A model for an early role of auxin in Arabidopsis gynoecium morphogenesis

Gene Regulatory Network Controlling Flower Development in Spinach (Spinacia oleracea L.)

Spinach (Spinacia oleracea L.) is a dioecious, diploid, wind-pollinated crop cultivated worldwide. Sex determination plays an important role in spinach breeding. Hence, this study aimed to understand the differences in sexual differentiation and floral organ development of dioecious flowers, as well as the differences in the regulatory mechanisms of floral organ development of dioecious and monoecious flowers. We compared transcriptional-level differences between different genders and identified differentially expressed genes (DEGs) related to spinach floral development, as well as sex-biased genes to investigate the flower development mechanisms in spinach. In this study, 9189 DEGs were identified among the different genders. DEG analysis showed the participation of four main transcription factor families, MIKC_MADS, MYB, NAC, and bHLH, in spinach flower development. In our key findings, abscisic acid (ABA) and gibberellic acid (GA) signal transduction pathways play major roles in male flower development, while auxin regulates both male and female flower development. By constructing a gene regulatory network (GRN) for floral organ development, core transcription factors (TFs) controlling organ initiation and growth were discovered. This analysis of the development of female, male, and monoecious flowers in spinach provides new insights into the molecular mechanisms of floral organ development and sexual differentiation in dioecious and monoecious plants in spinach.

Two orthogonal differentiation gradients locally coordinate fruit morphogenesis

Morphogenesis requires the coordination of cellular behaviors along developmental axes. In plants, gradients of growth and differentiation are typically established along a single longitudinal primordium axis to control global organ shape. Yet, it remains unclear how these gradients are locally adjusted to regulate the formation of complex organs that consist of diverse tissue types. Here we combine quantitative live imaging at cellular resolution with genetics, and chemical treatments to understand the formation of Arabidopsis thaliana female reproductive organ (gynoecium). We show that, contrary to other aerial organs, gynoecium shape is determined by two orthogonal, time-shifted differentiation gradients. An early mediolateral gradient controls valve morphogenesis while a late, longitudinal gradient regulates style differentiation. Local, tissue-dependent action of these gradients serves to fine-tune the common developmental program governing organ morphogenesis to ensure the specialized function of the gynoecium.

Comparative Transcriptome Analysis Points to the Biological Processes of Hybrid Incompatibility between Brassica napus and B. oleracea

Improving Brassica napus via introgression of the genome components from its parental species, B. oleracea and B. rapa, is an important breeding strategy. Interspecific hybridization between B. napus and B. rapa is compatible with high rate of survival ovules, while the hybridization between B. napus and B. oleracea is incompatible with the high occurrence of embryo abortion. To understand the diverse embryo fate in the two interspecific hybridizations, here, the siliques of B. napus pollinated with B. oleracea (AE) and B. rapa (NE) were employed for transcriptome sequencing at 8 and 16 days after pollination. Compared to NE and the parental line of B. napus, more specific differentially expressed genes (DEGs) (1274 and 1698) were obtained in AE and the parental line of B. napus at 8 and 16 days after pollination (DAP). These numbers were 51 and 5.8 times higher than the number of specific DEGs in NE and parental line of B. napus at 8 and 16 DAP, respectively, suggesting more complex transcriptional changes in AE. Most of DEGs in the terms of cell growth and cell wall formation exhibited down-regulated expression patterns (96(down)/131(all) in AE8, 174(down)/235(all) in AE16), while most of DEGs in the processes of photosynthesis, photorespiration, peroxisome, oxidative stress, and systemic acquired resistance exhibited up-regulated expression patterns (222(up)/304(all) in AE8, 214(up)/287(all) in AE16). This is in accordance with a high level of reactive oxygen species (ROS) in the siliques of B. napus pollinated with B. oleracea. Our data suggest that the disorder of plant hormone metabolism, retardation of cell morphogenesis, and the accumulation of ROS may be associated with hybrid incompatibility between B. napus and B. oleracea.

A model worker: Multifaceted modulation of AUXIN RESPONSE FACTOR3 orchestrates plant reproductive phases

The key phytohormone auxin is involved in practically every aspect of plant growth and development. Auxin regulates these processes by controlling gene expression through functionally distinct AUXIN RESPONSE FACTORs (ARFs). As a noncanonical ARF, ARF3/ETTIN (ETT) mediates auxin responses to orchestrate multiple developmental processes during the reproductive phase. The arf3 mutation has pleiotropic effects on reproductive development, causing abnormalities in meristem homeostasis, floral determinacy, phyllotaxy, floral organ patterning, gynoecium morphogenesis, ovule development, and self-incompatibility. The importance of ARF3 is also reflected in its precise regulation at the transcriptional, posttranscriptional, translational, and epigenetic levels. Recent studies have shown that ARF3 controls dynamic shoot apical meristem (SAM) maintenance in a non-cell autonomous manner. Here, we summarize the hierarchical regulatory mechanisms by which ARF3 is regulated and the diverse roles of ARF3 regulating developmental processes during the reproductive phase.

