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By changing their physiological conditions, higher plants protect themselves from various stresses such as pathogen attacks, wounding, application of chemicals including phytohormone and heavy metals, air pollutants like ozone, ultraviolet rays, and harsh growing conditions. These protective reactions are known as "defense responses" of higher plants (Fig. 1), and the proteins actively synthesized in accordance with this reaction are called "defense-related proteins" [1]. In particular, protective plant proteins specifically induced in pathological or related situations have been intensively studied from an agricultural perspective and are called "pathogenesis-related proteins" (PR proteins). Enzymes indispensable for biosynthesizing low-molecular-weight antibiotics (phytoalexin), isoflavone reductase, for example, can also be considered to be defense-related proteins. On the other hand, many of the reserve proteins accumulated in seeds and fruits are considered to have a constitutive defense function against microbial pathogens and invertebrate pests in addition to their storage function. These inducible or constitutive defense mechanisms of higher plants are relatively conserved during the course of evolution. Accordingly, most plants produce or accumulate structurally and functionally similar protective proteins under certain
situations, irrespective of their morphological differences. For instance, PR proteins, which have been found in many plant species to date, are classified into seventeen families (Table), regardless of the original plant species. The sequence similarities, serologic or immunologic relationships, and enzymatic properties are the basis of this classification [2].
Recently, protective proteins of higher plants have drawn much attention from plant breeders [3]. Because defense-related proteins usually provide a plant with resistance to stresses, varieties that are apt to intensively induce such proteins can be agriculturally valuable. Less toxic substances that cause crops to express defensive proteins are also being investigated as a new type of agrochemical. Moreover, some defense-related proteins will be constantly expressed in genetically modified plants [4-7]. However, several of these proteins have been proven to be latex allergens and cross-reactive allergens in fruits, vegetables, and pollen [8-10]. As such, it is probable that the crop content of allergenic proteins responsible for "oral allergy syndrome" (OAS) or similar syndromes has increased in response to traditional plant-breeding or the creation of stress-resistant varieties through the use of gene recombination techniques (Fig. 2). When a genetically modified new crop is brought to market as a food, its safety, including its possible allergenicity, must be strictly examined and reviewed in advance [11].
Table. Recommended Classification of Pathogenesis-Related Proteins (PRs)
| Family | Type member | Properties |
| PR-1 | tobacco PR-1a | antifungal?, 14-17kD |
| PR-2 | tobacco PR-2 | class I, II, and III endo-beta-1,3-glucanases, 25-35kD |
| PR-3 | tobacco P, Q | class I, II, IV, V, VI, and VII endochitinases, about 30kD |
| PR-4 | tobacco R | antifungal, win-like proteins, endochitinase activity, similar to prohevein C-terminal domain, 13-19kD |
| PR-5 | tobacco S | antifungal, thaumatin-like proteins, osmotins, zeamatins, permeatins, similar to alpha-amylase/trypsin inhibitors |
| PR-6 | tomato inhibitor I | protease inhibitors, 6-13kD |
| PR-7 | tomato P69 | endoproteases |
| PR-8 | cucumber chitinase | class III chitinases, chitinase/lysozyme |
| PR-9 | lignin-forming peroxidase | peroxidases, peroxidase-like proteins |
| PR-10 | parsley PR-1 | ribonucleases, Bet v 1-related proteins |
| PR-11 | tobacco class V chitinase | endochitinase activity |
| PR-12 | radish Ps-AFP3 | plant defensins |
| PR-13 | Arabidopsis THI2.1 | thionins |
| PR-14 | barley LTP4 | nonspecific lipid transfer proteins (ns-LTPs) |
| PR-15 | barley OxOa (germin) | oxalate oxidase |
| PR-16 | barley OxOLP | oxalate-oxidase-like proteins |
| PR-17 | tobacco PRp27 | unknown |
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