The PLETHORA genes mediate patterning of the Arabidopsis root stem cell niche

Surge and destroy: the role of auxin in plant embryogenesis

For nearly a century, the plant hormone auxin has been recognized for its effects on post-embryonic plant growth. Now recent insights into the molecular mechanism of auxin transport and signaling are uncovering fundamental roles for auxin in the earliest stages of plant development, such as in the development of the apical-basal (shoot-root) axis in the embryo, as well as in the formation of the root and shoot apical meristems and the cotyledons. Localized surges in auxin within the embryo occur through a sophisticated transcellular transport pathway causing the proteolytic destruction of key transcriptional repressors. As we discuss here, the resulting downstream gene activation, together with other, less well-understood regulatory pathways, establish much of the basic body plan of the angiosperm embryo.

hca: an Arabidopsis mutant exhibiting unusual cambial activity and altered vascular patterning

By screening a T-DNA population of Arabidopsis mutants for alterations in inflorescence stem vasculature, we have isolated a mutant with a dramatic increase in vascular tissue development, characterized by a continuous ring of xylem/phloem. This phenotype is the consequence of premature and numerous cambial cell divisions in both the fascicular and interfascicular regions that result in the loss of the alternate vascular bundle/fiber organization typically observed in Arabidopsis stems. The mutant was therefore designated high cambial activity (hca). The hca mutation also resulted in pleiotropic effects including stunting and a delay in developmental events such as flowering and senescence. The physiological characterization of hca seedlings in vitro revealed an altered auxin and cytokinin response and, most strikingly, an enhanced sensitivity to cytokinin. These results were substantiated by comparative microarray analysis between hca and wild-type plants. The genetic analysis of hca indicated that the mutant phenotype was not tagged by the T-DNA and that the hca mutation segregated as a single recessive locus, mapping to the long arm of chromosome 4. We propose that hca is involved in mechanisms controlling the arrangement of vascular bundles throughout the plant by regulating the auxin-cytokinin sensitivity of vascular cambial cells. Thus, the hca mutant is a useful model for examining the genetic and hormonal control of cambial growth and differentiation.

Stem cells: a plant biology perspective

A recent meeting at the Juan March Foundation in Madrid, Spain brought together plant biologists to discuss the characteristics of plant stem cells that are unique and those that are shared by stem cells from the animal kingdom.

Transcription switches for protoxylem and metaxylem vessel formation

Land plants evolved xylem vessels to conduct water and nutrients, and to support the plant. Microarray analysis with a newly established Arabidopsis in vitro xylem vessel element formation system and promoter analysis revealed the possible involvement of some plant-specific NAC-domain transcription factors in xylem formation. VASCULAR-RELATED NAC-DOMAIN6 (VND6) and VND7 can induce transdifferentiation of various cells into metaxylem- and protoxylem-like vessel elements, respectively, in Arabidopsis and poplar. A dominant repression of VND6 and VND7 specifically inhibits metaxylem and protoxylem vessel formation in roots, respectively. These findings suggest that these genes are transcription switches for plant metaxylem and protoxylem vessel formation.

Signals that regulate stem cell activity during plant development

Plant stem cells are used continuously to generate new structures during the entire life-span of the organism. In the adult plant, stem cells are found in specialized structures called meristems. The meristems contain the stem cell niche together with rapidly dividing daughter cells that will ultimately differentiate into specific cell types. Some of the master genes that orchestrate the establishment and maintenance of the stem cell niche have now been identified in both the root and the shoot. Recent results show that these genes also determine the fate of the stem cells and that feedback signals from differentiated cells are involved in stem cell specification. These advances have provided a framework to understand how short-range and long-range signals are integrated to specify and position the stem cell niche in the meristems, and how the differentiation potential of plant stem cells is controlled.

From genes to plants via meristems

The Society for Experimental Biology organised a "Plant Frontier" meeting, which was recently held at the University of Sheffield, UK. One of the sessions of this broad meeting was on plant meristems, which covered a range of topics, including stem cells, patterning, long distance signalling and epigenetic regulation of meristem development.

