Modification Of Plant Cell Walls Research Articles

Genetic Modification of Herbaceous Plants for Feed and Fuel

Referee: Dr. E. Charles Brummer, Forage Breeding and Genetics, 1204 Agromonomy, Iowa State University, Ames, IA 50011 Much of the research on the genetic modification of herbaceous plant cell walls has been conducted to improve the utilization of forages by ruminant livestock. The rumen of these animals is basically an anaerobic fermentation vat in which the micro flora break down the complex polysaccharides of plant cell walls into simpler compounds that can be further digested and absorbed by the mammalian digestive system. Research on improving the forage digestibility of switchgrass, Panicum virgatum L., and other herbaceous species has demonstrated that genetic improvements can be made in forage quality that can have significant economic value. To meet future energy needs, herbaceous biomass will need to be converted into a liquid fuel, probably ethanol, via conversion technologies still under development. If feedstock quality can be genetically improved, the economics and efficiency of the conversio...

Genetic Modification of Herbaceous Plants for Feed and Fuel

Referee: Dr. E. Charles Brummer, Forage Breeding and Genetics, 1204 Agromonomy, Iowa State University, Ames, IA 50011Much of the research on the genetic modification of herbaceous plant cell walls has been conducted to improve the utilization of forages by ruminant livestock. The rumen of these animals is basically an anaerobic fermentation vat in which the micro flora break down the complex polysaccharides of plant cell walls into simpler compounds that can be further digested and absorbed by the mammalian digestive system. Research on improving the forage digestibility of switchgrass, Panicum virgatum L., and other herbaceous species has demonstrated that genetic improvements can be made in forage quality that can have significant economic value. To meet future energy needs, herbaceous biomass will need to be converted into a liquid fuel, probably ethanol, via conversion technologies still under development. If feedstock quality can be genetically improved, the economics and efficiency of the conversion processes could be significantly enhanced. Improving an agricultural product for improved end product use via genetic modification requires knowledge of desired quality attributes, the relative economic value of the quality parameters in relation to yield, genetic variation for the desired traits, or for molecular breeding, knowledge of genes to suppress or add, and knowledge of any associated negative consequences of genetic manipulation. Because conversion technology is still under development, desirable plant feedstock characteristics have not been completely delineated. Some traits such as cellulose and lignin concentration will undoubtably be important. Once traits that affect biomass feedstock conversion are identified, it will be highly feasible to genetically modify the feedstock quality of herbaceous plants using both conventional and molecular breeding techniques. The use of molecular markers and transformation technology will greatly enhance the capability of breeders to modify the morphologic structure and cell walls of herbaceous species. It will be necessary to monitor gene flow to remnant wild populations of biomass plants and have strategies available to curtail gene flow if it becomes a potential problem. It will also be necessary to monitor plant survival and long-term productivity as affected by these genetic changes to herbaceous species.

The Role of Endoxyloglucan Transferase in the Organization of Plant Cell Walls

The plant cell wall plays a central role in morphogenesis as well as responsiveness to environmental signals. Xyloglucans are the principal component of the plant cell wall matrix and serve as cross-links between cellulose microfibrils to form the cellulose-xyloglucan framework. Endoxyloglucan transferase (EXGT), which was isolated and characterized in 1992, is an enzyme that mediates molecular grafting reaction between xyloglucan molecules. Structural studies on cDNAs encoding EXGT and its related proteins have disclosed the ubiquitous presence in the plant kingdom of a large multigene family of xyloglucan-related proteins (XRPs). Each XRP functions as either hydrolase or transferase acting on xyloglucans and is considered to be responsible for rearrangement of the cellulose-xyloglucan framework, the processes essential for the construction, modification, and degradation of plant cell walls. Different XRP genes exhibit potentially different expression profiles with respect to tissue specificity and responsiveness to hormonal and mechanical signals. The molecular approach to individual XRP genes will open a new path for exploring the controlling mechanisms by which the plant cell wall is constructed and reformed during plant growth and development.

