Phloem differentiation: an integrative model for cell specification

Bernhard Blob,Jung-Ok Heo,Yka Helariutta

doi:10.1007/s10265-017-0999-0

Bernhard Blob, Jung-Ok Heo + Show 1 more

Open Access

https://doi.org/10.1007/s10265-017-0999-0

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Abstract

Plant vasculature consists of two major conductive cell types, xylem tracheary elements and phloem sieve elements (SEs). Both cell types undergo a highly specialized differentiation process. The root meristem of Arabidopsis displays a stereotypical anatomy in which the central vasculature is surrounded by concentric layers of outer tissues. Each cell file is derived from stem cells located in the root tip. A series of formative and proliferative divisions take place in the meristem; these are followed by cell expansion and differentiation. Protophloem differentiation is unique in being complete only 20–25 cells away from the first stem cell, and during the differentiation process the cells lose several organelles, including the nucleus, while the remaining organelles are rearranged. Defects in SE development have been shown to result in impaired auxin transport and response and therefore systemically affect root growth. Although a few genes have been demonstrated to function in phloem development, detailed analyses and a comprehensive understanding of sieve element development (i.e. how often the stem cells divide, how frequently enucleation takes place, and how SE development is coordinated between cell division and differentiation on a molecular level) are still lacking. Advanced live-imaging techniques which enable prolonged time-lapse captures of root tip growth as well as single-cell transcriptomic analysis of the 20–25 cells in the SE file could help resolve these questions. In addition, understanding the interplay between the PLETHORA (PLT) gradient, which is known to govern the root zonation, and phloem development within the root meristem could shed light on the rapidity of SE differentiation and its importance to the meristem.

Highlights

In the vascular tissue of plants, the conductive cells in phloem and xylem are responsible for long distance transport
All of the regulatory mechanisms described so far are specific to the sieve elements (SE) or phloem; the role of general regulators or stem cell activity and cell division has not been taken into account, and very little is known about their specific role in SE differentiation
These include the further development and use of light sheet fluorescence microscopy for in vivo observations and the use of serial block phase scanning electron microscopy for 3D reconstructions at an extremely detailed level (Furuta et al 2014; Ovečka et al 2015). These new techniques will help answer questions that could not be addressed previously (e.g. How high is the rate of division in cells of the stem cell niche? How often does enucleation take place? How long does it take for a cell to progress through the full SE differentiation program from the stem cell touching the quiescent centre (QC) to enucleated cells?)

Summary

Introduction

In the vascular tissue of plants, the conductive cells in phloem and xylem are responsible for long distance transport. This interaction and the shootward plasma membrane localization appear not to be essential for the function of OPS in protophloem differentiation, since both are lost in plants with a hyperactive OPS protein resulting from a positive charge at a phosphosite without causing defects in phloem development (Breda et al 2017).

Results

Conclusion