Thomas Laux

Role of WUSCHEL in Regulating Stem Cell Fate in the Arabidopsis Shoot Meristem

The shoot meristem gives rise to the aerial parts of higher plants by continuously initiating new organs. The basis of this activity is its ability to maintain a pool of pluripotent stem cells, which are the ultimate source of all tissues of the shoot. In Arabidopsis plants mutant for the WUSCHEL (WUS) gene, the stem cells are misspecified and appear to undergo differentiation. Here, we show that WUS encodes a novel homeodomain protein which presumably acts as a transcriptional regulator. The pattern of WUS expression suggests that stem cells in the shoot meristem are specified by an underlying cell group which is established in the 16-cell embryo and becomes localized to its prospective domain of function by asymmetric cell divisions.

The Stem Cell Population of Arabidopsis Shoot Meristems Is Maintained by a Regulatory Loop between the CLAVATA and WUSCHEL Genes

The higher-plant shoot meristem is a dynamic structure whose maintenance depends on the coordination of two antagonistic processes, organ initiation and self-renewal of the stem cell population. In Arabidopsis shoot and floral meristems, the WUSCHEL (WUS) gene is required for stem cell identity, whereas the CLAVATA1, 2, and 3 (CLV) genes promote organ initiation. Our analysis of the interactions between these key regulators indicates that (1) the CLV genes repress WUS at the transcript level and that (2) WUS expression is sufficient to induce meristem cell identity and the expression of the stem cell marker CLV3. Our data suggest that the shoot meristem has properties of a self-regulatory system in which WUS/CLV interactions establish a feedback loop between the stem cells and the underlying organizing center.

Conserved factors regulate signalling in Arabidopsis thaliana shoot and root stem cell organizers

Throughout the lifespan of a plant, which in some cases can last more than one thousand years, the stem cell niches in the root and shoot apical meristems provide cells for the formation of complete root and shoot systems, respectively. Both niches are superficially different and it has remained unclear whether common regulatory mechanisms exist. Here we address whether root and shoot meristems use related factors for stem cell maintenance. In the root niche the quiescent centre cells, surrounded by the stem cells, express the homeobox gene WOX5 (WUSCHEL-RELATED HOMEOBOX 5), a homologue of the WUSCHEL (WUS) gene that non-cell-autonomously maintains stem cells in the shoot meristem. Loss of WOX5 function in the root meristem stem cell niche causes terminal differentiation in distal stem cells and, redundantly with other regulators, also provokes differentiation of the proximal meristem. Conversely, gain of WOX5 function blocks differentiation of distal stem cell descendents that normally differentiate. Importantly, both WOX5 and WUS maintain stem cells in either a root or shoot context. Together, our data indicate that stem cell maintenance signalling in both meristems employs related regulators.

Expression dynamics of WOX genes mark cell fate decisions during early embryonic patterning in Arabidopsis thaliana

During embryonic pattern formation, the main body axes are established and cells of different developmental fates are specified from a single-cell zygote. Despite the fundamental importance of this process, in plants, the underlying mechanisms are largely unknown. We show that expression dynamics of novel WOX (WUSCHEL related homeobox) gene family members reveal early embryonic patterning events in Arabidopsis. WOX2 and WOX8 are co-expressed in the egg cell and zygote and become confined to the apical and basal daughter cells of the zygote, respectively, by its asymmetric division. WOX2 not only marks apical descendants of the zygote, but is also functionally required for their correct development, suggesting that the asymmetric division of the plant zygote separates determinants of apical and basal cell fates. WOX9 expression is initiated in the basal daughter cell of the zygote and subsequently shifts into the descendants of the apical daughter apparently in response to signaling from the embryo proper. Expression of WOX5 shows that identity of the quiescent center is initiated very early in the hypophyseal cell, and highlights molecular and developmental similarities between the stem cell niches of root and shoot meristems. Together, our data suggest that during plant embryogenesis region-specific transcription programs are initiated very early in single precursor cells and that WOX genes play an important role in this process.

Termination of Stem Cell Maintenance in Arabidopsis Floral Meristems by Interactions between WUSCHEL and AGAMOUS

Floral meristems and shoot apical meristems (SAMs) are homologous, self-maintaining stem cell systems. Unlike SAMs, floral meristems are determinate, and stem cell maintenance is abolished once all floral organs are initiated. To investigate the underlying regulatory mechanisms, we analyzed the interactions between WUSCHEL (WUS), which specifies stem cell identity, and AGAMOUS (AG), which is required for floral determinacy. Our results show that repression of WUS by AG is essential for terminating the floral meristem and that WUS can induce AG expression in developing flowers. Together, this suggests that floral determinacy depends on a negative autoregulatory mechanism involving WUS and AG, which terminates stem cell maintenance.

Differential Expression of WOX Genes Mediates Apical-Basal Axis Formation in the Arabidopsis Embryo

Axis formation is one of the earliest patterning events in plant and animal embryogenesis. In Arabidopsis, the main axis of the embryo is evident at the asymmetric division of the zygote into a small, embryonic apical cell and a large extraembryonic basal cell. Here we show that the homeobox genes WOX2 and WOX8, which are initially coexpressed in the zygote, act as complementary cell fate regulators in the apical and basal lineage, respectively. Furthermore, WOX8 expression in the basal cell lineage is required for WOX2 expression and normal development of the proembryo, suggesting an inductive mechanism. The identified WOX cascade is required for normal expression of a reporter gene of the auxin efflux carrier PIN1 and for the formation of auxin response maxima in the proembryo. Thus, our results link the spatial separation of WOX transcription factors to localized auxin response and the formation of the main body axis in the embryo.