Case not closed: the mystery of the origin of the carpel

The carpel is a fascinating structure that plays a critical role in flowering plant reproduction and contributed greatly to the evolutionary success and diversification of flowering plants. The remarkable feature of the carpel is that it is a closed structure that envelopes the ovules and after fertilization develops into the fruit which protects, helps disperse, and supports seed development into a new plant. Nearly all plant-based foods are either derived from a flowering plant or are a direct product of the carpel. Given its importance it's no surprise that plant and evolutionary biologists have been trying to explain the origin of the carpel for a long time. Before carpel evolution seeds were produced on open leaf-like structures that are exposed to the environment. When the carpel evolved in the stem lineage of flowering plants, seeds became protected within its closed structure. The evolutionary transition from that open precursor to the closed carpel remains one of the greatest mysteries of plant evolution. In recent years, we have begun to complete a picture of what the first carpels might have looked like. On the other hand, there are still many gaps in our understanding of what the precursor of the carpel looked like and what changes to its developmental mechanisms allowed for this evolutionary transition. This review aims to present an overview of existing theories of carpel evolution with a particular emphasis on those that account for the structures that preceded the carpel and/or present testable developmental hypotheses. In the second part insights from the development and evolution of diverse plant organs are gathered to build a developmental hypothesis for the evolutionary transition from a hypothesized laminar open structure to the closed structure of the carpel.

The leaf polarity factors SGS3 and YABBYs regulate style elongation through auxin signaling in Mimulus lewisii

Style length is a major determinant of breeding strategies in flowering plants and can vary dramatically between and within species. However, little is known about the genetic and developmental control of style elongation. We characterized the role of two classes of leaf adaxial-abaxial polarity factors, SUPPRESSOR OF GENE SILENCING3 (SGS3) and the YABBY family transcription factors, in the regulation of style elongation in Mimulus lewisii. We also examined the spatiotemporal patterns of auxin response during style development. Loss of SGS3 function led to reduced style length via limiting cell division, and downregulation of YABBY genes by RNA interference resulted in shorter styles by decreasing both cell division and cell elongation. We discovered an auxin response minimum between the stigma and ovary during the early stages of pistil development that marks style differentiation. Subsequent redistribution of auxin response to this region was correlated with style elongation. Auxin response was substantially altered when both SGS3 and YABBY functions were disrupted. We suggest that auxin signaling plays a central role in style elongation and that the way in which auxin signaling controls the different cell division and elongation patterns underpinning natural style length variation is a major question for future research.

The Relationship between AGAMOUS and Cytokinin Signaling in the Establishment of Carpeloid Features

Gynoecium development is dependent on gene regulation and hormonal pathway interactions. The phytohormones auxin and cytokinin are involved in many developmental programs, where cytokinin is normally important for cell division and meristem activity, while auxin induces cell differentiation and organ initiation in the shoot. The MADS-box transcription factor AGAMOUS (AG) is important for the development of the reproductive structures of the flower. Here, we focus on the relationship between AG and cytokinin in Arabidopsis thaliana, and use the weak ag-12 and the strong ag-1 allele. We found that cytokinin induces carpeloid features in an AG-dependent manner and the expression of the transcription factors CRC, SHP2, and SPT that are involved in carpel development. AG is important for gynoecium development, and contributes to regulating, or else directly regulates CRC, SHP2, and SPT. All four genes respond to either reduced or induced cytokinin signaling and have the potential to be regulated by cytokinin via the type-B ARR proteins. We generated a model of a gene regulatory network, where cytokinin signaling is mainly upstream and in parallel with AG activity.

Comprehensive Analysis and Expression Profiling of PIN, AUX/LAX, and ABCB Auxin Transporter Gene Families in Solanum tuberosum under Phytohormone Stimuli and Abiotic Stresses

Simple Summary

In this study, we provide comprehensive information on auxin transporter gene families in potato, including basic parameters, chromosomal distribution, phylogeny, co-expression network analysis, gene structure, tissue-specific expression patterns, subcellular localization, transcription analysis under exogenous hormone stimuli and abiotic stresses, and cis-regulatory element prediction. The responsiveness of auxin transporter family genes to auxin and polar auxin transport inhibitors implied their possible roles in auxin homoeostasis and redistribution. Additionally, the differential expression levels of auxin transporter family genes in response to abscisic acid and abiotic stresses suggested their specific adaptive mechanisms on tolerance to various environmental stimuli. Promoter cis-regulatory element description analyses indicated that a number of cis-regulatory elements within the promoters of auxin transporter genes in potato were targeted by relevant transcription factors to respond to diverse stresses. We are confident that our results provide a foundation for a better understanding of auxin transporters in potato, as we have demonstrated the biological significance of this family of genes in hormone signaling and adaption to environmental stresses.

