Auxin signaling in Arabidopsis flower development?

Hormones regulate the flowering process in saffron differently depending on the developmental stage

Flowering in saffron is a highly complex process regulated by the synchronized action of environmental cues and endogenous signals. Hormonal regulation of flowering is a very important process controlling flowering in several plants, but it has not been studied in saffron. Flowering in saffron is a continual process completed in months with distinct developmental phases, mainly divided into flowering induction and flower organogenesis/formation. In the present study, we investigated how phytohormones affect the flowering process at different developmental stages. The results suggest that different hormones differentially affect flower induction and formation in saffron. The exogenous treatment of flowering competent corms with abscisic acid (ABA) suppressed both floral induction and flower formation, whereas some other hormones, like auxins (indole acetic acid, IAA) and gibberellic acid (GA), behaved contrarily at different developmental stages. IAA promoted flower induction, while GA suppressed it; however, GA promoted flower formation, whereas IAA suppressed it. Cytokinin (kinetin) treatment suggested its positive involvement in flower induction and flower formation. The expression analysis of floral integrator and homeotic genes suggests that ABA might suppress floral induction by suppressing the expression of the floral promoter (LFY, FT3) and promoting the expression of the floral repressor (SVP) gene. Additionally, ABA treatment also suppressed the expression of the floral homeotic genes responsible for flower formation. GA reduces the expression of flowering induction gene LFY, while IAA treatment upregulated its expression. In addition to these genes, a flowering repressor gene, TFL1-2, was also found to be downregulated in IAA treatment. Cytokinin promotes flowering induction by increasing the expression levels of the LFY gene and decreasing the TFL1-2 gene expression. Moreover, it improved flower organogenesis by increasing the expression of floral homeotic genes. Overall, the results suggest that hormones differently regulate flowering in saffron via regulating floral integrator and homeotic gene expression.

Transcriptome Analysis to Explore the Cause of the Formation of Different Inflorescences in Tomato

The number of inflorescence branches is an important agronomic character of tomato. The meristem differentiation and development pattern of tomato inflorescence is complex and its regulation mechanism is very different from those of other model plants. Therefore, in order to explore the cause of tomato inflorescence branching, transcriptome analysis was conducted on two kinds of tomato inflorescences (single racemes and compound inflorescences). According to the transcriptome data analysis, there were many DEGs of tomato inflorescences at early, middle, and late stages. Then, GO and KEGG enrichments of DEGs were performed. DEGs are mainly enriched in metabolic pathways, biohormone signaling, and cell cycle pathways. According to previous studies, DEGs were mainly enriched in metabolic pathways, and FALSIFLORA (FA) and ANANTHA (AN) genes were the most notable of 41 DEGs related to inflorescence branching. This study not only provides a theoretical basis for understanding inflorescence branching, but also provides a new idea for the follow-up study of inflorescence.

Genome–Wide Identification and Functional Characterization of Auxin Response Factor (ARF) Genes in Eggplant

Auxin response factors (ARFs) are important plant transcription factors that are differentially expressed in response to auxin and various abiotic stresses. ARFs play important roles in mediating plant growth and stress responses; however, these factors have not been studied in eggplants. In this study, genome–wide identification and the functional analysis of the ARF gene family in eggplants (Solanum melongena L.) were performed. A total of 20 ARF (SmARF) genes were identified and phylogenetically classified into three groups. Our analysis revealed four functional domains and 10 motifs in these proteins. Subcellular localization showed that the SmARFs localized in the nucleus. To investigate the biological functions of the SmARFs under 2,4–D and salt stress treatments, quantitative real–time RT–PCR (qRT–PCR) was conducted. Most SmARF genes exhibited changes in expression in response to 2,4–D treatments in the flowers, especially SmARF4 and 7B. All SmARF genes quickly responded to salt stress, except SmARF17 and 19 in leaves, SmARF1A and 7B in roots, and SmARF2A, SmARF7B, and SmARF16B in stems. These results helped to elucidate the role of ARFs in auxin signaling under 2,4–D and salt stress in eggplants.

