The Lateral suppressor (Ls) gene of tomato encodes a new member of the VHIID protein family

Genetic Regulation of Shoot Architecture

Shoot architecture is determined by the organization and activities of apical, axillary, intercalary, secondary, and inflorescence meristems and by the subsequent development of stems, leaves, shoot branches, and inflorescences. In this review, we discuss the unifying principles of hormonal and genetic control of shoot architecture including advances in our understanding of lateral branch outgrowth; control of stem elongation, thickness, and angle; and regulation of inflorescence development. We focus on recent progress made mainly in Arabidopsis thaliana, rice, pea, maize, and tomato, including the identification of new genes and mechanisms controlling shoot architecture. Key advances include elucidation of mechanisms by which strigolactones, auxins, and genes such as IDEAL PLANT ARCHITECTURE1 and TEOSINTE BRANCHED1 control shoot architecture. Knowledge now available provides a foundation for rational approaches to crop breeding and the generation of ideotypes with defined architectural features to improve performance and productivity.

From model to crop: functional characterization of SPL8 in M. truncatula led to genetic improvement of biomass yield and abiotic stress tolerance in alfalfa

Biomass yield, salt tolerance and drought tolerance are important targets for alfalfa (Medicago sativa L.) improvement. Medicago truncatula has been developed into a model plant for alfalfa and other legumes. By screening a Tnt1 retrotransposon-tagged M. truncatula mutant population, we identified three mutants with enhanced branching. Branch development determines shoot architecture which affects important plant functions such as light acquisition, resource use and ultimately impacts biomass production. Molecular analyses revealed that the mutations were caused by Tnt1 insertions in the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 8 (SPL8) gene. The M. truncatula spl8 mutants had increased biomass yield, while overexpression of SPL8 in M. truncatula suppressed branching and reduced biomass yield. Scanning electron microscopy (SEM) analysis showed that SPL8 inhibited branching by directly suppressing axillary bud formation. Based on the M. truncatula SPL8 sequence, alfalfa SPL8 (MsSPL8) was cloned and transgenic alfalfa plants were produced. MsSPL8 down-regulated or up-regulated alfalfa plants exhibited similar phenotypes to the M. truncatula mutants or overexpression lines, respectively. Specifically, the MsSPL8 down-regulated alfalfa plants showed up to 43% increase in biomass yield in the first harvest. The impact was even more prominent in the second harvest, with up to 86% increase in biomass production compared to the control. Furthermore, down-regulation of MsSPL8 led to enhanced salt and drought tolerance in transgenic alfalfa. Results from this research offer a valuable approach to simultaneously improve biomass production and abiotic stress tolerance in legumes.

Genome-wide identification and expression analysis of GRAS family transcription factors in tea plant (Camellia sinensis)

GRAS proteins are important transcription factors that play multifarious roles in regulating the growth and development as well as stress responses of plants. Tea plant is an economically important leaf -type beverage crop. Information concerning GRAS family transcription factors in tea plant is insufficient. In this study, 52 CsGRAS genes encoding GRAS proteins were identified from tea plant genome database. Phylogenetic analysis of the identified GRAS proteins from tea plant, Arabidopsis, and rice divided these proteins into at least 13 subgroups. Conserved motif analysis revealed that the gene structure and motif compositions of the proteins were considerably conserved among the same subgroup. Functional divergence analysis indicated that the shifted evolutionary rate might act as a major evolutionary force driving subfamily-specific functional diversification. Transcriptome analysis showed that the transcriptional levels of CsGRAS genes under non-stress conditions varied among different tea plant cultivars. qRT-PCR analysis revealed tissue and development stage-specific expression patterns of CsGRAS genes in tea plant. The expression patterns of CsGRAS genes in response to abiotic stresses and gibberellin treatment suggested the possible multiple functions of these genes. This study provides insights into the potential functions of GRAS genes.

Silencing SlMED18, tomato Mediator subunit 18 gene, restricts internode elongation and leaf expansion

Mediator complex, a conserved multi-protein, is necessary for controlling RNA polymerase II (Pol II) transcription in eukaryotes. Given little is known about them in tomato, a tomato Mediator subunit 18 gene was isolated and named SlMED18. To further explore the function of SlMED18, the transgenic tomato plants targeting SlMED18 by RNAi-mediated gene silencing were generated. The SlMED18-RNAi lines exhibited multiple developmental defects, including smaller size and slower growth rate of plant and significantly smaller compound leaves. The contents of endogenous bioactive GA3 in SlMED18 silenced lines were slightly less than that in wild type. Furthermore, qRT-PCR analysis indicated that expression of gibberellins biosynthesis genes such as SlGACPS and SlGA20x2, auxin transport genes (PIN1, PIN4, LAX1 and LAX2) and several key regulators, KNOX1, KNOX2, PHAN and LANCEOLATE(LA), which involved in the leaf morphogenesis were significantly down-regulated in SlMED18-RNAi lines. These results illustrated that SlMED18 plays an essential role in regulating plant internode elongation and leaf expansion in tomato plants and it acts as a key positive regulator of gibberellins biosynthesis and signal transduction as well as auxin proper transport signalling. These findings are the basis for understanding the function of the individual Mediator subunits in tomato.

A MADS-box transcription factor of 'Kuerlexiangli'(Pyrus sinkiangensis Yu) PsJOINTLESS gene functions in floral organ abscission

MADS-box proteins have been implicated in many biological processes. However, plant MADS-box proteins functioning in floral organ abscission and the underlying physiological mechanisms remain poorly understood. Here, we report the identification and functional characterization of PsJOINTLESS isolated from 'Kuerlexiangli'. PsJOINTLESS had a complete open reading frame of 672bp, encoding a 224 amino acid peptide, and shared high sequence identities with MADS-box from other plants. PsJOINTLESS was subcellularly targeted to the nucleus, supporting its role as a transcription factor. Expression levels of PsJOINTLESS in the calyx tube were strongly induced by calyx abscission treatment at 6d after full bloom. Overexpression of PsJOINTLESS in tomato enhanced the rate of pedicel abscission rate. Of special note, the transgenic plants increased the abscission zone cell layer compared with wild type. Furthermore, the tomato transgenic lines showed thinner flower pedicels, more cell number and small pedicel cell size. The cellulase activity in pedicel abscission zone of transgenic plants was higher than that of wild type. In addition, steady-state mRNA levels of five cell wall hydrolase genes coding for either functional or regulatory proteins were induced to higher levels in the transgenic lines. These results clearly demonstrate that PsJOINTLESS may affect pedicel abscission zone development.

Emerging role of the plant ERF transcription factors in coordinating wound defense responses and repair

Plants react to wounding through the activation of both defense and repair pathways, but how these two responses are coordinated is unclear. Here, we put forward the hypothesis that diverse members of the subfamily X of the plant-specific ethylene response factor (ERF) transcription factors coordinate stress signaling with the activation of wound repair mechanisms. Moreover, we highlight the observation that tissue repair is strongly boosted through the formation of a heterodimeric protein complex that comprises ERF and transcription factors of the GRAS domain type. This interaction turns ERFs into highly potent and stress-responsive activators of cell proliferation. The potency to induce stem cell identity suggests that these heterodimeric transcription factor complexes could become valuable tools to increase crop regeneration and transformation efficiency.

BrLAS, a GRAS Transcription Factor From Brassica rapa, Is Involved in Drought Stress Tolerance in Transgenic Arabidopsis

GRAS proteins belong to a plant-specific transcription factor family and play roles in diverse physiological processes and environmental signals. In this study, we identified and characterized a GRAS transcription factor gene in Brassica rapa, BrLAS, an ortholog of Arabidopsis AtLAS. BrLAS was primarily expressed in the roots and axillary meristems, and localized exclusively in the nucleus of B. rapa protoplast cells. qRT-PCR analysis indicated that BrLAS was upregulated by exogenous abscisic acid (ABA) and abiotic stress treatment [polyethylene glycol (PEG), NaCl, and H₂O₂]. BrLAS-overexpressing Arabidopsis plants exhibited pleiotropic characteristics, including morphological changes, delayed bolting and flowering time, reduced fertility and delayed senescence. Transgenic plants also displayed significantly enhanced drought resistance with decreased accumulation of ROS and increased antioxidant enzyme activity under drought treatment compared with the wild-type. Increased sensitivity to exogenous ABA was also observed in the transgenic plants. qRT-PCR analysis further showed that expression of several genes involved in stress responses and associated with leaf senescence were also modified. These findings suggest that BrLAS encodes a stress-responsive GRASs transcription factor that positively regulates drought stress tolerance, suggesting a role in breeding programs aimed at improving drought tolerance in plants.

Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies

BACKGROUND

In contrast to most animals, plants have an indeterminate body plan, which allows them to add new body parts during their lifetime. A plant's realized modular construction is the result of exogenous constraints and endogenous processes. This review focuses on endogenous processes that shape plant architectures and their evolution.

SCOPE

The phylogenetic distribution of plant growth forms across the phylogeny implies that body architectures have originated and been lost repeatedly, being shaped by a limited set of genetic pathways. We (1) synthesize concepts of plant architecture, so far captured in 23 models; (2) extend them to the fossil record; (3) summarize what is known about their developmental genetics; (4) use a phylogenetic approach in several groups to infer how plant architecture has changed and by which intermediate steps; and (5) discuss which macroecological factors may constrain the geographic and ecological distribution of plant architectures.

CONCLUSIONS

Dichotomously branching Paleozoic plants already encompassed a considerable diversity of growth forms, here captured in 12 new architectural models. Plotting the frequency of branching types through time based on an analysis of 58 927 land plant fossils revealed a decrease in dichotomous branching throughout the Devonian and Carboniferous, mirrored by an increase in other branching types including axillary branching. We suggest that the evolution of seed plant megaphyllous leaves enabling axillary branching contributed to the demise of dichotomous architectures. The developmental-genetic bases for key architectural traits underlying sympodial vs. monopodial branching, rhythmic vs. continuous growth, and axillary branching and its localization are becoming well understood, while the molecular basis of dichotomous branching and plagiotropy remains elusive. Three phylogenetic case studies of architecture evolution in conifers, Aloe and monocaulous arborescent vascular plants reveal relationships between architectural models and show that some are labile in given groups, whereas others are widely conserved, apparently shaped by ecological factors, such as intercepted sunlight, temperature, humidity and seasonality.

Genome-wide identification and characterization of GRAS transcription factors in tomato (Solanum lycopersicum)

Solanum lycopersicum, belonging to Solanaceae, is one of the commonly used model plants. The GRAS genes are transcriptional regulators, which play a significant role in plant growth and development, and the functions of several GRAS genes have been recognized, such as, axillary shoot meristem formation, radial root patterning, phytohormones (gibberellins) signal transduction, light signaling, and abiotic/biotic stress; however, only a few of these were identified and functionally characterized. In this study, a gene family was analyzed comprehensively with respect to phylogeny, gene structure, chromosomal localization, and expression pattern; the 54 GRAS members were screened from tomato by bioinformatics for the first time. The GRAS genes among tomato, Arabidopsis, rice, and grapevine were rebuilt to form a phylogenomic tree, which was divided into ten groups according to the previous classification of Arabidopsis and rice. A multiple sequence alignment exhibited the typical GRAS domain and conserved motifs similar to other gene families. Both the segmental and tandem duplications contributed significantly to the expansion and evolution of the GRAS gene family in tomato; the expression patterns across a variety of tissues and biotic conditions revealed potentially different functions of GRAS genes in tomato development and stress responses. Altogether, this study provides valuable information and robust candidate genes for future functional analysis for improving the resistance of tomato growth.

The miR156-SPL4 module predominantly regulates aerial axillary bud formation and controls shoot architecture

Grasses possess basal and aerial axillary buds. Previous studies have largely focused on basal bud (tiller) formation but scarcely touched on aerial buds, which may lead to aerial branch development. Genotypes with and without aerial buds were identified in switchgrass (Panicum virgatum), a dedicated bioenergy crop. Bud development was characterized using scanning electron microscopy. Microarray, RNA-seq and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were used to identify regulators of bud formation. Gene function was characterized by down-regulation and overexpression. Overexpression of miR156 induced aerial bud formation in switchgrass. Various analyses revealed that SQUAMOSA PROMOTER BINDING PROTEIN LIKE4 (SPL4), one of the miR156 targets, directly regulated aerial axillary bud initiation. Down-regulation of SPL4 promoted aerial bud formation and increased basal buds, while overexpression of SPL4 seriously suppressed bud formation and tillering. RNA-seq and RT-qPCR identified potential downstream genes of SPL4. Unlike all previously reported genes acting as activators of basal bud initiation, SPL4 acts as a suppressor for the formation of both aerial and basal buds. The miR156-SPL4 module predominantly regulates aerial bud initiation and partially controls basal bud formation. Genetic manipulation of SPL4 led to altered plant architecture with increased branching, enhanced regrowth after cutting and improved biomass yield.

Genome-wide characterization of GRAS family genes in Medicago truncatula reveals their evolutionary dynamics and functional diversification

The GRAS gene family is a large plant-specific family of transcription factors that are involved in diverse processes during plant development. Medicago truncatula is an ideal model plant for genetic research in legumes, and specifically for studying nodulation, which is crucial for nitrogen fixation. In this study, 59 MtGRAS genes were identified and classified into eight distinct subgroups based on phylogenetic relationships. Motifs located in the C-termini were conserved across the subgroups, while motifs in the N-termini were subfamily specific. Gene duplication was the main evolutionary force for MtGRAS expansion, especially proliferation of the LISCL subgroup. Seventeen duplicated genes showed strong effects of purifying selection and diverse expression patterns, highlighting their functional importance and diversification after duplication. Thirty MtGRAS genes, including NSP1 and NSP2, were preferentially expressed in nodules, indicating possible roles in the process of nodulation. A transcriptome study, combined with gene expression analysis under different stress conditions, suggested potential functions of MtGRAS genes in various biological pathways and stress responses. Taken together, these comprehensive analyses provide basic information for understanding the potential functions of GRAS genes, and will facilitate further discovery of MtGRAS gene functions.

The MADS-box gene SlMBP11 regulates plant architecture and affects reproductive development in tomato plants

MADS-domain proteins are important transcription factors that are involved in many biological processes of plants. In the present study, SlMBP11, a member of the AGL15 subfamily, was cloned in tomato plants (Solanum lycopersicon M.). SlMBP11 is ubiquitously expressed in all of the tissues we examined, whereas the SlMBP11 transcription levels were significantly higher in reproductive tissues than in vegetative tissues. Plants exhibiting increased SlMBP11 levels displayed reduced plant height, leaf size, and internode length as well as a loss of dominance in young seedlings, highly branched growth from each leaf axil, and increased number of nodes and leaves. Moreover, overexpression lines also exhibited reproductive phenotypes, such as those having a shorter style and split ovary, leading to polycarpous fruits, while the wild type showed normal floral organization. In addition, delayed perianth senescence was observed in transgenic tomatoes. These phenotypes were further confirmed by analyzing the morphological, anatomical and molecular features of lines exhibiting overexpression. These results suggest that SlMBP11 plays an important role in regulating plant architecture and reproductive development in tomato plants. These findings add a new class of transcription factors to the group of genes controlling axillary bud growth and illuminate a previously uncharacterized function of MADS-box genes in tomato plants.

