Repressing Factors

To date, only a few genes have been isolated that limit cell expansion during organ growth. BIGPETAL (BPE) encodes a putative transcriptional regulator of the basic helix-loop-helix class, and the BPE locus produces two distinct mRNAs through alternative splicing (Szecsi et al. 2006). One of these is present throughout the plant, whereas the other is found specifically in developing petals, and its accumulation is downstream of the known floral homeotic gene activities that specify petal identity. Downregulation of the petal-specific transcript leads to increased petal cell expansion and consequently to larger petals. How the petal-specific form of BPE protein limits cell expansion is currently unknown.

In addition, certain mutant alleles (rotunda2-1 and rotunda2-2) of the presumed transcriptional co-repressor LEUNIG (LUG, see below) form larger leaves with a wider lamina because of increased cell expansion, indicating that LUG is required to prevent excess cell expansion late in leaf growth (Cnops et al. 2004).

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