Rapid cell-to-cell expulsion completes phloem sieve element maturation

The plant vasculature transports sap through conduits formed by interconnected cells that undergo unique developmental programs. Whereas xylem vessel maturation culminates in programmed cell death, phloem sieve elements (PSEs) undergo selective organelle degradation, including enucleation, to accommodate symplastic mass flow. Despite insights into molecular mechanisms driving PSE development, the cytological details of PSE differentiation remain elusive. Here, we tracked PSE development at extraordinary spatiotemporal resolution using live imaging and focused ion beam scanning electron microscopy in Arabidopsis root tips. We found that enhanced calcium signaling and autophagy marker dynamics correlate with selective cytoplasmic clearing and shape unique cellular features, such as plasma membrane remodeling and a central endoplasmic reticulum sleeve. Real-time monitoring revealed rapid expulsion of PSE-specific markers into surrounding cells following enucleation, with filamentous actin (F-actin) dynamics emerging as a hallmark of PSE maturation. In summary, our experiments characterize a rapid developmental switch that radically remodels differentiating PSE precursors into functional PSEs.