When highly resistant wheat (Triticum aestivum L.) varieties are infected by an avirulent race of the stem rust fungus (Puccinia graminis Pers. f. sp. tritici Erics. and E. Henn.), penetrated host cells undergo rapid necrotization. This hypersensitive cell death is correlated with cellular lignification which efficiently restricts further fungal growth. Three competitive inhibitors of phenylalanine ammonia-lyase, the first enzyme of the general phenylpropanoid pathway and, thus, of lignin biosynthesis, namely a-aminooxyacetate, a-aminooxy-B-phenylpropionic acid, and (1-amino-2-phenylethyl)phosphonic acid, and two highly specific irreversible suicide inhibitors of the lignification-specific enzyme cinnamyl-alcohol dehydrogenase, namely N(O-aminophenyl)sulfinamoyl-tertiobutyl acetate and N(O-hydroxyphenyl)sulfinamoyltertiobutyl acetate, were applied to genetically resistant wheat plants prior to inoculation with stem rust. Treatment with any of these inhibitors decreased the frequency of lignified necrotic host cells and concomitantly led to increased fungal growth. The cinnamyl-alcohol dehydrogenase inhibitors were generally more effective than the phenylalanine ammonia-lyase inhibitors, occasionally allowing some sporulation to occur on the resistant wheat leaves. These results clearly point to a causal relationship between the formation of lignin precursors and the resistance of wheat to stem rust.The lignin content ofhigher plants has long been recognized as an important factor in the resistance response against potential pathogens. Lignin is extremely resistant to microbial degradation and thus constitutes one of the most effective barriers against pathogenic invasion (25). In addition to the role of lignin as a preformed resistance factor, induced lignification has been proposed as an active resistance mechanism of plants to fungi (15). However, it is still a matter of debate whether active lignification processes are causally involved in resistance (25).In the family Gramineae, active lignification appears to be of special importance in induced resistance mechanisms (28), possibly related to the near absence of phytoalexins in this family. In spite of intensive searches for infection-induced During the time of this study, B. M. M. received a Graduiertenstipendium des Landes Nordrhein Westfalen. This work was supported in part by the Deutsche Forschungsgemeinschaft.

Coniferyl alcohol was polymerized in pectin solution in order to mimic the lignification that is the final step of biosynthesis of plant cell wall. Dehydrogenated polymers (DHP = coniferyl alcohol polymers = synthetic lignin) interact with pectin to form hydrophobic clusters as monitored by pyrene fluorescence spectroscopy. The structure of these clusters was studied during the polymerization of synthetic lignin by static and quasielastic light scattering and small angle neutron scattering experiments. We show that synthetic lignin and pectin contribute to the same clusters, but the inner structure of these clusters is very heterogeneous and displays three phases. One observes a segregation between well separated pectin and lignin rich phases at length scales below approximately 30 nm. As a corollary of this segregation, clusters embody a large amount of solvent. On average, the density of the polymer rich phase (lignin plus pectin) inside clusters increases while its specific surface area decreases throughout the polymerization process. These results are discussed with respect to in vivo lignification of the plant cell wall.

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Structural analogues of endoperoxides belonging to the family of G factors have been synthesized under Mannich‐type conditions. The structures of the different diastereoisomers have been established from NMR spectroscopic data. Their cathodic peak potentials have been determined by thin layer electrochemistry under potentiostatic conditions, and compared to artemisinin. These endoperoxides were evaluated in vitro against Plasmodium falciparum and showed moderate to good activity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)