Parental effects on seed mass: seed coat but not embryo/endosperm effects

Many biologists studying environmentally induced parental effects have indirectly suggested that the parental environment alters seed mass by altering the amount of endosperm or embryo tissue in the seed. We tested this hypothesis by measuring the effects of parental temperature on total seed mass, seed coat mass, and embryo/endosperm mass in offspring of Plantago lanceolata. Parental temperature significantly affected total seed and coat mass but not endosperm/embryo mass. Thus, larger seeds do not contain more resources in the embryo or endosperm than do small seeds. Rather they have more coat mass, which probably strongly influences germination. These results suggest caution when making assumptions about the pathways by which environmentally induced parental effects are transmitted in plant species. We also observed that controlled crosses differed significantly in their response to parental temperature, which provides evidence for genetic variation in environmentally induced parental effects, i.e., intergenerational phenotypic plasticity, in natural populations of P. lanceolata.     

Genetic analysis of seed coat development in Arabidopsis

In the angiosperms, fertilization initiates the formation of the seed from the ovule, including the differentiation of the seed coat from the ovule integuments. Seed coat differentiation includes some of the most dramatic cellular changes of seed development and culminates in the death of the seed coat cells. Recently, genetic analyses in Arabidopsis have contributed substantially to our understanding of many aspects of seed coat biology and it might not be long before the entire differentiation pathway is understood. Such an advance would contribute substantially to our understanding of many important cellular events, including secondary cell wall synthesis, cell morphogenesis, vacuolar targeting and cell death, and would provide tools for the manipulation of seed dormancy and germination.     

Maternal control of integument cell elongation and zygotic control of endosperm growth are coordinated to determine seed size in Arabidopsis

We use Arabidopsis thaliana as a model to investigate coordination of cell proliferation and cell elongation in the three components that develop side by side in the seed. Two of these, the embryo and its nurturing annex, the endosperm, are placed under zygotic control and develop within the seed integument placed under maternal control. We show that integument cell proliferation and endosperm growth are largely independent from each other. By contrast, prevention of cell elongation in the integument by the mutation transparent testa glabra2 (ttg2) restricts endosperm and seed growth. Conversely, endosperm growth controlled by the HAIKU (IKU) genetic pathway modulates integument cell elongation. Combinations of TTG2 defective seed integument with reduction of endosperm size by iku mutations identify integument cell elongation and endosperm growth as the primary regulators of seed size. Our results strongly suggest that a cross talk between maternal and zygotic controls represents the primary regulator of the coordinated control of seed size in Arabidopsis.     

Arabidopsis haiku mutants reveal new controls of seed size by endosperm

In flowering plants, maternal seed integument encloses the embryo and the endosperm, which are both derived from double fertilization. Although the development of these three components must be coordinated, we have limited knowledge of mechanisms involved in such coordination. The endosperm may play a central role in these mechanisms as epigenetic modifications of endosperm development, via imbalance of dosage between maternal and paternal genomes, affecting both the embryo and the integument. To identify targets of such epigenetic controls, we designed a genetic screen in Arabidopsis for mutants that phenocopy the effects of dosage imbalance in the endosperm. The two mutants haiku 1 and haiku 2 produce seed of reduced size that resemble seed with maternal excess in the maternal/paternal dosage. Homozygous haiku seed develop into plants indistinguishable from wild type. Each mutation is sporophytic recessive, and double-mutant analysis suggests that both mutations affect the same genetic pathway. The endosperm of haiku mutants shows a premature arrest of increase in size that causes precocious cellularization of the syncytial endosperm. Reduction of seed size in haiku results from coordinated reduction of endosperm size, embryo proliferation, and cell elongation of the maternally derived integument. We present further evidence for a control of integument development mediated by endosperm-derived signals.     

The female gametophyte and the endosperm control cell proliferation and differentiation of the seed coat in Arabidopsis

Double fertilization of the female gametophyte produces the endosperm and the embryo enclosed in the maternal seed coat. Proper seed communication necessitates exchanges of signals between the zygotic and maternal components of the seed. However, the nature of these interactions remains largely unknown. We show that double fertilization of the Arabidopsis thaliana female gametophyte rapidly triggers sustained cell proliferation in the seed coat. Cell proliferation and differentiation of the seed coat occur in autonomous seeds produced in the absence of fertilization of the multicopy suppressor of ira1 (msi1) mutant. As msi1 autonomous seeds mostly contain autonomous endosperm, our results indicate that the developing endosperm is sufficient to enhance cell proliferation and differentiation in the seed coat. We analyze the effect of autonomous proliferation in the retinoblastoma-related1 (rbr1) female gametophyte on seed coat development. In contrast with msi1, supernumerary nuclei in rbr1 female gametophytes originate mainly from the endosperm precursor lineage but do not express an endosperm fate marker. In addition, defects of the rbr1 female gametophyte also reduce cell proliferation in the ovule integuments before fertilization and prevent further differentiation of the seed coat. Our data suggest that coordinated development of the seed components relies on interactions before fertilization between the female gametophyte and the surrounding maternal ovule integuments and after fertilization between the endosperm and the seed coat.     

