Abstract
ABSTRACTStem cell regeneration is crucial for both cell turnover and tissue healing in multicellular organisms. In Arabidopsis roots, a reduced group of cells known as the quiescent center (QC) act as a cell reservoir for surrounding stem cells during both normal growth and in response to external damage. Although cells of the QC have a very low mitotic activity, plant hormones such as brassinosteroids (BRs) can promote QC divisions. Here, we used a tissue-specific strategy to investigate the spatial signaling requirements of BR-mediated QC divisions. We generated stem cell niche-specific receptor knockout lines by placing an artificial microRNA against BRI1 (BRASSINOSTEROID INSENSITIVE 1) under the control of the QC-specific promoter WOX5. Additionally, QC-specific knock-in lines for BRI1 and its downstream transcription factor BES1 (BRI1-EMS-SUPPRESOR1) were also created using the WOX5 promoter. By analyzing the roots of these lines, we show that BES1-mediated signaling cell-autonomously promotes QC divisions, that BRI1 is essential for sensing nearby inputs and triggering QC divisions and that DNA damage promotes BR-dependent paracrine signaling in the stem cell niche as a prerequisite to stem cell replenishment.
Highlights
Brassinosteroids (BRs) are plant steroid hormones that were originally discovered in Brassica napus pollen for their ability to promote growth when exogenously applied to other vascular plants (Mitchell et al, 1970)
We demonstrate that: (1) active BES1 is necessary for cell-autonomous quiescent center (QC) divisions; (2) the BR hormone itself is the limiting factor for BR-induced QC divisions in the root apex; (3) BRI1 is required at the stem cell niche for mediating BRdependent QC divisions; and (4) upon stem cell death, paracrine BR signaling is required for QC divisions
The slow-dividing nature of the cells in the QC enable it to act as a cell reservoir and organizer for surrounding stem cells (Fulcher and Sablowski, 2009; Pi et al, 2015; Sarkar et al, 2007; van den Berg et al, 1997; Vilarrasa-Blasi et al, 2014)
Summary
Brassinosteroids (BRs) are plant steroid hormones that were originally discovered in Brassica napus pollen for their ability to promote growth when exogenously applied to other vascular plants (Mitchell et al, 1970). Upon BR binding, the heterodimerization of BRI1 with BAK1 (BRI1-ASSOCIATED RECEPTOR KINASE 1) is enhanced, and a cytoplasmic cascade of phosphorylation and dephosphorylation events is initiated (Li and Nam, 2002; Russinova et al, 2004) These events lead to the degradation of BIN2 (BRASSINOSTEROID INSENSITIVE 2) kinase (Li and Nam, 2002; Peng et al, 2008), and a consequential increase in the dephosphorylated forms of the BZR1 (BRASSINAZOLE RESISTANCE 1) (Wang et al, 2002) and BES1 (BRI1-EMS-SUPRESSOR 1) (Yin et al, 2002) transcription factors. Recent work has revealed that these transcription factors are subjected to post-transcriptional regulation in response to external stimuli such as light (Kim et al, 2014) and environmental stress (Nolan et al, 2017). BR-mediated transcriptional responses are controlled by an additional regulatory layer
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