FUNCTIONAL CELL BIOLOGY

Summary

Cell polarity plays important roles in plant development. For example, the main body axis is defined by the polarity of the single-celled zygote and the polar information is used for patterning and cell specification. ROP GTPases are key regulators of cellular polarity in plants. We aim to reveal the link of ROP GTPases to various kinases in order to have new insights into the regulation of cell polarity during morphogenesis, tip growth and fungal invasion of cells. We have shown previously that a specific class of RLCK kinases (VIA) is potential ROP effectors. As these kinases are implicated both in pollen tube growth as well as fungal susceptibility, they will be in the center of our studies. However, we also have data on the possible involvement of upstream kinases as potential ROP regulators. Therefore our investigations will include these kinases as well. We combine biochemical, structural, cellular and functional information using purified proteins, mutant and transgenic plants, yeast and chemical genomic screening systems, transient gene expression assays, confocal microscopy and in silico data analysis to compare ROP-centered kinase signaling during cell polarity (in vitro pollen tubes), morphogenesis (whole plant) and pathogenesis (fungi-infected cells).

Research questions

The main questions of the proposed research are: How the Rho-type ROP G-proteins are linked to kinase signaling in plants and what is the biological significance of these signaling pathways during cell polarity establishment related to morpho- and pathogenesis? In yeast/metazoa several types of Rho-activated kinases exist (e.g. PAKs) that are completely absent from plants. The link of Rho signaling to receptor tyrosine kinases through RhoGEFs was also revealed in these organisms. However, plants have unique Rho proteins (ROPs), unique receptor kinases (RLKs), unique RopGEFs, unique ROP effector kinases (RLCK_VIAs) and the signaling network involving these specific proteins is hardly known. Based on our preliminary data we hypothesize that all these proteins are interlinked and play together a central role in the regulation of ROP-related processes including cell polarity. We aim to prove this hypothesis by various experimental approaches. Together with our foreign partner, Ralph Hückelhoven (Technical University of Munich), we also hypothesize that compatible fungal pathogens use the plant cell polarity machinery to enter into plant cells. We aim to compare ROP- and kinase-related steps during polar cell growth (pollen tube) and fungal entrance into plant cells in addition to reveal the function of ROP-related kinases during normal morphogenesis.

Significance of the research

ROPs are the only and unique signaling-type small G-proteins in plants, but their integration into signaling networks is hardly known. In previous publications we provided the first experimental data that they can regulate kinases and they can also be regulated through phosphorylation. We aim to use the obtained knowledge to build up a general model for receptor signaling in plants; from RLKs to ROP effectors. Moreover, the data can serve as a basis for studies concerning the evolution of signaling mechanisms, as well as can give specific insights into the regulation of basic cellular processes such as polar cell growth, cell expansion, plant morphogenesis and the plants responses to fungal pathogens. This basic knowledge can be translated in long term into molecular markers and genetic technologies that can be used to alter plant shape/development and to coop with fungal plant pathogens.