Evolution of the <i>JULGI–SMXL4/5</i> module for phloem development in angiosperms

Bifacial cambium, which produces xylem and phloem, and monopodial architecture, characterized by apical dominance and lateral branching from axillary buds, are key developmental features of seed plants, consisting of angiosperms and gymnosperms. These allow seed plants to adapt to diverse environments by optimizing resource allocation and structural integrity. In seed plants, <i>SUPPRESSOR OF MAX2-LIKE</i> (<i>SMXL</i>) family members function in phloem development and strigolactone-induced inhibition of axillary bud outgrowth. Although strigolactone signaling regulates most <i>SMXL</i> family members, the only known regulator of <i>SMXL4</i> and <i>SMXL5</i> is the RNA-binding protein JULGI. We demonstrate that in angiosperms, by directly regulating <i>SMXL4/5</i> expression, JULGI uncouples SMXL4/5 activity from strigolactone signaling. <i>JULGI</i> and ancestral <i>SMXL</i>s from seedless vascular plants or <i>SMXL4/5</i> from seed plants are coexpressed in the phloem tissues of vascular plants, from lycophytes to angiosperms. Core angiosperm <i>SMXL4/5</i> mRNAs contain a G-rich element in the 5' untranslated region (UTR) that serves as a target sequence for JULGI to negatively regulate <i>SMXL4/5</i> expression. Heterologous expression of <i>JULGI</i>s from various angiosperms rescued the <i>Arabidopsis jul1 jul2</i> mutant. Expressing <i>SMXL4/5</i>s from seed plants and ancestral <i>SMXL</i>s rescued <i>Arabidopsis smxl4 smxl5</i>. Angiosperm SMXL4/5s lack an RGKT motif for proteasomal degradation. Indeed, treatment with the synthetic strigolactone analog <i>rac</i>-GR24 induced proteasomal degradation of SMXL from ferns and SMXL5a from gymnosperms, but not SMXL4/5 from angiosperms. These findings suggest that in ancestral angiosperms, the 5' UTR of <i>SMXL4/5</i> gained G-rich elements, creating a regulatory module with <i>JULGI</i> that allows the phloem development pathway to act independently of strigolactone signaling.