Our previous work (E. Shedletzky, M. Shmuel, D.P. Delmer, D.T.A. Lamport [19901 Plant Physiol 94:980-987) showed that suspension-cultured tomato cells adapted to growth on the cellulose synthesis inhibitor 2,6-dichlorobenzonitrile (DCB) have a markedly altered cell wall composition, most notably a markedly reduced level of the cellulose-xyloglucan network. This study compares the adaptation to DCB of two cell lines from dicots (tomato [ capacity to divide and expand under conditions where cellulose synthesis is inhibited. As a result, the cell walls of tomato cells adapted to growth on 12 to 25 gM DCB contain markedly reduced levels of cellulose. In addition, they show other unique characteristics including a significant enrichment in pectic polymers and a markedly reduced level of xyloglucan. Lacking cellulose, adapted cells secrete the xyloglucan to the medium, a result that also suggests the interesting possibility that much of the xyloglucan is not further linked to other polymers within the wall.These studies, as well as other recent studies with cells adapted to growth in NaCl (16-18), point out the remarkable flexibility of plant cells to tolerate induced changes in cell wall composition, and further show that the study of such induced changes can shed light on normal wall structure. The work presented here extends our analyses of the walls of DCB-adapted cells and addresses the following issues: (a) a demonstration that adapted tomato cells growing on DCB possess a normal capacity for cellulose synthesis immediately upon removal of DCB, and that the pathway has retained its sensitivity to DCB; (b) a comparison of altered wall composition between two dicot cell lines (tomato [Lycopersicon esculentum] and tobacco [Nicotiana tabacum]) and a monocot grass (barley [Hordeum bolbosum] endosperm). These analyses were undertaken because the grasses are known to have markedly different primary walls from those of the dicots, possessing reduced levels of galacturonic acid-rich polymers, xyloglucan, and HRGPs, elevated GAX, and a mixed-linked(1--3,1--*4)-#-glucan that is lacking in dicot walls (2). Although the ratio of GAX to noncellulosic glucan, as well as the extent of substitution of GAX with glucuronic acid residues, differs between walls of endosperm and vegetative barley tissues, the endosperm line used here is considered to be, in general, representative of the grasses. Thus, it seemed possible that the compensatory changes induced to provide strength to walls with reduced cellulose might be quite different between these cell types, and that an understanding of such changes might shed light on how the polymers of these different walls interact to provide tensile strength; and (c) a comparison of the tensile strength and porosity of dicot and monocot walls from cells grown in the presence and absence of DCB. MATERIALS AND METHODS Cell CulturesCell suspensions of tomato (Lycopersicon esculentum VF 36) were cultured and adapted to DCB as described previously 120 www.plantphysiol.org on May 9, 2018 -Pu...

Suspension-cultured cells of tomato (Lycopersicon esculentum VF 36) have been adapted to growth on high concentrations of 2,6-dichlorobenzonitrile, an herbicide which inhibits cellulose biosynthesis. The mechanism of adaptation appears to rest largely on the ability of these cells to divide and expand in the virtual absence of a cellulose-xyloglucan network. Walls of adapted cells growing on 2,6-dichlorobenzonitrile also differ from nonadapted cells by having reduced levels of hydroxyproline in protein, both in bound and salt-elutable form, and in having a much higher proportion of homogalacturonan and rhamnogalacturonan-like polymers. Most of these latter polymers are apparently crosslinked in the wall via phenolic-ester and/or phenolic ether linkages, and these polymers appear to represent the major loadbearing network in these unusual cell walls.

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SUMMARY Evidence from high‐resolution images of primary cell walls suggests that the cell wall is constructed from at least two independent yet coextensive fibrous networks, one based on cellulose/hemicellulose and the other on pectin. The ability to analyse the structure of each of these networks in isolation has been hampered by a lack of suitable biological material such as mutants. However, the recent use of the cellulose‐synthesis inhibitor 2,6‐dichlorobenzonitrile (DCB) that prevents the formation of the cellulose‐xyloglucan network while allowing the pectin network to form a functional wall offers the unique opportunity of studying at least the pectin network independently. A range of electron microscopy techniques and a novel spectroscopy method are used to study the walls from tomato suspension cells adapted to growth on DCB. Measurements of the minimum cell wall thickness derived from thin sections of dehydrated walls show that the marked reduction in level of the cellulose/hemicellulose network affects neither the thickness of the wall formed, nor the apparent spacing of pectin molecules. However, images obtained by the fast‐freeze, deep‐etch, rotary‐shadowed (FDR) replica technique show that the three‐dimensional architecture of these pectin‐rich walls is very different from that of nonadapted walls. Fourier transform infrared (FTIR) microspectroscopy data and immunogold‐labelling studies provide additional evidence that supports the previous biochemical data.

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