Bacterial spot, one of the most damaging diseases of pepper, is caused by Xanthomonas euvesicatoria. This pathogen has worldwide distribution and it is particularly devastating in tropical and sub-tropical regions where high temperatures and frequent precipitation provide ideal conditions for disease development. Three dominant resistance genes have been deployed singly and in combination in commercial cultivars, but have been rendered ineffectual by the high mutation rate or deletion of the corresponding cognate effector genes. These genes are missing in race P6, and their absence makes this race virulent on all commercial pepper cultivars. The breeding line ECW12346 is the only source of resistance to race P6 in Capsicum annuum, and displays a non-hypersensitive type of resistance. Characterization of this resistance has identified two recessive genes: bs5 and bs6. Individual analysis of these genes revealed that bs5 confers a greater level of resistance than bs6 at 25 degrees C, but in combination they confer full resistance to P6 indicating at least additive gene action. Tests carried out at 30 degrees C showed that both resistances are compromised to a significant extent, but in combination they provide almost full resistance to race P6 indicating a positive epistatic interaction at high temperatures. A scan of the pepper genome with restriction fragment length polymorphism and AFLP markers led to the identification of a set of AFLP markers for bs5. Allele-specific primers for a PCR-based bs5-marker have been developed to facilitate the genetic manipulation of this gene.
The endogenous localisation of peroxidase and hydrogen peroxide (H 2 O 2 ) was detected when gametophytes of the fern, Ceratopteris richardii, were exposed to the plant pathogenic fungi Sclerotium rolfsii and Sclerotinia sclerotiorum and Phytophthora infestans, an oomycete, in a gnotobiotic system. This was accomplished by light microscopy using 3,3#-diaminobenzidine, guaiacol and H 2 O 2 and starch potassium iodide (KI) staining procedures, which facilitated the observation of the reaction in vivo and in situ, without physically damaging the tissues. All three staining methods promoted staining at the rhizoid regions. Although most of the cells were destroyed when gametophytes were exposed to S. rolfsii and S. sclerotiorum, there was staining where mycelial growth was confluent with cell walls. A qualitative test confirmed that the colour change in starch KI agar medium, as well as in the histochemical test with starch KI, was because of H 2 O 2 secreted by S. rolfsii or S. sclerotiorum and not because of oxalic acid. When gametophytes were exposed to P. infestans, no infection occurred, but localisation of H 2 O 2 and peroxidase was detected irrespective of staining methods tested. Based on the observation on gametophytes grown in presence of P. infestans, it is possible that the peroxidase in plants coupled with H 2 O 2 may prevent the invasion of nonpathogens by functioning as a barrier. This fern-pathogen model system has potential for application as a tool to study the host-parasite interaction in a gnotobiotic system.
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