Plants possess an efficient nonself surveillance system triggering induced disease resistance mechanisms upon molecular recognition of microbial invaders. Successful pathogens have evolved strategies to evade or counteract these mechanisms, e.g., by the generation of suppressors. Pectic fragments produced during host cell wall degradation can act as endogenous suppressors of the hypersensitive response in wheat leaves. We have isolated and characterized homogalacturonans from cell walls of two wheat cultivars susceptible to the stem rust fungus, Puccinia graminis f. sp. tritici, namely cvs. Prelude and Marquis, and from near-isogenic lines of both cultivars containing the Sr5-gene for hypersensitive rust resistance. Two independent approaches were used to compare their methyl esterification: i) immunochemistry using the monoclonal antibodies JIM5, JIM7, PAM1, and LM7 and ii) chromatography of oligogalacturonides representing stretches of contiguous nonmethyl-esterified GalA residues. The results clearly indicate a significant difference in the homogalacturonans from susceptible and resistant wheat lines. The difference can best be explained by assuming a nonrandom and more blockwise distribution of the methyl esters in the homogalacturonans of susceptible wheat cultivars as compared with a presumably more random distribution in the near-isogenic resistant lines. Possible consequences of this difference for the enzymatic generation of endogenous suppressors are discussed.

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Protein accumulation and patterns during embryogenesis in the recalcitrant seeds of the gymnosperm species Araucaria angustifolia (Bert.) O. Kuntze were studied. Soluble seed proteins, chitinases, and arabinogalactan proteins (AGPs) were analyzed by means of 2-D gel electrophoresis, mass spectrometry, isoelectric focusing, Western blot, precipitation and staining with b-glucosyl Yariv reagent (b-Glc) 3 Y, and gas liquid chromatography. Despite the recalcitrant nature of the seeds, the electrophoretic patterns of A. angustifolia seed proteins showed similarities with orthodox seed types. Proteins showing chitinolytic activity were observed in all seed stages analyzed, but the expression of class IV chitinases was restricted to late stages of seed development. AGPs were prominent during stages of seed development characterized by intensive cell division and differentiation, and their decrease during seed maturation might be related to cell wall modifications during the deposition of storage compounds. Gas liquid chromatographic analyzes of AGPs did not show quantitative changes in their carbohydrate moieties during seed development. A low molecular weight protein specifically expressed in mature seeds was precipitated using (b-Glc) 3 Y. Amino acid sequences obtained from MS/MS analysis revealed peptides rich in valine and acidic amino acids, but devoid in amino acids normally found in AGPs polypeptides, suggesting that these peptides are not related to classical or non-classical AGPs. Possible implications of chitinases and AGPs during seed development in A. angustifolia are discussed.

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