Research

Overall Focus

The goal of the research in the Gallagher lab is to understand the mechanisms of intercellular movement of transcription factors. We primarily use the root of Arabidopsis thaliana as the system in which we examine protein movement, but have interests in the development of other root systems as well. Our research is predicated upon the conviction that a determination of how plant cells communicate is the key first step in understanding how protein movement contributes to plant development, cellular plasticity and function.

Background

Cells within an organism can communicate with each other through the secretion of soluble proteins. In plants, communication can also occur through specialized intercellular channels called plasmodesmata (PD). PD allow the direct exchange of proteins between cell. This means that proteins made by one cell can travel into the neighboring cell and influence the behavior and/or fate of the neighbor. The SHORT-ROOT (SHR) transcription factor is one of these proteins that can move between cells and change the behavior and cell fate of the recipient cells (Figure 1). The SHR protein is made in the stele cells of the root of A. thaliana; it moves from the stele into the neighboring cell layers, in particular the stem (or initial cells) that form the endodermis and cortex cell layers. In these cells SHR promotes an asymmetric division that separates the cortex cell fate from that of the endodermis. Likewise the CAPRICE protein is made in non-hair cells (N) in the root and moves into the hair cells (Figure 1) to initiate hair cell fate.

research

It is our goal, using the SHR and CPC (to a lesser extent) proteins as a model transcription factors for cell-to-cell transport, to understand how intercellular protein movement is controlled (both promoted and inhibited) and the consequences of this type of movement to overall plant development.

An intracellular shuttle controling intercellular protein movement? An important question in cell signaling is how cell-to-cell movement is controlled. Nearly all plant cells are connected by dynamic intercellular channels, plasmodesmata (PD) that allow for the transfer of macromolecular cargoes. If development is to be regulated, there must exist mechanisms to both promote and restrict movement through these PD. Recently we identified SIEL, (for SHORT-ROOT INTERACTING EMBRYONICLETHAL) one of the first plant proteins shown to promote the movement of an endogenously encoded plant transcription factor. We showed that SIEL interacts with and promotes the movement of SHR from the stele into the neighboring endodermis. In addition SIEL interacts with CAPRICE (CPC; encoded by; At2g46410), TARGET OF MONOPTEROUS 7 (TMO7; encoded by At1g74500) and AGAMOUS-LIKE 21 (AGL21; encoded by At4g37940) (24) all of which are mobile transcription factors. In cells of the A. thaliana root, the SIEL protein localizes to the nucleus and the cytoplasm, where it associates with endosomes. The localization of SIEL to endosomes and movement of SHR between cells requires intact microtubules. We propose that microtubules serve as a platform for the interaction of SHR and SIEL. Once assocaited SIEL acts as an endosome localized intracellular shuttle that promotes the intracellular movement of SHR to PD. As SIEL is an essential protein, it is likely that SIEL has additional functions. The elucidation of these functions is one of the primary projects in the lab.

Conservation of SHR function and movement outside of A. thalianaIn contrast to A. thaliana roots, the roots of Oryza sativa (rice), Zea mays and Brachypodium distachyon contain multiple ground tissue layers and the ground tissue and epidermis are clonally related.

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For example a typical rice root has one endodermal cell layer and multiple cortex cell layers (the number of cortex layers varies between the different root types). These cell layers are all derived from a common cortical epidermal initial cell (CEEI dark green Fig. 1B) that occupies the same cell layer as the QC in a position that is analogous to the CEI cell in A. thaliana. To form the ground tissue and epidermis, the CEEI divides anticlinally to produce a daughter cell that divides periclinally to generate the epidermis and the endodermal precursor cells. The resulting cell in the epidermal cell file divides anticlinally in a series of transit amplifying cell divisions to produce the epidermal cell lineage. In contrast the cell occupying the endodermal position divides periclinally several times to produce all of the specialized cortical cell layers of the root. Similar processes produce the ground tissue and epidermal cell layers in maize and B.distachyon.

It is not known what accounts for the differences in the number of ground tissue layers between A. thalianaand rice or maize, or what accounts for the variation in the number of cortex cell layers between different roots on the same rice or maize plant. In A. thaliana it is possible to induce additional ground tissue layers in the root by ectopically expressing SHR in the endodermis from the SCR promoter or by increasing the domain of SHR movement (via a reduction in SCR or loss of JACKDAW (JKD). However in both of these instances many of the ground tissue layers generated have characteristics of endodermis. Recently we have shown that one can also induce the formation of additional ground tissue layers in the A. thaliana root through a reduction in SHR movement or SHR function. When the ground tissue is examined in roots heterozygous for a null allele of SHR, shr-2, or homozygous for a hypomorphic allele, shr-eal1, there are additional layers of cortex. These extra cortex layers arise sporadically from the endodermis, which expressed the D-type cyclin, CYCD6;1. Regular additional layers of cortex can be induced in otherwise wild-type A. thaliana roots by significantly reducing SHR movement into the endodermis, suggesting that outside of the CEI and CED cells high levels of SHR are inhibitory to asymmetric cell division.

Based upon what is known about SHR function, we propose that differences in the regulation and execution of the SHR pathways in A. thaliana, maize and rice result in the differences in radial patterning of the ground tissue in these plants. Furthermore we propose that differences in the behavior of the SHR homologs across several different species (including distantly related proteins like SHR homologs from Selaginella) can be used to dissect out the factors that are involved in SHR movement and how SHR movement relates to root patterning. This is one of the pojects underway in the laboratory. Both the Perin laboratory (Cirad) and the Turgeon laboratory (Cornell) are collaborators on this project.