Adrienne H. K. Roeder

A MAPKK kinase gene regulates extra-embryonic cell fate in Arabidopsis.

The Arabidopsis zygote divides asymmetrically into an embryonic apical cell and a basal cell with mostly extra-embryonic fate. This fundamental asymmetry sets the stage for further embryonic development, but the events mediating it are poorly understood. We have identified a MAPKK kinase gene, named YODA, that promotes extra-embryonic cell fates in the basal lineage. In loss-of-function mutants, the zygote does not elongate properly, and the cells of the basal lineage are eventually incorporated into the embryo instead of differentiating the extra-embryonic suspensor. Gain-of-function alleles cause exaggerated growth of the suspensor and can suppress embryonic development to a degree where no recognizable proembryo is formed. Our results imply that a MAP kinase cascade acts as a molecular switch promoting extra-embryonic fate.

Control of Fruit Patterning in Arabidopsis by INDEHISCENT

The Arabidopsis seedpod opens through a spring-loaded mechanism known as pod shatter, which is essential for dispersal of the seeds. Here, we identify INDEHISCENT (IND), an atypical bHLH protein, that is necessary for fruit opening and is involved in patterning each of the three fruit cell types required for seed dispersal. Previous studies suggested that FRUITFULL (FUL), a member of the MADS-domain transcription factor family, is required for fruit growth since ful mutant fruit fail to undergo the dramatic enlargement that normally occurs after fertilization. Here we show, however, that FUL is not directly required for fruit elongation and instead is required to prevent ectopic activity of IND. Our molecular and genetic studies suggest a model for the regulatory interactions among the genes that control fruit development and the mechanism that results in the expression of IND in a narrow stripe of cells.

The Role of the REPLUMLESS Homeodomain Protein in Patterning the Arabidopsis Fruit

The outside of the Arabidopsis thaliana fruit consists of three principal tissues: the valves or seedpod walls, the replum or central ridge between the valves, and the valve margins where the valves separate from the replum to disperse the seeds. Previous studies have shown that valve margin formation is specified by the SHATTERPROOF MADS-box transcription factors and that valve development is controlled by the FRUITFULL MADS-box transcription factor. FRUITFULL negatively regulates SHATTERPROOF to prevent the valves from adopting a valve margin cell fate. Here we identify a gene called REPLUMLESS that is required for replum development. REPLUMLESS encodes a homeodomain protein that prevents replum cells from adopting a valve margin cell fate by negatively regulating expression of the SHATTERPROOF genes. Both REPLUMLESS and FRUITFULL are required to limit SHATTERPROOF expression to a narrow stripe of cells so that the valve margin differentiates precisely at the valve/replum boundary.

Variability in the Control of Cell Division Underlies Sepal Epidermal Patterning in Arabidopsis thaliana

Live cell imaging and computational modeling explains how variability in the timing of cell division generates a characteristic pattern of cell sizes during development.

