Jean-Claude Roland

The Wall of the Growing Plant Cell: Its Three-Dimensional Organization

Publisher Summary This chapter summarizes successive steps involved in the formation of a growing cell wall: (1) polymerization and release of wall subunits, (2) three-dimensional assembly of the subunits in the periplasm; positioning, and cross-linking of subunits probably involving self-assembly or transmembrane control, and (3) anisotropic surface expansion of the wall implying selective loosening and sliding of the subunits when the wall is under stress. The two cellular pathways necessary for growth are (1) the exocytic route, which contributes to plasmalemma enlargement and produces the subunits of the wall and (2) the endocytic route, which contributes to tonoplast enlargement and turgor pressure regulation. In this dichotomic flow, the endoplasmic reticulum and the Golgi body play a key role. Thus, the main elements of growth regulation and wall morphogenesis, though acting at different levels in the cell, should result in a homogeneous sequential process. In the future, closer cooperation among cytologists, biochemists, and physiologists seems highly desirable for a better understanding of the characteristic modalities of plant growth.

The relationship between the plasmalemma and plant cell wall.

Publisher Summary This chapter discusses the relationship between the plasmalemma and plant cell wall. In animals, the surface of the cell is composed of the plasma membrane, or plasmalemma, which in close association with a cell coat assures contact between the living cell and the extracellular medium. The plasmalemma and the cell wall are completely individual and distinct from each other and may be easily separated. However, in the living cell, there exists an extremely close structural and functional cooperation between the plasmalemma and the cell wall. Electron microscope examination shows that if the protoplasm and the cell wall differ significantly as regards several characteristics, numerous niorphologically individualized relationships are established between the two. Noncytological methods suggest physiological unity between the plasmalemma and cell wall. New techniques and tools are available for experimental investigation, and rapid progress can be expected.

Liquid crystal-type assembly of native cellulose-glucuronoxylans extracted from plant cell wall.

Summry— In numerous plant cell walls, the cellulose microfibrils are arranged in a helicoidal pattern which has been considered as an analog to a cholesteric order. Here, we report on the spontaneous helicoidal organization which occurs in acellular conditions from aqueous suspensions of cellulose. The cellulosic mucilage of mature seeds of quince (Cydonia oblonga L) was studied both in situ (pre‐release mucilage) and after water extraction and in in vitro re‐assembly (prolonged high speed ultracentrifugation, further progressive dehydration and embedding in LR White methacrylate or hydrosoluble melamine resin). The cellulosic component was characterized by the use of cellobiohydrolase (CBH1) bound to colloidal gold, and the glucuronic acid residues of the xylan matrix were characterized by the use of cationised gold. Inside the seeds, the pre‐release mucilage is mostly helicoidal, with the occurrence of more or less ordered domains, which indicate a fluid organization relevant to an actual liquid crystal state. Cytochemical tests revealed the tight association between cellulose and glucuronoxylans, the latter constituting a charged coat around each microfibril. Following the hydration of the seed, a cellulosic suspension was extracted in which microfibrils were totally dispersed. The progressive dehydration of the suspension gave rise to concentrated viscous drops. Ultrastructural observations revealed the occurrence of multidomain organization, from non‐ordered to cholesteric‐like regions, revealing that the mucilage is at the same time crystalline and liquid. This constitutes the first demonstration that liquid crystal type assemblies can arise from crystalline and biological cellulose in aqueous suspension. It strengthens the hypothesis that a transient liquid crystal state must occur during the cellulose ordering. The possible morphogenetic role of the glucuronoxylans in the cholesteric organization of the cellulose is discussed.

Dynamique du positionnement de la cellulose dans les parois des fibres textiles du lin (Linum usitatissimum)

Summary The cell wall of stem fibers of flax shows the classical cellulosic architecture with three main domains with constant slope (S1, S2- thickened and locked into an almost axial direction -, S3) within which the microfibril orientation remains parallel (unidirectionnal, the so-called “spiral texture”). The present paper focus on the fact that arced change of orientation occurs at the major wall layer interfaces. The nested arcs are cancelled or reversed by tilting of the goniometric stage at the electron microscopy level; they are transient cholesteric analog (multidirectional helicoids). Structural models are provided giving unified picture of the flax fiber wall according space and time. Data are consistant with a possible dual mechanism controlling the cellulosic framework morphogenesis: cooperation between a directed assembly (via the plasmalemma) and a liquid crystal-like self assembly. The secondary cell wall of fibers with intermittent cholesteric pattern is compared to the secondary cell wal...

