Studies on phenylalanine ammonia-lyase and peroxidases in cassava, Manihot esculenta Crantz, during root post-harvest deterioration and interaction with Xanthomonas
The main objective of this research was to initiate a molecular characterization of genes involved in the process of resistance of cassava to Xanthomonas and in the physiological post-harvest deterioration of the roots, which are two major problems for this crop. The studies reported in this thesis focused on characterization of genes for phenylalanine ammonia-lyase (PAL) and peroxidase which are involved in the initial steps of the phenyl-propanoid pathway and in the oxidation of the phenolic compounds produced by this pathway, respectively. PCR clones of cassava genes for PAL (MEPAL) and peroxidase (MEPX1) were isolated. MEPAL transcripts were detected in leaves, stems and petioles, and were especially prominently expressed in young tissues. The plant-pathogen portion of the studies characterized the interaction between two cultivars of cassava (MCol 22 and CM 523-7) and two pathogens (X. axonopodis pv. manihotis and X. cassavae). A resistant interaction was observed only when cultivar MCol 22 was inoculated with X. cassavae, wherein no disease symptoms were observed in the leaf and there was a reduction of bacterial growth and induction of both PAL and peroxidase enzymes. MEPAL mRNA was also detected during the resistance interaction between cultivar Mcol 22 and X. cassavae. Both clones showed potential as molecular markers relevant to breeding for resistance to bacterial blight. During post-harvest deterioration of the roots, it was demonstrated that there were differences in the enzyme activity of both PAL and peroxidase enzymes, concomitant with the level of deterioration of the roots, and probably related to the rate of oxidation which is higher in the air-exposed parts of the injured root, than in the inner layers. In both layers, however, there was an increase of PAL and peroxidase enzyme activity during deterioration, as well of the PAL mRNA transcription.