Comparative Genome Analysis of Genes Regulating Compound Inflorescences in Tomato

Inflorescences are the main factor affecting fruit yield. The quantity and quality of inflorescences are closely related to fruit quality and yield. The presence of compound inflorescences in cherry tomatoes is well established, and it has been discovered by chance that compound racemes also exist in tomatoes. To explore the formation of compound inflorescences in tomato, transcriptome sequencing was performed on Moneymaker (MM) and Compound Inflorescence (CI) plants. In-florescences were collected in three periods (early, middle and late) in three replicates, for a total of 18 samples. Data analysis showed that the DEGs were most enriched in metabolic pathways and plant hormone signal transduction pathways. The DEGs were also enriched in the cell cycle pathway, photosynthesis pathway, carbon metabolism pathway and circadian rhythm pathway. We found that the FALSIFLORA (FA), COMPOUND INFLORESCENCE (S) and ANANTHA (AN) genes were involved in compound inflorescence development, not only revealing novel genes but also providing a rich theoretical basis for compound inflorescence development.

Vitis flower types: from the wild to crop plants

Vitis vinifera can be divided into two subspecies, V. vinifera subsp. vinifera, one of the most important agricultural crops in the world, and its wild ancestor, V. vinifera subsp. sylvestris. Three flower types can be observed: hermaphrodite and female (on some varieties) in vinifera, and male or female flowers in sylvestris. It is assumed that the different flower types in the wild ancestor arose through specific floral patterns of organ abortion. A considerable amount of data about the diversity of sexual systems in grapevines has been collected over the past century. Several grapevine breeding studies led to the hypothesis that dioecy in vinifera is derived from a hermaphrodite ancestor and could be controlled by either, one or two linked genetic determinants following Mendelian inherence. More recently, experiments using molecular approaches suggested that these loci were located in a specific region of the chromosome 2 of vinifera. Based on the works published so far, its seems evident that a putative sex locus is present in chromosome 2. However, it is still not fully elucidated whether flower types are regulated by two linked loci or by one locus with three alleles. Nevertheless, several genes could contribute to sex determination in grapevine. This review presents the results from early studies, combined with the recent molecular approaches, which may contribute to the design of new experiments towards a better understanding of the sex inheritance in grapevine.

Down-regulation of AUXIN RESPONSE FACTORS 6 and 8 by microRNA 167 leads to floral development defects and female sterility in tomato

Auxin regulates the expression of diverse genes that affect plant growth and development. This regulation requires AUXIN RESPONSE FACTORS (ARFs) that bind to the promoter regions of these genes. ARF6 and ARF8 in Arabidopsis thaliana are required to promote inflorescence stem elongation and late stages of petal, stamen, and gynoecium development. All seed plants studied thus far have ARF6 and ARF8 orthologues as well as the microRNA miR167, which targets ARF6 and ARF8. Whether these genes have broadly conserved roles in flower development is not known. To address this question, the effects of down-regulation of ARF6 and ARF8 were investigated through transgenic expression of Arabidopsis MIR167a in tomato, which diverged from Arabidopsis before the radiation of dicotyledonous plants approximately 90-112 million years ago. The transgenic tomato plants overexpressing MIR167a exhibited reductions in leaf size and internode length as well as shortened petals, stamens, and styles. More significantly, the transgenic plants were female-sterile as a result of failure of wild-type pollen to germinate on the stigma surface and/or to grow through the style. RNA-Seq analysis identified many genes with significantly altered expression patterns, including those encoding products with functions in 'transcription regulation', 'cell wall' and 'lipid metabolism' categories. Putative orthologues of a subset of these genes were also differentially expressed in Arabidopsis arf6 arf8 mutant flowers. These results thus suggest that ARF6 and ARF8 have conserved roles in controlling growth and development of vegetative and flower organs in dicots.

