We use the Arabidopsis root as an experimental system to study vascular development, because here it is relatively easy to follow the processes of cell fate acquisition, tissue patterning, cell proliferation, and differentiation as the root is continuously growing and the tissue types are organised in a regular and simple pattern. Because the xylem tissue of the root is responsible for the uptake and transport of water from soil to the rest of the plant its development may be sensitive to availability of water in the surrounding environment of the root. We have analysed the effects of drought and ABA signalling on root vascular development and have recently shown that vascular patterning is indeed affected, resulting in increased number of xylem strands. We showed that this is a consequence of ABA signalling in the endodermis, resulting in elevated miR165 levels, targeting class III HD-ZIP transcription factors, that we previously have shown dose-dependently determine xylem cell identity (Figure 2; Prashanth et al., 2018; Carlsbecker & Lee, et al, Nature, 2010). Many questions remain to be answered, and we are currently addressing e.g. by what means ABA/drought affect miR165 levels, if there are non-miR165-mediated pathways by which ABA/drought affects vascular patterning, and how xylem differentiation is affected.
We are also interested in understanding to what extent the genetic and molecular mechanisms governing vascular patterning and development is evolutionary conserved among seed plants. We focus on primary vascular development and early secondary development in Norway spruce (Picea abies), and use e.g. transgenic and comparative transcriptomic approaches. Understanding how regulatory circuits governing early development compare to adult cambium activity during wood-formation will facilitate functional assessments of putative key regulators, and in the long run benefit breeding of conifers.
Figure 1. Comparison of the anatomy of the Arabidopsis and Norway spruce, Picea abies, primary root, with a focus on the vascular tissues. Arrows points at the xylem.
Figure 2. Model on how ABA non-cell autonomously affects xylem development. The levels of ABA determine the expression levels of MIR165A in the endodermis, producing miR165, which moves into the stele to restrict the activity domain of HD-ZIP III TFs, which in turn determine how xylem develops. Question marks highlight our current gaps in knowledge. Red, protoxylem; blue, metaxylem; yellow, discontinuous xylem strand; en, endodermis; xpp, xylem pole pericycle; pro, procambium
Augstein F, Carlsbecker A. Getting to the roots: A developmental genetic view of root anatomy and function from Arabidopsis to lycophytes. 2018. Front Plant Sci. 9:1410. doi.org/10.3389/fpls.2018.01410
Ramachandran P, Wang G, Augstein F, de Vries J, Carlsbecker A. Continuous root xylem formation and vascular acclimation to water deficit involves endodermal ABA signalling via miR165. 2018. Development. 145. doi: 10.1242/dev.159202, http://dev.biologists.org/content/145/3/dev159202
Ramachandran, P., Carlsbecker, A., Etchells, J. (2017). Class III HD- ZIPs govern vascular cell fate: an HD view on patterning and differentiation. Journal of Experimental Botany, 68(1): 55-69
Müller, C., Valdés, A., Wang, G., Ramachandran, P., Beste, L. Uddenberg, D., Carlsbecker, A. (2016). PHABULOSA Mediates an Auxin Signaling Loop to Regulate Vascular Patterning in Arabidopsis. Plant Physiology, 170(2): 956-970
Uddenberg, D., Akhter, S., Ramachandran, P., Sundström, J., Carlsbecker, A. (2015). Sequenced genomes and rapidly emerging technologies pave the way for conifer evolutionary developmental biology. Frontiers in Plant Science, 6
Ursache, R., Miyashima, S., Chen, Q., Vaten, A., Nakajima, K. Carlsbecker, A., Zhao, Y. Helariutta, Y., Dettmer, J. (2014). Tryptophan-dependent auxin biosynthesis is required for HD-ZIP III-mediated xylem patterning. Development, 141(6): 1250-1260
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. (2011). Callose Biosynthesis Regulates Symplastic Trafficking during Root Development. Developmental Cell, 21(6): 1144-1155
Carlsbecker#, A., Lee#, J.Y., Roberts, C., Dettmer, J., Lehesranta, S., Zhou, J., Lindgren, O., Moreno-Risueno, M.A., Vatén, A., Thitamadee, S., Campilho, A., Sebastian, J., Bowman, J.L., Helariutta, Y., and Benfey, P.N. (2010). Cell signalling by microRNA165/6 directs gene dose-dependent root cell fate. Nature, 465(7296): 316-321 # Equal contribution.