Biomedical subjects

Didier Reinhardt

Publications and source records attributed to Didier Reinhardt.

12 recordsLinked to original sources

PubMed2006

Programming good relations--development of the arbuscular mycorrhizal symbiosis.

The majority of plants live in symbiotic associations with fungi or bacteria that improve their nutrition. Critical steps in a symbiosis are mutual recognition and subsequently the establishment of an intimate association, which involves the penetration of plant tissues and, in many cases, the invasion of individual host cells by the microbial symbiont. Recent advances revealed that in the arbuscular mycorrhizal symbiosis with soil fungi of the order Glomeromycota, plant-derived signals attract fungal hyphae and stimulate their growth. Upon physical attachment of the fungal symbiont to the root surface, an active plant developmental program prepares the epidermal cells for penetration by the fungus. Thus, plants actively help symbiotic fungi to colonize their roots rather than just tolerating them.

Gene Expression Regulation, Fungal↗

PubMed2006

A plausible model of phyllotaxis.

A striking phenomenon unique to the kingdom of plants is the regular arrangement of lateral organs around a central axis, known as phyllotaxis. Recent molecular-genetic experiments indicate that active transport of the plant hormone auxin is the key process regulating phyllotaxis. A conceptual model based on these experiments, introduced by Reinhardt et al. [Reinhardt, D., Pesce, E. R., Stieger, P., Mandel, T., Baltensperger, K., et al. (2003) Nature 426, 255-260], provides an intuitively plausible interpretation of the data, but raises questions of whether the proposed mechanism is, in fact, capable of producing the observed temporal and spatial patterns, is robust, can start de novo, and can account for phyllotactic transitions, such as the frequently observed transition from decussate to spiral phyllotaxis. To answer these questions, we created a computer simulation model based on data described previously or in this paper and reasonable hypotheses. The model reproduces, within the standard error, the divergence angles measured in Arabidopsis seedlings and the effects of selected experimental manipulations. It also reproduces distichous, decussate, and tricussate patterns. The model thus offers a plausible link between molecular mechanisms of morphogenesis and the geometry of phyllotaxis.

Alleles↗

PubMed2005

Phyllotaxis--a new chapter in an old tale about beauty and magic numbers.

Phyllotaxis, the regular arrangement of leaves and flowers around the stem, is one of the most fascinating patterning phenomena in biology. Numerous theoretical models, that are based on biochemical, biophysical and other principles, have been proposed to explain the development of the patterns. Recently, auxin has been identified as the inducer of organ formation. An emerging model for phyllotaxis states that polar auxin transport in the plant apex generates local peaks in auxin concentration that determine the site of organ formation and thereby the different phyllotactic patterns found in nature. The PIN proteins play a primary role in auxin transport. These proteins are localized in a polar fashion, reflecting the directionality of polar auxin transport. Recent evidence shows that most aspects of phyllotaxis can be explained by the expression pattern and the dynamic subcellular localization of PIN1.

Body Patterning↗

PubMed2005

Regulation of phyllotaxis.

Plant architecture is characterized by a high degree of regularity. Leaves, flowers and floral organs are arranged in regular patterns, a phenomenon referred to as phyllotaxis. Regular phyllotaxis is found in virtually all higher plants, from mosses, over ferns, to gymnosperms and angiosperms. Due to its remarkable precision, its beauty and its accessibility, phyllotaxis has for centuries been the object of admiration and scientific examination. There have been numerous hypotheses to explain the nature of the mechanistic principle behind phyllotaxis, however, not all of them have been amenable to experimental examination. This is due mainly to the delicacy and small size of the shoot apical meristem, where plant organs are formed and the phyllotactic patterns are laid down. Recently, the combination of genetics, molecular tools and micromanipulation has resulted in the identification of auxin as a central player in organ formation and positioning. This paper discusses some aspects of phyllotactic patterns found in nature and summarizes our current understanding of the regulatory mechanism behind phyllotaxis.

Biological Evolution↗

PubMed2004

Microsurgical and laser ablation analysis of leaf positioning and dorsoventral patterning in tomato.

