Laboratory of developmental plant biology - Research topic
The goal of the LBDP is to elucidate key steps of the Arabidopsis phosphate signalling pathway. Elements involved in this signalling cascade remain mostly unknown despite its essential role for plant physiology.
We mainly (but not exclusively) focus our studies on the root. This plastic organ exhibits a simple structure which facilitates the cellular biology studies. This is also the main place of entry for ions in plants ,and it is very responsive (in terms of root architecture) to Pi modifications in the environment. We have used this property to set up various genetic screens. In addition, the use of the root system as a model allows us to take advantage of the many tools that were developed by our laboratory in the past (several collections of mutants and lines containing cellular markers).
The choice of phosphate (compared to other nutrients) is based on several factors. First, Pi is an essential macro-nutrient, and therefore its study affects a broad scientific audience. Pi deficiency promotes numerous physiological, developmental and biochemical modifications. It tightly controls the production of biomass and the synthesis of compounds, which can have several applications (for medical or energy production uses). As such, it can play the role of a crucial limiting factor. This is also an element which can be easily manipulated (radiotracers available), presenting a low mobility in soils. This resource is decreasing and there is an urgent need to find a solution. In the future, the remaining phosphate resources will be of lower quality (due to the capacities of Pi to fix many heavy metals) and this might impact health
Our studies suggest that a majority of plant responses associated to Pi deficiency are not direct consequence of metabolic limitations, but are rather resulting from the signalling cascade triggered by Pi deficiency. Our goal is to identify new key components involved in this signalling cascade.
1- The phosphate regulation pathway
contacts : marin"at"dsvsud.cea.fr
lnussaume"at"cea.fr
mcthibaud"at"cea.fr
Due to the low amount of phosphate present in the soils, plants have developed different regulatory system to optimize its absorption at the root level and its use at the plant level. Our laboratory is interested into the various steps regulating phosphate homeostasis in Arabidopsis, at the level of the cell, tissues and whole plant.
A transcriptomic analysis has shown a modulation of the main metabolic pathways regulated by phosphate starvation: remodelling of membrane lipids (phospho, galacto and sulfo-lipids), recovery of internal or external phosphate (induction of transporters, phosphatases, nucleases…), metal homeostasis... In addition, phosphate is heterogeneously distributed in the soil, and plants are able to distinguish between zones with different Pi contents. Using a « split root » experimental device mimicking this heterogeneity, we have identified genes regulated by local or systemic phosphate status.
We are also interested in understanding the role of the various root cell layers regarding phosphate absorption and signalization. Using a phf mutant exhibiting reduced phosphate transporters activity; we have created lines exhibiting specific complementation in various cell layers of the root (epidermis, cortex, pericycle, endodermis…). These lines are currently being studied and offer an interesting biological material to understand the specific roles of the various root tissues during Pi starvation.
Recently, we combined various approaches of proteomic, molecular and cellular biology to identify novel steps of posttranscriptional regulation modulating the level of high affinity phosphate transporters (AtPHT1 family).
We have also initiated several screens to identify mutations or drugs (chemical genetics) altering the Pi signalization pathway; several candidates (drugs, mutants) have been identified and are currently under study in the laboratory.
2 -Bioluminescence imaging of genes induced by Pi starvation
Contact : helene.javot"at"cea.fr
Our laboratory previously identified several genes expressed in the root, and whose expression is modulated in response to Pi starvation (Misson et al., 2005; Thibaud et al., 2010). Their expression pattern suggests that they may be differentially involved in local versus systemic regulation of the responses to changes of Pi status. We are currently developing several Promoter:Luciferase lines (a bioluminescent marker) for a subset of genes exhibiting distinct induction profiles based on transcriptomics data. These lines will be characterized in order to map precisely the kinetics of induction of these genes in response to changes in Pi conditions.
3 - Root growth and phosphate starvation
Contact: thierry.desnos"at"cea.fr
Phosphate starvation modifies the root architecture of many plant species. It seems this is an adaptive response promoting root exploration of the upper layers of soil that are richer in Pi, compared to deeper layers. By now we do not know the molecular mechanism underlying this response (1).
Our project is focused on the Arabidopsis early responses consisting in the primary root growth arrest when it encounters a low-Pi medium. We have shown that this fast growth arrest is triggered at the root tip. We are currently analyzing our transcriptomic data about this early response.
Few years ago, we used a quantitative genetics analysis to identify LPR1 as a major gene involved in this conditional growth response (2). LPR1 is a multicopper oxidase localized in the endoplasmic reticulum and its activity in the root tip is necessary for mediating the growth arrest (3, 4).
In order to discover other elements involved in this process, we have isolated 86 new lpr1-like mutants. In parallel, we have used a chemical genetics approach to identify two drugs mimicking the lpr1 phenotype and we are studying mutants displaying an altered sensibility to these drugs.
Altogether, the synergy between the genetics, the transcriptomic and the new tools of chemical genetics will allow us to better understand the molecular origin of the root growth arrest triggered by Pi starvation.
References:
1) Desnos T. Root branching responses to phosphate and nitrate. Curr Opin Plant Biol 11(1):82-7 (2008).
2) Reymond M., S. Svistoonoff, O. Loudet, L. Nussaume and T. Desnos. Identification of QTL controlling root growth response to phosphate starvation in Arabidopsis thaliana. Plant, Cell and Environment, 29:115-125 (2006).
3) Svistoonoff S, Creff A, Reymond M, Sigoillot-Claude C, Ricaud L, Blanchet A, Nussaume L, Desnos T. Root tip contact with low-phosphate media reprogrammes plant root architecture. Nature Genetics, 39:792-796 (2007).
4) Ticconi CA, Lucero RD, Sakhonwasee S, Adamson AW, Creff A, Nussaume L,
Desnos T, Abel S. ER-resident proteins PDR2 and LPR1 mediate the developmental response of root meristems to phosphate availability. Proc Natl Acad Sci U S A. 106(33):14174-9 (2009).