A broad competence to respond to SHORT ROOT revealed by tissue-specific ectopic expression
2004
In plants, cell fate specification depends primarily on position rather
than lineage. Recent results indicate that positional information can be
transmitted through intercellular trafficking of transcription factors. The
SHORT ROOT ( SHR ) gene, a member of the GRAS family of
putative transcription factors, is involved in root radial patterning in
Arabidopsis. Correct radial patterning depends on the positional
information transmitted through limited SHR intercellular movement and
translated into cell division and specification by competent target cells. To
investigate the regulation of SHR movement and the competence to respond to
it, we drove expression of a translational fusion SHR::GFP using four
different tissue-specific promoters. In a wild-type background, SHR::GFP was
not able to move from either phloem companion cells or epidermal cells, both
of which have been shown to support movement of other proteins, suggesting a
requirement for tissue-specific factors for SHR movement. When expressed from
its native promoter in plants with multiple endodermal layers, SHR::GFP was
not able to move beyond the first endodermal layer, indicating that movement
is not limited by a mechanism that recognizes boundaries between cell types.
Surprisingly, movement of SHR::GFP was observed when ectopic expression from
an epidermal promoter was placed in a scarecrow ( scr ) mutant
background, revealing a possible role for SCR in limiting movement. Analysis
of the competence to respond to SHR-mediated cell specification activity
indicated that it was broadly distributed in the epidermal lineage, while
competence to respond to the cell division activity of SHR appeared limited to
the initials and involved induction of SCR. The spatial distribution of
competence to respond to SHR highlights the importance of tightly regulated
movement in generating the root radial pattern.
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