Hans G. Edelmann

Ultrastructure and chemistry of the cell wall of the moss Rhacocarpus purpurascens (Rhacocarpaceae) : a puzzling architecture among plants

Abstract. The architectural, compositional and functional characteristics of the cell walls of the leaves of the moss Rhacocarpus purpurascens (Brid.) Par. have been analysed by scanning and transmission electron microscopy, wall-extraction methods, nuclear magnetic resonance (NMR) spectroscopy, and water-retention experiments. Four-layered cell walls with a peculiar architecture which, so far, appears to be unique among plants were apparent. The architecture of the walls was not affected by sequential wall-extraction procedures. Subsequent analysis of the residual pre-extracted walls by classical spectro-photometrical methods revealed that the walls are composed of mainly lignin, hemicellulose and cellulose in a ratio of about 9:8:5, determining their integrity. This was supported by NMR spectroscopy. The resonance spectrum showed various characteristics typical of lignin; however, some specific peaks associated with lignin were missing. The walls exhibited no particular properties for external water conduction but seem to be adapted to rapid absorption of fog, dew, or rain.

Unequal distribution of osmiophilic particles in the epidermal periplasmic space of upper and lower flanks of gravi-responding rye coleoptiles

In various studies, auxin (IAA)-induced coleoptile growth has been reported to be closely correlated with an increased occurrence of osmiophilic particles (OPs) at the inner surface of the outer, growth-limiting epidermal cell wall, indicating a possible function related to the mechanism of IAA-induced wall loosening. In order to test whether changes in cell elongation rates of upper and lower flanks (UFs, LFs, respectively) during gravi-responsive growth are reflected in appropriate changes in the occurrence of OPs, rye (Secale cereale L.) coleoptiles, either as segments or as part of intact seedlings, were gravitropically stimulated by positioning them horizontally for 2 h. Ultrastructural analyses within the UFs and LFs of the upward-bending coleoptiles revealed a distinct imbalance in the occurrence of OPs. The number of OPs per transverse epidermal cell section of the elongation-inhibited UF on average amounted to twice the number of OPs counted in epidermal cell sections of the faster-growing LF. As a hypothesis, the results lead us to suggest that OPs are involved in the mechanism of wall loosening and that temporary growth inhibition of epidermal cells of the UF during upward bending is mediated by inhibition of OP entry into the cell walls. Thereby, more OPs accumulate near the inner surface of the outer wall of epidermal cells of the UF compared with the LF.

Rapid Auxin-Induced Enhancement of Protein Biosynthesis in Rye Coleoptiles

Summary The effect of auxin (indole-3-acetic acid, IAA) on growth and incorporation of 3 H-leucine into cytoplasmic and cell-wall bound proteins of abraded coleoptile sections from rye seedlings ( Secale cereale L.) was investigated. In the absence of IAA (water control) a strong incorporation of tracer into both protein fractions was observed. Auxin rapidly increased the incorporation of 3 H-leucine into both cytoplasmic and cell-wall fractions of the coleoptiles in close temporal correspondence to the growth response of the sections. The uptake of tracer was unaffected by the hormone. However, auxin did not alter the pattern of protein synthesis as measured by 3 H-leucine labelling, i.e. we were unable to detect a specific polypeptide that is synthesized de novo in response to IAA.

Gravistimulated asymmetries in the outer epidermal cell walls of graviresponding coleoptiles

Abstract. Gravitropic plant growth is due to gravistimulated asymmetric extension rates of the affected flanks of the graviresponding organ. Differential growth of the upper and lower flanks (UFs, LFs) of graviresponding plant organs may in principle be achieved by various biochemical and/or biophysical asymmetries. The gravistimulated mechanism(s) by which the different growth rates are determined is still unresolved, as is the mechanism of IAA-regulated growth. The purpose of this brief review is to summarize and critically evaluate data concerning gravistimulated asymmetries, especially with respect to the interface of the plasma membranes and of the extension-restricting, load-bearing epidermal cell walls (Masuda and Yamamoto 1972 Physiol Plant 27: 109—115). In addition, recent results obtained by the author in experiments with rye coleoptiles will be presented and discussed in the context of a tentative model of gravistimulated wall asymmetries temporarily causing differential growth.