Identification, Classification, and Expression Analysis of GRAS Gene Family in Malus domestica

GRAS genes encode plant-specific transcription factors that play important roles in plant growth and development. However, little is known about the GRAS gene family in apple. In this study, 127 GRAS genes were identified in the apple (Malus domestica Borkh.) genome and named MdGRAS1 to MdGRAS127 according to their chromosomal locations. The chemical characteristics, gene structures and evolutionary relationships of the MdGRAS genes were investigated. The 127 MdGRAS genes could be grouped into eight subfamilies based on their structural features and phylogenetic relationships. Further analysis of gene structures, segmental and tandem duplication, gene phylogeny and tissue-specific expression with ArrayExpress database indicated their diversification in quantity, structure and function. We further examined the expression pattern of MdGRAS genes during apple flower induction with transcriptome sequencing. Eight higher MdGRAS (MdGRAS6, 26, 28, 44, 53, 64, 107, and 122) genes were surfaced. Further quantitative reverse transcription PCR indicated that the candidate eight genes showed distinct expression patterns among different tissues (leaves, stems, flowers, buds, and fruits). The transcription levels of eight genes were also investigated with various flowering related treatments (GA₃, 6-BA, and sucrose) and different flowering varieties (Yanfu No. 6 and Nagafu No. 2). They all were affected by flowering-related circumstance and showed different expression level. Changes in response to these hormone or sugar related treatments indicated their potential involvement during apple flower induction. Taken together, our results provide rich resources for studying GRAS genes and their potential clues in genetic improvement of apple flowering, which enriches biological theories of GRAS genes in apple and their involvement in flower induction of fruit trees.

The vascular plants: open system of growth

What is fascinating in plants (true also in sessile animals such as corals and hydroids) is definitely their open and indeterminate growth, as a result of meristematic activity. Plants as well as animals are characterized by a multicellular organization, with which they share a common set of genes inherited from a common eukaryotic ancestor; nevertheless, circa 1.5 billion years of evolutionary history made the two kingdoms very different in their own developmental biology. Flowering plants, also known as angiosperms, arose during the Cretaceous Period (145-65 million years ago), and up to date, they count around 235,000 species, representing the largest and most diverse group within the plant kingdom. One of the foundations of their success relies on the plant-pollinator relationship, essentially unique to angiosperms that pushed large speciation in both plants and insects and on the presence of the carpel, the structure devoted to seed enclosure. A seed represents the main organ preserving the genetic information of a plant; during embryogenesis, the primary axis of development is established by two groups of pluripotent cells: the shoot apical meristem (SAM), responsible for gene rating all aboveground organs, and the root apical meristem (RAM), responsible for producing all underground organs. During postembryonic shoot development, axillary meristem (AM) initiation and outgrowth are responsible for producing all secondary axes of growth including inflorescence branches or flowers. The production of AMs is tightly linked to the production of leaves and their separation from SAM. As leaf primordia are formed on the flanks of the SAM, a region between the apex and the developing organ is established and referred to as boundary zone. Interaction between hormones and the gene network in the boundary zone is fundamental for AM initiation. AMs only develop at the adaxial base of the leaf; thus, AM initiation is also strictly associated with leaf polarity. AMs function as new SAMs: form axillary buds with a few leaves and then the buds can either stay dormant or develop into shoot branches to define a plant architecture, which in turn affects assimilate production and reproductive efficiency. Therefore, the radiation of angiosperms was accompanied by a huge diversification in growth forms that determine an enormous morphological plasticity helping plants to environmental changes. In this review, we focused on the developmental processes of AM initiation and outgrowth. In particular, we summarized the primary growth of SAM, the key role of positional signals for AM initiation, and the dissection of molecular players involved in AM initiation and outgrowth. Finally, the interaction between phytohormone signals and gene regulatory network controlling AM development was discussed.

Molecular cloning, phylogenetic analysis, and expression patterns of LATERAL SUPPRESSOR-LIKE and REGULATOR OF AXILLARY MERISTEM FORMATION-LIKE genes in sunflower (Helianthus annuus L.)

The wild sunflower (Helianthus annuus) plants develop a highly branched form with numerous small flowering heads. The origin of a no branched sunflower, producing a single large head, has been a key event in the domestication process of this species. The interaction between hormonal factors and several genes organizes the initiation and outgrowth of axillary meristems (AMs). From sunflower, we have isolated two genes putatively involved in this process, LATERAL SUPPRESSOR (LS)-LIKE (Ha-LSL) and REGULATOR OF AXILLARY MERISTEM FORMATION (ROX)-LIKE (Ha-ROXL), encoding for a GRAS and a bHLH transcription factor (TF), respectively. Typical amino acid residues and phylogenetic analyses suggest that Ha-LSL and Ha-ROXL are the orthologs of the branching regulator LS and ROX/LAX1, involved in the growth habit of both dicot and monocot species. qRT-PCR analyses revealed a high accumulation of Ha-LSL transcripts in roots, vegetative shoots, and inflorescence shoots. By contrast, in internodal stems and young leaves, a lower amount of Ha-LSL transcripts was observed. A comparison of transcription patterns between Ha-LSL and Ha-ROXL revealed some analogies but also remarkable differences; in fact, the gene Ha-ROXL displayed a low expression level in all organs analyzed. In situ hybridization (ISH) analysis showed that Ha-ROXL transcription was strongly restricted to a small domain within the boundary zone separating the shoot apical meristem (SAM) and the leaf primordia and in restricted regions of the inflorescence meristem, beforehand the separation of floral bracts from disc flower primordia. These results suggested that Ha-ROXL may be involved to establish a cell niche for the initiation of AMs as well as flower primordia. The accumulation of Ha-LSL transcripts was not restricted to the boundary zones in vegetative and inflorescence shoots, but the mRNA activity was expanded in other cellular domains of primary shoot apical meristem as well as AMs. In addition, Ha-LSL transcript accumulation was also detected in leaves and floral primordia at early stages of development. These results were corroborated by qRT-PCR analyses that evidenced high levels of Ha-LSL transcripts in very young leaves and disc flowers, suggesting a role of Ha-LSL for the early outgrowth of lateral primordia.

ERAMOSA controls lateral branching in snapdragon

Plant forms display a wide variety of architectures, depending on the number of lateral branches, internode elongation and phyllotaxy. These are in turn determined by the number, the position and the fate of the Axillary Meristems (AMs). Mutants that affect AM determination during the vegetative phase have been isolated in several model plants. Among these genes, the GRAS transcription factor LATERAL SUPPRESSOR (Ls) plays a pivotal role in AM determination during the vegetative phase. Hereby we characterize the phylogenetic orthologue of Ls in Antirrhinum, ERAMOSA (ERA). Our data supported ERA control of AM formation during both the vegetative and the reproductive phase in snapdragon. A phylogenetic analysis combined with an analysis of the synteny of Ls in several species strongly supported the hypothesis that ERA is a phylogenetic orthologue of Ls, although it plays a broader role. During the reproductive phase ERA promotes the establishment of the stem niche at the bract axis but, after the reproductive transition, it is antagonized by the MADS box transcription factor SQUAMOSA (SQUA). Surprisingly double mutant era squa plants display a squa phenotype developing axillary meristems, which can eventually turn into inflorescences or flowers.

Evolutionary Analyses of GRAS Transcription Factors in Angiosperms

GRAS transcription factors (TFs) play critical roles in plant growth and development such as gibberellin and mycorrhizal signaling. Proteins belonging to this gene family contain a typical GRAS domain in the C-terminal sequence, whereas the N-terminal region is highly variable. Although, GRAS genes have been characterized in a number of plant species, their classification is still not completely resolved. Based on a panel of eight representative species of angiosperms, we identified 29 orthologous groups or orthogroups (OGs) for the GRAS gene family, suggesting that at least 29 ancestor genes were present in the angiosperm lineage before the "Amborella" evolutionary split. Interestingly, some taxonomic groups were missing members of one or more OGs. The gene number expansion usually observed in transcription factors was not observed in GRAS while the genome triplication ancestral to the eudicots (γ hexaploidization event) was detectable in a limited number of GRAS orthogroups. We also found conserved OG-specific motifs in the variable N-terminal region. Finally, we could regroup OGs in 17 subfamilies for which names were homogenized based on a literature review and described 5 new subfamilies (DLT, RAD1, RAM1, SCLA, and SCLB). This study establishes a consistent framework for the classification of GRAS members in angiosperm species, and thereby a tool to correctly establish the orthologous relationships of GRAS genes in most of the food crops in order to facilitate any subsequent functional analyses in the GRAS gene family. The multi-fasta file containing all the sequences used in our study could be used as database to perform diagnostic BLASTp to classify GRAS genes from other non-model species.