Primary cell wall texture and its relation to surface expansion

Recent data have revealed the frequent occurrence of helicoidal and cholesteric-like structures in various primary and secondary walls. The present article deals with the texture specificity and the texture changes in the primary wall during the rapidly growing phase. First we define the characteristics of the cholesteric pattern and the mesophases and discuss two possible sources of misinterpretations: one technical, the fragility of constructions, and one biological, the gradients, differentiation, and short-lived organization. Both features explain why the liquid crystal concept has emerged recently for the growing wall. Two examples of cells showing highly oriented surface growth are considered and compared. (a) Mung bean seedlings are often used for the study of expansion. In this model the external and growth-limiting tissues have primary walls with early and transient planar twisted assemblies. The latter are progressively sheared and dispersed, and growth stops when the helicoidal pattern is completely randomized. (b) In the differentiated cell wall of collenchyma the bundles of supporting cells assemble walls in which similar planar twists are found. The helicoids are likewise randomized during the surface growth. The difference is that they are vigorously renewed and the last arced layers are no longer involved in the elongation process and persist without apparent modification of their cholesteric arrangement. Finally, we discuss the question of the intermolecular interrelations and degree of freedom of the wall components through the helicoids.

Liquid crystal order and turbulence in the planar twist of the growing plant cell walls

During growth, the plant cell wall behaves as a dynamic structure that exemplifies a case of unstable balance between a capability to organize a characteristic order and a permanent tendency to destroy the order. This interpretation allows previous results to be complemented and reappraised; it also integrates the cell wall behavior into a unified frame. The wall thickenings are structured according to a crystal-like pattern (cholesteric mesophase). Laminar helicoids are locally deposited on cell facets and form actual flat and planar twists in which the causes of topological defects are minimized. The helicoidal pattern is short-lived and its occurrence results from a complex interplay between self-assembly and disassembly properties. Two alternative processes destroy this critical state: 1. Externally, the surface extension which produces a progressive degradation and a randomization by shearing and stretching; 2. Internally, aleatory and punctual instabilities which alter the genesis of the system at the plasmalemma/wall interface. As the time goes on, the twisting either resumes or, in contrast, the perturbation increases. The system tends to become unpredictable thus suggesting turbulence.

Distribution and Possible Morphogenetic Role of the Xylans Within the Secondary Vessel Wall of Linden Wood

The topochemistry of xylans was studied at the ultrastructural level in vessels of the wood of linden (Tilia plaryphyllos Scop.), by using an affinodetection method (xylan asegold labelling) and the cationic gold labelling for acidic charges.

Growth and extensibility of collenchyma cells

Abstract The extension of isolated collenchyma — of which the ultrastructure was described — was analysed by a relaxating traction technique. Auxin (NAA) pretreatment caused an increase in both cell elongation and cell extensibility. The relationships between growth, elasticity and plasticity were briefly discussed.

Croissance pluridirectionnelle des parois hélicoïdales: le raccourcissement cellulaire des racines tractrices

Summary The ability of cell to shorten has been studied in contractile roots of hyacinth bulb (Hyacinthus orientalis L.). It is characterized by a sequential (biphasic) change of the growth direction of internal cells of the cortical parenchyma. After an usual elongation phase, the cells undergo a specific contractile phase with radial surface expansion while the outer cortex is highly wrinkled. The contraction is a phase of intense synthesis of wall precursors (golgi activity and production of numerous exocytes with polysaccharides). The walls of the motor cells which support the changes of surface growth axis are of the primary and helicoidal type. Their texture is liquid crystal—like (cholesteric), able to multidirectionnally expand according to a precisely coordonated way. This new example shows the wide potentialities of cell wall analog to cholesteric assembly.