Multiple feedback loops through cytokinin signaling control stem cell number within the Arabidopsis shoot meristem

Abstract

A central unanswered question in stem cell biology, both in plants and in animals, is how the spatial organization of stem cell niches are maintained as cells move through them. We address this question for the shoot apical meristem (SAM) which harbors pluripotent stem cells responsible for growth of above-ground tissues in flowering plants. We find that localized perception of the plant hormone cytokinin establishes a spatial domain in which cell fate is respecified through induction of the master regulator WUSCHEL as cells are displaced during growth. Cytokinin-induced WUSCHEL expression occurs through both CLAVATA-dependent and CLAVATA-independent pathways. Computational analysis shows that feedback between cytokinin response and genetic regulators predicts their relative patterning, which we confirm experimentally. Our results also may explain how increasing cytokinin concentration leads to the first steps in reestablishing the shoot stem cell niche in vitro.clavata ͉ wuschel ͉ computational modeling P lants ranging from the small weed Arabidopsis to the giant sequoia tree, maintain growth of stems, leaves, flowers, and branches through the action of stem cells. In the model plant Arabidopsis, as in other flowering plants, stem cells which give rise to above-ground tissues reside in a structure termed the shoot apical meristem (SAM) (1, 2). The Arabidopsis SAM is composed of three functionally distinct zones. The central zone (CZ) at the tip of the SAM harbors pluripotent stem cells which are necessary for the indeterminate growth and development of the plant. As the plant grows, CZ cells become either multipotent peripheral zone (PZ) cells on the sides of the meristem, capable of differentiating to leaf and flower primordia, or multipotent rib meristem (RM) cells beneath, which can differentiate to the cell types of the stem (3). Positions of zones within the meristem are maintained even as individual cells are displaced from the CZ through the PZ and RM into differentiating tissues. Molecular mechanisms by which meristematic zones are maintained as cells comprising these domains change remains a fundamental question in plant biology (1, 4). One mechanism involves the transmembrane receptor kinase CLAVATA1 (CLV1), expressed in cells of the RM (5). Its ligand, the extracellular peptide product of the CLAVATA3 (CLV3) gene, is produced in the CZ (6), and when it signals the RM cells, they reduce the activity of the WUSCHEL (WUS) gene, which codes for a homeodomain transcription factor also expressed in the RM (7,8). WUS activity is nonautonomously necessary for the maintenance of the CZ cells as pluripotent stem cells, and therefore for persistence of the SAM (9). Loss of CLV3 activity causes enlargement of the CZ by conversion of PZ cells on the PZ-CZ border to CZ cells within hours, followed by enlargement of the SAM through increased cell division or reduced differentiation, or both, over days (10).Multiple lines of evidence show that the plant hormone cytokinin is involved in the CLV/WUS circuit, as well a...

RiceLHS1/OsMADS1Controls Floret Meristem Specification by Coordinated Regulation of Transcription Factors and Hormone Signaling Pathways

“…Interactions between cytokinin signaling and homeodomain transcription factors govern meristem maintenance in Arabidopsis, maize, and rice (Gordon et al, 2009;Tsuda et al, 2011;Bolduc et al, 2012). In OsMADS1 knockdown panicles, we detected an overall increased activity of cytokinin pathway genes, examples being LONELY GUY (LOG) family members, a predicted receptor, HKL1/CRL4, and several response regulator (RR) genes (Supplemental Table S4).…”

“…Cytokinin induces WUS expression by multiple pathways (Gordon et al, 2009;Chickarmane et al, 2012). WUS in turn directly represses the expression of some type A cytokinin response regulators in a feedback loop that regulates stem cell number in Arabidopsis shoot meristem (Leibfried et al, 2005;Gordon et al, 2009). The importance of cytokinin levels for the size and determinacy of rice FMs is clearly illustrated by the phenotypes of log and ckx2 mutants in cytokinin metabolism (Ashikari et al, 2005;Kurakawa et al, 2007).…”

“…Cytokinin induces WUS expression by multiple pathways (Gordon et al, 2009;Chickarmane et al, 2012). WUS in turn directly represses the expression of some type A cytokinin response regulators in a feedback loop that regulates stem cell number in Arabidopsis shoot meristem (Leibfried et al, 2005;Gordon et al, 2009).…”

Type-B ARABIDOPSIS RESPONSE REGULATORs Specify the Shoot Stem Cell Niche by Dual Regulation of WUSCHEL

“…Cytokinin signaling has been suggested to be a positional cue for WUS expression (Gordon et al, 2009). However, the regulatory pathways between cytokinin receptors and WUS remain to be elucidated.…”

“…Incubation on medium containing a high cytokinin-to-auxin ratio activates the expression of WUS in callus and lateral root primordia and induces the formation of the shoot meristem (Gordon et al, 2007;Chatfield et al, 2013). The expression of Arabidopsis His kinases4, which encodes a cytokinin receptor, precedes and subsequently overlaps with that of WUS (Gordon et al, 2009). Mutations in type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs), key regulators of primary cytokinin response genes, result in reduced shoot regeneration (Ishida et al, 2008;Hwang et al, 2012;Hill et al, 2013;Zhang et al, 2015).…”