To move or not to move?

My research interest is in studying the non-cell-autonomous protein TMO7, which represents the importance of cell-cell communication at early development of plants. Embryogenesis is the period when plants build up their basic body structure. The embryo development of the model plant Arabidopsis thaliana has a fixed and predictable pattern, which makes it a perfect model to study early events in pattern formation, cell-cell communication, and cell identification.

Determination of stem cells is one of the most important events during development. At early globular stage, plants start to set up all the tissues and stem cells. One of the extra-embryonic cells from the suspensor will be recruited to become the hypophysis, the founder cell of the root apical meristem (Figure 1). It is known that the phytohormone, auxin, will activate MONOPTEROS/ AUXIN RESPONSE FACTOR 5 (MP/ARF5) and generate a non-cell-autonomous signal to regulate hypophysis determination.

Figure 1. The embryogenesis of Arabidopsis theliana During the early globular stage, the upper-most suspensor cell (in green) will be recruited to become hypophysis (in orange). ac, apical cell, bc, basal cell, pe, pre-embryo, ut, upper tile cells, lt, lower tile cells, pd, protodermal cells, hy, hypophysis, Cot, cotyledons, SAM, soot apical meristem, Hyp, hypocotyl, RAM, root apical meristem.
Figure 1. The embryogenesis of Arabidopsis theliana During the early globular stage, the upper-most suspensor cell (in green) will be recruited to become hypophysis (in orange). ac, apical cell, bc, basal cell, pe, pre-embryo, ut, upper tile cells, lt, lower tile cells, pd, protodermal cells, hy, hypophysis, Cot, cotyledons, SAM, soot apical meristem, Hyp, hypocotyl, RAM, root apical meristem.

TARGET OF MONOPTEROS 7 (TMO7) is one of the direct targets of MP. The promoter activity of TMO7 is in cells adjacent to the suspensor while TMO7-GFP fusion protein can be detected in the upper-most suspensor cell, indicating the movement of TMO7 protein. The down regulation of TMO7 caused miss regulation of division plane in hypophysis and lead to rootless phenotype, demonstrating that TMO7 is important for hypophysis identification (Figure 2).

Figure 2. The movement of TMO7 and phenotypes of TMO7-RNAi transgenic plants pTMO7-3nGFP indicates the promoter activity of TMO7 is at cells neighboring suspensor (upper left panel) but TMO7-GFP fusion protein can be detected at hypophysis (lower left panel, with *). In TMO7-RNAi lines, the division of hypophysis is miss regulated (upper right panel, indicated by white arrow), and leading to rootless phenotype (lower right panel).
Figure 2. The movement of TMO7 and phenotypes of TMO7-RNAi transgenic plants pTMO7-3nGFP indicates the promoter activity of TMO7 is at cells neighboring suspensor (upper left panel) but TMO7-GFP fusion protein can be detected at hypophysis (lower left panel, with *). In TMO7-RNAi lines, the division of hypophysis is miss regulated (upper right panel, indicated by white arrow), and leading to rootless phenotype (lower right panel).

Two major directions I would like to explore is 1) how does TMO7 move from cell to cell? And 2) after TMO7 move into the upper-most suspensor cell, how dose it regulate the cell identification? To approach these two goals, I would like to combine the site-direct mutagenesis to study the cis-element on TMO7. In addition, by using biochemical methods to isolate the potential interacting partners can provide us more information about how the movement of TMO7 might be regulated. Furthermore, I will use the micro-array analysis to investigate the downstream targets of TMO7 to understand how TMO7 regulate the cell identity of hypophysis.


Reference:

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

2.    J. R. Wendrich and D. Weijers, The Arabidopsis embryo as a miniature morphogenesis model, New Phytologist (2013) 199: 14–25

3.    T. M. Burch-Smith and P. C. Zambryski, Plasmodesmata Paradigm Shift: Regulation from Without Versus Within, Annu. Rev. Plant Biol. (2012) 63:239–60