Control of root cap formation by MicroRNA-targeted auxin response factors in Arabidopsis

The plant root cap mediates the direction of root tip growth and protects internal cells. Root cap cells are continuously produced from distal stem cells, and the phytohormone auxin provides position information for root distal organization. Here, we identify the Arabidopsis thaliana auxin response factors ARF10 and ARF16, targeted by microRNA160 (miR160), as the controller of root cap cell formation. The Pro(35S):MIR160 plants, in which the expression of ARF10 and ARF16 is repressed, and the arf10-2 arf16-2 double mutants display the same root tip defect, with uncontrolled cell division and blocked cell differentiation in the root distal region and show a tumor-like root apex and loss of gravity-sensing. ARF10 and ARF16 play a role in restricting stem cell niche and promoting columella cell differentiation; although functionally redundant, the two ARFs are indispensable for root cap development, and the auxin signal cannot bypass them to initiate columella cell production. In root, auxin and miR160 regulate the expression of ARF10 and ARF16 genes independently, generating a pattern consistent with root cap development. We further demonstrate that miR160-uncoupled production of ARF16 exerts pleiotropic effects on plant phenotypes, and miR160 plays an essential role in regulating Arabidopsis development and growth.

Pattern formation in the monocot embryo as revealed by NAM and CUC3 orthologues from Zea mays L

All aerial parts of a higher plant originate from the shoot apical meristem (SAM), which is initiated during embryogenesis as a part of the basic body plan. In contrast to dicot species, the SAM in Zea mays is not established at an apico-central, but at a lateral position of the transition stage embryo. Genetic and molecular studies in dicots have revealed that members of the NAC gene family of plant-specific transcription factors such as NO APICAL MERISTEM (NAM) from Petunia or the CUP-SHAPED COTYLEDON (CUC) genes from Arabidopsis contribute essential functions to the establishment of the SAM and cotyledon separation. As an approach to the understanding of meristem formation in a monocot species, members of the maize NAC family highly related to the NAM/CUC genes were isolated and characterized. Our phylogenetic analysis indicates that two distinct NAM and CUC3 precursors already existed prior to the separation of mono- and dicot species. The allocation of the two maize paralogues, ZmNAM1 and ZmNAM2 together with PhNAM, AtCUC2 and AmCUP in one sub-branch and the corresponding expression patterns support their contribution to SAM establishment. In contrast, the ZmCUC3 orthologue is associated with boundary specification at the SAM periphery, where it visualizes which fraction of cells in the SAM is committed to a new leaf primordium. Other maize NAC gene family members are clearly positioned outside of this NAM/CUC3 branch and also exhibit highly cell type-specific expression patterns.

Transcriptional profile of the Arabidopsis root quiescent center

The self-renewal characteristics of stem cells render them vital engines of development. To better understand the molecular mechanisms that determine the properties of stem cells, transcript profiling was conducted on quiescent center (QC) cells from the Arabidopsis thaliana root meristem. The AGAMOUS-LIKE 42 (AGL42) gene, which encodes a MADS box transcription factor whose expression is enriched in the QC, was used to mark these cells. RNA was isolated from sorted cells, labeled, and hybridized to Affymetrix microarrays. Comparisons with digital in situ expression profiles of surrounding tissues identified a set of genes enriched in the QC. Promoter regions from a subset of transcription factors identified as enriched in the QC conferred expression in the QC. These studies demonstrated that it is possible to successfully isolate and profile a rare cell type in the plant. Mutations in all enriched transcription factor genes including AGL42 exhibited no detectable root phenotype, raising the possibility of a high degree of functional redundancy in the QC.

Auxin Distribution and Plant Pattern Formation: How Many Angels Can Dance on the Point of PIN?

The plant hormone auxin is central in patterning diverse plant tissues. The direction of auxin flow and the distribution of auxin within tissues are regulated by auxin efflux transporters that are polarly localized in cells. Feedback regulation between auxin and its transporters establishes homeostatic patterns of auxin accumulation but allows dynamic repatterning in response to developmental or environmental cues.

Signaling in stem cell niches: lessons from the Drosophila germline

Stem cells are cells that, upon division, can produce new stem cells as well as daughter cells that initiate differentiation along a specific lineage. Studies using the Drosophila germline as a model system have demonstrated that signaling from the stem cell niche plays a crucial role in controlling stem cell behavior. Surrounding support cells secrete growth factors that activate signaling within adjacent stem cells to specify stem cell self-renewal and block differentiation. In addition, cell-cell adhesion between stem cells and surrounding support cells is important for holding stem cells close to self-renewal signals. Furthermore, a combination of localized signaling and autonomously acting proteins might polarize stem cells in such a way as to ensure asymmetric stem cell divisions. Recent results describing stem cell niches in other adult stem cells, including hematopoietic and neural stem cells, have demonstrated that the features characteristic of stem cell niches in Drosophila gonads might be conserved.

