RBE is a C2H2 zinc finger transcription factor that acts as a transcriptional repressor. The corresponding loss-of-function mutant rbe displays defective petal development. RBE expression and protein accumulation in flowers is restricted to the petal precursor cells, which suggests that it acts cell-autonomously. It has a leucine-rich ethylene-responsive element binding factor-associated amphiphilic repression motif and represses AGAMOUS expression in the second whorl of flowers, which is required for proper petal development. In addition, RBE represses TEOSINTE BRANCHED1/CYCLOIDEA/PCF transcription factors and regulates the expression of microRNAs, which are involved in organ boundary specification. However, whether RBE also has functions beyond floral development has not been investigated before.
The researchers screened an Arabidopsis activation-tagging mutant population to find regulators of vascular cambium development and isolated a gain-of-function mutant with a loss of interfascicular cambium-derived tissue. This mutant also displayed stunted growth and an overall reduction in cambium and vascular cells. In this mutant line, RBE was overexpressed by more than 14-fold, whereas neighbouring genes were not affected. Ectopic overexpression of RBE mimicked the reduced number of interfascicular cambium-derived cells and the overall stunted growth to some extent, whereas rbe knockout mutants are taller and have more interfascicular cambium-derived cells as well as more xylem and phloem cells. In the gain-of-function mutants, WOX4 expression was reduced by 33-fold, suggesting that it is repressed by RBE. Indeed, RBE can directly bind to and repress the WOX4 promoter. Other factors involved in vascular stem cell division and differentiation, including the TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR–PHLOEM INTERCALATED WITH XYLEM signalling module, are also repressed by RBE. RBE is expressed in the cortex, phloem and distal cambial cells and suppresses WOX4 accumulation in both vascular and interfascicular cambium, which suggests that RBE might be involved in establishing and modulating the WOX4 expression domain boundary, possibly in response to hormone signalling, for which multiple cis elements are present in the RBE promoter.
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