Auxin regulation of Arabidopsis flower development involves members of the AINTEGUMENTA-LIKE/PLETHORA (AIL/PLT) family

Auxin is an important regulator of many aspects of plant growth and development. During reproductive development, auxin specifies the site of flower initiation and subsequently regulates organ growth and patterning as well as later events that determine reproductive success. Underlying auxin action in plant tissues is its uneven distribution, resulting in groups of cells with high auxin levels (auxin maxima) or graded distributions of the hormone (auxin gradients). Dynamic auxin distribution within the periphery of the inflorescence meristems specifies the site of floral meristem initiation, while auxin maxima present at the tips of developing floral organ primordia probably mediate organ growth and patterning. The molecular means by which auxin accumulation patterns are converted into developmental outputs in flowers is not well understood. Members of the AINTEGUMENTA-LIKE/PLETHORA (AIL/PLT) transcription factor family are important developmental regulators in both roots and shoots. In roots, the expression of two AIL/PLT genes is regulated by auxin and these genes feed back to regulate auxin distribution. Here, several aspects of flower development involving both auxin and AIL/PLT activity are described, and evidence linking AIL/PLT function with auxin distribution in reproductive tissues is presented.

Auxin flow in anther filaments is critical for pollen grain development through regulating pollen mitosis

It was well known that auxin is critical for anther/pollen grain development, however, the clear distribution and detailed effects of auxin during floral development are still unclear. We have shown here that, through analyzing GUS activities of Arabidopsis lines harboring auxin response elements DR5-GUS, auxin was mainly accumulated in the anther during flower stages 10-12. Further studies employing the indoleacetic acid-lysine synthetase (iaaL) coding gene from Pseudomonas syringae subsp. savastanoi under control of the promoter region of Arabidopsis phosphatidylinositol monophosphate 5-kinase 1 gene, which conducts the anther filament-specific expression, showed that block of auxin flow of filaments resulted in shortened filaments and significantly defective pollen grains. Similar phenotype was observed in tobacco plants transformed with the same construct, confirming the effects of auxin flow in filaments on anther development. Detailed studies further revealed that the meiosis process of pollen grain was normal while the mitosis at later stage was significantly defected, indicating the effects of auxin flow in filaments on pollen grain mitosis process. Analysis employing [(14)C]IAA, as well as the observation on the expression of AtPIN1, coding for auxin efflux carrier, demonstrated the presence of polar auxin transport in anther filaments and pollen grains.

Role of auxin in regulating Arabidopsis flower development

To elucidate the role of auxin in flower morphogenesis, its distribution patterns were studied during flower development in Arabidopsis thaliana (L.) Heynh. Expression of DR5::GUS was regarded to reflect sites of free auxin, while immunolocalization with auxin polyclonal antibodies visualized conjugated auxin distribution. The youngest flower bud was loaded with conjugated auxin. During development, the apparent concentration of free auxin increased in gradual patterns starting at the floral-organ tip. Anthers are major sites of high concentrations of free auxin that retard the development of neighboring floral organs in both the acropetal and basipetal directions. The IAA-producing anthers synchronize flower development by retarding petal development and nectary gland activity almost up to anthesis. Tapetum cells of young anthers contain free IAA which accumulates in pollen grains, suggesting that auxin promotes pollen-tube growth towards the ovules. High amounts of free auxin in the stigma induce a wide xylem fan immediately beneath it. After fertilization, the developing embryos and seeds show elevated concentrations of auxin, which establish their axial polarity. This developmental pattern of auxin production during floral-bud development suggests that young organs which produce high concentrations of free IAA inhibit or retard organ-primordium initiation and development at the shoot tip.

Auxin-induced reactive oxygen species production requires the activation of phosphatidylinositol 3-kinase

We recently reported that production of reactive oxygen species (ROS) is essential for auxin-induced gravitropic signaling. Here, we investigated the role of phosphatidylinositol 3-kinase and its product, PtdIns(3)P, in auxin-mediated ROS production and the root gravitropic response. Pretreatment with LY294002, an inhibitor of PtdIns 3-kinase activity, blocked auxin-mediated ROS generation, and reduced the sensitivity of root tissue to gravistimulation. The amount of PtdIns(3)P increased in response to auxin, and this effect was abolished by pretreatment with LY294002. In addition, sequestration of PtdIns(3)P by transient expression of the endosome binding domain in protoplasts abrogated IAA-induced ROS accumulation. These results indicate that activation of PtdIns 3-kinase and its product PtdIns(3)P are required for auxin-induced production of ROS and root gravitropism.

