Proline affects the size of the root meristematic zone in Arabidopsis

We reported previously that root elongation in Arabidopsis is promoted by exogenous proline, raising the possibility that this amino acid may modulate root growth. To evaluate this hypothesis we used a combination of genetic, pharmacological and molecular analyses, and showed that proline specifically affects root growth by modulating the size of the root meristem. The effects of proline on meristem size are parallel to, and independent from, hormonal pathways, and do not involve the expression of genes controlling cell differentiation at the transition zone. On the contrary, proline appears to control cell division in early stages of postembryonic root development, as shown by the expression of the G2/M-specific CYCLINB1;1 (CYCB1;1) gene. The overall data suggest that proline can modulate the size of root meristematic zone in Arabidopsis likely controlling cell division and, in turn, the ratio between cell division and cell differentiation

The COP9 SIGNALOSOME is required for postembryonic meristem maintenance in Arabidopsis thaliana

Cullin-RING E3 ligases (CRLs) regulate different aspects of plant development, and are activated by modification of their cullin subunit with the ubiquitin-like protein NEDD8 (NEural precursor cell expressed Developmentally Down-regulated 8) (neddylation) and deactivated by NEDD8 removal (deneddylation). The CONSTITUTIVELY PHOTOMORPHOGENIC9 (COP9) signalosome (CSN) acts as a molecular switch of CRLs activity by reverting their neddylation status, but its contribution to embryonic and early seedling development remains poorly characterized. Here, we analyzed the phenotypic defects of csn mutants and monitored the cullin deneddylation/neddylation ratio during embryonic and early seedling development. We show that while csn mutants can complete embryogenesis (albeit at a slower pace than wild type) and are able to germinate (albeit at a reduced rate), they progressively loose meristem activity upon germination, until they become unable to sustain growth. We also show that the majority of cullin proteins is progressively neddylated during the late stages of seed maturation and becomes deneddylated upon seed germination. This developmentally regulated shift in the cullin neddylation status is absent in csn mutants. We conclude that the CSN and its cullin deneddylation activity are required to sustain postembryonic meristem function in Arabidopsis

Fruit shape diversity in the Brassicaceae is generated by varying patterns of anisotropy

Fruits exhibit a vast array of different 3D shapes, from simple spheres and cylinders to more complex curved forms; however, the mechanism by which growth is oriented and coordinated to generate this diversity of forms is unclear. Here, we compare the growth patterns and orientations for two very different fruit shapes in the Brassicaceae: the heart-shaped Capsella rubella silicle and the near-cylindrical Arabidopsis thaliana silique. We show, through a combination of clonal and morphological analyses, that the different shapes involve different patterns of anisotropic growth during three phases. These experimental data can be accounted for by a tissue-level model in which specified growth rates vary in space and time and are oriented by a proximodistal polarity field. The resulting tissue conflicts lead to deformation of the tissue as it grows. The model allows us to identify tissue-specific and temporally specific activities required to obtain the individual shapes. One such activity may be provided by the valve-identity gene FRUITFULL, which we show through comparative mutant analysis to modulate fruit shape during post-fertilisation growth of both species. Simple modulations of the model presented here can also broadly account for the variety of shapes in other Brassicaceae species, thus providing a simplified framework for fruit development and shape diversity

Meristems, Stem Cells, and Stem Cell Niches in Vascular Land Plants-Chapter 6

All tissues and organs in vascular plants derive from a pool of undifferentiated, totipotent cells known as stem cells that divide repeatedly and asymmetrically to feed a growing organ with newly amplified cells. The evolution of stem cells and their organization within meristems of different types allowed the diversification of vascular land plant species, which have spread and diversified since the mid-Devonian period (about 400 Mya). Stem cells are set aside very early during de novo organogenesis to sustain the development of leaves, roots, and flowers throughout plant life cycles. The evolution of stem cells was essential for plant survival and integrating external/ exogenous stimuli with internal/endogenous mechanisms that allow coherent and plastic organ development and tissue replenishment. Stem cells are of pivotal importance for plant exploration of the surrounding space, both above and below the ground, for tissue repair and integration and to establish new generation during embryogenesis. This chapter highlights the basic principles of plant stem cell biology and their deployment in the evolution in vascular land plants. We discuss the advances made by studying model plants, particularly thale cress Arabidopsis thaliana, focusing on specification of plant meristems during early stages of embryogenesis and maintenance of meristem integrity during undetermined organ growth. Also, we examine the evolutionary appearance of stem cells and their organization in extinct and extant vascular land-plant phyla, the different types of meristematic structures in lycophytes, ferns, gymnosperms and angiosperms,1 and the importance of stem cells’ activity for root and shoot evolution and for strategies of branching morphogenesis

Cytokinin-auxin crosstalk

Post-embryonic plant growth and development are sustained by meristems, a source of undifferentiated cells that give rise to the adult plant structures. Two hormones, cytokinin and auxin, are known to act antagonistically in controlling meristem activities. Here, we review recent significant progress in elucidating the molecular mechanisms through which these hormones interact to control specific aspects of plant development. For example, in the root meristem of Arabidopsis thaliana, cytokinin promotes cell differentiation by repressing both auxin signalling and transport, whereas auxin sustains root meristem activity by promoting cell division. The coordinated action of these two hormones is essential for maintaining root meristem size and for ensuring root growth. © 2009 Elsevier Ltd. All rights reserved

