Metaphloem development in the Arabidopsis root tip.

The phloem transport network is a major evolutionary innovation that enabled plants to dominate terrestrial ecosystems. In the growth apices, the meristems, apical stem cells continuously produce early 'protophloem'. This is easily observed in Arabidopsis root meristems, in which the differentiation of individual protophloem sieve element precursors into interconnected conducting sieve tubes is laid out in a spatio-temporal gradient. The mature protophloem eventually collapses as the neighboring metaphloem takes over its function further distal from the stem cell niche. Compared with protophloem, metaphloem ontogenesis is poorly characterized, primarily because its visualization is challenging. Here, we describe the improved TetSee protocol to investigate metaphloem development in Arabidopsis root tips in combination with a set of molecular markers. We found that mature metaphloem sieve elements are only observed in the late post-meristematic root, although their specification is initiated as soon as protophloem sieve elements enucleate. Moreover, unlike protophloem sieve elements, metaphloem sieve elements only differentiate once they have fully elongated. Finally, our results suggest that metaphloem differentiation is not directly controlled by protophloem-derived cues but rather follows a distinct, robust developmental trajectory.