Abstract: The Arabidopsis WUSCHEL ( WUS ) protein , which plays an important role in the specification of the stem cells in the shoot apical meristem ( SAM ) , contains an atypical homeodomain ( HD ) with extra residues in its loop and turn regions .
We speculated that a WUS-type atypical HD protein might also be involved in the specification and maintenance of the root apical meristem ( RAM ) stem cells of rice .
To investigate this possibility , we isolated and characterized a rice WUS-type homeobox gene designated quiescent-center-specific homeobox ( QHB ) gene .
Using transformants carrying the QHB promoter-GUS and in situ hybridization , we found that QHB was specifically expressed in the central cells of a quiescent center ( QC ) of the root .
During embryogenesis and crown root formation , QHB expression was observed prior to the morphological differentiation of the root .
However , we detected different QHB expression patterns in the process of the RAM development , specifically between radicle and crown root formation , suggesting that the cell-fate determination of the QC may be controlled by different mechanisms .
We also produced transformants that overexpress QHB or Arabidopsis WUS .
These transformants did not form crown roots , but developed multiple shoots from ectopic SAMs with malformed leaves .
On the basis of these observations , we propose that the WUS-type homeobox gene is involved in the specification and maintenance of the stem cells ( QC cells ) in the RAM , by a mechanism similar to that for WUS in the SAM . |