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The evolution of a vascular distribution network enabled plants to conquer land and was an important driver of cellular specialization and body plan expansion. Among the plant vascular tissues, the phloem is particularly intriguing because of its complex functionality. The conducting phloem channels are called sieve tubes and transport nutrients as well as developmental signals. They are formed from interconnected individual sieve element cells, which represent a peculiar between-life-and-death state because they lack numerous organelles including the nucleus. Together with their neighboring companion cells, they form a functional unit that sustains phloem sap loading, transport and unloading. The unique developmental trajectory of sieve elements is laid out along a spatiotemporal gradient in the root meristem of Arabidopsis thaliana seedlings, where it is amenable to non-invasive investigation. Genetic, transcriptomic and cell biological analyses have revealed key steps in sieve element development at single-cell resolution. My lab has characterized a cell biological network that guides sieve element differentiation through the interplay between controlled auxin transport and receptor kinase signaling pathways. Key players in this network constitute a molecular rheostat that finetunes trans-cellular auxin flux and maintains its pronounced own polarity through a self-reinforcing mechanism. Its assembly is antagonized by the autocrine action of receptor kinase pathways. Their peptide ligands also act in a paracrine manner to safe-guard the phloem lineage by maintaining the developmental plasticity of neighboring cell files. Moreover, quantitative fine-tuning of antagonistic receptor kinase pathways is required to initiate the phloem lineage before transition toward differentiation. I will attempt to synthesize how this highly dosage-sensitive network guides the development of sieve elements from the inception of their precursors towards differentiation.