Abstract

Auxin is the only plant hormone that exhibits transport polarity mediated by three families: auxin resistant (AUX) 1/like AUX1 (LAX) influx carriers, pin-formed (PIN) efflux carriers, and ATP-binding cassette B (ABCB) influx/efflux carriers. Extensive studies about the biological functions of auxin transporter genes have been reported in model plants. Information regarding these genes in potato remains scarce. Here, we conducted a comprehensive analysis of auxin transporter gene families in potato to examine genomic distributions, phylogeny, co-expression analysis, gene structure and subcellular localization, and expression profiling using bioinformatics tools and qRT-PCR analysis. From these analyses, 5 StLAXs, 10 StPINs, and 22 StABCBs were identified in the potato genome and distributed in 10 of 18 gene modules correlating to the development of various tissues. Transient expression experiments indicated that three representative auxin transporters showed plasma membrane localizations. The responsiveness to auxin and auxin transport inhibitors implied their possible roles in mediating intercellular auxin homoeostasis and redistribution. The differential expression under abscisic acid and abiotic stresses indicated their specific adaptive mechanisms regulating tolerance to environmental stimuli. A large number of auxin-responsive and stress-related cis-elements within their promoters could account for their responsiveness to diverse stresses. Our study aimed to understand the biological significance of potato auxin transporters in hormone signaling and tolerance to environmental stresses.

Clonal sector analysis and cell ablation confirm a function for DORNROESCHEN-LIKE in founder cells and the vasculature in Arabidopsis

Inducible lineage analysis and cell ablation via conditional toxin expression in cells expressing the DORNRÖSCHEN-LIKE transcription factor represent an effective and complementary adjunct to conventional methods of functional gene analysis. Classical methods of functional gene analysis via mutational and expression studies possess inherent limitations, and therefore, the function of a large proportion of transcription factors remains unknown. We have employed two complementary, indirect methods to obtain functional information for the AP2/ERF transcription factor DORNRÖSCHEN-LIKE (DRNL), which is dynamically expressed in flowers and marks lateral organ founder cells. An inducible, two-component Cre-Lox system was used to express beta-glucuronidase GUS in cells expressing DRNL, to perform a sector analysis that reveals lineages of cells that transiently expressed DRNL throughout plant development. In a complementary approach, an inducible system was used to ablate cells expressing DRNL using diphtheria toxin A chain, to visualise the phenotypic consequences. These complementary analyses demonstrate that DRNL functionally marks founder cells of leaves and floral organs. Clonal sectors also included the vasculature of the leaves and petals, implicating a previously unidentified role for DRNL in provasculature development, which was confirmed in cotyledons by closer analysis of drnl mutants. Our findings demonstrate that inducible gene-specific lineage analysis and cell ablation via conditional toxin expression represent an effective and informative adjunct to conventional methods of functional gene analysis.

Auxin regulation involved in gynoecium morphogenesis of papaya flowers

The morphogenesis of gynoecium is crucial for propagation and productivity of fruit crops. For trioecious papaya (Carica papaya), highly differentiated morphology of gynoecium in flowers of different sex types is controlled by gene networks and influenced by environmental factors, but the regulatory mechanism in gynoecium morphogenesis is unclear. Gynodioecious and dioecious papaya varieties were used for analysis of differentially expressed genes followed by experiments using auxin and an auxin transporter inhibitor. We first compared differential gene expression in functional and rudimentary gynoecium at early stage of their development and detected significant difference in phytohormone modulating and transduction processes, particularly auxin. Enhanced auxin signal transduction in rudimentary gynoecium was observed. To determine the role auxin plays in the papaya gynoecium, auxin transport inhibitor (N-1-Naphthylphthalamic acid, NPA) and synthetic auxin analogs with different concentrations gradient were sprayed to the trunk apex of male and female plants of dioecious papaya. Weakening of auxin transport by 10 mg/L NPA treatment resulted in female fertility restoration in male flowers, while female flowers did not show changes. NPA treatment with higher concentration (30 and 50 mg/L) caused deformed flowers in both male and female plants. We hypothesize that the occurrence of rudimentary gynoecium patterning might associate with auxin homeostasis alteration. Proper auxin concentration and auxin homeostasis might be crucial for functional gynoecium morphogenesis in papaya flowers. These results will lead to further investigation on the auxin homeostasis and gynoecium morphogenesis in papaya.