The rate of cell differentiation controls the arabidopsis root meristem growth phase

Upon seed germination, apical meristems grow as cell division prevails over differentiation and reach their final size when division and differentiation reach a balance. In the Arabidopsis root meristem, this balance results from the interaction between cytokinin (promoting differentiation) [1-4] and auxin (promoting division) [2, 5] through a regulatory circuit whereby the ARR1 cytokinin-responsive transcription factor [6] activates the gene SHY2 [2, 6, 7], which negatively regulates the PIN genes encoding auxin transport facilitators [2, 5]. However, it remains unknown how the final meristem size is set, i.e., how a change in the relative rates of cell division and differentiation is brought about to cause meristem growth to stop. Here, we show that during meristem growth, expression of SHY2 is driven by another cytokinin-response factor, ARR12 [1], and that completion of growth is brought about by the upregulation of SHY2 caused by both ARR12 and ARR1: this leads to an increase in cell differentiation rate that balances it with division, thus setting root meristem size. We also show that gibberellins selectively repress expression of ARR1 at early stages of meristem development, and that the DELLA protein REPRESSOR OF GA 1-3 (RGA) [8] mediates this negative control. © 2010 Elsevier Ltd. All rights reserved

A SCARECROW-based regulatory circuit controls Arabidopsis thaliana meristem size from the root endodermis

Main conclusion: SCARECROW controls Arabidopsis root meristem size from the root endodermis tissue by regulating the DELLA protein RGA that in turn mediates the regulation ofARR1levels at the transition zone.Coherent organ growth requires a fine balance between cell division and cell differentiation. Intriguingly, plants continuously develop organs post-embryonically thanks to the activity of meristems that allow growth and environmental plasticity. In Arabidopsis thaliana, continued root growth is assured when division of the distal stem cell and their daughters is balanced with cell differentiation at the meristematic transition zone (TZ). We have previously shown that at the TZ, the cytokinin-dependent transcription factor ARR1 controls the rate of differentiation commitment of meristematic cells and that its activities are coordinated with those of the distal stem cells by the gene SCARECROW (SCR). In the stem cell organizer (the quiescent center, QC), SCR directly suppresses ARR1 both sustaining stem cell activities and titrating non-autonomously the ARR1 transcript levels at the TZ via auxin. Here, we show that SCR also exerts a fine control on ARR1 levels at the TZ from the endodermis by sustaining gibberellin signals. From the endodermis, SCR controls the RGA REPRESSOR OF ga1-3 (RGA) DELLA protein stability throughout the root meristem, thus controlling ARR1 transcriptional activation at the TZ. This guarantees robustness and fineness to the control of ARR1 levels necessary to balance cell division to cell differentiation in sustaining coherent root growth. Therefore, this work advances the state of the art in the field of root meristem development by integrating the activity of three hormones, auxin, gibberellin, and cytokinin, under the control of different tissue-specific activities of a single root key regulator, SCR.</p

P&F.3.The rate of cell differentiation controls the Arabidopsis root meristem growth phase

Upon seed germination, meristems rapidly grow due to a prevalence of cell division over cell differentiation and eventually reach their final size and a constant number of cells. At this stage, meristem maintenance and organ growth are ensured by the balance between cell division and cell differentiation. We have shown that in the Arabidopsis root meristem this balance is the result of the interaction between cytokinin (promoting differentiation) and auxin (promoting division) through a regulatory circuit where the ARR1 cytokinin-responsive transcription factor activates the gene SHY2 that negatively regulates the PIN genes encoding auxin transport facilitators. We have thus clarified how the size of the root meristem is maintained, but it is still unknown how the final meristem size is set, i.e. how a change in the relative rates of cell division and cell differentiation is brought about for meristem growth to stop. Here, we show that in allowing growth of the root meristem after seed germination and for the meristem to reach its final size, the ARR1/SHY2/PIN circuit necessary to maintain final root meristem size is integrated by two additional components: the cytokinin-responsive ARR12 transcription factor, and gibberellins. ARR12 drives a low level of expression of SHY2 during the growth phase for ensuring the prevalence of cell division over differentiation. ARR1 eventually joins ARR12 in increasing SHY2 expression, leading to an increase of cell differentiation that thus balances cell division and stops meristem growth. Gibberellin (GA), necessary for seed germination and radicle protrusion, represses transcription of the ARR1 gene during post-germination meristem growth, and a subsequent decrease in GA activity allows, via the REPRESSOR OF GA 1-3 (RGA) DELLA protein, ARR1 expression and the consequent up-regulation of SHY2

SCARECROW and SHORTROOT control the auxin/cytokinin balance necessary for embryonic stem cell niche specification

The root apical meristem is established during embryogenesis, when its organizer, the quiescent center, is specified and the stem cell niche is positioned. The SCARECROW-SHORTROOT heterodimer is essential for quiescent center specification and maintenance. As continuous post-embryonic root growth relies upon the SCARECROW-mediated control of the cytokinin/auxin balance, we investigated the role of SCARECROW and SHORTROOT in controlling cytokinin signaling during embryonic quiescent center specification. We found that from embryogenesis onward both SCARECROW and SHORTROOT antagonize cytokinin signaling, thus repressing the expression of the auxin biosynthetic enzyme ANTRANILATHE SYNTHASE BETA 1. This mechanism prevents detrimental and premature high auxin levels in the QC allowing the establishment of a functional embryonic root pole