MONOPTEROS isoform MP11ir role during somatic embryogenesis in Arabidopsis thaliana

Auxin is crucial for plant morphogenesis, including in vivo and in vitro embryo development. Exogenous auxin application is necessary for inducing embryogenic responses in in vitro cultured explants of Arabidopsis and other plants. Thus, components of auxin transport, signaling, and metabolism are key to somatic embryo formation. AUXIN RESPONSE FACTOR (ARF) transcription factors, which bind to auxin response elements to control the auxin-responsive gene expression, are vital in somatic embryo regeneration. ARFs are often repressed by AUXIN/INDOLE-3-ACETIC ACIDs (Aux/IAAs). MONOPTEROS (MP)/ARF5 is especially important in the embryogenic transition, being highly expressed during somatic embryogenesis; its mutant cannot develop somatic embryos. The MP11ir transcript, an alternatively spliced variant of MP/ARF5 , produces a truncated protein missing the Phox and Bem1p (PB1) domain, crucial for ARF-Aux/IAA dimerization. This makes the MP11ir isoform insensitive to Aux/IAA repression, suggesting auxin-independent regulation. High levels of MP11ir transcript are observed during auxin- and trichostatin A-dependent induction of somatic embryogenesis. Both MP/ARF5 and MP11ir are essential for embryo regeneration in the mpS319 mutant. However, overexpressing truncated MP/ARF5 protein (ΔARF5) lacking the PB1 domain inhibits somatic embryogenesis, leading to callus formation instead of somatic embryos. Overexpressing ΔARF5 , lacking MP/ARF5 protein ( mp/arf5 mutant), and blocking of MP/ARF5 action with auxin-resistant BODENLOS (BDL) protein affect the expression of genes involved in auxin biosynthesis, like TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 ( TAA1 ), TAA1-RELATED 1 ( TAR1 ), YUCCA3 ( YUC3 ), YUC5 and YUC8 , which may be potential targets of MP11ir and/or MP/ARF5. Consequently, ΔARF5 overexpression alters auxin homeostasis and endogenous auxin levels, hindering embryogenic transition.