Project

Discipline

cell wall; TOR; ROL5; flavonols; Arabidopsis

Lead
Lay summary
Plant cells are surrounded by a rigid yet elastic extracellular matrix, the cell wall. This structure, characteristic for plant cells, has protective functions and at the same time restricts cell growth. Hence, cell wall expansion is a critical step during cell enlargement. While the composition of the cell wall has been analyzed extensively, it is still not well understood how plants regulate growth of the cell wall. In particular, the regulatory mechanisms that influence wall expansion processes and hence regulate wall growth remain poorly understood. A major growth controller of eukaryotic cells in general is the TOR (Target of Rapamycin) pathway, a signaling network that senses environmental condition and growth factors and regulates growth processes accordingly. In mammals, targeting the TOR pathway via the cytotoxic drug rapamycin is being used to treat several conditions in humans. Recently, we have identified the TOR pathway as a regulator of cell wall development in Arabidopsis thaliana (thale cress). Interfering with the TOR pathway by rapamycin or by inactivating ROL5, a protein that is participating in the TOR network, induces changes in the cell wall structure. Plants with an inactive ROL5 protein (rol5 mutants) are not only affected in the TOR network but also fail to properly modify tRNAs, suggesting that the ROL5 protein influences several processes. One aim of this project is to characterize the ROL5 protein in more detail. We have evidence that the ROL5 protein indeed has a dual function in the TOR network and in tRNA modification. We are currently investigating the possibility that ROL5 is interacting with different proteins that are affected in either tRNA modification or in the TOR network. Plant affected in the genes encoding these proteins will be characterized for changes in cell wall structures. In a second part of this project, we are investigating the role of flavonols on plant development. Flavonols are secondary metabolites, can influence diverse cellular processes and thus have health-beneficial effects in humans. Yet, their mode of action in plants remains largely elusive. We have established plant lines (again Arabidopsis thaliana) that show changes in the flavonol accumulation pattern, which induces aberrant cell development. Through a genetic approach, we were able to identify several cellular processes that seem to be under the influence of flavonols. These findings will be analyzed in more detail in order to shed light on the function of flavonols during plant development.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

Cell walls of plant cells consist of a complex network of polysaccharides and structural proteins. The controlled expansion of the cell wall is a prerequisite for regulated cell growth to take place, which requires a cell wall-sensing machinery involved in the exchange of information between the different cellular compartments. The proposed project mainly aims at the elucidation of the TOR (Target of Rapamycin) pathway - a major controller of eukaryotic cell growth - as a regulator of cell wall development and in particular focuses on the interaction between different proteins involved in this pathway.LRR-extensins (LRXs) are extracellular proteins and likely candidates for a regulatory or signalling function during cell wall development. LRX1 and LRX2 of Arabidopsis thaliana are expressed in root hairs, and mutations in lrx1 and lrx2 induce a strong root hair cell wall formation phenotype. Two rol (repressor of lrx1) loci, rol1 and rol5, were identified that modify cell wall structures. The rol5 mutation leads to defects in tRNA modification which affects the TOR pathway. It is the modification of the TOR pathway that induces changes in cell wall structures, leading to suppression of the lrx1 root hair phenotype. At this point, it is not clear how the ROL5 protein and hence the process of tRNA modification connects to the TOR pathway. Our preliminary data presented in this proposal suggest that ROP6, a member of the Ras-related G protein family of plants is involved in the TOR pathway. We have obtained evidence for an interaction between ROP6 and ROL5 which is based on genetic analyses and yeast two hybrid experiments. Hence, the ROL5 protein appears to have a dual function in tRNA modification and in connecting this process to TOR signalling. We suggest to investigate this initial observation in more detail in order to i) elucidate the function of ROP6 in TOR signalling and ii) further characterize the interaction between ROL5 and ROP6. In parallel, we suggest to follow up Arabidopsis interactome data, according to which ROL5 interacts with two more proteins, the function of which is not clear at this point. Thirdly, ROL5-interacting proteins involved in the tRNA modification process have been identified and the analyses of which will be completed.Two subjects are proposed to be pursued that are also interesting but at this point a second priority. First, we have identified flavonols as modifiers of cell growth. We were able to identify an auxin-related process which seems to be the molecular target of flavonol action. These analyses will be finalized by a complementary in vitro analysis of flavonol action. Finally, the function of LRX proteins is not yet clear, and we are currently finalizing the characterization of lrx mutants that interfere with cell development in the shoot. The tissue affected by these mutations can be collected in large amounts and allows for relatively simple cytological analyses, which is a clear advantage over root hairs analyzed so far. Hence, a tool is being established that is crucial for the analysis of LRX gene function.