Kuan-Ju Lu: Intercellular transport and development & Pre-vascular function of TMO5 and LHW

1. Intercellular transport and development

Intercellular communication is the foundation for development and response to different internal and external factors (ten Hove et al., 2015). Unlike animal cells which move to their destination during development, plant cells are restricted in stiff cell walls and therefore the information exchanges between cells has extra importance in plant cells. Plasmodesmata (PD) are ER containing pores which connect two neighboring cells and create a continuous cytoplasm. Plant cells transport micro-molecules through PD by diffusion and transport macro-molecules, like proteins and RNAs by a selective transport. Several critical developmental processes are regulated by mobile transcription factors, in which, the earliest event occurs during embryogenesis.

The determination of hypophysis, the founder cells of quiescence center (QC) and columella cells, is critical for root formation. It is known that auxin regulates this process through activating AUXIN RESPONSE FACTOR 5/ MONOPTEROUS (ARF5/MP). MP further turns on its direct downstream target, TARGET OF MONOPTEROUS 7 (TMO7), in cells surrounding the hypophysis, and TMO7 further moves in the hypophysis to involve in its development (Figure 1) (Schlereth et al., 2010). “How does TMO7 move?” is the research question I am interested in. Interestingly, I also observed the same mobility of TMO7 in seedling root (Figure 2), which allow me to characterize the movement property of TMO7 easily.

KuanJu-1.png

Currently, I use site direct mutagenesis and combine with confocal microscopy to investigate whether TMO7 move through PD, and whether the transport of TMO7 is in a diffusion manner or a regulated manner. And how is the transport of TMO7 influence the hypophysis development by generating CRISPR-Cas9 mediated tmo7 mutant.

KuanJu-2.png

2. Pre-vascular function of TMO5 and LHW

The evolution of vasculature is a great leap which allows upright growth of plants. Vascular plants are known to evolve about 4.25 million years ago, based on the morphology, however, the molecular evolution of vascular-specific genes is largely unknown.

Studies in Arabidopsis reveal that another direct target of MP, TARGET OF MONOPTEROS 5 (TMO5), is turned on by MP at early embryogenesis (De Rybel et al., 2016; Schlereth et al., 2010). TMO5 further cooperate with another transcription factor called LONESOME HIGHWAY (LHW) and activate their downstream target LONELY GUY (LOG) (De Rybel et al., 2013). LOG is a key enzyme which involves in the final step of cytokinin activation. The up-regulation of LOG by TMO5-LHW complex triggers the periclinal cell division through activating cytokinin to thicken the vasculature (Figure 1) (De Rybel et al., 2014).

To answer the question how vascular tissue molecularly evolved, bioinformatics analysis was employed, and interestingly, TMO5-like and LHW-like were identified in non-vascular plants. What would be their function in the early land plants which lack vasculature? We take advantage of the liverwort, Marchantia polymorpha, which has all the tools for genome modification and its genome has been sequenced, to investigate what is the pre-vascular function of TMO5 and LHW.

I am currently focusing on investigating the expression pattern of MpTMO5 and MpLHW to gain information about when they might have physiological function. By using well-established agrobacterium mediated transformation, I managed to generate gain- and loss-of-function mutant Marchantia for getting information about their function.

De Rybel, B., Adibi, M., Breda, A. S., Wendrich, J. R., Smit, M. E., Novák, O., Yamaguchi, N., Yoshida, S., Van Isterdael, G. and Palovaara, J. (2014). Integration of growth and patterning during vascular tissue formation in Arabidopsis. Science 345, 1255215.

De Rybel, B., Mahonen, A. P., Helariutta, Y. and Weijers, D. (2016). Plant vascular development: from early specification to differentiation. Nat Rev Mol Cell Biol 17, 30-40.

De Rybel, B., Moller, B., Yoshida, S., Grabowicz, I., Barbier de Reuille, P., Boeren, S., Smith, R. S., Borst, J. W. and Weijers, D. (2013). A bHLH complex controls embryonic vascular tissue establishment and indeterminate growth in Arabidopsis. Dev Cell 24, 426-437.

Schlereth, A., Moller, B., Liu, W., Kientz, M., Flipse, J., Rademacher, E. H., Schmid, M., Jurgens, G. and Weijers, D. (2010). MONOPTEROS controls embryonic root initiation by regulating a mobile transcription factor. Nature 464, 913-916.

ten Hove, C. A., Lu, K. J. and Weijers, D. (2015). Building a plant: cell fate specification in the early Arabidopsis embryo. Development 142, 420-430.