Pea Border Cell Maturation and Release Involve Complex Cell Wall Structural Dynamics

Abstract

The adhesion of plant cells is vital for support and protection of the plant body and is maintained by a variety of molecular associations between cell wall components. In some specialized cases, though, plant cells are programmed to detach, and root cap-derived border cells are examples of this. Border cells (in some species known as border-like cells) provide an expendable barrier between roots and the environment. Their maturation and release is an important but poorly characterized cell separation event. To gain a deeper insight into the complex cellular dynamics underlying this process, we undertook a systematic, detailed analysis of pea (Pisum sativum) root tip cell walls. Our study included immunocarbohydrate microarray profiling, monosaccharide composition determination, Fourier-transformed infrared microspectroscopy, quantitative reverse transcription-PCR of cell wall biosynthetic genes, analysis of hydrolytic activities, transmission electron microscopy, and immunolocalization of cell wall components. Using this integrated glycobiology approach, we identified multiple novel modes of cell wall structural and compositional rearrangement during root cap growth and the release of border cells. Our findings provide a new level of detail about border cell maturation and enable us to develop a model of the separation process. We propose that loss of adhesion by the dissolution of homogalacturonan in the middle lamellae is augmented by an active biophysical process of cell curvature driven by the polarized distribution of xyloglucan and extensin epitopes.

A protocol for combining fluorescent proteins with histological stains for diverse cell wall components

“…Both dyes can be used in a fashion similar to Calcofluor White for generic staining of primary cell walls. Direct Red 23 has the additional advantage of emitting light preferentially in the presence of cellulose and has been used to image the architecture and dynamics of cellulose in expanding Arabidopsis root cells, as well as in describing border cell maturation in Pisum sativum (Anderson et al ., ; Wallace and Anderson, ; Liesche et al ., ; Mravec et al ., ). Direct Yellow 96 exhibits a characteristic shift in its fluorescence emission maximum in the presence of xyloglucan, but not cellulose, and could potentially serve as an orthogonal fluorescent label for xyloglucan because its excitation and emission maxima do not overlap with those of Direct Red 23 (Anderson et al ., ; Wallace and Anderson, ).…”

Border cell release: Cell separation without cell wall degradation?

“…The root cap has to be rigid to allow for the penetration through the substrate and to protect the meristem and on the other hand it continuously grows and its surface is a site for a recurring cell separation. 5,6 In our recent publication, 7 we showed that the pea root cap cell walls become structurally different and enriched in various cell wall epitopes once they reach the surface of the root cap. Both in situ analyses and tests on cellwall-component-degrading enzyme activities suggested that no extensive cell wall degradation is involved in this process.…”

“…Both in situ analyses and tests on cellwall-component-degrading enzyme activities suggested that no extensive cell wall degradation is involved in this process. 7 Brachypodium distachyon is already a well-established model organism for grasses. 8 It is generally accepted that grasses produce primary cell walls distinct from dicots.…”

“…The Fig. 1 (B,C) presents the outcome of such an experiment using the pectin methylesterase (PME)-inhibitor (-)-epigallocatechin gallate 13 (EGCG) which, in similar manner to pea, 7 inhibited the border cell release to the media. Sectioning of resin embedded and toluidine stained Brachypodium root apex shows that the Brachypodium root cap is a triangular structure which does not fully cover the root meristem ( Fig.…”

“…Compelling observations from different models 7,14 form the base for my hypothesis about possible mechanism of detachment. I propose that the elevated PME activity on the secreted HG act as a calcium chelating agent disassembling the already formed egg boxes (gluing complexes of HG with calcium) within the middle lamella.…”