A partial PCR clone of a PAL gene (MEPAL) was amplified from genomic DNA of cassava (Manihot escuknta). PAL enzyme activity and MEPAL mRNA levels were measured in leaves of cultiiar MCOL 22 following mechanical wounding or inoculation with 2 bacterial pathogens. MCOL 22 is resistant to Xanthomonas c~s s~v a e , which causes cassava bacterial necrosis and susceptible to X. axonopodis pv. manihotis, which causes cassava bacterial blight. PAL enzyme activity in the resistant interaction was significantly higher than in the susceptible interaction or the control. Similarly, MEPAL transcripts were detected in leaves during the resistant interaction, but not during the Susceptible reaction. RFLP analysis with MEPAL revealed a divergence between South American and African/Asian cultivars and showed promise for MEPAL as a marker for resistance to Xanthomonads.

Phenolic compounds have been implicated in the resist! ance of cassava "Manihot esculenta# to xanthomonads[ Cassava cultivars MCOL 11 and CM 412Ð6 were inocu! lated with Xanthomonas axonopodis pv[ manihotis and Xanthomonas cassavae[ CM412Ð6 was susceptible to both pathogens\ whereas MCOL 11 was susceptible to X[ a[ pv[ manihotis and resistant to X[ cassavae[ In the resistant interaction\ no disease symptoms were observed in the leaf\ bacterial growth was greatly reduced\ and there was a twofold increase in cell wall!bound peroxidase activity\ which may be related to lignin deposition[ Zusammenfassung Peroxidaseaktivita Ãt wa Ãhrend anfa Ãlliger und resistenter Inter! aktionen zwischen Maniok "Manihot esculenta# und Xan! thomonas axonopodis pv[ manihotis sowie X[ cassavae Phenolische Verbindungen gelten als ein ursa Ãchlicher Faktor fu à r die Resistenz von Maniok "Manihot esculenta# gegen Xanthomonaden[ Die Manioksorten MCOL 11 und CM 412Ð6 wurden mit Xanthomonas axonopodis pv[ manihotis und Xanthomonas cassavae inokuliert[ CM 412Ð6 war fu à r beide Pathogene anfa Ãllig\ MCOL 11 rea! gierte anfa Ãllig gegenu à ber X[ a[ pv manihotis und resistent gegenu à ber X[ cassavae[ Bei der resistenten Interaktion blieben die Bla Ãtter symptomfrei\ das Bakterienwachstum war stark verringert\ und die Aktivita Ãt zellwandgebunde! ner Peroxidasen stieg auf den doppelten Wert\ was mit einer Ligninablagerung in Zusammenhang stehen ko à nnte[