Practice question

TOPLESS encodes Groucho/Tup1-type transcriptional corepressor that directly binds EAR motif of AUX/IAA repressors and other EAR-containing factors recruiting HISTONE DEACETYLASE 19 to silence auxin-responsive genes via chromatin compaction. tpl topless mutants exhibit dramatic embryonic patterning defects where shoot apical domain loses shoot identity and transforms into second root pole producing seedlings with double roots, ectopic expression of PLT and WOX5 throughout upper axis, loss of shoot markers STM and WUS. Shoot-to-root conversion reveals that TPL-mediated active repression of root programs in upper embryo is essential for stabilizing apical fate.