Prof Yrjö Helariutta
Prof Helariutta has had longstanding collaboration with Prof Jim Murray and Dr Walter Dewitte at the School of Biosciences through two ERA-NET programmes in Plant Genomics (The Plant Stem Cell Network: Integrated analysis of stem cell function in plant growth and development) and in Systems Biology (Integrated Analysis of the Shoot Apical Meristem)
His research focuses on understanding the development of vascular tissue and the cambium, the vascular meristem, of plants, studying both Arabidopsis as a model systems and, since these tissues are responsible for wood development, trees such as birch and poplar. A recent focus has been on understanding the hormonal signals, particularly cytokinin, involved in controlling both stem cell identity and the proliferation of cells during secondary growth or thickening processes.
Helariutta has defined some major regulatory principles for formation of the vascular pattern during root development in Arabidopsis. In the Arabidopsis root the vascular tissue has a central axis of xylem cell file, flanked by the pluripotent procambial tissue and phloem. Helariutta has shown that cytokinins promote the stem cell identity in the procambial domains by regulating polar auxin transport. Decrease in cytokinin activity causes all vascular cells to accumulate auxin and differentiate into protoxylem cells (Mähönen et al. 2006a; Bishopp et al. 2011). AHP6, an inhibitory protein, counteracts cytokinin signalling in a spatially specific manner, thereby allowing protoxylem formation. The CRE1/WOL cytokinin receptor is a bifunctional kinase/ phosphatase, and elimination of the negatively regulating phosphatase activity of CRE1/WOL results in stimulation of proliferation of vascular cell files (Mähönen et al. 2006b). This indicates that in addition to specifying vascular cell identity, cytokinins have a further role in controlling the rate of proliferation of vascular cell files. Following new genetic screens, he has more recently identified some novel components that appear to regulate the auxin-cytokinin interaction during root vascular development.
On the other hand, within the xylem axis two cell identities, the peripheral protoxylem and the central metaxylem, can be distinguished. In collaboration with other colleagues Helariutta has determined that the protoxylem-metaxylem pattern is based on bidirectional signaling of two mobile signals, the SHR protein that moves out from the vascular tissue to activate transcription of miR165/6 species that move back to the vascular tissue (Carlsbecker et al. 2011). There they regulate the dosage of the class III HD-ZIP genes that code for homeodomain transcription factors. As the peripheral protoxylem position is close to the ground tissue source for the microRNA species, the dosage of the class III HD-ZIP genes is lower compared to the centrally located metaxylem cells. Quite recently, Prof. Helariutta identified gain-of-function mutations in one of the CALLOSE SYNTHASE isoforms (CALS3; Vaten et al. 2011). As callose is known to be an important polymer regulating the size exclusion limit of plasmodesmata, he has subsequently introduced these mutations in an inducible system under various promoter specificities (icals3m). This system can be used to control the symplastic trafficking through the plasmodesmata between the neighboring cells in a time and space specific manner.
Helariutta has also identified the first component (APL) of the regulatory machinery that defines whether the cell takes the wood or phloem vascular tissue fate (Bonke et al. 2003). Consequently, he has been analyzing transcriptional processes upstream and downstream of APL.