Is Chara corallina a suitable model plant for studying cell-failure mechanisms in fruit skins?

Abstract

The freshwater alga, Chara corallina, is often used as a model system in studies of cell-wall mechanics. Chara has a unique and very simple architecture, in which a number of node cells form the interface between two giant internode cells. Given this architecture, when investigating a specimen comprising a single internode cell, a tensile test stresses just its cell walls. However, when investigating a specimen comprising two internode cells and their intervening node, the stress is imposed on both the cellulosic cell wall components but also on the (in-series) pectin middle lamellae between the cells comprising the node. This setup should allow comparative study of the properties of the cellulose of the cell walls in the absence of middle lamellae and also of the pectin layers of the middle lamellae in the node. Such information would be of particular interest in the case of fruit skins that in tension typically fail along the middle lamellae causing separation of adjacent cells. The objectives were to establish whether the mechanical properties of the pectin middle lamellae between the Chara node cells may be inferred by comparing tensile properties of an internode cell and of a node. Chara cell walls are composed of cellulose and de-esterified pectins as indexed by staining with calcofluorwhite and LM19. Staining with 2F4 indicates the presence of homogalacturonans crosslinked by Ca. Compared with internodes, nodes had lower stiffness, lower maximum force at failure and lower maximum strain. For the internode+node+internode samples, failure in the internode was more frequent than failure in the node. Lowering the medium pH had no effect on stiffness, maximum force at fracture or maximum strain of either internodes or nodes but it did increase the frequency of node vs. internode failure. Incubation in EGTA extracted Ca and decreased stiffness, maximum force at fracture and maximum strain of both internode and node samples. There was little effect of EGTA on the frequency of node vs. internode failure. Incubating internodes and nodes in pectinase decreased stiffness, maximum force at failure and maximum strain. The pectin components of Chara cells are almost identical to those of fruit skin cells. The tensile properties of both internodes and nodes are affected by pectins. This makes it difficult to infer those of the interfacing middle lamellae.