Floral stem cell termination involves the direct regulation of AGAMOUS by PERIANTHIA

In Arabidopsis, the population of stem cells present in young flower buds is lost after the production of a fixed number of floral organs. The precisely timed repression of the stem cell identity gene WUSCHEL (WUS) by the floral homeotic protein AGAMOUS (AG) is a key part of this process. In this study, we report on the identification of a novel input into the process of floral stem cell regulation. We use genetics and chromatin immunoprecipitation assays to demonstrate that the bZIP transcription factor PERIANTHIA (PAN) plays a role in regulating stem cell fate by directly controlling AG expression and suggest that this activity is spatially restricted to the centermost region of the AG expression domain. These results suggest that the termination of floral stem cell fate is a multiply redundant process involving loci with unrelated floral patterning functions

Pattern formation during de novo assembly of the Arabidopsis shoot meristem

Most multicellular organisms have a capacity to regenerate tissue after wounding. Few, however, have the ability to regenerate an entire new body from adult tissue. Induction of new shoot meristems from cultured root explants is a widely used, but poorly understood, process in which apical plant tissues are regenerated from adult somatic tissue through the de novo formation of shoot meristems. We characterize early patterning during de novo development of the Arabidopsis shoot meristem using fluorescent reporters of known gene and protein activities required for shoot meristem development and maintenance. We find that a small number of progenitor cells initiate development of new shoot meristems through stereotypical stages of reporter expression and activity of CUP-SHAPED COTYLEDON 2 (CUC2), WUSCHEL (WUS), PIN-FORMED 1 (PIN1), SHOOT-MERISTEMLESS (STM), FILAMENTOUS FLOWER (FIL, also known as AFO), REVOLUTA (REV), ARABIDOPSIS THALIANA MERISTEM L1 LAYER (ATML1) and CLAVATA 3 (CLV3). Furthermore, we demonstrate a functional requirement for WUS activity during de novo shoot meristem initiation. We propose that de novo shoot meristem induction is an easily accessible system for the study of patterning and self-organization in the well-studied model organism Arabidopsis

Anomalous size dependent rheological behavior of alumina based nanofluids

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Rheological behaviour of Alumina (Al2O3) based nanofluids (NFs) has been studied and found to be exhibit unexpected behaviour. Two base-fluids viz, water and ethylene glycols (EG). Three particle sizes (11, 45 and 150 nm), varying over an order of magnitude, were used to analyze the effect of particle size. The experimental data has shown typical Newtonian behavior for both W based and EG based alumina NFs The viscosity of EG based NFs is found to be anomalously reduced compared to the base fluid. This reduction in viscosity may be due to hygroscopic nature of EG or due to the presence of water in as-received high concentration sample also, as told by some researchers. However, this phenomenon was absent for water based NFs. The inter-related effects of particle size, concentration and mode of dispersion (mono or poly-dispersed) were investigated. To eliminate the effect of size variation, mono dispersed NFs are obtained by centrifuging and re-suspension of parent NFs. Particle migration under shear is attributed to the reduction of viscosity. The increase in bulk viscosity with particle size reduction is attributed to the surface forces acting between the particles and the medium in a suspension

Towards identifying the dynamic cellular patterns underlying early Arabidopsis floral development

A major challenge in developmental biology is to understand how multicellular tissues are organised into complex shapes. The various genetic, hormonal and mechanical events that drive morphogenesis must act by altering the properties of groups of cells in a coordinated manner through time and space. To gain a clear understanding of these events, we generate digital reconstructions of growing Arabidopsis owers, and use these to precisely quantify cellular properties during the early stages of ower development. Our goal is to then analyse these data with the appropriate mathematical methodology, in order to determine precisely which cellular properties (and in what measure) best characterises the emerging differentiation states that are essential to morphogenesis. We propose to identify and characterize cellular patterns based on measured cellular properties, linking their spatio-temporal behaviour to the observed changes induced by organogenesis. Our experiments begin with imaging the ower from multiple angles. These images are then fused to generate a high-resolution reconstruction, which is then segmented. By imaging the same ower bud over several days and by identifying cell lineages between time points, we generate 4-D data on the development of the ower. From these data, several spatial and spatio-temporal cell properties like volume, volumetric growth or strain were extracted. We have adopted a graph- based approach to organise the data. The 3D tissue observed at successive dates/time points is transformed into a graph whose vertices represent cells and edges represent either cell spatial neighborhood or temporal relations between cells based on lineage. This graph thus contains all the spatio-temporal information about the ower under study. While developing the methods to analyse these growing tissues, we have opted to initially simplify the task in two ways, by restricting the study to the earliest morphogenetic events in the ower (stages 1-3), and by examining only the outermost cell layer, the L1. This rich spatio-temporal information is used in two ways to study the events underlying ower development. Firstly, we applied a supervised approach to explore the properties of specific groups of cells, identified either by morphology or by gene expression. Secondly, we applied an unsupervised approach to identify cellular patterns. More specifically, a clustering method was applied to all the cells of one (or several) observed tissue sequence in order to identify homogeneous regions corresponding to specific cell behaviours. One important long-term goal is to determine how these cellular patterns diverge in mutants presenting a different phenotype, and to identify shifts or changes in properties within these groups or the presence or absence of certain groups. I will detail my approach and present preliminary results of my analyses. (Résumé d'auteur