Plant development: auxin in loops

Concentration gradients of the hormone auxin are associated with various patterning events in plants. Recent work has refined our picture of the complex and dynamic system of auxin transport underlying the formation of these gradients.

Root development: the embryo within?

Root growth is sustained by stem cells maintained at the root tip. Recent evidence suggests that genes required to maintain root stem cells also specify root identity in the early embryo.

AINTEGUMENTA-like (AIL) genes are expressed in young tissues and may specify meristematic or division-competent states

Although several members of the AP2/ERF family of transcription factors are important developmental regulators in plants, many genes in this large protein family remain uncharacterized. Here, we present a phylogenetic analysis of the 18 genes that make up the AP2 subgroup of this family. We report expression analyses of seven Arabidopsis genes most closely related to the floral development gene AINTEGUMENTA (ANT) and show that all AINTEGUMENTA-like (AIL) genes are transcribed in multiple tissues during development. They are expressed primarily in young actively dividing tissues of a plant and not in mature leaves or stems. The spatial distribution of AIL5, AIL6, and AIL7 mRNA in inflorescences was characterized by in situ hybridization. Each of these genes is expressed in a spatially and temporally distinct pattern within inflorescence meristems and flowers. Ectopic expression of AIL5 resulted in a larger floral organ phenotype, similar to that resulting from ectopic expression of ANT. Our results are consistent with AIL genes having roles in specification of meristematic or division-competent states.

Stepwise understanding of root development

Recent studies using Arabidopsis propose a framework of root development and pattern formation that can be divided to three processes. First, a positional signal that is delivered from neighboring cells controls the fate of undifferentiated cells. Then, cell fate is fixed through a protein-network that includes various transcription factors. Finally, the expression of a particular gene-set leads to fate-dependent cell differentiation, resulting in oriented cell division, cell specification and cell elongation. In addition, these processes could be modified by chromatin stabilization and protein degradation. We focus on three fundamental patterns of root development, circumferential pattern, radial pattern and proximo-distal pattern, and on novel approaches to identify genes that are responsible for the spatiotemporal regulation of root development.

Intercellular signalling in the transition from stem cells to organogenesis in meristems

Meristems continuously produce new cells to sustain plant growth. Stem cells are maintained in the centre of the meristem and provide the precursor cells for the initiation of new organs and tissues in the periphery. The structure of the meristem is maintained while cells are constantly displaced by new divisions. Recent advances have been made in understanding the intercellular signals that maintain meristem structure by adjusting gene expression according to cell position. In addition to refinements in our understanding of how the position and size of the stem-cell population is regulated, there have been advances in understanding how the location of new organ primordia is controlled and how the meristem influences organ polarity.

The PIN auxin efflux facilitator network controls growth and patterning in Arabidopsis roots

Local accumulation of the plant growth regulator auxin mediates pattern formation in Arabidopsis roots and influences outgrowth and development of lateral root- and shoot-derived primordia. However, it has remained unclear how auxin can simultaneously regulate patterning and organ outgrowth and how its distribution is stabilized in a primordium-specific manner. Here we show that five PIN genes collectively control auxin distribution to regulate cell division and cell expansion in the primary root. Furthermore, the joint action of these genes has an important role in pattern formation by focusing the auxin maximum and restricting the expression domain of PLETHORA (PLT) genes, major determinants for root stem cell specification. In turn, PLT genes are required for PIN gene transcription to stabilize the auxin maximum at the distal root tip. Our data reveal an interaction network of auxin transport facilitators and root fate determinants that control patterning and growth of the root primordium.

Mechanisms of Pattern Formation in Plant Embryogenesis

Many of the patterning mechanisms in plants were discovered while studying postembryonic processes and resemble mechanisms operating during animal development. The emergent role of the plant hormone auxin, however, seems to represent a plant-specific solution to multicellular patterning. This review summarizes our knowledge on how diverse mechanisms that were first dissected at the postembryonic level are now beginning to provide an understanding of plant embryogenesis.