Tissue pressure and cell-wall metabolism in auxin-mediated growth of sunflower hypocotyls

Summary The effect of auxin (indole-3-acetic acid, IAA) on growth, tissue pressure and cell turgor of hypocotyl segments from etiolated sunflower seedlings ( Helianthus annuus L.) was investigated. The pressure exerted by the inner tissues (cortex, vascular tissues and pith) on the growth-limiting peripheral cell walls (longitudinal tissue pressure) was almost identical with the average turgor pressure of the cortical cells, as measured with the pressure probe. The change in length during 6h of growth in water or IAA was inversely correlated with a corresponding decrease in tissue pressure and cell turgor. These results indicate that the turgor (tissue) pressure of the internal cells maintains the peripheral cell walls under tension. This tensile stress in the walls of the outer cell layers (tissue tension) decreases during auxin-mediated longterm growth of the organ segment. In a second set of experiments we determined the effect of IAA on biosynthesis and breakdown of cell wall material in the outer and inner tissues of abraded segments. During the first h of IAA-mediated growth no promotive effect on incorporation of 3 H-glucose into the cellulose and matrix fractions of the cell wall poysaccharides was observed. Likewise, IAA had no effect on incorporation of 3 H-leucine into the protein fraction of the cell walls. To determine whether IAA induces a breakdown of cell wall polysaccharides abraded segments were given a pulse of 3 H-glucose. Thereafter the labelled segments were incubated in water (without tracer or sugar) in the presence or absence of IAA. The amount of 3 H-glucose retained in the cell walls during the chase period was unaffected by IAA, i.e. auxin does not induce a breakdown of cell-wall material. Finally, we determined the pattern of polypeptides synthesized during the first 2h of IAA-mediated growth. The sections were labelled with 3 H-leucine, and protein patterns were analyzed by one-dimensional polyacrylamid gel electrophoresis. Auxin treatment did not change the pattern of protein synthesis. However, we detected one major protein that was more abundant in the peripheral cell layers than in the inner tissues of the hypocotyl.

Characterization of Hydration-Dependent Wall-Extensible Properties of Rye Coleoptiles: Evidence for Auxin-induced Changes of Hydrogen Bonding

Summary The magnitude of changes of wall-extensible properties due to changes of the composition or concentration of wall solutions may reflect changes in polymer interactions relevant for growth regulation. Employing this approach, the present study demonstrates that in rye coleoptiles ( Secale cereale L.) turgor-mediated wall stress can, in principle, be borne by non-covalent polymer interactions. Transfer of turgid coleoptile segments to methanol fixed the immediate tension-stressed wall length dimensions despite turgor elimination. Length dimensions of methanol-incubated, previously turgid walls of coleoptile segments could reversibly be fixed or relaxed to defined extension states depending on wall water content. Previously auxin-incubated segments exhibited an increased stress relaxation by this method, i.e. they exhibited stronger shrinkage in dependence of defined hydration conditions as compared with control. Methanol-mediated fixation of wall length dimensions, inclusive therein conserved wall stresses, is due to removal of the water from the walls and thereby increased hydrogen bonding between wall polymers. Auxin-incubated segments therefore are characterized by a decrease of hydrogen bonding. Denaturation of wall protein by boiling segments in water, depectination or extraction of non-covalently bound (glyco)protein did not impair methanol-induced fixability of wall lengths tension-stressed in the extensiometer. Regulation of auxin-induced growth, therefore, in principle may also depend on polymer interactions as the ones affected by changes of hydration conditions due to methanol treatment, i.e. changes in hydrogen bonding.

Coleoptiles are gravi-guiding systems vital for gravi-insensitive shoots of germinating grass seedlings

Surgical removal of the coleoptiles from germinating seedlings of Secale cereale L., Avena sativa L., and Zea mays L. leads to a total loss of gravitropic response of the shoot without affecting growth. The principal function of coleoptiles is therefore to guide the gravi-insensitive shoot to the soil surface.