gene modules in patterning boundaries throughout the life cycle.

Down-regulation of a LBD-like gene, OsIG1, leads to occurrence of unusual double ovules and developmental abnormalities of various floral organs and megagametophyte in rice

The indeterminate gametophyte1 (ig1) mutation was first characterized to modulate female gametophyte development in maize (Zea mays). However, the function of its rice orthologue, OsIG1, remains unknown. For this, we first analysed OsIG1 localization from differential tissues in rice. Real-time quantitative PCR (qRT-PCR) and histochemical staining results demonstrated that the expression signal of OsIG1 was strongly detected in young inflorescence, moderately in mature flower and weakly in leaf. Furthermore, RNA in situ hybridization analyses exhibited that OsIG1 was strongly expressed in inflorescence meristems, floral meristems, empty-glume- and floret- primordia, especially in the primordia of stamens and immature ovules, and the micropylar side of the mature ovary. In OsIG1-RNAi lines, wrinkled blade formation was accompanied by increased leaf inclination angle. Cross-section further showed that the number of bulliform cells located between the vasculatures was significantly increased, indicating that OsIG1 is involved in division and differentiation of bulliform cell and lateral growth during leaf development. OsIG1-RNAi suppression lines showed pleiotropic phenotypes, including degenerated palea, glume-like features and open hull. In addition, a single OsIG1-RNAi floret is characterized by frequently developing double ovules with abnormal embryo sac development. Additionally, down-regulation of OsIG1 differentially affected the expression of genes associated with the floral organ development including EG1, OsMADS6 and OsMADS1. Taken together, these results demonstrate that OsIG1 plays an essential role in the regulation of empty-glume identity, floral organ number control and female gametophyte development in rice.

The genetic basis of natural variation in seed size and seed number and their trade-off using Arabidopsis thaliana MAGIC lines

Offspring number and size are key traits determining an individual's fitness and a crop's yield. Yet, extensive natural variation within species is observed for these traits. Such variation is typically explained by trade-offs between fecundity and quality, for which an optimal solution is environmentally dependent. Understanding the genetic basis of seed size and number, as well as any possible genetic constraints preventing the maximization of both, is crucial from both an evolutionary and applied perspective. We investigated the genetic basis of natural variation in seed size and number using a set of Arabidopsis thaliana multiparent advanced generation intercross (MAGIC) lines. We also tested whether life history affects seed size, number, and their trade-off. We found that both seed size and seed number are affected by a large number of mostly nonoverlapping QTL, suggesting that seed size and seed number can evolve independently. The allele that increases seed size at most identified QTL is from the same natural accession, indicating past occurrence of directional selection for seed size. Although a significant trade-off between seed size and number is observed, its expression depends on life-history characteristics, and generally explains little variance. We conclude that the trade-off between seed size and number might have a minor role in explaining the maintenance of variation in seed size and number, and that seed size could be a valid target for selection.

Interaction between the GROWTH-REGULATING FACTOR and KNOTTED1-LIKE HOMEOBOX families of transcription factors

KNOTTED1-LIKE HOMEOBOX (KNOX) genes are important regulators of meristem function, and a complex network of transcription factors ensures tight control of their expression. Here, we show that members of the GROWTH-REGULATING FACTOR (GRF) family act as players in this network. A yeast (Saccharomyces cerevisiae) one-hybrid screen with the upstream sequence of the KNOX gene Oskn2 from rice (Oryza sativa) resulted in isolation of OsGRF3 and OsGRF10. Specific binding to a region in the untranslated leader sequence of Oskn2 was confirmed by yeast and in vitro binding assays. ProOskn2:β-glucuronidase reporter expression was down-regulated by OsGRF3 and OsGRF10 in vivo, suggesting that these proteins function as transcriptional repressors. Likewise, we found that the GRF protein BGRF1 from barley (Hordeum vulgare) could act as a repressor on an intron sequence in the KNOX gene Hooded/Barley Knotted3 (Bkn3) and that AtGRF4, AtGRF5, and AtGRF6 from Arabidopsis (Arabidopsis thaliana) could repress KNOTTED-LIKE FROM ARABIDOPSIS THALIANA2 (KNAT2) promoter activity. OsGRF overexpression phenotypes in rice were consistent with aberrant meristematic activity, showing reduced formation of tillers and internodes and extensive adventitious root/shoot formation on nodes. These effects were associated with down-regulation of endogenous Oskn2 expression by OsGRF3. Conversely, RNA interference silencing of OsGRF3, OsGRF4, and OsGRF5 resulted in dwarfism, delayed growth and inflorescence formation, and up-regulation of Oskn2. These data demonstrate conserved interactions between the GRF and KNOX families of transcription factors in both monocot and dicot plants.

AINTEGUMENTA-LIKE proteins: hubs in a plethora of networks

Members of the AINTEGUMENTA-LIKE (AIL) family of APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain transcription factors are expressed in all dividing tissues in the plant, where they have central roles in developmental processes such as embryogenesis, stem cell niche specification, meristem maintenance, organ positioning, and growth. When overexpressed, AIL proteins induce adventitious growth, including somatic embryogenesis and ectopic organ formation. The Arabidopsis (Arabidopsis thaliana) genome contains eight AIL genes, including AINTEGUMENTA, BABY BOOM, and the PLETHORA genes. Studies on these transcription factors have revealed their intricate relationship with auxin as well as their involvement in an increasing number of gene regulatory networks, in which extensive crosstalk and feedback loops have a major role.

GIP1 may act as a coactivator that enhances transcriptional activity of LBD18 in Arabidopsis

The LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) gene family encodes a class of transcription factors harboring a conserved plant-specific lateral organ boundaries domain and plays a key role in lateral organ development of plants. Recent studies have revealed developmental functions of some LBD genes in Arabidopsis, rice, and maize. We have shown previously that LBD18/ASL20 promotes the emergence of lateral roots in Arabidopsis. LBD18 induces EXPANSIN14 (EXP14) expression by binding to a specific region of the EXP14 promoter. To further understand the molecular mechanism of LBD18 acting as a transcription factor, we isolated a protein interacting with LBD18 by screening an Arabidopsis cDNA library using the yeast two-hybrid system with LBD18 as bait. We found that GBF INTERACTING PROTEIN1 (GIP1) interacts with LBD18 in yeast and Arabidopsis protoplasts. Reverse-transcription-polymerase chain reaction analysis showed overlapping expression of GIP1 and LBD18 in various tissues of Arabidopsis such as roots, aerial parts, and rosette leaves. Transient gene expression assay results with Arabidopsis protoplasts indicated that GIP1 enhances transcriptional activity of LBD18 in the EXP14 promoter fused to the GUS reporter gene. These results show that GIP1 may act as a transcriptional coactivator of LBD18.