Manipulation of flowering time and branching by overexpression of the tomato transcription factor SlZFP2

Flowering of higher plants is orchestrated by complex regulatory networks through integration of various environmental signals such as photoperiod, temperature, light quality and developmental cues. In Arabidopsis, transcription of the flowering integrator gene FLOWERING LOCUS T (FT) that several flowering pathways converge to is directly regulated by more than ten transcription factors. However, very little is known about the transcriptional regulation of the FT homolog SINGLE FLOWER TRUESS (SFT) in the day-neutral plant tomato (Solanum lycopersicum). Previously, we showed that the zinc finger transcription factor SlZFP2 plays important roles in regulation of seed germination and fruit ripening in tomato and also found that overexpression of SlZFP2 impacted flowering and branching. Here, we characterized in detail the early flowering and high branching phenotypes by overexpression of this transcription factor. Our data showed that overexpression of SlZFP2 accelerated flowering in an SFT-dependent manner as demonstrated by elevated SFT expression in the leaves and the transcription factor's binding ability to SFT promoter in vitro and in vivo. Furthermore, overexpression of the SlZFP2 gene in the sft plants failed to rescue the mutant's late flowering. Through analysis of grafting phenotype, growth response of branches to auxin application and transcriptome profiling by RNA sequencing, we also showed that overexpression of SlZFP2 affected shoot apical dominance through multiple regulatory pathways. Our results suggest that the transcription factor SlZFP2 has potential applications in genetic modification of plant architecture and flowering time for tomato production and other crops as well.

The historical role of species from the Solanaceae plant family in genetic research

This article evaluates the main contributions of tomato, tobacco, petunia, potato, pepper and eggplant to classical and molecular plant genetics and genomics since the beginning of the twentieth century. Species from the Solanaceae family form integral parts of human civilizations as food sources and drugs since thousands of years, and, more recently, as ornamentals. Some Solanaceous species were subjects of classical and molecular genetic research over the last 100 years. The tomato was one of the principal models in twentieth century classical genetics and a pacemaker of genome analysis in plants including molecular linkage maps, positional cloning of disease resistance genes and quantitative trait loci (QTL). Besides that, tomato is the model for the genetics of fruit development and composition. Tobacco was the major model used to establish the principals and methods of plant somatic cell genetics including in vitro propagation of cells and tissues, totipotency of somatic cells, doubled haploid production and genetic transformation. Petunia was a model for elucidating the biochemical and genetic basis of flower color and development. The cultivated potato is the economically most important Solanaceous plant and ranks third after wheat and rice as one of the world's great food crops. Potato is the model for studying the genetic basis of tuber development. Molecular genetics and genomics of potato, in particular association genetics, made valuable contributions to the genetic dissection of complex agronomic traits and the development of diagnostic markers for breeding applications. Pepper and eggplant are horticultural crops of worldwide relevance. Genetic and genomic research in pepper and eggplant mostly followed the tomato model. Comparative genome analysis of tomato, potato, pepper and eggplant contributed to the understanding of plant genome evolution.

Identification and molecular characterization of an IDA-like gene from litchi, LcIDL1, whose ectopic expression promotes floral organ abscission in Arabidopsis

Unexpected abscission of flowers or fruits is a major limiting factor for crop productivity. Key genes controlling abscission in plants, especially in popular fruit trees, are largely unknown. Here we identified a litchi (Litchi chinensis Sonn.) IDA-like (INFLORESCENCE DEFICIENT IN ABSCISSION-like) gene LcIDL1 as a potential key regulator of abscission. LcIDL1 encodes a peptide that shows the closest homology to Arabidopsis IDA, and is localized in cell membrane and cytoplasm. Real-time PCR analysis showed that the expression level of LcIDL1 accumulated gradually following flower abscission, and it was obviously induced by fruit abscission-promoting treatments. Transgenic plants expressing LcIDL1 in Arabidopsis revealed a role of LcIDL1 similar to IDA in promoting floral organ abscission. Moreover, ectopic expression of LcIDL1 in Arabidopsis activated the expression of abscission-related genes. Taken together, our findings provide evidence that LcIDL1 may act as a key regulator in control of abscission.

Apoplastic H2 O2 plays a critical role in axillary bud outgrowth by altering auxin and cytokinin homeostasis in tomato plants

Although phytohormones such as indole-3-acetic acid (IAA), cytokinin (CK) and strigolactone are important modulators of plant architecture, it remains unclear whether reactive oxygen species are involved in the regulation of phytohormone-dependent axillary bud outgrowth in plants. We used diverse techniques, including transcriptional suppression, HPLC-MS, biochemical methodologies and gene transcript analysis to investigate the signaling pathway for apoplastic hydrogen peroxide (H2 O2 )-induced axillary bud outgrowth. Silencing of tomato RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1) and WHITEFLY INDUCED 1 (WFI1), two important genes involved in H2 O2 production in the apoplast, enhanced bud outgrowth, decreased transcript of FZY - a rate-limiting gene in IAA biosynthesis and IAA accumulation in the apex - and increased the transcript of IPT2 involved in CK biosynthesis and CK accumulation in the stem node. These effects were fully abolished by the application of exogenous H2 O2 . Both decapitation and the silencing of FZY promoted bud outgrowth, and downregulated and upregulated the transcripts for IAA3 and IAA15, and IPT2, respectively. However, these effects were not blocked by treatment with exogenous H2 O2 but by napthaleneacetic acid (NAA) treatment. These results suggest that RBOHs-dependent apoplastic H2 O2 promotes IAA biosynthesis in the apex, which, in turn, inhibits CK biosynthesis and subsequent bud outgrowth in tomato plants.

Two-Step Regulation of a Meristematic Cell Population Acting in Shoot Branching in Arabidopsis

Shoot branching requires the establishment of new meristems harboring stem cells; this phenomenon raises questions about the precise regulation of meristematic fate. In seed plants, these new meristems initiate in leaf axils to enable lateral shoot branching. Using live-cell imaging of leaf axil cells, we show that the initiation of axillary meristems requires a meristematic cell population continuously expressing the meristem marker SHOOT MERISTEMLESS (STM). The maintenance of STM expression depends on the leaf axil auxin minimum. Ectopic expression of STM is insufficient to activate axillary buds formation from plants that have lost leaf axil STM expressing cells. This suggests that some cells undergo irreversible commitment to a developmental fate. In more mature leaves, REVOLUTA (REV) directly up-regulates STM expression in leaf axil meristematic cells, but not in differentiated cells, to establish axillary meristems. Cell type-specific binding of REV to the STM region correlates with epigenetic modifications. Our data favor a threshold model for axillary meristem initiation, in which low levels of STM maintain meristematic competence and high levels of STM lead to meristem initiation.

In seed plants, branches arise from axillary meristems (AMs), which form in the crook between the leaf and the stem. How AMs initiate to produce branches remains unclear. In this study, we show that a group of meristematic cells maintain expression of the meristem marker SHOOT MERISTEMLESS (STM); the progeny of these cells form the axillary buds. Our results suggest that low-level STM expression is required (but not sufficient) for AM initiation, and that high-level STM expression induces initiation of the AM. The initial expression of STM requires the auxin minimum in the leaf axil and the transcription factor REVOLUTA directly up-regulates STM expression.

Genome-Wide Identification, Evolutionary Analysis, and Stress Responses of the GRAS Gene Family in Castor Beans

Plant-specific GRAS transcription factors play important roles in regulating growth, development, and stress responses. Castor beans (Ricinus communis) are important non-edible oilseed plants, cultivated worldwide for its seed oils and its adaptability to growth conditions. In this study, we identified and characterized a total of 48 GRAS genes based on the castor bean genome. Combined with phylogenetic analysis, the castor bean GRAS members were divided into 13 distinct groups. Functional divergence analysis revealed the presence of mostly Type-I functional divergence. The gene structures and conserved motifs, both within and outside the GRAS domain, were characterized. Gene expression analysis, performed in various tissues and under a range of abiotic stress conditions, uncovered the potential functions of GRAS members in regulating plant growth development and stress responses. The results obtained from this study provide valuable information toward understanding the potential molecular mechanisms of GRAS proteins in castor beans. These findings also serve as a resource for identifying the genes that allow castor beans to grow in stressful conditions and to enable further breeding and genetic improvements in agriculture.