A galling insect activates plant reproductive programs during gall development

Many insect species have acquired the ability to redirect plant development to form unique organs called galls, which provide these insects with unique, enhanced food and protection from enemies and the elements. Many galls resemble flowers or fruits, suggesting that elements of reproductive development may be involved. We tested this hypothesis using RNA sequencing to quantify the transcriptional responses of wild grapevine (Vitis riparia) leaves to a galling parasite, phylloxera (Daktulosphaira vitifoliae). If development of reproductive structures is part of gall formation, we expected to find significantly elevated expression of genes involved in flower and/or fruit development in developing galls as opposed to ungalled leaves. We found that reproductive gene ontology categories were significantly enriched in developing galls, and that expression of many candidate genes involved in floral development were significantly increased, particularly in later gall stages. The patterns of gene expression found in galls suggest that phylloxera exploits vascular cambium to provide meristematic tissue and redirects leaf development towards formation of carpels. The phylloxera leaf gall appears to be phenotypically and transcriptionally similar to the carpel, due to the parasite hijacking underlying genetic machinery in the host plant.

Histone deacetylases HDA6 and HDA9 coordinately regulate valve cell elongation through affecting auxin signaling in Arabidopsis

Both Histone Deacetylases HDA6 and HDA9 belong to class I subfamily of RPD3/HDA1 HDACs. Loss-of-function mutants of HDA9 form slightly blunt siliques. However, the involvement of HDA6 in regulating silique tip growth is unclear. In this study, we show that HDA6 acts redundantly with HDA9 in regulating the elongation of valve cells in the silique tip. Although the hda6 single mutant does not exhibit a detectable silique phenotype, the silique tip of hda6 hda9 double mutant displays a more severe bulge, a morphology we termed as "nock-shaped". The valve cells of the silique tip of hda9 are longer than wild-type, and loss of HDA6 in hda9 enhances the valve cell elongation phenotype. The transcript levels of auxin-signaling-related genes are mis-regulated in hda9 and hda6 hda9 siliques, and the GFP reporter driven by the auxin response promoter DR5 is weaker in hda9 or hda6 hda9 than wild-type or hda6. Thus, our findings reveal that HDA6 and HDA9 coordinately control the elongation of silique valve cells through regulating the expression of auxin-related genes in silique tips.

Class XI Myosins Contribute to Auxin Response and Senescence-Induced Cell Death in Arabidopsis

The integrity and dynamics of actin cytoskeleton is necessary not only for plant cell architecture but also for membrane trafficking-mediated processes such as polar auxin transport, senescence, and cell death. In Arabidopsis, the inactivation of actin-based molecular motors, class XI myosins, affects the membrane trafficking and integrity of actin cytoskeleton, and thus causes defective plant growth and morphology, altered lifespan and reduced fertility. To evaluate the potential contribution of class XI myosins to the auxin response, senescence and cell death, we followed the flower and leaf development in the triple gene knockout mutant xi1 xi2 xik (3KO) and in rescued line stably expressing myosin XI-K:YFP (3KOR). Assessing the development of primary inflorescence shoots we found that the 3KO plants produced more axillary branches. Exploiting the auxin-dependent reporters DR5::GUS and IAA2::GUS, a significant reduction in auxin responsiveness was found throughout the development of the 3KO plants. Examination of the flower development of the plants stably expressing the auxin transporter PIN1::PIN1-GFP revealed partial loss of PIN1 polarization in developing 3KO pistils. Surprisingly, the stable expression of PIN1::PIN1-GFP significantly enhanced the semi-sterile phenotype of the 3KO plants. Further we investigated the localization of myosin XI-K:YFP in the 3KOR floral organs and revealed its expression pattern in floral primordia, developing pistils, and anther filaments. Interestingly, the XI-K:YFP and PIN1::PIN1-GFP shared partially overlapping but distinct expression patterns throughout floral development. Assessing the foliar development of the 3KO plants revealed increased rosette leaf production with signs of premature yellowing. Symptoms of the premature senescence correlated with massive loss of chlorophyll, increased cell death, early plasmolysis of epidermal cells, and strong up-regulation of the stress-inducible senescence-associated gene SAG13 in 3KO plants. Simultaneously, the reduced auxin responsiveness and premature leaf senescence were accompanied by significant anthocyanin accumulation in 3KO tissues. Collectively, our results provide genetic evidences that Arabidopsis class XI myosins arrange the flower morphogenesis and leaf longevity via contributing to auxin responses, leaf senescence, and cell death.

Evidence for the Regulation of Gynoecium Morphogenesis by ETTIN via Cell Wall Dynamics

ETTIN (ETT) is an atypical member of the AUXIN RESPONSE FACTOR family of transcription factors that plays a crucial role in tissue patterning in the Arabidopsis (Arabidopsis thaliana) gynoecium. Though recent insights have provided valuable information on ETT's interactions with other components of auxin signaling, the biophysical mechanisms linking ETT to its ultimate effects on gynoecium morphology were until now unknown. Here, using techniques to assess cell-wall dynamics during gynoecium growth and development, we provide a coherent body of evidence to support a model in which ETT controls the elongation of the valve tissues of the gynoecium through the positive regulation of pectin methylesterase (PME) activity in the cell wall. This increase in PME activity results in an increase in the level of demethylesterified pectins and a consequent reduction in cell wall stiffness, leading to elongation of the valves. Though similar biophysical mechanisms have been shown to act in the stem apical meristem, leading to the expansion of organ primordia, our findings demonstrate that regulation of cell wall stiffness through the covalent modification of pectin also contributes to tissue patterning within a developing plant organ.

