Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot

Mutation of the SCARECROW (SCR) gene results in a radial pattern defect, loss of a ground tissue layer, in the root. Analysis of the shoot phenotype of scr mutants revealed that both hypocotyl and shoot inflorescence also have a radial pattern defect, loss of a normal starch sheath layer, and consequently are unable to sense gravity in the shoot. Analogous to its expression in the endodermis of the root, SCR is expressed in the starch sheath of the hypocotyl and inflorescence stem. The SCR expression pattern in leaf bundle sheath cells and root quiescent center cells led to the identification of additional phenotypic defects in these tissues. SCR expression in a pin-formed mutant background suggested the possible origins of the starch sheath in the shoot inflorescence. Analysis of SCR expression and the mutant phenotype from the earliest stages of embryogenesis revealed a tight correlation between defective cell divisions and SCR expression in cells that contribute to ground tissue radial patterning in both embryonic root and shoot. Our data provides evidence that the same molecular mechanism regulates the radial patterning of ground tissue in both root and shoot during embryogenesis as well as postembryonically.     

Cloning of Pinus sylvestris SCARECROW gene and its expression pattern in the pine root system, mycorrhiza and NPA-treated short roots

The SCARECROW (SCR) gene is central to root radial patterning. Its expression has not been investigated in conifers with morphologically different root types. Additional interest in SCR functions in the Pinus sylvestris root system comes from the effect of ectomycorrhiza formation on the short root apical structure. Here, the P. sylvestris SCR gene (PsySCR) was cloned and its expression investigated by northern blot and in situ hybridization of primary, lateral and short roots and mycorrhiza. Short root dichotomization was induced by auxin transport inhibitor (N-1-naphthylphthalamic acid (NPA)). PsySCR has conserved GRAS family protein motifs at the C-terminus and a variable N-terminus. PsySCR expression occurred in young root tissue and mycorrhiza. In root sections the PsySCR signal runs through the tip in initials for stele and root cap column and becomes upwards-restricted to endodermis in all root types. The PsySCR expression pattern suggests for the first time a regulatory role for SCR in maintaining the endodermal characteristics and radial patterning of roots with open meristem organization. The specific PsySCR localization is also an excellent marker for investigation of the dichotomization process in short roots.     

The expression patterns of arabinogalactan-protein AtAGP30 and GLABRA2 reveal a role for abscisic acid in the early stages of root epidermal patterning

In the Arabidopsis root, patterning of the epidermal cell types is position-dependent. The epidermal cell pattern arises early during root development, and can be visualized using reporter genes driven by the GLABRA (GL)2 promoter as markers. The GL2 gene is preferentially expressed in the differentiating hairless cells (atrichoblasts) during a period in which epidermal cell identity is believed to be established. We show that AtAGP30 is also expressed in atrichoblasts. This gene encodes an arabinogalactan-protein (AGP) that is known to play a role in root regeneration and increases abscisic acid (ABA)-response rates. Although the expression level of this gene is regulated by the plant growth factors ABA and ethylene, only ABA was found to affect the tissue-specific pattern of expression. ABA also disrupts the expression pattern of the GL2::GUS (beta-glucuronidase) reporter gene. Our results indicate that ABA regulates epidermal cell-type-specific gene expression in the meristematic zone of the Arabidopsis root, while ethylene is known to act at later stages of epidermal differentiation. Despite its effects on the early stages of root epidermal patterning, ABA does not affect root hair formation on mature wild-type epidermal cells, suggesting that other developmental cues, like positional information, can progressively over-ride the ABA-mediated disruption of early epidermal patterning.     

Interplay between SCARECROW, GA and LIKE HETEROCHROMATIN PROTEIN 1 in ground tissue patterning in the Arabidopsis root

Regulated cell division is critical for the development of multi-cellular organisms. In the Arabidopsis root, SCARECROW (SCR) is required for the first cell division, but represses the subsequent, longitudinal asymmetric cell divisions that generate the two cell types of the ground tissue – cortex and endodermis. To elucidate the molecular basis of the role of SCR in ground tissue patterning, we screened for SCR-interacting proteins using the yeast two-hybrid method. A number of putative SCR-interacting proteins were identified, among them LIKE HETEROCHROMATIN PROTEIN 1 (LHP1). In lhp1 mutants, a second longitudinal asymmetric cell division occurs in the ground tissue earlier than in wild-type plants. Similar to the scr mutant, this premature middle cortex phenotype is suppressed by the phytohormone gibberellin (GA). We provide evidence that the N-terminal domain of SCR is required for the interaction between SCR and LHP1 as well as with other interacting partners, and that this domain is essential for repression of asymmetric cell divisions. Consistent with a role for GA in cortex proliferation, mutants of key GA signaling components produce a middle cortex precociously. Intriguingly, we found that the spindly (spy) mutant has a similar middle cortex phenotype. As SPY homologs in animals physically interact with histone deacetylase, we examined the role of histone deacetylation in middle cortex formation. We show that inhibition of histone deacetylase activity causes premature middle cortex formation in wild-type roots. Together, these results suggest that epigenetic regulation is probably the common basis for SCR and GA activity in cortex cell proliferation.     

The SCARECROW gene's role in asymmetric cell divisions in rice plants

Asymmetric cell division is one of the most important mechanisms in the diversification of cell function and fate. In Arabidopsis, SCARECROW (SCR) is essential for the asymmetric division of the cortex/endodermis progenitor cell in the root. To learn more about how SCR is involved in asymmetric division, we analyzed the rice SCR (OsSCR) expression. In the root tip, OsSCR expression was observed in the endodermal cell layer and downregulated in the daughter cortex cell after asymmetric division, just as with Arabidopsis SCR. In leaf primordia, expression of OsSCR was observed in stomatal and ligule formation. In stomatal development, OsSCR was specifically expressed in the stomatal cell files before formation of guard mother cells (GMCs), and then, its expression was localized in GMCs, when the first asymmetric division occurred to generate the GMCs. Before the second asymmetric division of subsidiary mother cells (SMCs), localized OsSCR expression was observed in SMCs in the area close to the GMCs. Before these asymmetric divisions, the localization of OsSCR mRNA in GMC-forming cells and SMCs was observed in the area of the daughter GMC and subsidiary cells. OsSCR expression was also observed in the initiation area of ligule formation, and its downregulation occurred in the inner L2 cells generated by asymmetric division. Based on these observations, we proposed that OsSCR is involved not only in the asymmetric division of the cortex/endodermis progenitor cell but also during stomata and ligule formation by establishing the polarization of cytoplasm.     

FASCIATA genes for chromatin assembly factor-1 in arabidopsis maintain the cellular organization of apical meristems

Postembryonic development of plants depends on the activity of apical meristems established during embryogenesis. The shoot apical meristem (SAM) and the root apical meristem (RAM) have similar but distinct cellular organization. Arabidopsis FASCIATA1 (FAS1) and FAS2 genes maintain the cellular and functional organization of both SAM and RAM, and FAS gene products are subunits of the Arabidopsis counterpart of chromatin assembly factor-1 (CAF-1). fas mutants are defective in maintenance of the expression states of WUSCHEL (WUS) in SAM and SCARECROW (SCR) in RAM. We suggest that CAF-1 plays a critical role in the organization of SAM and RAM during postembryonic development by facilitating stable maintenance of gene expression states.