Crystal Structure of the GRAS Domain of SCARECROW-LIKE7 in Oryza sativa

GRAS proteins belong to a plant-specific protein family with many members and play essential roles in plant growth and development, functioning primarily in transcriptional regulation. Proteins in the family are minimally defined as containing the conserved GRAS domain. Here, we determined the structure of the GRAS domain of Os-SCL7 from rice (Oryza sativa) to 1.82 Å. The structure includes cap and core subdomains and elucidates the features of the conserved GRAS LRI, VHIID, LRII, PFYRE, and SAW motifs. The structure is a dimer, with a clear groove to accommodate double-stranded DNA. Docking a DNA segment into the groove to generate an Os-SCL7/DNA complex provides insight into the DNA binding mechanism of GRAS proteins. Furthermore, the in vitro DNA binding property of Os-SCL7 and model-defined recognition residues are assessed by electrophoretic mobility shift analysis and mutagenesis assays. These studies reveal the structure and preliminary DNA interaction mechanisms of GRAS proteins and open the door to in-depth investigation and understanding of the individual pathways in which they play important roles.

MicroRNAs in Control of Plant Development

In the long evolutionary history, plant has evolved elaborate regulatory network to control functional gene expression for surviving and thriving, such as transcription factor-regulated transcriptional programming. However, plenty of evidences from the past decade studies demonstrate that the 21-24 nucleotides small RNA molecules, majorly microRNAs (miRNAs) play dominant roles in post-transcriptional gene regulation through base pairing with their complementary mRNA targets, especially prefer to target transcription factors in plants. Here, we review current progresses on miRNA-controlled plant development, from miRNA biogenesis dysregulation-caused pleiotropic developmental defects to specific developmental processes, such as SAM regulation, leaf and root system regulation, and plant floral transition. We also summarize some miRNAs that are experimentally proved to greatly affect crop plant productivity and quality. In addition, recent reports show that a single miRNA usually displays multiple regulatory roles, such as organ development, phase transition, and stresses responses. Thus, we infer that miRNA may act as a node molecule to coordinate the balance between plant development and environmental clues, which may shed the light on finding key regulator or regulatory pathway for uncovering the mysterious molecular network.

Overexpression of VaPAT1, a GRAS transcription factor from Vitis amurensis, confers abiotic stress tolerance in Arabidopsis

VaPAT1 functions as a stress-inducible GRAS gene and enhanced cold, drought and salt tolerance in transgenic Arabidopsis via modulation of the expression of a series of stress-related genes. The plant-specific GRAS transcription factor family regulates diverse processes involved in plant growth, development and stress responses. In this study, VaPAT1, a GRAS gene from Vitis amurensis was isolated and functionally characterized. Sequence alignment and phylogenetic analysis showed that VaPAT1 has a high sequence identity to CmsGRAS and OsCIGR1, which belong to PAT1 branch of GRAS family and function in stress resistance. The transcription of VaPAT1 was markedly induced by stress-related phytohormone abscisic acid (ABA) and various abiotic stress treatments such as cold, drought and high salinity, however, it was repressed by exogenous gibberellic acid (GA) application. Overexpression of VaPAT1 increased the cold, drought and high salinity tolerance in transgenic Arabidopsis. When compared with wild type (WT) seedlings, the VaPAT1-overexpression lines accumulated higher levels of proline and soluble sugar under these stress treatments. Moreover, stress-related genes such as AtSIZ1, AtCBF1, AtATR1/MYB34, AtMYC2, AtCOR15A, AtRD29A and AtRD29B showed higher expression levels in VaPAT1 transgenic lines than in WT Arabidopsis under normal growth conditions. Together, our results indicated that VaPAT1 functions as a positive transcriptional regulator involved in grapevine abiotic stress responses.

Genome-Wide Analysis Indicates Lineage-Specific Gene Loss during Papilionoideae Evolution

Gene loss is the driving force for changes in genome and morphology; however, this particular evolutionary event has been poorly investigated in leguminous plants. Legumes (Fabaceae) have some lineage-specific and diagnostic characteristics that are distinct from other angiosperms. To understand the potential role of gene loss in the evolution of legumes, we compared six genome-sequenced legume species of Papilionoideae, the largest representative clade of Fabaceae, such as Glycine max, with 34 nonlegume plant species, such as Arabidopsis thaliana. The results showed that the putative orthologs of the 34 Arabidopsis genes belonging to 29 gene families were absent in these legume species but these were conserved in the sequenced nonlegume angiosperm lineages. Further evolutionary analyses indicated that the orthologs of these genes were almost completely lost in the Papillionoideae ancestors, thus designated as the legume lost genes (LLGs), and these underwent purifying selection in nonlegume plants. Most LLGs were functionally unknown. In Arabidopsis, two LLGs were well-known genes that played a role in plant immunity such as HARMLESS TO OZONE LAYER 1 and HOPZ-ACTIVATED RESISTANCE 1, and 16 additional LLGs were predicted to participate in plant-pathogen interactions in in silico expression and protein-protein interaction network analyses. Most of these LLGs' orthologs in various plants were also found to be associated with biotic stress response, indicating the conserved role of these genes in plant defense. The evolutionary implication of LLGs during the development of the ability of symbiotic nitrogen fixation involving plant and bacterial interactions, which is a well-known characteristic of most legumes, is also discussed. Our work sheds light on the evolutionary implication of gene loss events in Papilionoideae evolution, as well as provides new insights into crop design to improve nitrogen fixation capacity.

A characterization of grapevine of GRAS domain transcription factor gene family

GRAS domain genes are a group of important plant-specific transcription factors that have been reported to be involved in plant development. In order to know the roles of GRAS genes in grapevine, a widely cultivated fruit crop, the study on grapevine GRAS (VvGRAS) was carried out, and from which, 43 were identified from 12× assemble grapevine genomic sequences. Further, the genomic structures, synteny, phylogeny, expression profiles in different tissues of these genes, and their roles in response to stress were investigated. Among the genes, two potential target genes (VvSCL15 and VvSCL22) for VvmiR171 were experimentally verified by PPM-RACE and RLM-RACE, in that not only the cleavage sites of miR171 on the target mRNA were mapped but also the cleaved fragments and their expressing patterns were detected. Transgenic Arabidopsis plants over expression VvSCL15 showed lower tolerance to drought and salt treatments.

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.

Structural and Functional Analysis of the GRAS Gene Family in Grapevine Indicates a Role of GRAS Proteins in the Control of Development and Stress Responses

GRAS transcription factors are involved in many processes of plant growth and development (e.g., axillary shoot meristem formation, root radial patterning, nodule morphogenesis, arbuscular development) as well as in plant disease resistance and abiotic stress responses. However, little information is available concerning this gene family in grapevine (Vitis vinifera L.), an economically important woody crop. We performed a model curation of GRAS genes identified in the latest genome annotation leading to the identification of 52 genes. Gene models were improved and three new genes were identified that could be grapevine- or woody-plant specific. Phylogenetic analysis showed that GRAS genes could be classified into 13 groups that mapped on the 19 V. vinifera chromosomes. Five new subfamilies, previously not characterized in other species, were identified. Multiple sequence alignment showed typical GRAS domain in the proteins and new motifs were also described. As observed in other species, both segmental and tandem duplications contributed significantly to the expansion and evolution of the GRAS gene family in grapevine. Expression patterns across a variety of tissues and upon abiotic and biotic conditions revealed possible divergent functions of GRAS genes in grapevine development and stress responses. By comparing the information available for tomato and grapevine GRAS genes, we identified candidate genes that might constitute conserved transcriptional regulators of both climacteric and non-climacteric fruit ripening. Altogether this study provides valuable information and robust candidate genes for future functional analysis aiming at improving the quality of fleshy fruits.

