Development in multicellular organisms is a process of cell divisions, followed by differentiation of the daughter cells forming the various tissue types. Key questions are how the balance of cell proliferation and differentiation is genetically controlled and how the cells communicate with each other for proper tissue patterning. Our focus is to understand these processes in vascular development of plants.
The plant vasculature originates from two types of stem cells. In primary development from the procambium within the shoot or root apical meristems, in secondary development from the cambium. The (pro-)cambium is positioned between the two conducting cell types, the phloem and the xylem. The activity of the cambium gives rise to radial growth, and secondary phloem and xylem. Secondary xylem is the major constituent of wood. Apart form sugar, nutrients and water, both xylem and phloem also transports various signalling molecules throughout the plant. Hence, it is essential that the formation of these cell types is properly connected to the development of the various plant organs, to ensure continuous connections between all plant parts.
We are using the Arabidopsis root as an experimental system to study vascular development, be- cause here it is relatively easy to follow the processes of cell fate acquisition, tissue patterning, cell proliferation, and differen- tiation as the root is continuously growing and the tissue types are organised in a regular and simple pattern. The vascular cells can be readily studied in the Arabidopsis primary root tip through the use of various staining techniques and visual molecular markers (Fig. 1).
In collaboration with the groups of Ykä Helariutta, University of Helsinki, where I did my post doc, Philip Benfey, Duke University and Ji-Young Lee, Boyce Thompson Institute, we have recently uncovered a bidirectional signaling pathway that controls the development of vascular tissues in the Arabidopsis root (Fig. 2, Carlsbecker et al, Nature, 2010). The pathway involves the movement of a microRNA (miRNA) and is the first well-documented case of a movement of a microRNA between cells and the biological relevance of this movement. We have also participated in a study showing that the movement between cells of the miRNA occurs through plasmodesmata (Vatén et al, in press). The miRNA moves from an outer cell layer into the vasculature to restrict the mRNA domains of class III homeodomain leucine zipper (HD-ZIP III) factors predominantly at the stele periphery. The levels of HD-ZIP III factors then determine the identity of the xylem cells: high levels at the stele centre direct metaxylem cell differentiation and low levels at the periphery direct protoxylem differentiation. We are continuing by using pharmacological, genetic and molecular methods to further understand the means of HD-ZIP III regulation of vascular development
Vatén A., Dettmer J., Wu S., Stierhof Y., Miyashima S., Yadav S.R., Roberts C.J., Campilho A., Bulone V., Lichtenberger R., Sevilem I., Lehesranta S., Jokitalo E., Mähönen A.P., J.-Y., Sauer N., Scheres B., Nakajima K., Carlsbecker A.#, Gallagher K.L.#, Helariutta Y., Callose biosynthesis regulates symplastic trafficking during root development.
Accepted for publication in Developmental Cell.
# Equal contribution.
Carlsbecker A.,1 Lee J.-Y.,1 J. Roberts C. J.,2 Dettmer J.,2 Lehesranta S.,2 Zhou J.,2 Lindgren O.,3 Moreno-Risueno M. A.,3 Vatén A.,3 Thitamadee S., Campilho A., Sebastian J., Bowman J. L., Helariutta Y.,4 and Benfey P. N.4 (2010) Cell signalling by microRNA165/6 directs gene dose dependent root cell fate. Nature, DOI 10.1038/nature08977
1-4 These authors contributed equally to this work.
Carlsbecker A., and Helariutta Y. (2005) Phloem and xylem specification: pieces of the puzzle emerging. Current Opinion in Plant Biology 8:512-517.