(M.P.) Pectin, one of the main components of plant cell wall, is secreted in a highly methylesterified form and is demethylesterified in muro by pectin methylesterase (PME). The action of PME is important in plant development and defense and makes pectin susceptible to hydrolysis by enzymes such as endopolygalacturonases. Regulation of PME activity by specific protein inhibitors (PMEIs) can, therefore, play a role in plant development as well as in defense by influencing the susceptibility of the wall to microbial endopolygalacturonases. To test this hypothesis, we have constitutively expressed the genes AtPMEI-1 and AtPMEI-2 in Arabidopsis (Arabidopsis thaliana) and targeted the proteins into the apoplast. The overexpression of the inhibitors resulted in a decrease of PME activity in transgenic plants, and two PME isoforms were identified that interacted with both inhibitors. While the content of uronic acids in transformed plants was not significantly different from that of wild type, the degree of pectin methylesterification was increased by about 16%. Moreover, differences in the fine structure of pectins of transformed plants were observed by enzymatic fingerprinting. Transformed plants showed a slight but significant increase in root length and were more resistant to the necrotrophic fungus Botrytis cinerea. The reduced symptoms caused by the fungus on transgenic plants were related to its impaired ability to grow on methylesterified pectins.Pectin is a structurally complex polysaccharide that accounts for nearly 35% of the dicot and nongraminaceous monocot primary cell wall. A main component of pectin is homogalacturonan (HGA) consisting of a backbone of 1,4-linked a-D-GalUA units, with variable amounts of methylester in the C 6 position. Pectins are secreted into the cell wall in a highly methylesterified form and, soon thereafter, are deesterified in muro by pectin methylesterase (PME; Brummell and Harpster, 2001;Willats et al., 2001). Demethylesterification produces free carboxyl groups and modifies the pH and charge of the wall, allowing the aggregation of polyuronides into a calcium-linked gel structure and increasing the wall firmness (Willats et al., 2001). In addition, the action of PMEs makes HGA susceptible to degradation by hydrolases such as endopolygalacturonases (endoPGs), contributing to the softening of the cell wall (Brummell and Harpster, 2001;Wakabayashi et al., 2003).Plant PMEs are involved in important physiological processes such as microsporogenesis, pollen growth, pollen separation, seed germination, root development, polarity of leaf growth, stem elongation, fruit ripening, and loss of tissue integrity

The cell wall is a dynamic structure that often determines the outcome of the interactions between plants and pathogens. It is a barrier that pathogens need to breach to colonize the plant tissue. While fungal necrotrophs extensively destroy the integrity of the cell wall through the combined action of degrading enzymes, biotrophic fungi require a more localized and controlled degradation of the cell wall in order to keep the host cells alive and utilize their feeding structures. Also bacteria and nematodes need to degrade the plant cell wall at a certain stage of their infection process, to obtain nutrients for their growth. Plants have developed a system for sensing pathogens and monitoring the cell wall integrity, upon which they activate defense responses that lead to a dynamic cell wall remodeling required to prevent the disease. Pathogens, on the other hand, may exploit the host cell wall metabolism to support the infection. We review here the strategies utilized by both plants and pathogens to prevail in the cell wall battleground.

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Plant cell walls represent an abundant, renewable source of biofuel and other useful products. The major bottleneck for the industrial scale-up of their conversion to simple sugars (saccharification), to be subsequently converted by microorganisms into ethanol or other products, is their recalcitrance to enzymatic saccharification. We investigated whether the structure of pectin that embeds the cellulose-hemicellulose network affects the exposure of cellulose to enzymes and consequently the process of saccharification. Reduction of de-methyl-esterified homogalacturonan (HGA) in Arabidopsis plants through the expression of a fungal polygalacturonase (PG) or an inhibitor of pectin methylesterase (PMEI) increased the efficiency of enzymatic saccharification. The improved enzymatic saccharification efficiency observed in transformed plants could also reduce the need for acid pretreatment. Similar results were obtained in PG-expressing tobacco plants and in PMEI-expressing wheat plants, indicating that reduction of de-methyl-esterified HGA may be used in crop species to facilitate the process of biomass saccharification.biofuel | pectin | plant cell wall | pectin methylesterase inhibitor | polygalacturonase P lant biomass has been a source of energy for most part of human history and, due to the increasing demand for renewable materials and industrial products, is reconsidered today as a possible strategic resource. Plant cell walls comprise a significant proportion of the lignocellulosic biomass (1) and are a potentially abundant substrate for bioconversion to ethanol and other industrial products (2). They are composed of a heterogeneous polysaccharidic matrix associated with components like lignin and proteins. Saccharification, a key process for the production of ethanol, is the degradation of the wall polysaccharides into fermentable sugars. Enzymatic hydrolysis is the most promising and environmentally friendly technology available for saccharification (3, 4), but the recalcitrance of cell walls to hydrolysis is the major bottleneck for the industrial scale-up of this process (2). Thermochemical pretreatments using high temperature, toxic acids, peroxides, and ammonia, often along with some form of mechanical disruption, are currently required to make biomass accessible to cell wall-degrading enzymes and represent up to 30% of the cost of biofuel production (2).Modification of the cell wall structure may be useful for reducing pretreatments and improving the overall saccharification process. For example, it has been shown that reducing the lignin content in transgenic alfalfa plants improves saccharification efficiency, although it can reduce biomass yield (5). A cell wall component that, particularly in dicots, is critical for tissue integrity and accessibility to cell wall-degrading enzymes is the cohesive pectin matrix embedding the cellulose-hemicellulose network, which in turn contains the major strength-conferring elements. It is well known that intermolecular bonds of pectin, mediated by acidic homogalac...