It is generally believed that plants "evolved a strategy of defending themselves from a phytopathogen attack" during evolution. This metaphor is used frequently, but it does not facilitate understanding of the mechanisms providing plant resistance to the invasion of foreign organisms and to other unfavorable external factors, as well as the role of these mechanisms in plant growth and development. Information on processes involving one of the plant resistance factors--polygalacturonase-inhibiting protein (PGIP)--is considered in this review. The data presented here indicate that PGIP, being an extracellular leucine-rich repeat-containing protein, performs important functions in the structure of plant cell wall. Amino acid residues participating in PGIP binding to homogalacturonan in the cell wall have been determined. The degree of methylation and the mode of distribution of homogalacturonan methyl groups are responsible for the formation of a complex structure, which perhaps determines the specificity of PGIP binding to pectin. PGIP is apparently one of the components of plant cell wall determining some of its mechanical properties; it is involved in biochemical processes related to growth, expansion, and maceration, and it influences plant morphology. Polygalacturonase (PG) is present within practically all plant tissues, but the manifestation of its activity varies significantly depending on physiological conditions in the tissue. Apparently, the regulation of PG functioning in apoplast significantly affects the development of processes associated with the modification of the structure of plant cell wall. PGIP can regulate PG activity through binding to homogalacturonan. The genetically determined structure of PGIP in plants determines the mode of its interaction with an invader and perhaps is one of the factors responsible for the set of pathogens causing diseases in a given plant species.

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The enzyme polygalacturonase (poly-[1,4-α -Dgalacturonide] glycanohydrolase, EC 3.2.1.15) plays a key role in the life of plants. It is involved in the transformations of pectin substances during the growth of plants and ripening of fruit [1]. Phytopathogenic organisms secrete polygalacturonases into the environment, which enables their intrusion into plant tissues due to degradation of pectin substances contained in the cell wall of plants [2,3]. In bananas and some other fleshy fruit, the activity of polygalacturonase increases in the course of fruit ripening. It is believed that this enzyme is responsible for the maceration of fruit tissues [4]. In the course of fruit ripening, the rate of ethylene production increases, which is accompanied by changes in a complex of processes, including those that are related to tissue maceration as a result of pectin degradation [5]. Polygalacturonase inhibiting protein (PGIP) is an important component of the system of transformation of pectin substances. In addition, it plays a key role in the development of resistance of plants to infection with phytopathogenic microorganisms, because it significantly decreases the effectiveness of polygalacturonase, which is secreted by pathogens and represents one of the main pathogenicity factors [6]. The content of PGIP in fruit is obviously correlated with the degree of their maceration during ripening; however, no direct effect of PGIP of a certain plant on its own polygalacturonase has been found [7]. The studies of the characteristic features of the effect of PGIP are also topical in terms of development of procedures for testing the physiological state of plant tissues. Data on PGIP activity in banana fruit are absent.The goal of this study was to detect PGIP in banana fruit and to determine its effect on polygalacturonase from different plants and pathogenic fungi infecting banana fruit. METHODSThis study was performed with ripe banana ( Musa acuminata L.) fruit of cultivars Cavendish and Korolevskii (Vietnam). To determine the rate of ethylene production, fruit were placed in desiccators equipped with a gas-escape tube made from silicon rubber and hermetically closed [8]. After 24 h of incubation, gas samples for analysis were taken with a 1-cm 3 polyethylene syringe. Ethylene content in samples was determined by GLC on a Chrom-4 instrument (Czech Republic) equipped with a flame ionization detector. GLC analysis was performed in a glass column ( 150 × 0.3 cm) packed with Porapack N (Serva, Germany) under the following conditions: carrier gas (nitrogen) flow rate, 25 ml/min; column temperature, 75°C ; and evaporator temperature, 130°C . The amount of ethylene was determined using a calibration curve and expressed in µ l per kg produced in one day.In experiments we also used cabbage ( Brassica oleracea L.), potato ( Solanum tuberosum L.) and Jerusalem artichoke ( Helianthus tuberosus L.) tubers, and pepper ( Capsicum annuum L.) and apple ( Malus domestica Borkh.) fruit. To obtain PGIP preparations, plant tissues were homogenized i...

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