Chiara Mizzotti

The MADS box genes SEEDSTICK and ARABIDOPSIS Bsister play a maternal role in fertilization and seed development

The haploid generation of flowering plants develops within the sporophytic tissues of the ovule. After fertilization, the maternal seed coat develops in a coordinated manner with formation of the embryo and endosperm. In the arabidopsis bsister (abs) mutant, the endothelium, which is the most inner cell layer of the integuments that surround the haploid embryo sac, does not accumulate proanthocyanidins and the cells have an abnormal morphology. However, fertility is not affected in abs single mutants. SEEDSTICK regulates ovule identity redundantly with SHATTERPROOF 1 (SHP1) and SHP2 while a role in the control of fertility was not reported previously. Here we describe the characterization of the abs stk double mutant. This double mutant develops very few seeds due to both a reduced number of fertilized ovules and seed abortions later during development. Morphological analysis revealed a total absence of endothelium in this double mutant. Additionally, massive starch accumulation was observed in the embryo sac. The phenotype of the abs stk double mutant highlights the importance of the maternal-derived tissues, particularly the endothelium, for the development of the next generation.

Genetic regulation and structural changes during tomato fruit development and ripening

Fruits are an important evolutionary acquisition of angiosperms, which afford protection for seeds and ensure their optimal dispersal in the environment. Fruits can be divided into dry or fleshy. Dry fruits are the more ancient and provide for mechanical seed dispersal. In contrast, fleshy fruits develop soft tissues in which flavor compounds and pigments accumulate during the ripening process. These serve to attract animals that eat them and disseminate the indigestible seeds. Fruit maturation is accompanied by several striking cytological modifications. In particular, plastids undergo significant structural alterations, including the dedifferentiation of chloroplasts into chromoplasts. Chloroplast biogenesis, their remodeling in response to environmental constraints and their conversion into alternative plastid types are known to require communication between plastids and the nucleus in order to coordinate the expression of their respective genomes. In this review, we discuss the role of plastid modifications in the context of fruit maturation and ripening, and consider the possible involvement of organelle-nucleus crosstalk via retrograde (plastid to nucleus) and anterograde (nucleus to plastid) signaling in the process.

SEEDSTICK is a Master Regulator of Development and Metabolism in the Arabidopsis Seed Coat

The role of secondary metabolites in the determination of cell identity has been an area of particular interest over recent years, and studies strongly indicate a connection between cell fate and the regulation of enzymes involved in secondary metabolism. In Arabidopsis thaliana, the maternally derived seed coat plays pivotal roles in both the protection of the developing embryo and the first steps of germination. In this regard, a characteristic feature of seed coat development is the accumulation of proanthocyanidins (PAs - a class of phenylpropanoid metabolites) in the innermost layer of the seed coat. Our genome-wide transcriptomic analysis suggests that the ovule identity factor SEEDSTICK (STK) is involved in the regulation of several metabolic processes, providing a strong basis for a connection between cell fate determination, development and metabolism. Using phenotypic, genetic, biochemical and transcriptomic approaches, we have focused specifically on the role of STK in PA biosynthesis. Our results indicate that STK exerts its effect by direct regulation of the gene encoding BANYULS/ANTHOCYANIDIN REDUCTASE (BAN/ANR), which converts anthocyanidins into their corresponding 2,3-cis-flavan-3-ols. Our study also demonstrates that the levels of H3K9ac chromatin modification directly correlate with the active state of BAN in an STK-dependent way. This is consistent with the idea that MADS-domain proteins control the expression of their target genes through the modification of chromatin states. STK might thus recruit or regulate histone modifying factors to control their activity. In addition, we show that STK is able to regulate other BAN regulators. Our study demonstrates for the first time how a floral homeotic gene controls tissue identity through the regulation of a wide range of processes including the accumulation of secondary metabolites.

The Developmental Regulator SEEDSTICK Controls Structural and Mechanical Properties of the Arabidopsis Seed Coat

A molecular network controlled by STK regulates cell wall properties of the seed coat, showing that, in addition to specifying organ identity, the role of STK in seed development includes modification of the cell wall structure. Although many transcription factors involved in cell wall morphogenesis have been identified and studied, it is still unknown how genetic and molecular regulation of cell wall biosynthesis is integrated into developmental programs. We demonstrate by molecular genetic studies that SEEDSTICK (STK), a transcription factor controlling ovule and seed integument identity, directly regulates PMEI6 and other genes involved in the biogenesis of the cellulose-pectin matrix of the cell wall. Based on atomic force microscopy, immunocytochemistry, and chemical analyses, we propose that structural modifications of the cell wall matrix in the stk mutant contribute to defects in mucilage release and seed germination under water-stress conditions. Our studies reveal a molecular network controlled by STK that regulates cell wall properties of the seed coat, demonstrating that developmental regulators controlling organ identity also coordinate specific aspects of cell wall characteristics.