Regulation of Axillary Meristem Initiation by Transcription Factors and Plant Hormones

One distinctive feature of plant post-embryonic development is that plants can undergo reiterative growth and continuous organogenesis throughout their lifetimes. Axillary meristems (AMs) in leaf axils play a central role in this growth and differences in meristem initiation and development produce the diversity of plant architecture. Studies in the past 15 years have shown that several transcription factors (TFs) and phytohormones affect AM initiation. In this review, we highlight recent research using systems biology approaches to examine the regulatory hierarchies underlying AM initiation and the role of auxins and cytokinins in AM initiation and development. This research revealed a developmental mechanism in which phytohormone signals act with a gene regulatory network containing multiple TFs to contribute to the initiation of AMs.

An Ethylmethane Sulfonate Mutant Resource in Pre-Green Revolution Hexaploid Wheat

Mutagenesis is a powerful tool used for studying gene function as well as for crop improvement. It is regaining popularity because of the development of effective and cost efficient methods for high-throughput mutation detection. Selection for semi-dwarf phenotype during green revolution has reduced genetic diversity including that for agronomically desirable traits. Most of the available mutant populations in wheat (Triticum aestivum L.) were developed in post-green revolution cultivars. Besides the identification and isolation of agronomically important alleles in the mutant population of pre-green revolution cultivar, this population can be a vital resource for expanding the genetic diversity for wheat breeding. Here we report an Ethylmethane Sulfonate (EMS) generated mutant population consisting of 4,180 unique mutant plants in a pre-green revolution spring wheat cultivar ‘Indian’. Released in early 1900s, ‘Indian’ is devoid of any known height-reducing mutations. Unique mutations were captured by proceeding with single M₂ seed from each of the 4,180 M₁ plants. Mutants for various phenotypic traits were identified by detailed phenotyping for altered morphological and agronomic traits on M₂ plants in the greenhouse and M₃ plants in the field. Of the 86 identified mutants, 75 (87%) were phenotypically stable at the M₄ generation. Among the observed phenotypes, variation in plant height was the most frequent followed by the leaf morphology. Several mutant phenotypes including looped peduncle, crooked plant morphology, ‘gritty’ coleoptiles, looped lower internodes, and burnt leaf tips are not reported in other plant species. Considering the extent and diversity of the observed mutant phenotypes, this population appears to be a useful resource for the forward and reverse genetic studies. This resource is available to the scientific community.

RNA-sequencing Reveals Global Transcriptomic Changes in Nicotiana tabacum Responding to Topping and Treatment of Axillary-shoot Control Chemicals

Removal of terminal buds (topping) and control of the formation of axillary shoots (suckers) are common agronomic practices that significantly impact the yield and quality of various crop plants. Application of chemicals (suckercides) to plants following topping is an effective method for sucker control. However, our current knowledge of the influence of topping, and subsequent suckercide applications, to gene expression is limited. We analyzed the differential gene expression using RNA-sequencing in tobacco (Nicotiana tabacum) that are topped, or treated after topping by two different suckercides, the contact-localized-systemic, Flupro(®) (FP), and contact, Off-Shoot-T(®). Among the differentially expressed genes (DEGs), 179 were identified as common to all three conditions. DEGs, largely related to wounding, phytohormone metabolism and secondary metabolite biosynthesis, exhibited significant upregulation following topping, and downregulation after suckercide treatments. DEGs related to photosynthetic processes were repressed following topping and suckercide treatments. Moreover, topping and FP-treatment affect the expression of auxin and cytokinin signaling pathway genes that are possibly involved in axillary shoot formation. Our results provide insights into the global change of plant gene expression in response to topping and suckercide treatments. The regulatory elements of topping-inducible genes are potentially useful for the development of a chemical-free sucker control system.

The WRKY Transcription Factor WRKY71/EXB1 Controls Shoot Branching by Transcriptionally Regulating RAX Genes in Arabidopsis

Plant shoot branching is pivotal for developmental plasticity and crop yield. The formation of branch meristems is regulated by several key transcription factors including REGULATOR OF AXILLARY MERISTEMS1 (RAX1), RAX2, and RAX3. However, the regulatory network of shoot branching is still largely unknown. Here, we report the identification of EXCESSIVE BRANCHES1 (EXB1), which affects axillary meristem (AM) initiation and bud activity. Overexpression of EXB1 in the gain-of-function mutant exb1-D leads to severe bushy and dwarf phenotypes, which result from excessive AM initiation and elevated bud activities. EXB1 encodes the WRKY transcription factor WRKY71, which has demonstrated transactivation activities. Disruption of WRKY71/EXB1 by chimeric repressor silencing technology leads to fewer branches, indicating that EXB1 plays important roles in the control of shoot branching. We demonstrate that EXB1 controls AM initiation by positively regulating the transcription of RAX1, RAX2, and RAX3. Disruption of the RAX genes partially rescues the branching phenotype caused by EXB1 overexpression. We further show that EXB1 also regulates auxin homeostasis in control of shoot branching. Our data demonstrate that EXB1 plays pivotal roles in shoot branching by regulating both transcription of RAX genes and auxin pathways.

A GRAS-like gene of sunflower (Helianthus annuus L.) alters the gibberellin content and axillary meristem outgrowth in transgenic Arabidopsis plants

The GRAS proteins belong to a plant transcriptional regulator family that function in the regulation of plant growth and development. Despite their important roles, in sunflower only one GRAS gene (HaDella1) with the DELLA domain has been reported. Here, we provide a functional characterisation of a GRAS-like gene from Helianthus annuus (Ha-GRASL) lacking the DELLA motif. The Ha-GRASL gene contains an intronless open reading frame of 1,743 bp encoding 580 amino acids. Conserved motifs in the GRAS domain are detected, including VHIID, PFYRE, SAW and two LHR motifs. Within the VHII motif, the P-H-N-D-Q-L residues are entirely maintained. Phylogenetic analysis reveals that Ha-GRASL belongs to the SCARECROW LIKE4/7 (SCL4/7) subfamily of the GRAS consensus tree. Accumulation of Ha-GRASL mRNA at the adaxial boundaries from P6/P7 leaf primordia suggests a role of Ha-GRASL in the initiation of median and basal axillary meristems (AMs) of sunflower. When Ha-GRASL is over-expressed in Arabidopsis wild-type plants, the number of lateral bolts increases differently from untransformed plants. However, Ha-GRASL slightly affects the lateral suppressor (las-4-) mutation. Therefore, we hypothesise that Ha-GRASL and LAS are not functionally equivalent. The over-expression of Ha-GRASL reduces metabolic flow of gibberellins (GAs) in Arabidopsis and this modification could be relevant in AM development. Phylogenetic analysis includes LAS and SCL4/7 in the same major clade, suggesting a more recent separation of these genes with respect to other GRAS members. We propose that some features of their ancestor, as well as AM initiation and outgrowth, are partially retained in both LAS and SCL4/7.

ClRTL1 Encodes a Chinese Fir RNase III-Like Protein Involved in Regulating Shoot Branching

Identification of genes controlling shoot branching is crucial for improving plant architecture and increasing crop yield or biomass. A branching mutant of Chinese fir named “Dugansha” (Cunninghamia lanceolata var. dugan.) has been isolated in our laboratory. We chose the cDNA-AFLP technique and an effective strategy to screen genes that potentially regulate shoot branching in Chinese fir using this mutant. An RNase III-like1 cDNA fragment named ClRTL1 was identified as a potential positive regulator. To investigate the function of ClRTL1 in regulating shoot branching, we cloned the full-length cDNA sequence from C. lanceolata (Lamb.) Hook, deduced its secondary structure and function, and overexpressed the coding sequence in Arabidopsis. The ClRTL1 cDNA is 1045 bp and comprises an open reading frame of 705 bp. It encodes a protein of 235 amino acids. The deduced secondary structure of the ClRTL1 indicates that it is a mini-RNase III-like protein. The expression analysis and phenotypes of 35S: ClRTL1 in A. thaliana implies that ClRTL1 plays a role in promoting shoot branching in Chinese fir.