Leaves are arranged according to regular patterns, a phenomenon referred to as phyllotaxis. Important determinants of phyllotaxis are the divergence angle between successive leaves, and the size of the leaves relative to the shoot axis. Young leaf primordia are thought to provide positional information to the meristem, thereby influencing the positioning of new primordia and hence the divergence angle. On the contrary, the meristem signals to the primordia to establish their dorsoventral polarity, which is a prerequisite for the formation of a leaf blade. These concepts originate from classical microsurgical studies carried out between the 1920s and the 1970s. Even though these techniques have been abandoned in favor of genetic analysis, the resulting insights remain a cornerstone of plant developmental biology. Here, we employ new microsurgical techniques to reassess and extend the classical studies on phyllotaxis and leaf polarity. Previous experiments have indicated that the isolation of an incipient primordium by a tangential incision caused a change of divergence angle between the two subsequent primordia, indicating that pre-existing primordia influence further phyllotaxis. Here, we repeat these experiments and compare them with the results of laser ablation of incipient primordia. Furthermore, we explore to what extent the different pre-existing primordia influence the size and position of new organs, and hence phyllotaxis. We propose that the two youngest primordia (P1 and P2) are sufficient for the approximate positioning of the incipient primordium (I1), and therefore for the perpetuation of the generative spiral, whereas the direct contact neighbours of I1 (P2 and P3) control its delimitation and hence its exact size and position. Finally, we report L1-specific cell ablation experiments suggesting that the meristem L1 layer is essential for the dorsoventral patterning of leaf primordia.

Gene Expression Regulation, Plant↗

PubMed2003

Regulation of phyllotaxis by polar auxin transport.

The regular arrangement of leaves around a plant's stem, called phyllotaxis, has for centuries attracted the attention of philosophers, mathematicians and natural scientists; however, to date, studies of phyllotaxis have been largely theoretical. Leaves and flowers are formed from the shoot apical meristem, triggered by the plant hormone auxin. Auxin is transported through plant tissues by specific cellular influx and efflux carrier proteins. Here we show that proteins involved in auxin transport regulate phyllotaxis. Our data indicate that auxin is transported upwards into the meristem through the epidermis and the outermost meristem cell layer. Existing leaf primordia act as sinks, redistributing auxin and creating its heterogeneous distribution in the meristem. Auxin accumulation occurs only at certain minimal distances from existing primordia, defining the position of future primordia. This model for phyllotaxis accounts for its reiterative nature, as well as its regularity and stability.

Arabidopsis↗

PubMed2003

Arabidopsis AXR6 encodes CUL1 implicating SCF E3 ligases in auxin regulation of embryogenesis.

The AXR6 gene is required for auxin signaling in the Arabidopsis embryo and during postembryonic development. One of the effects of auxin is to stimulate degradation of the Aux/IAA auxin response proteins through the action of the ubiquitin protein ligase SCF(TIR1). Here we show that AXR6 encodes the SCF subunit CUL1. The axr6 mutations affect the ability of mutant CUL1 to assemble into stable SCF complexes resulting in reduced degradation of the SCF(TIR1) substrate AXR2/IAA7. In addition, we show that CUL1 is required for lateral organ initiation in the shoot apical meristem and the inflorescence meristem. These results indicate that the embryonic axr6 phenotype is related to a defect in SCF function and accumulation of Aux/IAA proteins such as BDL/IAA12. In addition, we show that CUL1 has a role in auxin response throughout the life cycle of the plant.

Amino Acid Sequence↗

PubMed2003

Vascular patterning: more than just auxin?

The plant hormone auxin has long been known to play a pivotal role in vascular patterning and differentiation. But auxin is not the whole story: recent genetic analyses have identified additional factors required for vascular patterning, one of them involving sterols.

Arabidopsis Proteins↗

PubMed2003

The plant multidrug resistance ABC transporter AtMRP5 is involved in guard cell hormonal signalling and water use.