The backstage of the ABC model: The Antirrhinum majus contribution

At the beginning of the 1990s, a simple genetic model that explained flower development was presented based on Arabidopsis thaliana and Antirrhinum majus floral homeotic mutants. According to this model, which is a milestone in plant development studies, flower development can be explained by three classes of genes (A, B and C), each one controlling the identity of organs in two adjacent whorls. Intriguingly, more than 20 years later, there are still some unanswered questions, in particular regarding the universality of the class A-function genes. Class A genes are well characterised in A. thaliana, but so far no A mutants have been described in other plant species nor in Antirrhinum majus. Here, we retrace the story that led to the proposal of the ABC model focusing on the contribution of A. majus to this model. Although fewer groups are still using A. majus as a model system, this plant was a master contributor to our comprehension of the molecular networks controlling flower development.

SUPPRESSOR OF FRIGIDA (SUF4) supports gamete fusion via regulating arabidopsis EC1 gene expression

In Arabidopsis, gamete fusion requires the C2H2 transcription factor SUF4, which regulates the expression of the EGG CELL1 gene family. The EGG CELL1 (EC1) gene family of Arabidopsis (Arabidopsis thaliana) comprises five members that are specifically expressed in the egg cell and redundantly control gamete fusion during double fertilization. We investigated the activity of all five EC1 promoters in promoter-deletion studies and identified SUF4 (SUPPRESSOR OF FRIGIDA4), a C2H2 transcription factor, as a direct regulator of the EC1 gene expression. In particular, we demonstrated that SUF4 binds to all five Arabidopsis EC1 promoters, thus regulating their expression. The down-regulation of SUF4 in homozygous suf4-1 ovules results in reduced EC1 expression and delayed sperm fusion, which can be rescued by expressing SUF4-β-glucuronidase under the control of the SUF4 promoter. To identify more gene products able to regulate EC1 expression together with SUF4, we performed coexpression studies that led to the identification of MOM1 (MORPHEUS’ MOLECULE1), a component of a silencing mechanism that is independent of DNA methylation marks. In mom1-3 ovules, both SUF4 and EC1 genes are down-regulated, and EC1 genes show higher levels of histone 3 lysine-9 acetylation, suggesting that MOM1 contributes to the regulation of SUF4 and EC1 gene expression.

Breeding for grapevine downy mildew resistance: a review of “omics” approaches

Downy mildew (DM) is one of the most destructive diseases affecting viticulture, especially in temperate-humid climates. This pathogen is able to differentially attack leaves and grapes, and is currently controlled with repeated applications of fungicides that lead to environmental pollution, development of resistant strains, residual toxicity, and pathogen pressure. The use of resistant grapevine varieties, obtained through breeding programs, represents a potential alternative to control grapevine DM, although this approach is constrained by the limits of natural resistance. In this review, we describe the latest breakthroughs achieved in the exploitation of the host–pathogen interaction, which have led to the identification of DM resistance loci, with associated molecular markers, that can be employed to efficiently screen hybrid grapevines and improve (pre-)breeding programs. The development of controlled infection protocols and contemporary “omics” approaches (next generation sequencing/genomics, QTLomics, transcriptomics, proteomics and metabolomics) integrated with comparative studies are shedding light onto the early host responses to DM attack and the complex plant defence mechanisms that are triggered.

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.

Trans-splicing of plastid rps12 transcripts, mediated by AtPPR4, is essential for embryo patterning in Arabidopsis thaliana

Main conclusionAtPPR4-mediated trans-splicing of plastid rps12 transcripts is essential for key embryo morphogenetic events such as development of cotyledons, determination of provascular tissue, and organization of the shoot apical meristem (SAM), but not for the formation of the protodermal layer.Members of the pentatricopeptide repeat (PPR) containing protein family have emerged as key regulators of the organelle post-transcriptional processing and to be essential for proper plant embryo development. In this study, we report the functional characterization of the AtPPR4 (At5g04810) gene encoding a plastid nucleoid PPR protein. In-situ hybridization analysis reveals the presence of AtPPR4 transcripts already at the transition stage of embryo development. As a consequence, embryos lacking the AtPPR4 protein arrest their development at the transition/early-heart stages and show defects in the determination of the provascular tissue and organization of SAM. This complex phenotype is due to the specific role of AtPPR4 in the trans-splicing of the plastid rps12 transcripts, as shown by northern and slot-blot hybridizations, and the consequent defect in 70S ribosome accumulation and plastid protein synthesis, in agreement with the role proposed for the maize orthologue, ZmPPR4.

Time-course transcriptome analysis of Arabidopsis siliques discloses genes essential for fruit development and maturation

Dynamic changes in the transcriptome of developing seedless fruits reveals key regulators of fruit growth and maturation. Fruits protect the developing seeds of angiosperms and actively contribute to seed dispersion. Furthermore, fruit and seed development are highly synchronized and require exchange of information between the mother plant and the developing generations. To explore the mechanisms controlling fruit formation and maturation, we performed a transcriptomic analysis on the valve tissue of the Arabidopsis (Arabidopsis thaliana) silique using RNA sequencing. In doing so, we have generated a data set of differentially regulated genes that will help to elucidate the molecular mechanisms that underpin the initial phase of fruit growth and, subsequently, trigger fruit maturation. The robustness of our data set has been tested by functional genomic studies. Using a reverse genetics approach, we selected 10 differentially expressed genes and explored the consequences of their disruption for both silique growth and senescence. We found that genes contained in our data set play essential roles in different stages of silique development and maturation, indicating that our transcriptome-based gene list is a powerful tool for the elucidation of the molecular mechanisms controlling fruit formation in Arabidopsis.