Fruit Development in Arabidopsis

Luscious cherries, sweet peaches, creamy avocados, and tropical papayas are just a few of the tasty treats that come to mind when we think of fruit. Indeed, fruit come in all shapes and sizes, from gigantic pumpkins to the tiny fruit of the duckweed Wolffia angusta, which are as small as a grain of salt. Fruit range in texture from soft and fleshy to dry and papery with each design optimized for a different seed dispersal strategy. Fleshy fruit are often sweet, brightly colored, and are generally adapted to be eaten by vertebrates, which carry the seeds to a new location before depositing them in a pile of fertilizer. In contrast, wind, water, and the force generated by the opening of the seedpod commonly distribute the seeds of dry fruit. Of course there are many exceptions, such as the spiked, barbed, dry fruit that snag a ride by adhering to the fur of passing animals. Dry fruit are classified as either dehiscent, in which the walls of the ovary open to release the seeds into the environment, or indehiscent, in which the seeds remain enclosed in the fruit and the fruit is shed from the plant. Many important crops including peas, beans, lentils, soybeans and canola have dehiscent fruit. Both crops with fleshy fruit and with dehiscent fruit are of such importance to agriculture and the human diet that fruit have been the focus of extensive research in recent years. Research on fleshy fruit has focused primarily on tomato and great progress has been made in understanding the genes that control the size and ripening of tomato fruit (for reviews see Giovannoni, 2004; Tanksley, 2004; Adams-Phillips, et al., 2004). Research on dehiscent fruit has focused on Arabidopsis thaliana, which will be the focus of this chapter (for additional reviews see Dinneny and Yanofsky, 2004; Ferrandiz, et al., 1999; Bowman et al., 1999). In this chapter, we will first discuss wild-type fruit development and then turn to the genes and hormones that are known to regulate fruit formation in Arabidopsis. Specifically, we will examine the genes that are involved in specifying the development of the different tissue types within the fruit, the genes that control the formation of axes within the fruit, and the processes that regulate fruit development after fertilization (see Table 1 for a list of genes involved in fruit development). The fruit is arguably the most complex plant organ and its development is just beginning to be understood, making fruit development a ripe field for many years to come. Table 1. Genes involved in fruit development 1.1 Wild-type Fruit Structure The fruit is defined as the mature ovary (and, in some types of fruit, additional floral tissues) that forms a specialized structure designed to protect the seeds while they develop and disperse them at maturity. The fruit develops from the gynoecium after fertilization. The gynoecium is the female reproductive structure including the ovary and is usually formed from one or more fused carpels at the center of the flower. A carpel is a single ovule bearing structural unit of the gynoecium and is thought to have originated from a modified bract or leaf (Bowman et al., 1999). The Arabidopsis gynoecium is composed of two fused carpels, each of which consists of a seedpod wall and surrounding tissues. The fusion of the carpels is congenital, meaning that the gynoecium arises as a single primordium. The Arabidopsis fruit develops from the fertilized gynoecium to form a silique, or seedpod, which dries and dehisces at maturity, releasing the seeds. The Arabidopsis fruit consists of many distinct cell types, which are derived from the gynoecium. From top to bottom there are four different regions of the gynoecium and subsequently the fruit: the stigma, style, ovary, and gynophore (Figure 1; see Figure 2 for definitions of axes in the fruit and other terms). Figure 1. Structure of the wild-type fruit. (A) Scanning electron micrograph (SEM) of a Landsberg erecta (Ler) fruit at stage 17. The fruit has been false colored to distinguish the different parts and this color code has been used throughout the review. At the ... Figure 2. Terminology. (A) Picture of a stage 17 fruit with axes labeled. (B) Cross section of a stage 17 fruit with axes labeled. The scale bar in A represents 1 mm and the scale bar in B represents 100 µm. Stigma At the top of the gynoecium, the stigma is comprised of a single layer of elongated papillar cells specialized for the germination of pollen (Figure 1B). The stigma is the first component of the transmitting tract, a set of cells that secrete a polysaccharide-rich extracellular matrix, which-forms a pathway for the growth and guidance of pollen tubes (Sessions and Zambryski, 1995; for reviews of pollen tube guidance see Lord and Russell, 2002 and Palanivelu and Preuss, 2000).

MorphoGraphX: A platform for quantifying morphogenesis in 4D

Morphogenesis emerges from complex multiscale interactions between genetic and mechanical processes. To understand these processes, the evolution of cell shape, proliferation and gene expression must be quantified. This quantification is usually performed either in full 3D, which is computationally expensive and technically challenging, or on 2D planar projections, which introduces geometrical artifacts on highly curved organs. Here we present MorphoGraphX (www.MorphoGraphX.org), a software that bridges this gap by working directly with curved surface images extracted from 3D data. In addition to traditional 3D image analysis, we have developed algorithms to operate on curved surfaces, such as cell segmentation, lineage tracking and fluorescence signal quantification. The softwares modular design makes it easy to include existing libraries, or to implement new algorithms. Cell geometries extracted with MorphoGraphX can be exported and used as templates for simulation models, providing a powerful platform to investigate the interactions between shape, genes and growth. DOI: http://dx.doi.org/10.7554/eLife.05864.001

