The vascular network (xylem and phloem) provides plants with structural support and long-distance transport, meaning that vascular tissues must predictably differentiate to form continuous strands. To uncover the mechanisms that establish and maintain the vascular pattern, we take advantage of the Arabidopsis root tip, which has a stereotypical arrangement with procambial cells separating opposite phloem poles from a central xylem axis. Earlier, we have shown that phytohormones cytokinin and auxin interactively specify vascular patterning during root development (1). To identify novel factors that may participate in the cytokinin and auxin signaling pathways, and modify vascular patterning, we perform genetic screens for mis-expression of specific markers in the Arabidopsis primary root. One such marker for vascular patterning is ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6), a spatially specific inhibitor of cytokinin signalling that is also regulated by auxin (2). Based on this approach, we have recently identified two loci that are involved in RNA methylation. One of these novel mutants, yet unpublished, exhibits an expanded AHP6 expression domain in a cytokinin hyposensitive background, and defects in growth and cytokinin contents. Interestingly, the mutation can induce the production of a storage root -like structure during secondary development in Arabidopsis. Thus, we are for the first-time contemplating Arabidopsis as a model system for storage root morphogenesis, which may contribute to understanding novel molecular mechanisms that eventually translate into enhanced food production in tuberous root crops, such as sweet potato and cassava.
We also employ Populus spp. and Betula spp. as model organisms to study plant development in species with pronounced secondary growth. Using a tree species as a model organism in genetic studies pose a challenge in contrast to annual plants such as Arabidopsis thaliana. This is due to several years long juvenile stage which trees usually require before they start to flower and subsequently enable crossings. Conveniently, Betula spp. can be induced to flower within a year when introduced into special accelerated flowering conditions (3). We utilize accelerated flowering and forward genetics approach to study developmental mechanisms underlying distinct Betula spp. shoot system phenotypes.
A recent study from our group displayed that cambial cell division rate and biomass production can be stimulated dramatically in hybrid aspen trees through overexpression of cytokinin biosynthesis gene, ISOPENTENYLTRANSFERASE 7 (IPT7) (4). We are interested in understanding how cytokinin orchestrates the cambium activity and wood formation. For this purpose, we collected genome-wide profiling data from wood-forming regions of wild-type and mutant trees with enhanced cytokinin production, and from the stem of wild-type trees treated with cytokinin. As a result, several new regulators of cambium development in hybrid aspen were identified. Currently we are studying the functions of these candidate genes in Populus spp. through transgenic approach.
- Bishopp A, Help H, El-Showk S, Weijers D, Scheres B, Friml J, Benková E, Mähönen AP, Helariutta Y. (2011) A mutually inhibitory interaction between auxin and cytokinin specifies vascular pattern in roots. Current Biology. doi:10.1016/j.cub.2011.04.017
- Mähönen AP, Bishopp A, Higuchi M, Nieminen K, Kinoshita K, Törmäkangas K, Ikeda Y, Oka A, Kakimoto T, Helariutta Y (2006). Cytokinin Signaling and Its Inhibitor AHP6 Regulate Cell Fate During Vascular Development. Science. doi: 10.1126/science.1118875
- Longman, K. A. & Wareing, P.F. (1959). Early Induction of Flowering in Birch Seedlings. Nature. doi:10.1038/1842037b0
- Immanen J, Nieminen K, Smolander OP, Kojima M, Alonso Serra J, Koskinen P, Zhang J, Elo A, Mähönen AP, Street N, Bhalerao RP, Paulin L, Auvinen P, Sakakibara H, Helariutta Y. (2016). Cytokinin and Auxin Display Distinct but Interconnected Distribution and Signaling Profiles to Stimulate Cambial Activity. Current Biology. doi: 10.1016/j.cub.2016.05.053.