Spatiotemporal regulation of cell-cycle genes by SHORTROOT links patterning and growth

Abstract

The development of multicellular organisms relies on the coordinated control of cell divisions leading to proper patterning and growth [1][2][3] . The molecular mechanisms underlying pattern formation, particularly the regulation of formative cell divisions, remain poorly understood. In Arabidopsis, formative divisions generating the root ground tissue are controlled by SHORTROOT (SHR) and SCARECROW (SCR) 4-6. Here we show, using cell-type-specific transcriptional effects of SHR and SCR combined with data from chromatin immunoprecipitation-based microarray experiments, that SHR regulates the spatiotemporal activation of specific genes involved in cell division. Coincident with the onset of a specific formative division, SHR and SCR directly activate a D-type cyclin; furthermore, altering the expression of this cyclin resulted in formative division defects. Our results indicate that proper pattern formation is achieved through transcriptional regulation of specific cellcycle genes in a cell-type-and developmental-stage-specific context. Taken together, we provide evidence for a direct link between developmental regulators, specific components of the cell-cycle machinery and organ patterning.Growth and patterning are key processes that govern the development of multicellular organisms. In some cases, like early Drosophila embryogenesis 7 , these are independent. However, in many animals and plants, proper development frequently relies on tight coordination of growth and patterning. Disruption of this coordination can lead to unchecked cell growth, resulting in tumorigenesis or misshapen organs 8 . Although the molecular mechanisms involved in pattern formation 9-11 and in cell-cycle control [12][13][14][15] 5,6,23,24,25 . To gain insight into the role of the SHR/SCR network in controlling formative cell divisions, we expressed an inducible version of either SHR or SCR in its respective mutant background and characterized the timing of formative divisions after induction.Before induction, SHR-and SCR-inducible plants had a single mutant ground tissue layer 4,5 (Supplementary Fig. 1). After SHR induction, SCR expression was observed within 3 h, indicating that SHR rapidly activates its targets ( Supplementary Fig. 1). The first periclinal (parallel to the direction of growth) division in the mutant ground tissue layer occurred 6 h after SHR induction (Fig. 1a , Supplementary Fig. 1 and Supplementary Movie 1) and earlier after SCR induction (Fig. 1b). Two layers of ground tissue with SCR expression in the quiescent centre and endodermis (Supplementary Fig. 1), along with a nearly complete Casparian band 5 , were detected 24 h after SHR induction ( Supplementary Fig. 2). This underlines the combinatorial role of SHR and SCR in regulating formative cell divisions and also indicates that the two inducible systems have slightly different kinetics.To understand the dynamics of the SHR/SCR regulatory network, we sorted ground tissue cells at several time points after SHR and SCR induction and performed microarray a...

SHORT-ROOT Regulates Primary, Lateral, and Adventitious Root Development in Arabidopsis

“…2B; Ferreira et al, 1994). Consistent with a role as a key root mitotic regulator, Sozzani et al (2010) recently reported that SHR controls the expression of a number of genes encoding components of the cell cycle machinery. Direct targets of SHR include the D-type cyclin gene CYCD6;1 and the cyclin-dependent kinase genes CDKB2;1 and CDK2;2 (Sozzani et al, 2010).…”

“…Consistent with a role as a key root mitotic regulator, Sozzani et al (2010) recently reported that SHR controls the expression of a number of genes encoding components of the cell cycle machinery. Direct targets of SHR include the D-type cyclin gene CYCD6;1 and the cyclin-dependent kinase genes CDKB2;1 and CDK2;2 (Sozzani et al, 2010). While ectopic expression of two of these genes in shr was able to partially restore formative divisions, it was not able to rescue mutant root growth (Sozzani et al, 2010).…”

“…Direct targets of SHR include the D-type cyclin gene CYCD6;1 and the cyclin-dependent kinase genes CDKB2;1 and CDK2;2 (Sozzani et al, 2010). While ectopic expression of two of these genes in shr was able to partially restore formative divisions, it was not able to rescue mutant root growth (Sozzani et al, 2010). Hence, further genes are likely to be required.…”

“…S2). Mutant roots exhibit QC defects and loss of activity of surrounding stem (initial) cells, resulting in the disruption of formative cell division events (Sozzani et al, 2010) and the depletion of proliferating cells in the RAM (Sabatini et al, 2003).…”

“…SHR and SCR genes encode closely related transcription factors belonging to the GRAS gene family (Di Laurenzio et al, 1996;Helariutta et al, 2000). SHR has been demonstrated to directly regulate the expression of genes including SCR (Levesque et al, 2006) and a number of cell cycle components including the D-type cyclin, CYCD6;1 (Sozzani et al, 2010).…”