Abstract
In Arabidopsis shoot apical meristems (SAMs), a well-characterized regulatory loop between WUSCHEL (WUS) and CLAVATA3 (CLV3) maintains stem cell homeostasis by regulating the balance between cell proliferation and cell differentiation. WUS proteins, translated in deep cell layers, move into the overlaying stem cells to activate CLV3. The secreted peptide CLV3 then regulates WUS levels through a ligand-receptor mediated signaling cascade. CLV3 is specifically expressed in the stem cells and repressed in the deep cell layers despite presence of the WUS activator, forming an apical-basal polarity along the axis of the SAM. Previously, we proposed and validated a hypothesis that the HAIRY MERISTEM (HAM) family genes regulate this polarity, keeping the expression of CLV3 off in interior cells of the SAM. However, the specific role of each individual member of the HAM family in this process remains to be elucidated. Combining live imaging and molecular genetics, we have dissected the conserved and distinct functions of different HAM family members in control of CLV3 patterning in the SAMs and in the de novo shoot stem cell niches as well.
Highlights
Pluripotent stem cells in plant shoot apical meristems (SAMs) can continuously divide and initiate new leaves and flowers
A single mutation of HAM1, HAM2, or HAM3 does not result in any obvious defects (Engstrom et al, 2011) and the ham123 triple loss of function mutant leads to the ectopic activation of CLV3 in the rib meristem of SAMs (Schulze et al, 2010; Zhou et al, 2018), suggesting that HAM1-3 likely have shared functions
In contrast to pHAM1:: YPET-HAM1 (Figures 1A–F) or HAM2::YPET-HAM2 (Figures 2A–F), we found that the pHAM3::YPET-HAM3 translational reporter is only expressed at the boundary between the meristem and primordia and in a few cells of the peripheral zone (PZ) of the SAM (Figures 3A–F)
Summary
Pluripotent stem cells in plant shoot apical meristems (SAMs) can continuously divide and initiate new leaves and flowers. Along the axis of the SAM, a regulatory feedback loop involving CLAVATA3 (CLV3) and WUSCHEL (WUS) controls stem cell homeostasis through cell-cell communication between these two cell types (Laux et al, 1996; Mayer et al, 1998; Fletcher et al, 1999; Brand et al, 2000; Schoof et al, 2000). CLV3 mRNAs are expressed in the stem cells at the central zone but not expressed in the rib meristem cells that are located beneath the stem cells (Fletcher et al, 1999; Brand et al, 2000). WUS protein, a homeodomain transcription factor, can move
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