Genome-wide identification, phylogeny and expression analysis of GRAS gene family in tomato

BACKGROUND

GRAS transcription factors usually act as integrators of multiple growth regulatory and environmental signals, including axillary shoot meristem formation, root radial pattering, phytohormones, light signaling, and abiotic/biotic stress. However, little is known about this gene family in tomato (Solanum lycopersicum), the most important model plant for crop species with fleshy fruits.

RESULTS

In this study, 53 GRAS genes were identified and renamed based on tomato whole-genome sequence and their respective chromosome distribution except 19 members were kept as their already existed name. Multiple sequence alignment showed typical GRAS domain in these proteins. Phylogenetic analysis of GRAS proteins from tomato, Arabidopsis, Populus, P.mume, and Rice revealed that SlGRAS proteins could be divided into at least 13 subfamilies. SlGRAS24 and SlGRAS40 were identified as target genes of miR171 using5'-RACE (Rapid amplification of cDNA ends). qRT-PCR analysis revealed tissue-/organ- and development stage-specific expression patterns of SlGRAS genes. Moreover, their expression patterns in response to different hormone and abiotic stress treatments were also investigated.

CONCLUSIONS

This study provides the first comprehensive analysis of GRAS gene family in the tomato genome. The data will undoubtedly be useful for better understanding the potential functions of GRAS genes, and their possible roles in mediating hormone cross-talk and abiotic stress in tomato as well as in some other relative species.

Beyond the meristems: similarities in the CLAVATA3 and INFLORESCENCE DEFICIENT IN ABSCISSION peptide mediated signalling pathways

Plants form new organs throughout their lives; this requires a balance between cell proliferation and differentiation, and between the generation and loss of organs. To do this, plants must maintain a population of stem cells within the meristems, and at the same time, closely control the identity and position of cells at the meristem boundaries as they differentiate to new leaf or flower primordia. Once developed, organs may need to be shed, either as a controlled developmental decision-such as floral abscission after pollination, or as a response to disease, environmental stress, and predators. Cell wall degradation at specialized abscission zone (AZ) cells needs to occur for this to take place, but since there is little cell rearrangement in plants, cell separation events are also important for plant architecture. In this Opinion paper we discuss the role of two peptide ligand signalling systems that control stem cell homeostasis and cell separation, respectively. We draw parallels between the signalling pathways and explore on the commonalities of the downstream components activated and controlled by the signalling peptides. We provide evidence for AZ cells having a meristem identity and discuss the role of identical KNOTTED-LIKE HOMEOBOX (KNOX) transcription factors in meristem maintenance and abscission. Lastly we explore the evolutionary relationship between the pathways.

Novel function of a putative MOC1 ortholog associated with spikelet number per spike in common wheat

Axillary meristems play an important role in determining final plant architecture and floral structures. Tomato Ls, Arabidopsis LAS and rice MOC1 are orthologous genes regulating axillary meristem initiation and outgrowth. Their functions are generally conserved but the functional specificities are divergent among species. Obvious differences between rice panicles and wheat spikes suggest the divergent functions of MOC1 and its wheat ortholog. We show that TaMOC1 might be involved in wheat spikelet development. TaMOC1 is a typical nucleus localized protein with transcriptional activation abilities. The variable N-termini of TaMOC1 protein is necessary for transcriptional activation. TaMOC1 is highly expressed in ears with length of 2, 3 and 6 cm. Significant associations between the TaMOC1-7A haplotype and spikelet number per spike were observed in ten environments over 3 years and 2 sites. TaMOC1-7A HapH, a favored haplotype acquired during wheat polyploidization, may make a positive contribution to spikelet number per spike. Based on evolutionary analysis, geographic distribution and frequency changes, TaMOC1-7A HapH might be associated with wheat domestication and Chinese wheat breeding history. The pyramiding favorable alleles of TaMOC1-7A HapH and TaSnRK2.10 (C, associated with higher TGW) can improve both spikelet number per spike and TGW simultaneously.

Axillary Meristem Formation in Rice Requires the WUSCHEL Ortholog TILLERS ABSENT1

Axillary shoot formation is a key determinant of plant architecture. Formation of the axillary shoot is regulated by initiation of the axillary meristem or outgrowth of the axillary bud. Here, we show that rice (Oryza sativa) TILLERS ABSENT1 (TAB1; also known as Os WUS), an ortholog of Arabidopsis thaliana WUS, is required to initiate axillary meristem development. We found that formation of the axillary meristem in rice proceeds via a transient state, which we term the premeristem, characterized by the expression of OSH1, a marker of indeterminate cells in the shoot apical meristem. In the tab1-1 (wus-1) mutant, however, formation of the axillary meristem is arrested at various stages of the premeristem zone, and OSH1 expression is highly reduced. TAB1/WUS is expressed in the premeristem zone, where it shows a partially overlapping pattern with OSH1. It is likely, therefore, that TAB1 plays an important role in maintaining the premeristem zone and in promoting the formation of the axillary meristem by promoting OSH1 expression. Temporal expression patterns of WUSCHEL-RELATED HOMEOBOX4 (WOX4) indicate that WOX4 is likely to regulate meristem maintenance instead of TAB1 after establishment of the axillary meristem. Lastly, we show that the prophyll, the first leaf in the secondary axis, is formed from the premeristem zone and not from the axillary meristem.

Association mapping in sunflower (Helianthus annuus L.) reveals independent control of apical vs. basal branching

BACKGROUND

Shoot branching is an important determinant of plant architecture and influences various aspects of growth and development. Selection on branching has also played an important role in the domestication of crop plants, including sunflower (Helianthus annuus L.). Here, we describe an investigation of the genetic basis of variation in branching in sunflower via association mapping in a diverse collection of cultivated sunflower lines.

RESULTS

Detailed phenotypic analyses revealed extensive variation in the extent and type of branching within the focal population. After correcting for population structure and kinship, association analyses were performed using a genome-wide collection of SNPs to identify genomic regions that influence a variety of branching-related traits. This work resulted in the identification of multiple previously unidentified genomic regions that contribute to variation in branching. Genomic regions that were associated with apical and mid-apical branching were generally distinct from those associated with basal and mid-basal branching. Homologs of known branching genes from other study systems (i.e., Arabidopsis, rice, pea, and petunia) were also identified from the draft assembly of the sunflower genome and their map positions were compared to those of associations identified herein. Numerous candidate branching genes were found to map in close proximity to significant branching associations.

CONCLUSIONS

In sunflower, variation in branching is genetically complex and overall branching patterns (i.e., apical vs. basal) were found to be influenced by distinct genomic regions. Moreover, numerous candidate branching genes mapped in close proximity to significant branching associations. Although the sunflower genome exhibits localized islands of elevated linkage disequilibrium (LD), these non-random associations are known to decay rapidly elsewhere. The subset of candidate genes that co-localized with significant associations in regions of low LD represents the most promising target for future functional analyses.

Lateral suppressor and Goblet act in hierarchical order to regulate ectopic meristem formation at the base of tomato leaflets

In seed plants, new axes of growth are established by the formation of meristems, groups of pluripotent cells that maintain themselves and initiate the formation of lateral organs. After embryonic development, secondary shoot meristems form in the boundary zones between the shoot apical meristem and leaf primordia, the leaf axils. In addition, many plant species develop ectopic meristems at different positions of the plant body. In the compound tomato leaf, ectopic meristems can initiate at the base of leaflets, which are delimited by two distinct boundary zones, referred to as the proximal (PLB) and distal (DLB) leaflet boundaries. We demonstrate that the two leaflet boundaries differ from each other and that ectopic meristem formation is strictly limited to the DLB. Our data suggest that the DLB harbours a group of pluripotent cells that seems to be the launching pad for meristem formation. Initiation of these meristems is dependent on the activities of the transcriptional regulators Goblet (Gob) and Lateral suppressor (Ls), specifically expressed in the DLB. Gob and Ls act in hierarchical order, because Ls transcript accumulation is dependent on Gob activity, but not vice versa. Ectopic meristem formation at the DLB is also observed in other seed plants, like Cardamine pratensis, indicating that it is part of a widespread developmental program. Ectopic meristem formation leads to an increase in the number of buds, enhances the capacity for survival and opens the route to vegetative propagation.

