The PR1 family of pathogenesis-related proteins from tobacco (Nicotiana tabacum L.) leaves is induced by a variety of pathogenic and chemical agents and is associated with resistance to tobacco mosaic virus. The majority of the PR1 proteins did not copurify with mesophyll protoplasts (the major cell type of the leaf) isolated from tobacco mosaic virus-infected N. tabacum cv. Xanthi-nc leaves. However, these isolated protoplasts were capable of synthesizing and selectively secreting the PR1 proteins. Using monoclonal antibodies for immunofluorescence microscopy, we localized these proteins to the extracellular spaces predominantly in regions adjacent to viral lesions as well as in xylem elements of infected leaves.Infection of tobacco leaves with tobacco mosaic virus (TMV) results in one of two distinct disease processes that depend upon both the genetic background of the host and the viral strain involved. Many tobacco cultivars undergo a "systemic" infection in which TMV spreads from the original point of entry to many parts of the host with the potential of causing widespread damage, especially in younger leaves. In contrast, some tobacco cultivars are able to restrict (localize) the spread of TMV to a small zone of tissue around the point of entry where a necrotic lesion later appears. In tobacco, this "hypersensitive" reaction is accompanied by the induction throughout the plant of acquired resistance: the appearance of smaller and often fewer necrotic lesions in response to a second exposure to TMV (16,17). Virus localization and the induction of acquired resistance are accompanied by the synthesis of abundant amounts of pathogenesis-related (PR) proteins. The production of these proteins may be part of a general defense mechanism against pathogenic attack, since their synthesis can also be induced by certain bacteria and fungi (9,19). Alternatively, their induction by a number of chemical agents, such as acetylsalicylic acid (20), suggests that they are synthesized by the plant in response to stress.The polypeptides of the PR1 gene family, PRla, PRlb, and PRlc, with molecular weights of approximately 15,000, are biochemically and genetically the best characterized PR proteins (1,3,6,13). Their synthesis is regulated predominantly at the level of mRNA accumulation and occurs on membrane-bound polysomes (5, 6, 11). The observation that these proteins can be extracted from TMV-infected leaves by vacuum infiltration suggests that they accumulate in the intercellular fluid of leaves (15) infected whole leaf tissue or from mesophyl protoplasts prepared from this tissue were subjected to immunoblot analysis with a combination of polyclonal rabbit antisera against PR1 and the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase. Although the LSU was enriched in the mesophyll protoplast preparation as expected, less than 20% of the PR1 proteins found in the whole leaf preparation copurified with the mesophyll protoplasts (Fig. 1). Similar results were obtained with mesophyll protoplasts isolated from leave...

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l h e expression of a-and 8-tubulin proteins in developing fibers and severa1 other tissues of cotton (Gossypium hirsutum, cv Texas Marker 1) have been analyzed by immunoblots of one-and twodimensional gels utilizing anti-tubulin antibodies as probes. As a percentage of total protein, fibers had greater amounts of tubulin than did hypocotyls, roots, leaves, or cotyledons. Both a-and 8-tubulin, having apparent molecular masses of approximately 50 kD and isoelectric points between p H 5 and pH 6, were resolved on a single two-dimensional gel. Under the conditions used, atubulin was less acidic in the isoelectric focusing dimension and migrated slightly faster in the sodium dodecyl sulfate dimension than did 8-tubulin. Nine a-tubulin isotypes that formed two distind groups were identified on immunoblots of two-dimensional gels. l h e three most abundant a-tubulin isotypes were common to all tissues examined. Seven distind 8-tubulin isotypes were also iden- Microtubules in association with other components of the cytoskeleton have a central function in many important processes in eukaryotic cells, including cell division, intracellular transport, cell motility, and cell morphogenesis. In plants, microtubules have a unique role in morphogenesis. Microtubules appear to have a direct influence on the morphology of individual cells as well as an indirect influence on the morphology of the entire plant, since overall plant morphology is collectively determined by the shape of individual cells (Lloyd, 1991; Shibaoka, 1991). This shaping influence of microtubules in plants is a result of their involvement in establishing cellular division planes and in determining the axes of cellular elongation. The influence of microtubules in determining axes of cellular elongation appears to be associated with the deposition of cellulose microfibrils in the cell wall. For example, in plant cells that are expanding isodiametrically, microtubules and wall microfibrils are oriented randomly and are believed to allow expansion of the cell in a11 directions. In contrast, during plant cell elongation, both microtubules and wall microfibrils are oriented transversely

The pathogenesis-related (PR) proteins of tobacco plants are induced in response to a variety of pathogenic and chemical agents. Although the function of these proteins is unknown, they are associated with resistance to multiplication and/or spread of tobacco mosaic virus. We report that functional mRNAs encoding PR proteins are present only when synthesis of these proteins has been induced, suggesting that their synthesis is controlled in part at the level of mRNA accumulation. In addition PR proteins appear to be synthesized and processed in a manner analogous to proteins destined for the endoplasmic reticulum since (i) the in vitro translation products synthesized in the wheat-germ cell-free system are slightly larger than the in vivo products, (ii) translation of PR mRNAs in the rabbit reticulocyte lysate system is blocked unless that system is supplemented with dog pancreas microsomes, and (iii) mRNAs for PR proteins are associated predominantly with membrane-bound polysomes in vivo. This pathway of synthesis and posttranslational modification suggests possible sites of action of these proteins.