Stem cell homeostasis in the Arabidopsis shoot meristem is regulated by intercellular movement of CLAVATA3 and its sequestration by CLAVATA1

Stem cell maintenance in the Arabidopsis shoot meristem is regulated by communication between the apical stem cells and the underlying organizing centre. Expression of the homeobox gene WUSCHEL in the organizing centre induces stem cell identity in the overlying neighbours, which then express the CLAVATA3 gene whose activity in turn restricts the size of the WUSCHEL expression domain. We have analyzed how the stem cells and the organizing centre communicate, by studying the mode of action of CLAVATA3 protein. We provide direct evidence that CLAVATA3 protein functions as a mobile intercellular signal in the shoot apical meristem that spreads laterally from the stem cells and acts both on their lateral neighbours and on the stem cells themselves to repress WUSCHEL transcription. We also show that the spread and range of action of CLAVATA3 can be limited by binding to its receptor CLAVATA1, which offers an explanation for how CLAVATA3 is prevented from entering the organizing centre and repressing WUSCHEL transcription there. This regulated spread of a secreted signalling molecule enables the shoot meristem to permit the onset of cell differentiation in the periphery, but at the same time to maintain a stable niche for its stem cells in the center.

The E3 Ubiquitin Ligase BIG BROTHER Controls Arabidopsis Organ Size in a Dosage-Dependent Manner

Organ growth up to a species-specific size is tightly regulated in plants and animals. Final organ size is remarkably constant within a given species, suggesting that a species-specific size checkpoint terminates organ growth in a coordinated and timely manner. Phytohormones influence plant organ size, but their precise functions in size control are unclear because of their pleiotropic and complex developmental roles. The Arabidopsis transcription factors AINTEGUMENTA and JAGGED promote organ growth by maintaining cellular proliferation potential. Loss of the Antirrhinum transcription factor CINCINNATA causes leaf overgrowth, yet also leads to a highly abnormal leaf shape. Thus, no dedicated factor that limits the final size of plant organs has been isolated. Here, we identify the novel RING-finger protein BIG BROTHER (BB) as a repressor of plant organ growth. Small changes in BB expression levels substantially alter organ size, indicating a central regulatory role for BB in growth control. Recombinant BB protein has E3 ubiquitin-ligase activity that is essential for its in vivo function, suggesting that BB acts by marking cellular proteins for degradation. Our data indicate that plants limit the duration of organ growth and ultimately organ size by actively degrading critical growth stimulators.

Mechanical induction of lateral root initiation in Arabidopsis thaliana

Lateral roots are initiated postembryonically in response to environmental cues, enabling plants to explore efficiently their underground environment. However, the mechanisms by which the environment determines the position of lateral root formation are unknown. In this study, we demonstrate that in Arabidopsis thaliana lateral root initiation can be induced mechanically by either gravitropic curvature or by the transient bending of a root by hand. The plant hormone auxin accumulates at the site of lateral root induction before a primordium starts to form. Here we describe a subcellular relocalization of PIN1, an auxin transport protein, in a single protoxylem cell in response to gravitropic curvature. This relocalization precedes auxin-dependent gene transcription at the site of a new primordium. Auxin-dependent nuclear signaling is necessary for lateral root formation; arf7/19 double knock-out mutants normally form no lateral roots but do so upon bending when the root tip is removed. Signaling through arf7/19 can therefore be bypassed by root bending. These data support a model in which a root-tip-derived signal acts on downstream signaling molecules that specify lateral root identity.

Embryogenesis: A New Start in Life.

Embryogenesis effects the transition from the fertilized egg to the new multicellular generation, the seedling, which displays the basic body plan and organization of the plant. An apical-basal pattern along the main body axis of the embryo consists of a linear array of distinct elements, including the shoot meristem, cotyledons, hypocotyl, root, and root meristem. A radial pattern around the apical-basal axis is represented by the concentric arrangement of the primary tissues: epidermis at the periphery, ground tissue underneath, and conductive tissue in the center. During postembryonic development, the two primary meristems give rise to the elaborate structures of the adult plant (see Clark, 1997; Kerstetter and Hake, 1997; Schiefelbein et al., 1997, in this issue). The basic body plan is established within the first onethird of embryogenesis and becomes fully apparent by the time dicot embryos reach the heart stage. Subsequent events include further growth of the embryo, morphogenesis, activity of the primary meristems, cell differentiation, and preparation of both embryo and seed for dormancy. Previous reviews have covered various aspects of embryogenesis, including the formation of embryo initials (Mordhorst et al., 1997), fertilization (Russell, 1993), endosperm development (Lopes and Larkins, 1993), somatic embryogenesis (Zimmerman, 1993; Emons, 1994), axis formation (Jürgens, 1995), gene expression (Thomas, 1993), and related topics (Goldberg et al., 1994; Laux and Jürgens, 1994; Yadegari and Goldberg, 1997). In this review, we discuss how pattern formation generates different cell fates during embryogenesis. By drawing mainly on recent genetic and molecular studies in Arabidopsis, we first summarize what is known about the successive generation of cell fates and then discuss mechanisms that may underlie the establishment of diverse cell identities.