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

Proline is a very unusual amino acid, in that the side-chain is cyclized back on to the backbone amide position. This has three important consequences. First, the backbone conformation of proline itself is very restricted. The available backbone sb dihedral angles are limited to a small range around = -65 [1,2] (Figure 1). Similar restrictions do not apply to i/, which is able to populate either the a-helical region (VY -40 0) or the f-sheet region (Q approx. + 150 0). Surveys of prolines in crystal structures show that roughly 44% of prolines are in the a region and 560% are in the ,3 region [4,5]. Second, the bulkiness of the N-CH2 group places restrictions on the conformation of the residue preceding proline [6], disfavouring the a-helix conformation [4,5]. Third, because the amide proton is replaced by

The structure and function of proline-rich regions in proteins.

The ability to produce morphologically and developmentally normal embryos and, indeed, whole plants from undifferentiated somatic cells in culture, through the process of somatic embryogenesis, resides uniquely within the plant kingdom. Since the initial description of somatic embryo production from carrot callus cells more than 35 years ago (Steward et al., 1958), this unique developmental potential has been recognized both as an important pathway for the regeneration of plants from cell culture systems and as a potential model for studying early regulatory and morphogenetic events in plant embryogenesis. The last 5 to 10 years have witnessed an explosion in the number of species that can now be regenerated from cell culture into whole plants through somatic embryogenesis. The literature contains hundreds of references describing the specific manipulations required to effect somatic embryo development from a variety of agronomically and horticulturally important plants. Although this is obviously an extremely important application of the process of somatic embryogenesis, it is not the focus of this review. Rather, this review will focus on the use of somatic embryogenesis as a model system for understanding the regulation of gene expression required for the earliest developmental events in the life of a higher plant: the development of the fertilized zygote into a mature embryo. The events of fertilization and subsequent embryo development normally occur deep within maternal tissues. The early embryo is minute and is surrounded by both endosperm and maternal cells. Although the morphological description of embryo development has been extensively recorded through microscopy, molecular and biochemical analyses of early embryogenesis have been hampered significantly by this physical inaccessibility. As a consequence, we know very little about the genes that are necessary for early embryogenesis in higher plants and even less about their regulation. This is beginning to be remedied by recent intensive efforts to genetically identify genes required for early embryogenesis in model systems such as Arabidopsis and maize (see West and Harada, 1993, this issue); many interesting mutants have been identified that may provide entry points into molecular analyses of major morphogenetic events in embryogenesis. These analyses would be greatly enhanced by the availability of cell, tissue, and developmental stage-specific markers of important events in the differentiation of cells and the establishment of the major tissue systems of the plant, which occur early in embryogenesis. In addition, once genes have been identified that are essential for embryogenesis, the subsequent analysis of their regulation would be greatly facilitated by the availability of an appropriate in vitro model system that is not limited in tissue quantity or accessibility. The somatic embryo system represents just such a model system. This review will summarize the process of somatic embryogenesis and will address the strengths and limitations of somatic embryos as potential models for studying early events in plant embryo development.

https://www.jstor.org/stable/info/3869792

Somatic Embryogenesis: A Model for Early Development in Higher Plants.

Infection of plants with pathogens like viruses or fungi often leads to a response of the host ranging from very mild to very severe. A very mild response usually does not result in considerable changes in the patterns of gene expression of the plant. However, a hypersensitive reaction, e.g., in the case of infection of Samsun NN tobacco with tobacco mosaic virus, is accompanied by a massive increase in the amounts of a large number of different proteins. Among these induced proteins are members of a group of so‐called pathogenesis‐related (PR) proteins. Frequently, the hypersensitively reacting host becomes resistant to subsequent pathogenic attack. The correlation of this phenomenon of acquired resistance and the induced protein synthesis suggests a direct involvement of these proteins in the observed replication deficiency of the pathogen. This has initiated studies by several research groups toward a better understanding of the role of these PR‐proteins in the hypersensitively reacting plant....

Pathogenesis-related proteins of plants

▪ Abstract The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Plant cell wall proteins

Extensins are hydroxyproline-rich glycoproteins found in many plant cell walls as a major protein component The peptide Ser-Hyp-Hyp-Hyp-Hyp is abundant in the extensins Using extensin cDNA clones as probes, we isolated six different clones from carrot genomic libraries One of the genomic clones, pDC5A1, was characterized and found to contain an open reading frame encoding extensin and a single intron in the 3'-non-coding region The derived amino acid sequence contains a signal peptide sequence and 25 Ser-Pro-Pro-Pro-Pro repetitive sequences Two extensin transcripts were found corresponding to pDC5A1 with different 5' start sites These transcripts increase in abundance after wounding This is consistent with the reported extensin accumulation in the cell wall upon wounding

An extracellular matrix protein in plants: characterization of a genomic clone for carrot extensin.

Extensins are hydroxyproline-rich glycoproteins associated with most dicotyledonous plant cell walls. To isolate cDNA clones encoding extensin, we started by isolating poly(A)+ RNA from carrot root tissue, and then translating the RNA in vitro, in the presence of tritiated leucine or proline. A 33-kDa peptide was identified in the translation products as a putative extensin precursor because: (i) it is rich in proline and poor in leucine, and (ii) the message appears to be more abundant when carrot tissue is wounded. From a cDNA library constructed with poly(A)+ RNA from wounded carrots, one cDNA clone (pDC5) was identified that specifically hybridized to poly(A)+ RNA encoding this 33-kDa peptide. We isolated three cDNA clones (pDC11, pDC12, and pDC16) from another cDNA library using pDC5 as a probe. DNA sequence data, RNA hybridization analysis, and hybrid released in vitro translation indicate that the cDNA clone pDC11 encodes extensin and that cDNA clones pDC12 and pDC16 encode the 33-kDa peptide, which as yet has an unknown identity and function. The assumption that the 33-kDa peptide was an extensin precursor was invalid. RNA hybridization and DNA sequence analysis indicate that pDC5 is a hybrid clone corresponding to two RNA species. RNA hybridization analysis showed that RNA encoded by both clone types is accumulated upon wounding.

Isolation and characterization of cDNA clones for carrot extensin and a proline-rich 33-kDa protein

Mechanical dissociation of nerve and glial cells from 16-day embryos of the cockroach Periplaneta americana and their subsequent culture in a Co2-conditioned, chemically defined medium results in the survival of neurons but not of glial cells The dissociated nerve cells remain alive and in excellent condition for many months and build a dense fibrillar network in presence of foregut explants from the same embryos These explants are invaded by numerous and large fiber bundles emerging from the fibrillar network The morphological characteristics of nerve cells and the structural and ultrastructural features of nerve bundles in long-term glia-free culture are described

Axonal growth from insect neurons in glia-free cultures.

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An extracellular matrix protein in plants: characterization of a genomic clone for carrot extensin.

Isolation and characterization of cDNA clones for carrot extensin and a proline-rich 33-kDa protein

Axonal growth from insect neurons in glia-free cultures.