Grierson Lab:Further information

Major new collaborations are being established and are summarised below (website still in constuction). In the mean time you can learn about our current work on the Lab Members page.

• Summary: We are pioneering an integrative, predictive biology approach to describing the mechanisms of differentiation, morphogenesis and directed elongation of an individual cell. Root hair (RH) cells are an ideal model for this, since their development is highly reproducible, and many of the pieces of the root hair developmental jigsaw are already known for the model organism Arabidopsis. We will bring together these pieces and use novel experiments, mathematical models and biological computation to fill in missing pieces and to begin to understand the system as a whole. This will boost the global research effort on a “virtual” plant by linking to the virtual root model being developed at the Centre for Plant Integrative Biology (CPIB) at Nottingham (Nottingham CPIB homepage). Root hair development presents a unique set of challenges that are beyond the scope of the CPIB, and are outlined here. The research will build on skills already at Bristol: a world-leading root hair biology group (Grierson Lab frontpage), expert mathematical modelling in dynamical systems (Prof. Champneys, University of BristolDr Payne, University of Bristol), mechanics (Dr Chenchiah, University of Bristol), bioinformatics [], and computational (Prof. Flach, University of Bristol), and statistical (Prof. Green FRS, University of Bristol) modelling plus leading edge techniques in light (cell imaging facilities, University of Bristol) and atomic force (Prof. Miles, University of Bristol) microscopy and image analysis (Dr Mirmehdi, University of Bristol). Other UK expertise in modelling and experimentation at Sheffield (Dr Monk, University of Sheffield), Norwich (Prof Dolan, John Innes Centre) and Nottingham (Nottingham CPIB homepage).

• Rationale: Root hairs are agronomically important. They make up the majority of the root surface area of many crops, where they play an essential role in taking up nutrients and water from the soil, in interacting with pathogens and symbionts, and in anchorage. Research into optimisation of root hair properties is vital, since current agricultural usage levels of fertiliser and fresh water are not sustainable. Historically root hair research has been multidisciplinary and has involved developmental, genetic and cellular approaches to investigate the network controlling formation of the cell. More is known about the mechanism underpinning root hair cell development than any other plant cell type.

  

  Figure 1. Cross-section of an Arabidopsis root tip showing the two types of cell in the epidermis. Hair cells (yellow) overly the junction between two cortical cells in the layer below, whereas non-hair cells (blue) overly a single cortical cell. Specific transcription factors including WER, CPC, TRY, EGL3/GL3, and GL2 establish this pattern by regulating each other’s activity. Nick Monk [1], Liam Dolan [2] and Takuji Wada [3] are researching and developing an ODE model of these interactions (funded by a Human Frontiers grant).

  Hair cells are an exemplary experimental system: they develop in a predictable spatial pattern (Fig.1), allowing cells to be imaged throughout their development, and they develop cylindrical “hairs” that grow away from the surface of the plant into surrounding medium, and which are transparent thus facilitating quality imaging. Genetic knowledge of root hairs is excellent and there are many viable mutants and transgenic lines available, along with other outstanding international resources. These mutants are often characterised by specific aberrant morphologies, and the ability to explain these mutant forms will be a key bench-mark of our work. Root hairs are also an outstanding system for generic modelling of plant cell development differentiation and growth, posing a series of profound biological questions that are relevant to many other biological systems.

• Objectives: Our research focuses on key objectives, each approximately linked to a developmental stage.

1. Understanding which epidermal cells become root hair cells (Teams A [hotlink to Team A], B [hotlink to Team B]). Monk et al. have developed a preliminary model of epidermal cell differentiation, which describes how gene expression and cell-cell movement of transcription factors specifies newly formed cells as RH cells or non-RH. We are joining forces with them to extend this model to account for longer-range influences. This will incorporate developing knowledge of the roles of two new genes (see Objective 2), and of ethylene and auxin, which strongly influence commitment to hair formation.
2. Understanding when and how root hair formation is triggered (Teams A [hotlink to Team A], B [hotlink to Team B], D [hotlink to Team D]). Auxin transport is a key regulator of RH development. Each new RH cell elongates (along the axis of the root) as it moves away from the root tip, before producing a single root hair. The position on the root where RHs form is influenced, inter alia, by auxin-responsive transcription factors. New results from the Grierson lab (Fig. 2) show RH cells express very different levels of the auxin influx carrier AUX1 from non-hair cells. Interpretation based on Kramer’s [4] model of auxin flow through the root suggests this should result in root hair cells containing very different levels of auxin from non-hair cells. We are testing this prediction by measuring the auxin content of hair and non-hair cells in collaboration with Ljung [5].

   

   Figure 2 Surface of an Arabidopsis root expressing a green fluorescent protein fused with the auxin transporter AUX1 (AUX1::AUX1:YFP). Micro-array data from three independent laboratories show that AUX1 mRNA levels are similar in N and H cells, but our results presented here show that AUX1 protein levels are much higher in non-hair cells than in hair cells. This suggests that there is post-transcriptional control of AUX1 protein levels. (Harry Jones Lab Members)

   We are also ascertaining the contributions that auxin and ethylene make to hair development using a combination of hormone treatments, fluorescent imaging, genetic manipulation, and measurements of auxin content and response. In parallel we will build a mathematical model of hair morphogenesis using modified reaction-diffusion equations, and test whether the robustness of the morphogenesis is explained by a Turing-like process. Preliminary results from Dolan’s lab [6] indicate that the ability of auxin and ethylene to trigger root hair formation depends on two new transcription factors, whose relation to the transcription factors alluded to in Figure 1 is being examined using microarray data, transgenic reporters, triple mutants and hormone treatments. Bioinformatics tools could identify similarities between the promoter sequences of putative targets and suppressors, look for likely transcription factor binding sites, and identify possible mechanistic links with auxin and ethylene. The results will also feed into Objective 3.

Grierson Lab frontpage