Auxin is crucial for plant morphogenesis, includingin vivoandin vitroembryo development. Exogenous auxin application is necessary for inducing embryogenic responses inin vitrocultured 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. TheMP11irtranscript, an alternatively spliced variant ofMP/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 ofMP11irtranscript are observed during auxin- and trichostatin A-dependent induction of somatic embryogenesis. Both MP/ARF5 and MP11ir are essential for embryo regeneration in thempS319mutant. However, overexpressing truncated MP/ARF5 protein (ΔARF5) lacking the PB1 domain inhibits somatic embryogenesis, leading to callus formation instead of somatic embryos. OverexpressingΔARF5, lacking MP/ARF5 protein (mp/arf5mutant), and blocking of MP/ARF5 action with auxin-resistant BODENLOS (BDL) protein affect the expression of genes involved in auxin biosynthesis, likeTRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1(TAA1),TAA1-RELATED 1(TAR1),YUCCA3(YUC3),YUC5andYUC8, which may be potential targets of MP11ir and/or MP/ARF5. Consequently,ΔARF5overexpression alters auxin homeostasis and endogenous auxin levels, hindering embryogenic transition.
