ARGONAUTE10 and ARGONAUTE1 Regulate the Termination of Floral Stem Cells through Two MicroRNAs in Arabidopsis

Abstract

Stem cells are crucial in morphogenesis in plants and animals. Much is known about the mechanisms that maintain stem cell fates or trigger their terminal differentiation. However, little is known about how developmental time impacts stem cell fates. Using Arabidopsis floral stem cells as a model, we show that stem cells can undergo precise temporal regulation governed by mechanisms that are distinct from, but integrated with, those that specify cell fates. We show that two microRNAs, miR172 and miR165/166, through targeting APETALA2 and type III homeodomain-leucine zipper (HD-Zip) genes, respectively, regulate the temporal program of floral stem cells. In particular, we reveal a role of the type III HD-Zip genes, previously known to specify lateral organ polarity, in stem cell termination. Both reduction in HD-Zip expression by over-expression of miR165/166 and mis-expression of HD-Zip genes by rendering them resistant to miR165/166 lead to prolonged floral stem cell activity, indicating that the expression of HD-Zip genes needs to be precisely controlled to achieve floral stem cell termination. We also show that both the ubiquitously expressed ARGONAUTE1 (AGO1) gene and its homolog AGO10, which exhibits highly restricted spatial expression patterns, are required to maintain the correct temporal program of floral stem cells. We provide evidence that AGO10, like AGO1, associates with miR172 and miR165/166 in vivo and exhibits “slicer” activity in vitro. Despite the common biological functions and similar biochemical activities, AGO1 and AGO10 exert different effects on miR165/166 in vivo. This work establishes a network of microRNAs and transcription factors governing the temporal program of floral stem cells and sheds light on the relationships among different AGO genes, which tend to exist in gene families in multicellular organisms.

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

“…Recent data reveal an unexpected but crucial contribution of HD-Zip III factors in the precise regulation of Arabidopsis floral stem cells, which suggests additional functions for these factors (Ji et al, 2011). Our microarray data showed that four rice HD-Zip III genes (OsHB1-OsHB4) of the REVOLUTA Table I.…”

“…between WUSCHEL (WUS), AGAMOUS (AG), AGL24, and SVP; Liu et al, 2009;Grandi et al, 2012) and the combinatorial activity of three BLH proteins and SHOOT MERISTEMLESS (STM), a KNOX HD protein, regulate Arabidopsis FM initiation and maintenance (Rutjens et al, 2009). Recently, the miR165/166-targeted HD-Zip III members PHAVO-LUTA and PHABULOSA were found to have an unexpected crucial role in Arabidopsis FM stem cell regulation (Ji et al, 2011), aside from their role in specifying organ polarity. Our data suggest that OsMADS1 aids rice FM development and determinacy possibly by regulating genes like OsHB4/HD-Zip III and OsYABBY5/TONGARI-BOUSHI1 (TOB1).…”

Plant MicroRNAs Display Differential 3' Truncation and Tailing Modifications That Are ARGONAUTE1 Dependent and Conserved Across Species

“…The probe for detecting miR171a was 59-GATATTG-GCGCGGCTCAATCA-39 (21 nucleotides); the probe for detecting miR158 was59-GCTTTGTCTACATTTGGGA-39 (19 nucleotides). IP was performed as previously described (Ji et al, 2011). The AGO1 mouse polyclonal antibody was generated in the Cao lab (Institute of Genetics and Developmental Biology, Chinese Academy of Sciences).…”

AGAMOUSTerminates Floral Stem Cell Maintenance inArabidopsisby Directly RepressingWUSCHELthrough Recruitment of Polycomb Group Proteins

“…The termination of floral stem cell fate is coincident with the development of carpel primordia, the final organs to be made from the floral meristem. However, the termination of floral stem cell maintenance is not simply the differentiation of stem cells into carpel cells because we isolated mutants that uncouple carpel identity specification and the termination of floral stem cell maintenance (Ji et al, 2011;this study). Instead, a mechanism independent of, but coordinated with, organ identity specification is responsible for the precise termination of floral stem cell fate.…”