Natural variation in the degree of autonomous endosperm formation reveals independence and constraints of embryo growth during seed development in Arabidopsis thaliana

Seed development in flowering plants is a paradigm for the coordination of different tissues during organ growth. It requires a tight interplay between the two typically sexually produced structures: the embryo, developing from the fertilized egg cell, and the endosperm, originating from a fertilized central cell, along with the surrounding maternal tissues. Little is known about the presumptive signal transduction pathways administering and coordinating these different tissues during seed growth and development. Recently, a new signal has been identified emanating from the fertilization of the egg cell that triggers central cell proliferation without prior fertilization. Here, we demonstrate that there exists a large natural genetic variation with respect to the outcome of this signaling process in the model plant Arabidopsis thaliana. By using a recombinant inbred line population between the two Arabidopsis accessions Bayreuth-0 and Shahdara, we have identified two genetic components that influence the development of unfertilized endosperm. Exploiting this natural variation, we could further dissect the interdependence of embryo and endosperm growth during early seed development. Our data show an unexpectedly large degree of independence in embryo growth, but also reveal the embryo's developmental restrictions with respect to endosperm size. This work provides a genetic framework for dissection of the interplay between embryo and endosperm during seed growth in plants.     

Sulfurtransferases 1 and 2 play essential roles in embryo and seed development in Arabidopsis thaliana

Sulfurtransferases (STRs) catalyze the transfer of a sulfur atom from a donor to a suitable acceptor molecule. The Arabidopsis thaliana genome encodes 20 putative STR proteins. The biological functions of most are unclear. We found that STR1 and STR2 play important roles in embryo/seed development. Mutation of STR1 alone resulted in a shrunken seed phenotype, although growth and development of vegetative and reproductive organs were not affected. The shrunken seed phenotype was associated with the delayed/arrested embryo development, in most cases, at the heart stage. The embryo defect of str1 mutant is not fully penetrant. Approximately 12.5% of embryos developed further and formed normal looking seeds. In severely shrunken seeds, no embryo could be identified after seed collection. Partially shrunken seeds that contained viable embryos could still germinate. However, cotyledons of the seedlings from such seeds were abnormal. An STR1-GUS fusion reporter revealed that the STR1 gene was universally expressed, with high levels of expression in specific tissues/organs including embryos. The incomplete penetrance of str1 embryo/seed phenotype is a result of functional STR2. Single str2 mutant had no phenotype. However, no str1(-/-)/str2(-/-) double mutant embryos were able to develop past the heart stage. Furthermore, STR2 is haplo-insufficient in str1 mutant background, and str1(-/-)/str2(+/-) embryos were 100% lethal. These data provide new insights into the biological functions of the ubiquitous sulfurtransferase in Arabidopsis embryogenesis and seed development.     

西瓜胚和胚乳的发育

应用显微技术对西瓜胚和胚乳的发育过程进行了观察并分析了西瓜胚珠败育的原因。西瓜胚发育属紫菀型。合子第一次分裂为不均等分裂 ,形成的基细胞体积明显较顶细胞大 ,两细胞均含有多个液泡。原胚发育过程中没有明显的胚柄。最外层的原胚细胞 ,与胚乳细胞相邻的壁上被胼胝质物质包围 ,且无外连丝存在 ;与胚囊壁相接的壁上无壁内突结构。胚的子叶体积增长的同时 ,子叶细胞内积累蛋白质和脂类物质 ,多糖物质的含量下降。胚乳发育属核型 ,在球形胚期开始自珠孔端向合点端细胞化 ,胚子叶分化出后开始自珠孔端向合点端退化。胚乳合点端在球形胚早期形成发达的胚乳吸器 ,开始呈游离核状态 ,后细胞化 ,在心型胚期之后退化。     

Modelling epistatic effects of embryo and endosperm QTL on seed quality traits

Coordinated expression of embryo and endosperm tissues is required for proper seed development. The coordination among these two tissues is controlled by the interaction between multiple genes expressed in the embryo and endosperm genomes. In this article, we present a statistical model for testing whether quantitative trait loci (QTL) active in different genomes, diploid embryo and triploid endosperm, epistatically affect a trait expressed on the endosperm tissue. The maximum likelihood approach, implemented with the EM algorithm, was derived to provide the maximum likelihood estimates of the locations of embryo- and endosperm-specific QTL and their main effects and epistatic effects. This model was used in a real example for rice in which two QTL, one from the embryo genome and the other from the endosperm genome, exert a significant interaction effect on gel consistency on the endosperm. Our model has successfully detected Waxy, a candidate gene in the embryo genome known to regulate one of the major steps of amylose biosynthesis in the endosperm. This model will have great implications for agricultural and evolutionary genetic research.