Developmental cartography: coordination via hormonal and genetic interactions during gynoecium formation

Development in multicellular organisms requires the establishment of tissue identity through polarity cues. The Arabidopsis gynoecium presents an excellent model to study this coordination, as it comprises a complex tissue structure which is established through multiple polarity systems. The gynoecium is derived from the fusion of two carpels and forms in the centre of the flower. Many regulators of carpel development also have roles in leaf development, emphasizing the evolutionary origin of carpels as modified leaves. The gynoecium can therefore be considered as having evolved from a simple setup followed by adjustment in tissue polarity to facilitate efficient reproduction. Here, we discuss concepts to understand how hormonal and genetic systems interact to pattern the gynoecium.

Cytokinin-Auxin Crosstalk in the Gynoecial Primordium Ensures Correct Domain Patterning

The Arabidopsis (Arabidopsis thaliana) gynoecium consists of two congenitally fused carpels made up of two lateral valve domains and two medial domains, which retain meristematic properties and later fuse to produce the female reproductive structures vital for fertilization. Polar auxin transport (PAT) is important for setting up distinct apical auxin signaling domains in the early floral meristem remnants allowing for lateral domain identity and outgrowth. Crosstalk between auxin and cytokinin plays an important role in the development of other meristematic tissues, but hormone interaction studies to date have focused on more accessible later-stage gynoecia and the spatiotemporal interactions pivotal for patterning of early gynoecium primordia remain unknown. Focusing on the earliest stages, we propose a cytokinin-auxin feedback model during early gynoecium patterning and hormone homeostasis. Our results suggest that cytokinin positively regulates auxin signaling in the incipient gynoecial primordium and strengthen the concept that cytokinin regulates auxin homeostasis during gynoecium development. Specifically, medial cytokinin promotes auxin biosynthesis components [YUCCA1/4 (YUC1/4)] in, and PINFORMED7 (PIN7)-mediated auxin efflux from, the medial domain. The resulting laterally focused auxin signaling triggers ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN6 (AHP6), which then represses cytokinin signaling in a PAT-dependent feedback. Cytokinin also down-regulates PIN3, promoting auxin accumulation in the apex. The yuc1, yuc4, and ahp6 mutants are hypersensitive to exogenous cytokinin and 1-napthylphthalamic acid (NPA), highlighting their role in mediolateral gynoecium patterning. In summary, these mechanisms self-regulate cytokinin and auxin signaling domains, ensuring correct domain specification and gynoecium development.

Fine-tuning of auxin homeostasis governs the transition from floral stem cell maintenance to gynoecium formation

To ensure successful plant reproduction and crop production, the spatial and temporal control of the termination of the floral meristem must be coordinated. In Arabidopsis, the timing of this termination is determined by AGAMOUS (AG). Following its termination, the floral meristem underdoes gynoecium formation. A direct target of AG, CRABS CLAW (CRC), is involved in both floral meristem determinacy and gynoecium development. However, how floral meristem termination is coordinated with gynoecium formation is not understood. Here, we identify a mechanistic link between floral meristem termination and gynoecium development through fine-tuning of auxin homeostasis by CRC. CRC controls auxin homeostasis in the medial region of the developing gynoecium to generate proper auxin maxima. This regulation partially occurs via direct transcriptional repression of TORNADO2 (TRN2) by CRC. Plasma membrane-localized TRN2 modulates auxin homeostasis. We propose a model describing how regulation of auxin homeostasis mediates the transition from floral meristem termination to gynoecium development.

In Arabidopsis, the timing of floral meristem termination is determined by AGAMOUS. Here, the authors show that the CRC transcription factor, itself a direct target of AGAMOUS, coordinates meristem termination with subsequent gynoecium formation partly by repressing TRN2 expression and regulating auxin homeostasis.

