Alternative splicing generates a MONOPTEROS isoform required for ovule development (Curr. Biol.)

Auxin shapes the plant body and drives the formation of lateral organs such as roots and ovules. In the current model, AUXIN RESPONSE FACTORs (ARFs) are transcription factors that regulate plant growth by activating signaling in an auxin dose-dependent manner, regulated by Aux/IAA proteins. Here, Cucinotta and coworkers demonstrate a new twist on this paradigm, by showing that MONOPTEROS (MP) is an ARF with regulatory roles independent of auxin concentrations. The authors found that an alternative splice variant of MP, characterized by retention of intron 11 (MP11ir), is specifically expressed in inflorescences and accounts for 6% of total MP transcripts. MP11ir encodes a truncated isoform that retains the DNA-binding domain but lacks the Aux/IAA interaction domain. To functionally characterize the two isoforms, vectors expressing the full-length sequence (pMP:MP) or the splice variant (pMP:MP11ir) were introduced into plants carrying the weak allele mpS319, which displays defects including abnormal pistils and ovule abortion. The full-length construct more efficiently rescued the patterning defects of mpS319 pistils, whereas the splice variant better complemented impaired development of mpS319 ovules. These findings revealed a tissue-specific function of the non-canonical MP11ir, and showed that it can function in spite of low auxin levels (Summary and image adaptation by Michela Osnato @michela_osnato) Curr. Biol. 10.1016/j.cub.2020.11.026