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 embryogenesis. AUXIN RESPONSE FACTOR (ARF) transcription factors bind to auxin response elements to control auxin-responsive gene expression and 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, produces a truncated protein missing the Phox and Bem1p (PB1) domain, crucial for ARF-Aux/IAA dimerization. This renders the MP11ir isoform insensitive to Aux/IAA repression, suggesting an auxin-independent regulation. High levels of MP11ir transcript are observed during auxin- and trichostatin A-dependent induction of somatic embryogenesis. Both MP and MP11ir are essential for embryo regeneration in the mpS319 mutant. However, overexpression of a truncated MP protein ({Delta}ARF5) lacking the PB1 domain inhibits somatic embryogenesis, resulting in callus formation instead of somatic embryos. Overexpression of {Delta}ARF5, lack of MP protein (mp mutant), and interference with MP action by the auxin-resistant BODENLOS (BDL) protein affect the expression of the genes TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 (TAA1), TAA1-RELATED 1 (TAR1), YUCCA3 (YUC3), YUC5 and YUC8, which are involved in auxin biosynthesis. The partial complementation of mpS319 by the expression of MP11ir may be due to the restoration of the expression of TAA1, TAR1, YUC3, YUC5, and YUC8 to the levels observed in the wild-type genotype Col-0. The presence of MP protein alone or in combination with MP11ir in mpS319 restored the expression of all analyzed genes compared to Col-0. Our results suggest that the analyzed auxin-related genes could be potential targets of MP11ir and/or MP. Consequently, {Delta}ARF5 overexpression alters auxin homeostasis and endogenous auxin levels, hindering embryogenic transition.