De novo Transcriptome Profiling of Flowers, Flower Pedicels and Pods of Lupinus luteus (Yellow Lupine) Reveals Complex Expression Changes during Organ Abscission

Yellow lupine (Lupinus luteus L., Taper c.), a member of the legume family (Fabaceae L.), has an enormous practical importance. Its excessive flower and pod abscission represents an economic drawback, as proper flower and seed formation and development is crucial for the plant's productivity. Generative organ detachment takes place at the basis of the pedicels, within a specialized group of cells collectively known as the abscission zone (AZ). During plant growth these cells become competent to respond to specific signals that trigger separation and lead to the abolition of cell wall adhesion. Little is known about the molecular network controlling the yellow lupine organ abscission. The aim of our study was to establish the divergences and similarities in transcriptional networks in the pods, flowers and flower pedicels abscised or maintained on the plant, and to identify genes playing key roles in generative organ abscission in yellow lupine. Based on de novo transcriptome assembly, we identified 166,473 unigenes representing 219,514 assembled unique transcripts from flowers, flower pedicels and pods undergoing abscission and from control organs. Comparison of the cDNA libraries from dropped and control organs helped in identifying 1,343, 2,933 and 1,491 differentially expressed genes (DEGs) in the flowers, flower pedicels and pods, respectively. In DEG analyses, we focused on genes involved in phytohormonal regulation, cell wall functioning and metabolic pathways. Our results indicate that auxin, ethylene and gibberellins are some of the main factors engaged in generative organ abscission. Identified 28 DEGs common for all library comparisons are involved in cell wall functioning, protein metabolism, water homeostasis and stress response. Interestingly, among the common DEGs we also found an miR169 precursor, which is the first evidence of micro RNA engaged in abscission. A KEGG pathway enrichment analysis revealed that the identified DEGs were predominantly involved in carbohydrate and amino acid metabolism, but some other pathways were also targeted. This study represents the first comprehensive transcriptome-based characterization of organ abscission in L. luteus and provides a valuable data source not only for understanding the abscission signaling pathway in yellow lupine, but also for further research aimed at improving crop yields.

Development of Incompletely Fused Carpels in Maize Ovary Revealed by miRNA, Target Gene and Phytohormone Analysis

Although the molecular basis of carpel fusion in maize ovary development remains largely unknown, increasing evidence suggests a critical role of microRNAs (miRNAs). In this study, a combination of miRNA sequencing, degradome and physiological analyses was used to characterize carpel fusion development in maize ovaries showing incompletely (IFC) and completely fused carpels (CFC). A total of 162 known miRNAs distributed across 33 families were identified, of which 20 were differentially expressed. In addition, 53 miRNA candidates were identified, of which 10 were differentially expressed in the IFC and CFC ovaries. In degradome analysis, a total of 113 and 11 target genes were predicted for the known and novel miRNAs, respectively. Moreover, 24 (60%) target genes of the differentially expressed known miRNAs were found to code transcription factors, including auxin response factor (ARF), TB1-CYC-PCFs (TCP), APETALA2 (AP2), growth regulating factor (GRF), MYB, NAC, and NF-YA, all of which have been shown to play a role in carpel fusion development. Correlation analysis of these differentially expressed known miRNAs and their targets with phytohormone signals revealed significant correlations with at least one phytohormone signal, the main regulator of carpel fusion development. These results suggest that incomplete carpel fusion is partly the result of differential expression of certain miRNAs and their targets. Overall, these findings improve our knowledge of the effect of miRNA regulation on target expression, providing a useful resource for further analysis of the interactions between miRNAs, target genes and phytohormones during carpel fusion development in maize.

Genome-wide analysis of auxin transport genes identifies the hormone responsive patterns associated with leafy head formation in Chinese cabbage

Auxin resistant 1/like aux1 (AUX/LAX), pin-formed (PIN) and ATP binding cassette subfamily B (ABCB/MDR/PGP) are three families of auxin transport genes. The development-related functions of the influx and efflux carriers have been well studied and characterized in model plants. However, there is scant information regarding the functions of auxin genes in Chinese cabbage and the responses of exogenous polar auxin transport inhibitors (PATIs). We conducted a whole-genome annotation and a bioinformatics analysis of BrAUX/LAX, BrPIN, and BrPGP genes in Chinese cabbage. By analyzing the expression patterns at several developmental stages in the formation of heading leaves, we found that most auxin-associate genes were expressed throughout the entire process of leafy head formation, suggesting that these genes played important roles in the development of heads. UPLC was used to detect the distinct and uneven distribution of auxin in various segments of the leafy head and in response to PATI treatment, indicated that the formation of the leafy head depends on polar auxin transport and the uneven distribution of auxin in leaves. This study provides new insight into auxin polar transporters and the possible roles of the BrLAX, BrPIN and BrPGP genes in leafy head formation in Chinese cabbage.

MicroRNA 157-targeted SPL genes regulate floral organ size and ovule production in cotton

BACKGROUND

microRNAs (miRNAs) have been involved in regulation of diverse spectrum of plant development processes in many species. In cotton, few miRNAs have been well characterised in floral organ development. Floral organ, which should be finely tuned, is a crucial factor affecting the yield of cotton. Therefore, it is well worth revealing the function of miRNAs in regulation of floral organ development. Here, we report the role of miRNA156/157 in regulation of floral organ size in cotton.