Carbon dioxide uptake and water release through stomata, controlling the opening and closure of stomatal pore size in the leaf surface, is critical for optimal plant performance. Stomatal movements are regulated by multiple signalling pathways involving guard cell ion channels. Using reverse genetics, we recently isolated a T-DNA insertion mutant for the Arabidopsis ABC-transporter AtMRP5 (mrp5-1). Guard cells from mrp5-1 mutant plants were found to be insensitive to the sulfonylurea compound glibenclamide, which in the wild type induces stomatal opening in the dark. Here, we report that the knockout in AtMRP5 affects several signalling pathways controlling stomatal movements. Stomatal apertures of mrp5-1 and wild-type Ws-2 were identical in the dark. In contrast, opening of stomata of mrp5-1 plants was reduced in the light. In the light, stomatal closure of mrp5-1 was insensitive to external calcium and abscisic acid, a phytohormone responsible for stomatal closure during drought stress. In contrast to Ws-2, the phytohormone auxin could not stimulate stomatal opening in the mutant in darkness. All stomatal phenotypes were complemented in transgenic mrp5-1 plants transformed with a cauliflower mosaic virus (CaMV) 35S-AtMRP5 construct. Both whole-plant and single-leaf gas exchange measurements demonstrated a reduced transpiration rate of mrp5-1 in the light. Excised leaves of mutant plants exhibited reduced water loss, and water uptake was strongly decreased at the whole-plant level. Finally, if plants were not watered, mrp5-1 plants survived much longer due to reduced water use. Analysis of CO2 uptake and transpiration showed that mrp5-1 plants have increased water use efficiency. Mutant plants overexpressing AtMRP5 under the control of the CaMV 35S promoter again exhibited wild-type characteristics. These results demonstrate that multidrug resistance-associated proteins (MRPs) are important components of guard cell functioning.

Arabidopsis↗

PubMed2003

Microsurgical and laser ablation analysis of interactions between the zones and layers of the tomato shoot apical meristem.

Plants exhibit life-long organogenic and histogenic activity in a specialised organ, the shoot apical meristem. Leaves and flowers are formed within the ring-shaped peripheral zone, which surrounds the central zone, the site of the stem cells. We have undertaken a series of high-precision laser ablation and microsurgical tissue removal experiments to test the functions of different parts of the tomato meristem, and to reveal their interactions. Ablation of the central zone led to ectopic expression of the WUSCHEL gene at the periphery, followed by the establishment of a new meristem centre. After the ablation of the central zone, organ formation continued without a lag. Thus, the central zone does not participate in organogenesis, except as the ultimate source of founder cells. Microsurgical removal of the external L(1) layer induced periclinal cell divisions and terminal differentiation in the subtending layers. In addition, no organs were initiated in areas devoid of L(1), demonstrating an important role of the L(1) in organogenesis. L(1) ablation had only local effects, an observation that is difficult to reconcile with phyllotaxis theories that invoke physical tension operating within the meristem as a whole. Finally, regeneration of L(1) cells was never observed after ablation. This shows that while the zones of the meristem show a remarkable capacity to regenerate after interference, elimination of the L(1) layer is irreparable and causes terminal differentiation.

Cell Communication↗

PubMed2002

The auxin influx carrier is essential for correct leaf positioning.

Auxin is of vital importance in virtually every aspect of plant growth and development, yet, even after almost a century of intense study, major gaps in our knowledge of its synthesis, distribution, perception, and signal transduction remain. One unique property of auxin is its polar transport, which in many well-documented cases is a critical part of its mode of action. Auxin is actively transported through the action of both influx and efflux carriers. Inhibition of polar transport by the efflux inhibitor N-1-naphthylphthalamic acid (NPA) causes a complete cessation of leaf initiation, a defect that can be reversed by local application of the auxin, indole-3-acetic acid (IAA), to the responsive zone of the shoot apical meristem. In this study, we address the role of the auxin influx carrier in the positioning and outgrowth of leaf primordia at the shoot apical meristem of tomato. By using a combination of transport inhibitors and synthetic auxins, we demonstrate that interference with auxin influx has little effect on organ formation as such, but prevents proper localization of leaf primordia. These results suggest the existence of functional auxin concentration gradients in the shoot apical meristem that are actively set up and maintained by the action of efflux and influx carriers. We propose a model in which efflux carriers control auxin delivery to the shoot apical meristem, whereas influx and efflux carriers regulate auxin distribution within the meristem.

Carrier Proteins↗

PubMed2002

Plant architecture.

Plant architecture is species specific, indicating that it is under strict genetic control. Although it is also influenced by environmental conditions such as light, temperature, humidity and nutrient status, here we wish to focus only on the endogenous regulatory principles that control plant architecture. We summarise recent progress in the understanding of the basic patterning mechanisms involved in the regulation of leaf arrangement, the genetic regulation of meristem determinacy, i.e. the decision to stop or continue growth, and the control of branching during vegetative and generative development. Finally, we discuss the basis of leaf architecture and the role of cell division and cell growth in morphogenesis.

Gene Expression Regulation, Plant↗