Metaphloem development in the Arabidopsis root tip

Abstract

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, so-called protophloem. This is easily observed in Arabidopsis root meristems, where 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 to protophloem, metaphloem ontogenesis is poorly characterized, primarily because its visualization is challenging. Here we describe an improved protocol to investigate metaphloem development in Arabidopsis root tips in combination with a set of new 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.Summary statementMetaphloem sieve element differentiation in Arabidopsis roots follows a robust developmental trajectory.

An Arabidopsis root phloem pole cell atlas reveals PINEAPPLE genes as transitioners to autotrophy

“…While we have end points for our PSE, CC and PPP trajectories, we don’t expect to have an endpoint for MSE, since this cell type differentiates further away from the meristem 13 . Out of the 7 genes identified to be expressed in both sieve elements 13 , we detected all the genes in PSE clusters but only DESIGUAL2 ( DEAL2, At4g21310) in cluster 1 and CC, confirming we have not sampled mature MSE cells. However, we are convinced we have identified the first stages of MSE development in clusters 10 and 1.…”

“…For this reason, we interpret that cluster 1 is a more developed MSE, since we find early SE genes like PEAR1 and S32 (At2g18380) expressed in this cluster, which is mainly contributed by the APL sorting. While we have end points for our PSE, CC and PPP trajectories, we don't expect to have an endpoint for MSE, since this cell type differentiates further away from the meristem 13 . Out of the 7 genes identified to be expressed in both sieve elements 13 , we detected all the genes in PSE clusters but only DESIGUAL2 (DEAL2, At4g21310) in cluster 1 and CC, confirming we have not sampled mature MSE cells.…”

“…An exception are the partially redundant homologs OPS and OPL2 , identified as important for MSE entry into differentiation 14 . Despite some commonalities between PSE and MSE, a recent study highlighted MSE differentiation is independent of adjacent or preceding PSE 13 , underlining the peculiarities of this cell type. The conducting cell types and CC originate from different progenitors in the Arabidopsis root 12 .…”

“…In the Arabidopsis root, both conductive elements (MSE and PSE) derive from the same stem cell 12 but MSE differentiates later, when PSE cells are no longer functional. Despite having a similar function to PSE, MSE ontogeny is less well characterized 13 and few factors have been directly related to MSE development. An exception are the partially redundant homologs OPS and OPL2 , identified as important for MSE entry into differentiation 14 .…”

Identification of new proteins in mature sieve elements

“…In their recent study, Graeff and Hardtke (2021) generated new meta‐ and protophloem sieve element makers based on promoters from genes in the Brady et al (2007) S32 set. They report three new sieve element genes, SIEVE ELEMENT MARKER 1–3 (SEMA1‐3) .…”