RESULTS

Over-expression of the GhmiRNA157 precursor in cotton (Gossypium hirsutum) resulted in smaller floral organs, fewer ovules and decreased seed production due to suppression of cell proliferation and cell elongation. Five SQUAMOSA promoter-binding protein-like (SPL) genes were identified as targets of GhmiRNA157 using a RNA ligase-mediated rapid amplification of cDNA end approach, and the expression level of miR157-targeted GhSPLs decreased in the miR157 over-expression lines, indicating the presence of the miR157/SPL axis in cotton. Two MADS-box genes, orthologs of AtAGL6 and SITDR8, which are associated with floral organ development and reproductive production, were repressed in the miR157 over-expression lines. In addition, auxin-inducible genes were also down-regulated, and auxin signal visualized by a DR5::GUS reporter was attenuated in the miR157 over-expression lines.

CONCLUSIONS

Our results indicate that the miR157/SPL axis controls floral organ growth and ovule production by regulating MADS-box genes and auxin signal transduction. The work further elucidates the mechanism of floral organ development and provides helpful molecular basis for improvement of cotton yield.

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.

A common miRNA160-based mechanism regulates ovary patterning, floral organ abscission and lamina outgrowth in tomato

Plant microRNAs play vital roles in auxin signaling via the negative regulation of auxin response factors (ARFs). Studies have shown that targeting of ARF10/16/17 by miR160 is indispensable for various aspects of development, but its functions in the model crop tomato (Solanum lycopersicum) are unknown. Here we knocked down miR160 (sly-miR160) using a short tandem target mimic (STTM160), and investigated its roles in tomato development. Northern blot analysis showed that miR160 is abundant in developing ovaries. In line with this, its down-regulation perturbed ovary patterning as indicated by the excessive elongation of the proximal ends of mutant ovaries and thinning of the placenta. Following fertilization, these morphological changes led to formation of elongated, pear-shaped fruits reminiscent of those of the tomato ovate mutant. In addition, STTM160-expressing plants displayed abnormal floral organ abscission, and produced leaves, sepals and petals with diminished blades, indicating a requirement for sly-miR160 for these auxin-mediated processes. We found that sly-miR160 depletion was always associated with the up-regulation of SlARF10A, SlARF10B and SlARF17, of which the expression of SlARF10A increased the most. Despite the sly-miR160 legitimate site of SlARF16A, its mRNA levels did not change in response to sly-miR160 down-regulation, suggesting that it may be regulated by a mechanism other than mRNA cleavage. SlARF10A and SlARF17 were previously suggested to function as inhibiting ARFs. We propose that by adjusting the expression of a group of ARF repressors, of which SlARF10A is a primary target, sly-miR160 regulates auxin-mediated ovary patterning as well as floral organ abscission and lateral organ lamina outgrowth.

Transcriptional Responses to the Auxin Hormone

Auxin is arguably the most important signaling molecule in plants, and the last few decades have seen remarkable breakthroughs in understanding its production, transport, and perception. Recent investigations have focused on transcriptional responses to auxin, providing novel insight into the functions of the domains of key transcription regulators in responses to the hormonal cue and prominently implicating chromatin regulation in these responses. In addition, studies are beginning to identify direct targets of the auxin-responsive transcription factors that underlie auxin modulation of development. Mechanisms to tune the response to different auxin levels are emerging, as are first insights into how this single hormone can trigger diverse responses. Key unanswered questions center on the mechanism for auxin-directed transcriptional repression and the identity of additional determinants of auxin response specificity. Much of what has been learned in model plants holds true in other species, including the earliest land plants.

Hormonal control of the development of the gynoecium

Flowering plants are called angiosperms and most of their flowers produce at their center a pistil or a gynoecium, which is the female reproductive structure. After a double fertilization event, the gynoecium develops into a fruit with great importance for the plant because it protects and helps the dispersion of a new generation, and, for humans is a key nutritional source. Over 20 years, Arabidopsis thaliana has been used to discover important genes for gynoecium development, and in the early years, auxin was already proposed to play a role. More recently, new discoveries are unveiling the importance of other hormones, particularly cytokinins, and providing insights about the action of these hormones in gynoecium development, which is the focus of this review. One of the next challenges is to further refine the knowledge about the mechanisms by which hormones shape the gynoecium, understand the communication among them and their interactions with transcription factors that altogether guide gynoecium development.

