Cell-to-cell movement of the CAPRICE protein inArabidopsisroot epidermal cell differentiation

Abstract

CAPRICE (CPC), a small, R3-type Myb-like protein, is a positive regulator of root hair development in Arabidopsis.Cell-to-cell movement of CPC is important for the differentiation of epidermal cells into trichoblasts (root hair cells). CPC is transported from atrichoblasts (hairless cells), where it is expressed, to trichoblasts, and generally accumulates in their nuclei. Using truncated versions of CPC fused to GFP, we identified a signal domain that is necessary and sufficient for CPC cell-to-cell movement. This domain includes the N-terminal region and a part of the Myb domain. Amino acid substitution experiments indicated that W76 and M78 in the Myb domain are critical for targeted transport, and that W76 is crucial for the nuclear accumulation of CPC:GFP. To evaluate the tissuespecificity of CPC movement, CPC:GFP was expressed in the stele using the SHR promoter and in trichoblasts using the EGL3 promoter. CPC:GFP was able to move from trichoblasts to atrichoblasts but could not exit from the stele, suggesting the involvement of tissue-specific regulatory factors in the intercellular movement of CPC. Analyses with a secretion inhibitor, Brefeldin A, and with an rhd3 mutant defective in the secretion process in root epidermis suggested that intercellular CPC movement is mediated through plasmodesmata. Furthermore, the fusion of CPC to tandem-GFPs defined the capability of CPC to increase the size exclusion limit of plasmodesmata.

Analysis of Arabidopsis Transcription Factor Families Revealed Extensive Capacity for Cell-to-Cell Movement as Well as Discrete Trafficking Patterns

“…A B however, one member, CPC was earlier shown to engage in selective trafficking within the epidermal cell layer (Kurata et al, 2005). So again, the size of a NCATF cannot be used as a predictor of the mechanism underlying cell-to-cell movement.…”

“…However, when expressed in cells of the stele, using the SHR promoter, CPC-GFP was unable to traffic into endodermal/cortical cells (Kurata et al, 2005). This suggested a selectivity of CPC movement between different cell layers as well as a directional movement in the same endodermal and pericycle cell boundary.…”

“…Protein size and subcellular localization affect intercellular movement Within the list of identified NCATFs were STM, CPC and SCL23 that were earlier suggested to move through PD by a selective trafficking pathway (Gallagher and Benfey, 2009;Kim et al, 2005;Kurata et al, 2005). However, other NCATFs, like LFY have been suggested to move cell to cell via a diffusionbased non-selective mode in which the NCATF does not need a direct interaction with PD components (Wu et al, 2003).…”

“…This suggested a selectivity of CPC movement between different cell layers as well as a directional movement in the same endodermal and pericycle cell boundary. Furthermore, a selective mode for CPC movement between epidermal cells was supported by the fact that CPC carrying a specific mutation (CPC M78A ) retained its function but was defective in cell-to-cell movement (Kurata et al, 2005). To test whether CPC M78A was also dysfunctional in movement from endodermal/cortical cells, UAS::CPC M78A -mCherry was introduced into J0571 plants.…”

“…Other well-characterized NCATFs include the floral homeotic proteins, LEAFY (Sessions et al, 2000;Wu et al, 2003), AGAM-OUS (Urbanus et al, 2010), DEFICIENS and GLOBOSA (Perbal et al, 1996), as well as TFs involved in root development, such as the Arabidopsis GRAS family member, SHORT ROOT (SHR) (Nakajima et al, 2001), CAPRICE (CPC), a Myb-like DNA-binding domain protein (Kurata et al, 2005;Wada et al, 2002), basic helix-loop-helix proteins GLABRA 3 (GL3) and ENHANCED GLABRA 3 (EGL3) (Bernhardt et al, 2005).…”