Development of pollinated and unpollinated ovules in Ginkgo biloba: unravelling pollen's role in ovule tissue maturation.

In gymnosperms such as Ginkgo biloba, the pollen's arrival plays a key role in ovule development, before fertilization occurs. Accordingly, G. biloba female plants geographically isolated from male plants aborted all their ovules after the pollination drop emission, which is the event that allows the ovule to capture pollen grains. To decipher the mechanism induced by pollination required to avoid ovule senescence and then abortion, we compared the transcriptomic of pollinated and unpollinated ovules at three time points after the end of the emission of pollination drops. Transcriptomic and in situ expression analyses revealed that several key genes involved in programmed cell death such as senescence and apoptosis, DNA replication, and cell cycle were differentially expressed in unpollinated ovules compared to pollinated ones. Interestingly, we provided evidence that the pollen captured by the pollination drop affects auxin local accumulation and might cause the deregulation of key genes required for ovule's programmed cell death and activating both the cell cycle and DNA replication genes.

Maternal control of triploid seed development by the TRANSPARENT TESTA 8 (TT8) transcription factor in Arabidopsis thaliana.

The balance between parental genome dosage is critical to offspring development in both animals and plants. In some angiosperm species, despite the imbalance between maternally and paternally inherited chromosome sets, crosses between parental lines of different ploidy may result in viable offspring. However, many plant species, like Arabidopsis thaliana, present a post-zygotic reproductive barrier, known as triploid block which results in the inability of crosses between individuals of different ploidy to generate viable seeds but also, in defective development of the seed. Several paternal regulators have been proposed as active players in establishing the triploid block. Maternal regulators known to be involved in this process are some flavonoid biosynthetic (FB) genes, expressed in the innermost layer of the seed coat. Here we explore the role of selected flavonoid pathway genes in triploid block, including TRANSPARENT TESTA 4 (TT4), TRANSPARENT TESTA 7 (TT7), SEEDSTICK (STK), TRANSPARENT TESTA 16 (TT16), TT8 and TRANSPARENT TESTA 13 (TT13). This approach allowed us to detect that TT8, a bHLH transcription factor, member of this FB pathway is required for the paternal genome dosage, as loss of function tt8, leads to complete rescue of the triploid block to seed development.

The Genetic Control of SEEDSTICK and LEUNIG-HOMOLOG in Seed and Fruit Development: New Insights into Cell Wall Control.

Although much is known about seed and fruit development at the molecular level, many gaps remain in our understanding of how cell wall modifications can impact developmental processes in plants, as well as how biomechanical alterations influence seed and fruit growth. Mutants of Arabidopsis thaliana constitute an excellent tool to study the function of gene families devoted to cell wall biogenesis. We have characterized a collection of lines carrying mutations in representative cell wall-related genes for seed and fruit size developmental defects, as well as altered germination rates. We have linked these studies to cell wall composition and structure. Interestingly, we have found that disruption of genes involved in pectin maturation and hemicellulose deposition strongly influence germination dynamics. Finally, we focused on two transcriptional regulators, SEEDSTICK (STK) and LEUNIG-HOMOLOG (LUH), which positively regulate seed growth. Herein, we demonstrate that these factors regulate specific aspects of cell wall properties such as pectin distribution. We propose a model wherein changes in seed coat structure due to alterations in the xyloglucan-cellulose matrix deposition and pectin maturation are critical for organ growth and germination. The results demonstrate the importance of cell wall properties and remodeling of polysaccharides as major factors responsible for seed development.

Cell wall modifications by α-XYLOSIDASE1 are required for control of seed and fruit size in Arabidopsis.

Cell wall modifications are of pivotal importance during plant development. Among cell wall components, xyloglucans are the major hemicellulose polysaccharide in primary cell walls of dicots and non-graminaceous monocots. They can connect the cellulose microfibril surface to affect cell wall mechanical properties. Changes in xyloglucan structure are known to play an important role in regulating cell growth. Therefore, the degradation of xyloglucan is an important modification that alters the cell wall. The α-XYLOSIDASE1 (XYL1) gene encodes the only α-xylosidase acting on xyloglucans in Arabidopsis thaliana. Here, we showed that mutation of XYL1 strongly influences seed size, seed germination, and fruit elongation. We found that the expression of XYL1 is directly regulated in developing seeds and fruit by the MADS-box transcription factor SEEDSTICK. We demonstrated that XYL1 complements the stk smaller seed phenotype. Finally, by atomic force microscopy, we investigated the role of XYL1 activity in maintaining cell stiffness and growth, confirming the importance of cell wall modulation in shaping organs.

The role of pollination in controlling Ginkgo biloba ovule development.

Generally, in gymnosperms, pollination and fertilization events are temporally separated and the developmental processes leading the switch from ovule integument into seed coat are still unknown. The single ovule integument of Ginkgo biloba acquires the typical characteristics of the seed coat long before the fertilization event. In this study, we investigated whether pollination triggers the transformation of the ovule integument into the seed coat. Transcriptomics and metabolomics analyses performed on ovules just prior and after pollination lead to the identification of changes occurring in Ginkgo ovules during this specific time. A morphological atlas describing the developmental stages of ovule development is presented. The metabolic pathways involved in the lignin biosynthesis and in the production of fatty acids are activated upon pollination, suggesting that the ovule integument starts its differentiation into a seed coat before the fertilization. Omics analyses allowed an accurate description of the main changes that occur in Ginkgo ovules during the pollination time frame, suggesting the crucial role of the pollen arrival on the progression of ovule development.