Transcriptional programs regulated by both LEAFY and APETALA1 at the time of flower formation

Two key regulators of the switch to flower formation and of flower patterning in Arabidopsis are the plant-specific helix-turn-helix transcription factor LEAFY (LFY) and the MADS box transcription factor APETALA1 (AP1). The interactions between these two transcriptional regulators are complex. AP1 is both a direct target of LFY and can act in parallel with LFY. Available genetic and molecular evidence suggests that LFY and AP1 together orchestrate the switch to flower formation and early events during flower morphogenesis by altering transcriptional programs. However, very little is known about target genes regulated by both transcription factors. Here, we performed a meta-analysis of public datasets to identify genes that are likely to be regulated by both LFY and AP1. Our analyses uncovered known and novel direct LFY and AP1 targets with a role in the control of onset of flower formation. It also identified additional families of proteins and regulatory pathways that may be under transcriptional control by both transcription factors. In particular, several of these genes are linked to response to hormones, to transport and to development. Finally, we show that the gibberellin catabolism enzyme ELA1, which was recently shown to be important for the timing of the switch to flower formation, is positively feedback-regulated by AP1. Our study contributes to the elucidation of the regulatory network that leads to formation of a vital plant organ system, the flower.

Auxin and cytokinin act during gynoecial patterning and the development of ovules from the meristematic medial domain

The gynoecium is the female reproductive structure of flowering plants, and is the site of ovule and seed development. The gynoecium is critical for reproductive competence and for agricultural productivity in many crop plants. In this review we focus on molecular aspects of the development of the Arabidopsis thaliana gynoecium. We briefly introduce gynoecium structure and development and then focus on important research advances published within the last year. We highlight what has been learned recently with respect to: (1) the role of auxin in the differential development of the medial and lateral domains of the Arabidopsis gynoecium; (2) the interaction between cytokinin and auxin during gynoecial development; (3) the role of auxin in the termination of the floral meristem and in the transition of floral meristem to gynoecium; and (4) recent studies that suggest a degree of evolutionary conservation of auxin mechanisms during gynoecial development in other eudicots.

Control of patterning, growth, and differentiation by floral organ identity genes

In spite of the different morphologies of sepals, petals, stamens, and carpels, all these floral organs are believed to be modified versions of a ground-state organ similar to the leaf. Modifications of the ground-state developmental programme are orchestrated by different combinations of MADS-domain transcription factors encoded by floral organ identity genes. In recent years, much has been revealed about the gene regulatory networks controlled by the floral organ identity genes and about the genetic pathways that control leaf development. This review examines how floral organ identity is connected with the control of morphogenesis and differentiation of shoot organs, focusing on the model species Arabidopsis thaliana. Direct links have emerged between floral organ identity genes and genes involved in abaxial-adaxial patterning, organ boundary formation, tissue growth, and cell differentiation. In parallel, predictive models have been developed to explain how the activity of regulatory genes can be coordinated by intercellular signalling and constrained by tissue mechanics. When combined, these advances provide a unique opportunity for revealing exactly how leaf-like organs have been 'metamorphosed' into floral organs during evolution and showing crucial regulatory points in the generation of plant form.

The yin-yang of hormones: cytokinin and auxin interactions in plant development

The phytohormones auxin and cytokinin interact to regulate many plant growth and developmental processes. Elements involved in the biosynthesis, inactivation, transport, perception, and signaling of these hormones have been elucidated, revealing the variety of mechanisms by which signal output from these pathways can be regulated. Recent studies shed light on how these hormones interact with each other to promote and maintain plant growth and development. In this review, we focus on the interaction of auxin and cytokinin in several developmental contexts, including its role in regulating apical meristems, the patterning of the root, the development of the gynoecium and female gametophyte, and organogenesis and phyllotaxy in the shoot.

Polar auxin transport is essential for medial versus lateral tissue specification and vascular-mediated valve outgrowth in Arabidopsis gynoecia

Although it is generally accepted that auxin is important for the patterning of the female reproductive organ, the gynoecium, the flow as well as the temporal and spatial actions of auxin have been difficult to show during early gynoecial development. The primordium of the Arabidopsis (Arabidopsis thaliana) gynoecium is composed of two congenitally fused, laterally positioned carpel primordia bisected by two medially positioned meristematic regions that give rise to apical and internal tissues, including the ovules. This organization makes the gynoecium one of the most complex plant structures, and as such, the regulation of its development has remained largely elusive. By determining the spatiotemporal expression of auxin response reporters and localization of PINFORMED (PIN) auxin efflux carriers, we have been able to create a map of the auxin flow during the earliest stages of gynoecial primordium initiation and outgrowth. We show that transient disruption of polar auxin transport (PAT) results in ectopic auxin responses, broadened expression domains of medial tissue markers, and disturbed lateral preprocambium initiation. Based on these results, we propose a new model of auxin-mediated gynoecial patterning, suggesting that valve outgrowth depends on PIN1-mediated lateral auxin maxima as well as subsequent internal auxin drainage and provascular formation, whereas the growth of the medial domains is less dependent on correct PAT. In addition, PAT is required to prevent the lateral domains, at least in the apical portion of the gynoecial primordium, from obtaining medial fates.