Interactions between cassava mealybug and cassava: cytochemical aspects of plant cell wall modifications

The cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae), a serious pest of cassava in Africa, is an oligophagous insect mainly living on cassava (Manihot esculenta Crantz) (Matile-Ferrero, 1976). Following the demonstration of an aphid-type phloem-feeding behaviour of cassava mealybug with an aphid-like predominance of extracellular pathways of stylets (Calatayud et al., 1994), host-plant resistance of cassava to P. manihoti has been investigated. The hypothesis was that P. manihoti secretes enzymes to facilitate intercellular stylet penetration by digestion of plant middle lamellae and primary cell walls. We suggested that pectinesterase isolated from mealybugs could be involved in plant pectin degradation; however, no polygalacturonase was detected (Calatayud, 1993).

Programmed cell death in plants: A pathogen-triggered response activated coordinately with multiple defense functions

In plants, the hypersensitive response (HR) to pathogens involves rapid cell death, which is hypothesized to arise from the activation of a cell death program. We describe mutant A. thaliana plants that contain lesions in a single accelerated cell death ( ACD) gene called ACD2 and that bypass the need for pathogen exposure to induce the HR. acd2 plants that develop spontaneous lesions show typical HR characteristics both within the necrotic tissue and within the healthy part of the plant, including: modification of plant cell walls, resistance to bacterial pathogens, and accumulation of defense-related gene transcripts, the signal molecule salicylic acid and an antimicrobial compound. We propose that the ACD2 gene is involved in a pathway(s) that negatively regulates a genetically programmed HR.

Immunogold localization of pathogenesis-related protein P14 in tomato root cells infected by Fusarium oxysporum f. sp. radicis-lycopersici

The accumulation of soluble pathogenesis-related (PR) P14 protein in tomato plants has only been reported in green tissue. An antiserum raised against purified PR P14 protein was used to study the spatial and temporal distribution of this protein in tomato root tissues infected by Fusarium oxysporum f. sp. radicis-lycopersici . Within healthy root tissue, PR P14 protein was found to be present in variable amount in cell walls, depending apparently on cell age and on the degree of tissue differentiation. Cell walls of infected tomato root tissues were markedly enriched in PR P14 protein. Although differences in the timing of PR P14 protein accumulation were observed between susceptible and resistant cultivars, the spatial distribution of the protein was similar in both interactions. Its predominant accumulation in host primary and secondary cell walls contrasted with its lack of association with the middle lamella. Deposition of PR P14 protein was also noticeable in intercellular spaces and within wall appositions. Its absence in the fungal wall provides support for a possible role in plant cell wall modification. This is contrary to other extracellular PR proteins such as chitinases and β-1,3-glucanases which are found associated with walls of invading fungal pathogens.

Ultrastructure of interactions between pseudomonads and oat leaves

Ultrastructural studies were made on interactions between oat leaves and the compatible pathogen Pseudomonas coronafaciens and the incompatible bacteria P. coronafaciens var. atropurpurea, P. fluorescens and P. tabaci. Ruthenium red, a cytochemical stain for acidic polysaccharides, was used to enhance resolution of bacterial extracellular polysaccharide (EPS) in vivo. EPS accumulation and the attachment of bacteria to oat cell walls were assessed quantitatively. Two types of association between bacteria and cell walls were distinguished, (1) the encapsulation of bacteria by fibrillar materials on to smooth areas of plant wall (physical entrapment) and (2) attachment involving localized but pronounced modification (often degradation) of the underlying wall. Quantitative analyses showed that EPS accumulated rapidly around P. coronafaciens but not incompatible bacteria. Production of EPS by the pathogen appeared to prevent attachment to oat cell walls. The proportion of incompatible bacteria attached to plant cell walls increased between 2·5 and 12 h after inoculation, but only with P. fluorescens were more than 35% of attached bacteria associated with pronounced plant cell wall modifications. Our results suggest that attachment per se is unlikely to restrict bacterial multiplication in oats but may allow the transfer of factors causing the production of antibacterial compounds by challenged cells. Examination of EPS accumulation in tobacco gave results closely comparable to those recorded in oats. The compatible bacterium, in this case, P. tabaci, was surrounded by EPS whereas the incompatible P. coronafaciens was not, but was often attached to cell walls.