Abstract
The phloem tissue mediates long-distance transport of energy metabolites along plant bodies and, therefore, is central for plant performance. However, mechanisms initiating the transition of undifferentiated stem cells to cells specialized in metabolite transport are unknown. Here we identify the ubiquitously expressed PHD-finger protein OBERON3 (OBE3) to be essential for phloem formation. We show that OBE3 directly interacts with the SUPPRESSOR OF MAX2 1-LIKE 5 (SMXL5) protein specifically expressed during early phloem development. Both proteins co-localize in nuclei of phloem stem cells and, together with the SMXL5 homologs SMXL3 and SMXL4, promote the establishment of phloem-specific cellular signatures in a cell-autonomous manner. These signatures include expression of OCTOPUS (OPS), BREVIS RADIX (BRX), BARELY ANY MERISTEM3 (BAM3), and COTYLEDON VASCULAR PATTERN2 (CVP2) genes acting as mediators of phloem differentiation. Consistently, genetic analyses show that SMXL5 acts upstream and independently of OPS and BRX functions. Based on our findings, we conclude that the formation of an OBE3/SMXL5 protein complex specifically in nuclei of early phloem cells is essential for establishing a phloem-specific developmental program.