Bordered Pit Formation in Cell Walls of Spruce Tracheids.

Dmitry G Chukhchin,Evgeniy Novozhilov,Ksenia Vashukova

doi:10.3390/plants10091968

Dmitry G Chukhchin, Evgeniy Novozhilov + Show 1 more

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https://doi.org/10.3390/plants10091968

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Abstract

The process of pit formation in plants still has various questions unaddressed and unknown, which opens up many interesting and new research opportunities. The aim of this work was elucidation of the mechanism for the formation of bordered pits of the spruce (Picea abies (L.) Karst.) tracheid with exosomes participation and mechanical deformation of the cell wall. Sample sections were prepared from spruce stem samples after cryomechanical destruction with liquid nitrogen. The study methods included scanning electron microscopy and enzymatic treatment. Enzymatic treatment of the elements of the bordered pit made it possible to clarify the localization of cellulose and pectin. SEM images of intermediate stages of bordered pit formation in the radial and tangential directions were obtained. An asynchronous mechanism of formation of bordered-pit pairs in tracheids is proposed. The formation of the pit pair begins from the side of the initiator cell and is associated with enzymatic hydrolysis of the secondary cell wall and subsequent mechanical deformation of the primary cell walls. Enzymatic hydrolysis of the S1 layer of the secondary cell wall is carried out by exosome-delivered endoglucanases.

Highlights

Gymnosperm wood is superficially simple, composed mostly of single-celled tracheid
The formation of the pit pair begins from the side of the initiator cell and is associated with enzymatic hydrolysis of the secondary cell wall and subsequent mechanical deformation of the primary cell walls
The exosomes are released onto the inner surface of the cell wall where multivesicular bodies come into contact with the cell membrane

Summary

Introduction

Gymnosperm wood is superficially simple, composed mostly of single-celled tracheid. Tracheids perform conductive and mechanical functions. During the thickening of the cell wall, a secondary wall is formed, consisting of three layers: outer S1, middle S2, and inner S3. These layers differ in the orientation of microfibrils, which is estimated by the angle of inclination in relation to the longitudinal axis of the cell; the microfibrils lie almost parallel to each other inside each layer. In the S1 layer, cellulose microfibrils are located almost perpendicular to the longitudinal axis of the cell, which limits its radial extension. The thickness of this layer is comparable to the thickness of the primary cell wall—approximately 0.10–0.35 μm (5–10% of the total cell wall thickness). The prevalence of its structure and stability during evolution are most likely indicative of its main task—to withstand enormous mechanical stresses while maintaining flexibility [3]

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