Phloem differentiation: an integrative model for cell specification

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.

Keywords: Auxin; Phloem sieve elements; Plethora; Single-cell transcriptomics.

Fig. 1

Overview of protophloem SE differentiation within the root meristem of Arabidopsis thaliana. 4 zones in the root tip can be distinguished: Meristem, Transition, Elongation and Differentiation zone (Ivanov and Dubrovsky 2013). The gradient of protein abundance of the PLTs is shown on the left side (Mähönen et al. 2014). The protophloem SE cell file is fully differentiated before the end of the transition zone as shown in the enlargement. The timing of differentiation progress has not been investigated yet, thus indicated zones within the SE cell stand are approximations. The first cell of the expression domains of known SE regulators is indicated with arrow heads. The yellow and red cell mark the first and second periclinal cell division, respectively. The transit amplifying cells divide several times anticlinally. The differentiating cells elongate and undergo cell wall thickening, sieve plate morphogenesis and autolysis. The green cell is enucleating