Regulation of cellular distribuion of homoedomain proteins

Plant cells differentiate depending on their position and not according to their ancestry or lineage. Several signals in the form of hormones, systemic secondary messengers, small peptides, RNA molecules and non-cell- autonomous acting proteins can provide the means to exchange positional information. Recently, a number of plant transcription factors involved in cell fate regulation were shown to act non-cell-autonomously and to move from cell-to-cell. Transcription factors involved in meristem initiation and/or maintenance such as KNOTTED1 or conferring and maintaining floral meristem identity such as LEAFY are thought to move like viral movement proteins to adjacent cells via intercellular pores formed by plasmodesmata. A number of studies suggest that specific and regulated cell-to-cell transport of proteins through plasmodesmata depends on cellular receptors. Transcription factors function within the nucleus, thus, a decision between non-cell-autonomy versus nuclear import has to be made within the cytoplasm. Next, actively transported transcription factors have to interact with specific receptors regulating access to the intercellular transport machinery, which in turn transfers the non-cell- autonomous proteins to neighboring cells. Our recent results indicate that entry to the intercellular transport pathway is negatively regulated by a novel microtubules-associated protein named MPB2C. Structurally and functionally distinct proteins such as the tobacco mosaic virus movement protein (TMV-MP), KNOTTED1 as well as the KNOTTED1 A. thaliana homologue SHOOT MERISTEMLESS interact with MPB2C. Ample presence of the MPB2C prevents cell-to-cell movement of the homeobox transcription factor KNOTTED1 and TMV-MP. This observation suggests that MPB2C binds a range of structurally distinct proteins and decides/regulates their intracellular distribution and, consequently, also access to the non-cell-autonomous transport pathway. In addition, we isolated a novel KNOTTED1 binding protein, KNB36, which binds also to MPB2C. Experiments are proposed to characterize the potential role of MPB2C and a novel KNOTTED1 binding protein named KNB36 in the delivery of non-cell-autonomous proteins into neighboring cells or in intracellular distribution. Expression profiling, phenotypic analysis of mutant plants, and in vivo interaction assays will confirm the observed negative transport effects exerted by MPB2C and address the role of KNB36. Additional, functional studies on protein motifs essential for MPB2C and KNB36 interaction will potentially allow us to classify proteins as non-cell- autonomous. The characterization of mutant and transgenic plants will evaluate the importance of the cell-to-cell transport of homeobox transcription factors in exchange of positional and developmental information.

Plant cells differentiate depending on their position and not according to their ancestry or lineage. Signals in the form of hormones, systemic secondary messengers, small peptides, and non-cell-autonomous acting RNAs and proteins can provide the means to exchange positional information. In the last years, a number of plant transcription factors regulating production of proteins and by this means control plant development were shown to move from cell to cell. Transported transcription factors are involved in meristem (plant stem cells) initiation and/or maintenance giving rise to all plant tissues such as stem, leaves, flowers, and roots. A subset of transcription factors regulating the plant body formation are named homeodomain proteins and are thought to move like viruses to neighboring cells. Transcription factors function within the nucleus by binding to the genomic DNA, thus, a decision between non-cell-autonomy versus nuclear localization has to be made within the cytoplasm surrounding the nucleus. Next, transcription factors have to interact with specific receptors regulating access to the intercellular transport machinery, which in turn facilitates the transfer to neighboring cells via small channels called plasmodesmata, We study the mechanism regulating cell-to-cell transport of homeodomain transcription factors. In course of our FWF funded project we confirmed that the entry of a meristem identity transcription factor into the intercellular transport pathway is regulated by a microtubules-associated complex. This complex also limits the spread of specific plant viruses and submits these to degradation and, thus, acts as a selective gatekeeper limiting viral infections. The very same complex limits the transport of shoot apex identity factors regulating plant development. In addition, we found evidence that this complex contains a factor regulating cell cycle progression and, thus, cell division. In contrast to animal systems, no reports exist for plant systems that homeodomain transcription proteins associate to core components of the cell cycle machinery and by this means regulate cell cycle progression. The project aim was to understand the developmental mechanism of tissue differentiation and pattern formation in plants. In the long term we expect that our insights will allow us to find new ways to control the production, time, and spatial arrangement of plant organs such as flowers, leaves, and roots.