Comprehensive Analysis of CLE Polypeptide Signaling Gene Expression and Overexpression Activity in Arabidopsis

Intercellular signaling is essential for the coordination of growth and development in higher plants. Although hundreds of putative receptors have been identified in Arabidopsis (Arabidopsis thaliana), only a few families of extracellular signaling molecules have been discovered, and their biological roles are largely unknown. To expand our insight into the developmental processes potentially regulated by ligand-mediated signal transduction pathways, we undertook a systematic expression analysis of the members of the Arabidopsis CLAVATA3/ESR-RELATED (CLE) small signaling polypeptide family. Using reporter constructs, we show that the CLE genes have distinct and specific patterns of promoter activity. We find that each Arabidopsis tissue expresses at least one CLE gene, indicating that CLE-mediated signaling pathways are likely to play roles in many biological processes during the plant life cycle. Some CLE genes that are closely related in sequence have dissimilar expression profiles, yet in many tissues multiple CLE genes have overlapping patterns of promoter-driven reporter activity. This observation, plus the general absence of detectable morphological phenotypes in cle null mutants, suggest that a high degree of functional redundancy exists among CLE gene family members. Our work establishes a community resource of CLE-related biological materials and provides a platform for understanding and ultimately manipulating many different plant signaling systems.

What determines cell size

AbstractFirst paragraph (this article has no abstract) For well over 100 years, cell biologists have been wondering what determines the size of cells. In modern times, we know all of the molecules that control the cell cycle and cell division, but we still do not understand how cell size is determined. To check whether modern cell biology has made any inroads on this age-old question, BMC Biology asked several heavyweights in the field to tell us how they think cell size is controlled, drawing on a range of different cell types. The essays in this collection address two related questions - why does cell size matter, and how do cells control it.

Local Cues and Asymmetric Cell Divisions Underpin Body Plan Transitions in the Moss Physcomitrella patens

BACKGROUND Land plants evolved from aquatic algae more than 450 million years ago. Algal sisters of land plants grow through the activity of apical initial cells that cleave either in one plane to generate filaments or in two planes to generate mats. Acquisition of the capacity for cell cleavage in three planes facilitated the formation of upright bushy body plans and enabled the invasion of land. Evolutionary transitions between filamentous, planar, and bushy growth are mimicked within moss life cycles. RESULTS We have developed lineage analysis techniques to assess how transitions between growth forms occur in the moss Physcomitrella patens. We show that initial cells giving rise either to new filaments or bushy shoots are frequently juxtaposed on a single parent filament, suggesting a role for short-range cues in specifying differences in cell fate. Shoot initials cleave four times to establish a tetrahedral shape and subsequently cleave in three planes, generating bushy growth. Asymmetric and self-replacing divisions from the tetrahedral initial generate leaf initials that divide asymmetrically to self-replace and to produce daughter cells with restricted fate. The cessation of division in the leaf is distributed unevenly and contributes to final leaf shape. CONCLUSIONS In contrast to flowering plants, changes in body plan in P. patens are regulated by cues acting at the level of single cells and are mediated through asymmetric divisions. Genetic mechanisms regulating shoot and leaf development in P. patens are therefore likely to differ substantially from mechanisms operating in plants with more recent evolutionary origins.

Computational morphodynamics of plants: integrating development over space and time

The emerging field of computational morphodynamics aims to understand the changes that occur in space and time during development by combining three technical strategies: live imaging to observe development as it happens; image processing and analysis to extract quantitative information; and computational modelling to express and test time-dependent hypotheses. The strength of the field comes from the iterative and combined use of these techniques, which has provided important insights into plant development.