Genes involved in phosphoinositide signalling are conserved across eukaryotes, yet their role in plant development remains unclear. Now (p. 1437), Christian Hardtke and colleagues reveal that balanced phosphatidylinositol-4,5-bisphosphate (PIP2) levels are required for differentiation of the Arabidopsis protophloem, a specialised vascular tissue found in the root. The researchers analyse plants harbouring mutations in COTYLEDON VASCULAR PATTERN 2 (CVP2) and its partially redundant homolog CVP2-LIKE 1 (CVL1), which encode phosphoinositide 5-phosphatases that convert PIP2 into phosphatidylinositolphosphate (PIP). They reveal that a second site mutation in cvp2 partially rescues previously identified mutants with impaired protophloem development, suggesting that PIP2 levels modulate protophloem differentiation. In line with this, they demonstrate that CVP2 hyperactivation impairs protophloem differentiation and overall root growth. The researchers further show that, while cvp2 and cvl1 single mutants display no apparent root defects, double mutants paradoxically also exhibit protophloem differentiation defects and a skewed PIP to PIP2 ratio. Finally, they report, this impaired protophloem differentiation systemically alters the auxin response in the root system and, hence, lateral root emergence. In summary, these findings highlight a crucial role for tightly regulated PIP2 levels in the Arabidopsis root and suggest that activity in the primary root protophloem shapes root architecture.