A KNOTTED1-LIKE HOMEOBOX protein regulates abscission in tomato by modulating the auxin pathway

A gene encoding a KNOTTED1-LIKE HOMEOBOX PROTEIN1 (KD1) is highly expressed in both leaf and flower abscission zones. Reducing the abundance of transcripts of this gene in tomato (Solanum lycopersicum) by both virus-induced gene silencing and stable transformation with a silencing construct driven by an abscission-specific promoter resulted in a striking retardation of pedicel and petiole abscission. In contrast, Petroselinum, a semidominant KD1 mutant, showed accelerated pedicel and petiole abscission. Complementary DNA microarray and quantitative reverse transcription-polymerase chain reaction analysis indicated that regulation of abscission by KD1 was associated with changed abundance of genes related to auxin transporters and signaling components. Measurement of auxin content and activity of a DR5::β-glucuronidase auxin reporter assay showed that changes in KD1 expression modulated the auxin concentration and response gradient in the abscission zone.

De novo Transcriptome Sequencing and Development of Abscission Zone-Specific Microarray as a New Molecular Tool for Analysis of Tomato Organ Abscission

Abscission of flower pedicels and leaf petioles of tomato (Solanum lycopersicum) can be induced by flower removal or leaf deblading, respectively, which leads to auxin depletion, resulting in increased sensitivity of the abscission zone (AZ) to ethylene. However, the molecular mechanisms that drive the acquisition of abscission competence and its modulation by auxin gradients are not yet known. We used RNA-Sequencing (RNA-Seq) to obtain a comprehensive transcriptome of tomato flower AZ (FAZ) and leaf AZ (LAZ) during abscission. RNA-Seq was performed on a pool of total RNA extracted from tomato FAZ and LAZ, at different abscission stages, followed by de novo assembly. The assembled clusters contained transcripts that are already known in the Solanaceae (SOL) genomics and NCBI databases, and over 8823 identified novel tomato transcripts of varying sizes. An AZ-specific microarray, encompassing the novel transcripts identified in this study and all known transcripts from the SOL genomics and NCBI databases, was constructed to study the abscission process. Multiple probes for longer genes and key AZ-specific genes, including antisense probes for all transcripts, make this array a unique tool for studying abscission with a comprehensive set of transcripts, and for mining for naturally occurring antisense transcripts. We focused on comparing the global transcriptomes generated from the FAZ and the LAZ to establish the divergences and similarities in their transcriptional networks, and particularly to characterize the processes and transcriptional regulators enriched in gene clusters that are differentially regulated in these two AZs. This study is the first attempt to analyze the global gene expression in different AZs in tomato by combining the RNA-Seq technique with oligonucleotide microarrays. Our AZ-specific microarray chip provides a cost-effective approach for expression profiling and robust analysis of multiple samples in a rapid succession.

Development and regulation of pedicel abscission in tomato

To shed unfertilized flowers or ripe fruits, many plant species develop a pedicel abscission zone (AZ), a specialized tissue that develops between the organ and the main body of the plant. Regulation of pedicel abscission is an important agricultural concern because pre-harvest abscission can reduce yields of fruit or grain crops, such as apples, rice, wheat, etc. Tomato has been studied as a model system for abscission, as tomato plants develop a distinct AZ at the midpoint of the pedicel and several tomato mutants, such as jointless, have pedicels that lack an AZ. This mini-review focuses on recent advances in research on the mechanisms regulating tomato pedicel abscission. Molecular genetic studies revealed that three MADS-box transcription factors interactively play a central role in pedicel AZ development. Transcriptome analyses identified activities involved in abscission and also found novel transcription factors that may regulate AZ activities. Another study identified transcription factors mediating abscission pathways from induction signals to activation of cell wall hydrolysis. These recent findings in tomato will enable significant advances in understanding the regulation of abscission in other key agronomic species.

Seed shattering: from models to crops

Seed shattering (or pod dehiscence, or fruit shedding) is essential for the propagation of their offspring in wild plants but is a major cause of yield loss in crops. In the dicot model species, Arabidopsis thaliana, pod dehiscence necessitates a development of the abscission zones along the pod valve margins. In monocots, such as cereals, an abscission layer in the pedicle is required for the seed shattering process. In the past decade, great advances have been made in characterizing the genetic contributors that are involved in the complex regulatory network in the establishment of abscission cell identity. We summarize the recent burgeoning progress in the field of genetic regulation of pod dehiscence and fruit shedding, focusing mainly on the model species A. thaliana with its close relatives and the fleshy fruit species tomato, as well as the genetic basis responsible for the parallel loss of seed shattering in domesticated crops. This review shows how these individual genes are co-opted in the developmental process of the tissues that guarantee seed shattering. Research into the genetic mechanism underlying seed shattering provides a premier prerequisite for the future breeding program for harvest in crops.

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.

Shaping plant architecture

Plants exhibit phenotypical plasticity. Their general body plan is genetically determined, but plant architecture and branching patterns are variable and can be adjusted to the prevailing environmental conditions. The modular design of the plant facilitates such morphological adaptations. The prerequisite for the formation of a branch is the initiation of an axillary meristem. Here, we review the current knowledge about this process. After its establishment, the meristem can develop into a bud which can either become dormant or grow out and form a branch. Many endogenous factors, such as photoassimilate availability, and exogenous factors like nutrient availability or shading, have to be integrated in the decision whether a branch is formed. The underlying regulatory network is complex and involves phytohormones and transcription factors. The hormone auxin is derived from the shoot apex and inhibits bud outgrowth indirectly in a process termed apical dominance. Strigolactones appear to modulate apical dominance by modification of auxin fluxes. Furthermore, the transcription factor BRANCHED1 plays a central role. The exact interplay of all these factors still remains obscure and there are alternative models. We discuss recent findings in the field along with the major models. Plant architecture is economically significant because it affects important traits of crop and ornamental plants, as well as trees cultivated in forestry or on short rotation coppices. As a consequence, plant architecture has been modified during plant domestication. Research revealed that only few key genes have been the target of selection during plant domestication and in breeding programs. Here, we discuss such findings on the basis of various examples. Architectural ideotypes that provide advantages for crop plant management and yield are described. We also outline the potential of breeding and biotechnological approaches to further modify and improve plant architecture for economic needs.

Unusual tandem expansion and positive selection in subgroups of the plant GRAS transcription factor superfamily

BACKGROUND

GRAS proteins belong to a plant transcription factor family that is involved with multifarious roles in plants. Although previous studies of this protein family have been reported for Arabidopsis, rice, Chinese cabbage and other species, investigation of expansion patterns and evolutionary rate on the basis of comparative genomics in different species remains inadequate.

RESULTS

A total of 289 GRAS genes were identified in Arabidopsis, B. distachyon, rice, soybean, S. moellendorffii, and P. patens and were grouped into seven subfamilies, supported by the similarity of their exon-intron patterns and structural motifs. All of tandem duplicated genes were found in group II except one cluster of rice, indicating that tandem duplication greatly promoted the expansion of group II. Furthermore, segment duplications were mainly found in the soybean genome, whereas no single expansion pattern dominated in other plant species indicating that GRAS genes from these five species might be subject to a more complex evolutionary mechanism. Interestingly, branch-site model analyses of positive selection showed that a number of sites were positively selected under foreground branches I and V. These results strongly indicated that these groups were experiencing higher positive selection pressure. Meanwhile, the site-specific model revealed that the GRAS genes were under strong positive selection in P. patens. DIVERGE v2.0 was used to detect critical amino acid sites, and the results showed that the shifted evolutionary rate was mainly attributed to the functional divergence between the GRAS genes in the two groups. In addition, the results also demonstrated the expression divergence of the GRAS duplicated genes in the evolution. In short, the results above provide a solid foundation for further functional dissection of the GRAS gene superfamily.

CONCLUSIONS

In this work, differential expression, evolutionary rate, and expansion patterns of the GRAS gene family in the six species were predicted. Especially, tandem duplication events played an important role in expansion of group II. Together, these results contribute to further functional analysis and the molecular evolution of the GRAS gene superfamily.