MADS-Box and bHLH Transcription Factors Coordinate Transmitting Tract Development in Arabidopsis thaliana.

The MADS-domain transcription factor SEEDSTICK (STK) controls several aspects of plant reproduction. STK is co-expressed with CESTA (CES), a basic Helix-Loop-Helix (bHLH) transcription factor-encoding gene. CES was reported to control redundantly with the brassinosteroid positive signaling factors BRASSINOSTEROID ENHANCED EXPRESSION1 (BEE1) and BEE3 the development of the transmitting tract. Combining the stk ces-4 mutants led to a reduction in ovule fertilization due to a defect in carpel fusion which, caused the formation of holes at the center of the septum where the transmitting tract differentiates. Combining the stk mutant with the bee1 bee3 ces-4 triple mutant showed an increased number of unfertilized ovules and septum defects. The transcriptome profile of this quadruple mutant revealed a small subset of differentially expressed genes which are mainly involved in cell death, extracellular matrix and cell wall development. Our data evidence a regulatory gene network controlling transmitting tract development regulated directly or indirectly by a STK-CES containing complex and reveal new insights in the regulation of transmitting tract development by bHLH and MADS-domain transcription factors.

Gynoecium size and ovule number are interconnected traits that impact seed yield.

Angiosperms form the largest group of land plants and display an astonishing diversity of floral structures. The development of flowers greatly contributed to the evolutionary success of the angiosperms as they guarantee efficient reproduction with the help of either biotic or abiotic vectors. The female reproductive part of the flower is the gynoecium (also called pistil). Ovules arise from meristematic tissue within the gynoecium. Upon fertilization, these ovules develop into seeds while the gynoecium turns into a fruit. Gene regulatory networks involving transcription factors and hormonal communication regulate ovule primordium initiation, spacing on the placenta, and development. Ovule number and gynoecium size are usually correlated and several genetic factors that impact these traits have been identified. Understanding and fine-tuning the gene regulatory networks influencing ovule number and pistil length open up strategies for crop yield improvement, which is pivotal in light of a rapidly growing world population. In this review, we present an overview of the current knowledge of the genes and hormones involved in determining ovule number and gynoecium size. We propose a model for the gene regulatory network that guides the developmental processes that determine seed yield.

SEEDSTICK Controls Arabidopsis Fruit Size by Regulating Cytokinin Levels and FRUITFULL.

Upon fertilization, the ovary increases in size and undergoes a complex developmental process to become a fruit. We show that cytokinins (CKs), which are required to determine ovary size before fertilization, have to be degraded to facilitate fruit growth. The expression of CKX7, which encodes a cytosolic CK-degrading enzyme, is directly positively regulated post-fertilization by the MADS-box transcription factor STK. Similar to stk, two ckx7 mutants possess shorter fruits than wild type. Quantification of CKs reveals that stk and ckx7 mutants have high CK levels, which negatively control cell expansion during fruit development, compromising fruit growth. Overexpression of CKX7 partially complements the stk fruit phenotype, confirming a role for CK degradation in fruit development. Finally, we show that STK is required for the expression of FUL, which is essential for valve elongation. Overall, we provide insights into the link between CKs and molecular pathways that control fruit growth.

New roles of NO TRANSMITTING TRACT and SEEDSTICK during medial domain development in Arabidopsis fruits.

The gynoecium, the female reproductive part of the flower, is key for plant sexual reproduction. During its development, inner tissues such as the septum and the transmitting tract tissue, important for pollen germination and guidance, are formed. In Arabidopsis, several transcription factors are known to be involved in the development of these tissues. One of them is NO TRANSMITTING TRACT (NTT), essential for transmitting tract formation. We found that the NTT protein can interact with several gynoecium-related transcription factors, including several MADS-box proteins, such as SEEDSTICK (STK), known to specify ovule identity. Evidence suggests that NTT and STK control enzyme and transporter-encoding genes involved in cell wall polysaccharide and lipid distribution in gynoecial medial domain cells. The results indicate that the simultaneous loss of NTT and STK activity affects polysaccharide and lipid deposition and septum fusion, and delays entry of septum cells to their normal degradation program. Furthermore, we identified KAWAK, a direct target of NTT and STK, which is required for the correct formation of fruits in Arabidopsis These findings position NTT and STK as important factors in determining reproductive competence.

Seed abscission and fruit dehiscence required for seed dispersal rely on similar genetic networks.

Seed dispersal is an essential trait that enables colonization of new favorable habitats, ensuring species survival. In plants with dehiscent fruits, such as Arabidopsis, seed dispersal depends on two processes: the separation of the fruit valves that protect the seeds (fruit dehiscence) and the detachment of the seeds from the funiculus connecting them to the mother plant (seed abscission). The key factors required to establish a proper lignin pattern for fruit dehiscence are SHATTERPROOF 1 and 2 (SHP1 and SHP2). Here, we demonstrate that the SHP-related gene SEEDSTICK (STK) is a key factor required to establish the proper lignin pattern in the seed abscission zone but in an opposite way. We show that STK acts as a repressor of lignin deposition in the seed abscission zone through the direct repression of HECATE3, whereas the SHP proteins promote lignin deposition in the valve margins by activating INDEHISCENT. The interaction of STK with the SEUSS co-repressor determines the difference in the way STK and SHP proteins control the lignification patterns. Despite this difference in the molecular control of lignification during seed abscission and fruit dehiscence, we show that the genetic networks regulating these two developmental pathways are highly conserved.