HD Protein Cell-To-Cell Transport & Cell Cycle Regulation

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 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 the homeodomain (HD) proteins KNOTTED1 (KN1), SHOOT MERISTEMLESS (STM), and KNAT1, are thought to move like viral movement proteins to adjacent cells. A number of studies suggest that intercellular transcription factor movement is specific and depends on transport regulators. Transcription factors function within the nucleus, thus, a decision between non-cell-autonomy versus nuclear localization 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. In course of our previous FWF funded project we confirmed that the entry of the meristem identity factor KN1 into the intercellular transport pathway is negatively regulated by a microtubules-associated complex containing the Tobacco Mosaic Virus movement protein binding factor MPB2C and the novel HD protein binding factor KNB36. Ectopically expressed MPB2C prevents the HD transcription factor KN1 from cell-to-cell movement by transferring it to microtubules. Here KN1 seems to be degraded in a KNB36 triggered manner via the proteasome pathway. Thus, MPB2C and KNB36 appear to act as selective gatekeepers limiting availability of HD proteins to enter the cell-to-cell transport pathway. This notion is consistent with our finding that KNB36 and MPB2C expression in meristematic and expanding tissues overlaps with expression of the Arabidopsis KN1 homologues STM and KNAT1. In addition, we could show that KNB36 is a non-cell-autonomous protein and interacts with the cell cycle factor cyclin B. Interestingly, in contrast to animal systems, no reports exist for plant systems indicating that HD proteins associate to core components of the cell cycle machinery and by this means regulate cell cycle progression. Yet, KNB36 interacting with HD proteins and cyclin B may well constitute such a physical link in plants. We suggest that binding of KNB36 to a HD protein regulated by yet unknown factor(s) decides whether HD proteins function non-cell-autonomous, regulate cell cycle progression/proliferation or control cell-fate. Experiments are proposed to advance our insights regarding the role of KNB36 and MPB2C in cell cycle progression and in controlling HD protein availability for movement. Phenotypic analysis of mutant plants, in vivo interaction assays and identification of proteins present in KNB36--MPB2C complex(es) will substantiate the observed effects exerted by KNB36 and MPB2C and should further substantiate the role of microtubules in cell-to- cell transport. The intended characterization of mutant and transgenic plants should demonstrate the importance of cell-to-cell transport and involvement of HD transcription factors in cell cycle progression.

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 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 the homeodomain (HD) proteins KNOTTED1 (KN1), SHOOT MERISTEMLESS (STM), and KNAT1, are thought to move like viral movement proteins to adjacent cells. A number of studies suggest that intercellular transcription factor movement is specific and depends on transport regulators. Transcription factors function within the nucleus, thus, a decision between non-cell-autonomy versus nuclear localization 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. In course of our previous FWF funded project we confirmed that the entry of the meristem identity factor KN1 into the intercellular transport pathway is negatively regulated by a microtubules-associated complex containing the Tobacco Mosaic Virus movement protein binding factor MPB2C and the novel HD protein binding factor KNB36. Ectopically expressed MPB2C prevents the HD transcription factor KN1 from cell-to-cell movement by transferring it to microtubules. Here KN1 seems to be degraded in a KNB36 triggered manner via the proteasome pathway. Thus, MPB2C and KNB36 appear to act as selective gatekeepers limiting availability of HD proteins to enter the cell-to-cell transport pathway. This notion is consistent with our finding that KNB36 and MPB2C expression in meristematic and expanding tissues overlaps with expression of the Arabidopsis KN1 homologues STM and KNAT1. In addition, we could show that KNB36 is a non-cell-autonomous protein and interacts with the cell cycle factor cyclin B. Interestingly, in contrast to animal systems, no reports exist for plant systems indicating that HD proteins associate to core components of the cell cycle machinery and by this means regulate cell cycle progression. Yet, KNB36 interacting with HD proteins and cyclin B may well constitute such a physical link in plants. We suggest that binding of KNB36 to a HD protein regulated by yet unknown factor(s) decides whether HD proteins function non-cell-autonomous, regulate cell cycle progression/proliferation or control cell-fate. Experiments are proposed to advance our insights regarding the role of KNB36 and MPB2C in cell cycle progression and in controlling HD protein availability for movement. Phenotypic analysis of mutant plants, in vivo interaction assays and identification of proteins present in KNB36--MPB2C complex(es) will substantiate the observed effects exerted by KNB36 and MPB2C and should further substantiate the role of microtubules in cell-to- cell transport. The intended characterization of mutant and transgenic plants should demonstrate the importance of cell-to-cell transport and involvement of HD transcription factors in cell cycle progression.