The PIN and LAX families of auxin transport genes in Medicago truncatula

Auxin transport proteins may be involved in nodule development. As a prelude to investigating the roles of these proteins in nodule development, we took advantage of the genetic and molecular resources available in the legume Medicago truncatula to characterize the gene families encoding auxin efflux and influx carriers. We identified ten auxin efflux carrier sequences (MtPINs) and five auxin influx/permease sequences (MtLAXs). The genomic sequence of each of these fifteen genes was determined, the genes were mapped on the publicly available map of M. truncatula, and their expression was examined in shoot and root tissue of nodulating plants. With one exception, transcripts of all MtPIN genes were detected. The expression of MtPIN2 was limited to nodulating roots, while transcripts of all other expressed genes were detected in both shoots and roots. Both the PIN and LAX gene families contain more members in M. truncatula than in Arabidopsis, but the gene families are not significantly expanded. Sequence comparison of the M. truncatula PIN and LAX genes with PIN and LAX genes from other dicots and monocots indicates that both gene families share a common overall structure, with areas of high homology both within M. truncatula and across species boundaries. Molecular phylogenies of both the PIN and LAX gene families were constructed. Combined with intron position and expression data, the phylogenies were used to assign relationships between MtPIN and MtLAX genes and the orthologous Arabidopsis PIN and LAX genes. MtPIN2 and MtPIN7 appear to be the result of a recent gene duplication with subsequent divergence of expression patterns. These results set the stage for the use of these genes in research on the role of auxin in nodulation.

The tobacco bZIP transcription factor BZI-1 binds the GH3 promoter in vivo and modulates auxin-induced transcription

Summary In order to establish the biological function of the tobacco basic leucine zipper (bZIP) transcription factor BZI-1 in hormone signalling, we have analysed transgenic plants which were altered with respect to the protein level or the activation potential of BZI-1. Overexpression of a dominant-negative derivative of BZI-1, lacking the N-terminal activation domain, resulted in plants displaying reduced internode size, enhanced lateral shoot formation and small, curly leaves. The response to auxin monitored with reference to root organogenesis, epinastic leaf curvature and transcription of the auxin-induced GH3 gene was reduced. In vitro, BZI-1 specifically binds to ACGT elements (ACEs) present in the GH3 promoter. In vivo, binding to the GH3 promoter was confirmed by chromatin immunoprecipitation (ChIP). Overexpression of BZI-1 in transgenic plants did not lead to a significant activation of the GH3 target gene. In contrast, plants expressing a VP16 (Herpes simplex virion protein 16)-BZI-1 fusion protein showed enhanced auxin-induced GH3 transcription. However, VP16-BZI-1 is insufficient to trigger GH3 expression independently of the auxin stimulus. Whereas auxin responsiveness has been shown to be mediated by ARF (auxin response factor) transcription factors, we discuss a function of BZI-1 assisting in fine-tuning of auxin-induced transcription.

DEVELOPMENT OF SYMMETRY IN PLANTS

Plant development involves specification and elaboration of axes of asymmetry. The apical-basal and inside-outside axes arise in embryogenesis, and are probably oriented maternally. They are maintained during growth post-germination and interact to establish novel axes of asymmetry in flowers and lateral organs (such as leaves). Whereas the genetic control of axis elaboration is now partially understood in embryos, floral meristems, and organs, the underlying mechanisms of axis specification remain largely obscure. Less functionally significant aspects of plant asymmetry (e.g. the handedness of spiral phyllotaxy) may originate in random events and therefore have no genetic control.

PIN-pointing the molecular basis of auxin transport

Significant advances in the genetic dissection of the auxin transport pathway have recently been made. Particularly relevant is the molecular analysis of mutants impaired in auxin transport and the subsequent cloning of genes encoding candidate proteins for the elusive auxin efflux carrier. These studies are thought to pave the way to the detailed understanding of the molecular basis of several important facets of auxin action.