Advances in determination of polymer structure and in preservation of structure for electron microscopy provide the best view to date of how polysaccharides and structural proteins are organized into plant cell walls. The walls that form and partition dividing cells are modified chemically and structurally from the walls expanding to provide a cell with its functional form. In grasses, the chemical structure of the wall differs from that of all other flowering plant species that have been examined. Nevertheless, both types of wall must conform to the same physical laws. Cell expansion occurs via strictly regulated reorientation of each of the wall's components that first permits the wall to stretch in specific directions and then lock into final shape. This review integrates information on the chemical structure of individual polymers with data obtained from new techniques used to probe the arrangement of the polymers within the walls of individual cells. We provide structural models of two distinct types of walls in flowering plants consistent with the physical properties of the wall and its components.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.1993.tb00007.x

Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth

Pectins: structure, biosynthesis, and oligogalacturonide-related signaling.

Plants are constantly being challenged by aspiring pathogens, but disease is rare. Why? Broadly, there are three reasons for pathogen failure. Either (1) the plant is unable to support the niche requirements of a potential pathogen and is thus a non? host; or (2) the plant possesses preformed structural barriers or toxic compounds that confine successful infections to specialized pathogen species; or (3) upon recognition of the attacking pathogen, defense mechanisms are elaborated and the invasion remains localized. All three types of interaction are said to be incompatible, but only the latter resistance mech? anism depends on induced responses. Successful pathogen invasion and disease (compatibility) ensue if the preformed plant defenses are inappropriate, the plant does not detect the pathogen, or the activated defense responses are ineffective. In this review, we examine the essential prerequisites for patho? gen recognition and the induction of localized defense responses. Preformed defenses are considered elsewhere in this issue (see Osbourn, 1996, in this issue). Race-specific pathogen recognition is hypothesized to re? sult from the direct or indirect interaction of the product of a dominant or semidominant plant resistance (R) gene with a product derived from the corresponding dominant pathogen avirulence (Avr) gene (Keen, 1992; Staskawicz et al., 1995). Subsequent signal transduction events are assumed to coordinate the activation of an array of defense responses. This "simple" model appears to explain much but begs many questions. For example, R gene products are likely to provide key components for recognition, but how do the distinct classes of R proteins characterized to date (see Bent, 1996, in this is? sue) activate the defense response? Do different R gene classes activate distinct responses? The regulation of some components of defense mechanisms has been studied in plant cell cultures in response to non-race-specific elicitors, but to what extent do such studies provide a model for R gene func? tion? Plant resistance is often correlated with the activation of specific defense responses, but which (if any) are required to abolish or retard pathogen growth, and how? Which are pri? mary responses and which are secondary? Does the first response involve transcriptional regulation, the activation of preformed enzymes, and/or the opening of ion channels, or

A. Bacic

Papers

8 – Structure and Function of Plant Cell Walls