Retain in the membrane: Tinkering with the BRX-PAX-PIP5K auxin efflux machinery affects vascular tissue differentiation

The dawn of vasculature in plants more than 400 million years ago was a game changer.This newfound ability of plants to transport water and nutrients over long distances meant that they could grow larger and taller.As young Arabidopsis seedlings develop, two protophloem poles harboring protophloem sieve elements (PPSE) and protophloem companion cells are among the first to differentiate (Figure ,A) (Roszak et al. 2021).The timely accumulation of the phytohormone auxin in these precursor cells is essential for their differentiation into the phloem of the developing vasculature.Necessary auxin gradients that drive physiological functions are generated by plasma membrane (PM)-integral auxin efflux carrier PIN-FORMED (PIN) as well as the kinases at the PM that affect their polar localization, auxin-efflux capacity, and turnover.Of the PM phospholipids, phosphoinositide phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] produced by PHOSPHATIDYLINOSITOL-4-PHOSPHATE-5-KINASE (PIP5K) is important for the recruitment of PM proteins and PIN endocytosis (Barbosa et al., 2016).