Genetics of Erwinia resistance in Zantedeschia: impact of plastome-genome incompatibility

Soft rot caused by Erwinia carotovora subsp. carotovora ( Pectobacterium carotovorum subsp. carotovorum ) is the most important disease of Zantedeschia spp. Cultivation measures can protect the crop partially, but a combination with resistant cultivars could result in better control. Resistant cultivars are not available, however. It is determined in this thesis that it is possible to breed for resistance caused by Erwinia. Resistance tests as well as a collection of plant material were developed to determine the genetic variation for resistance to Erwinia carotovora subsp. carotovora . The results of four resistance tests were in general concordance with experiences from practice. Although the tuber tests were most reproducible, the leaf disk test was used further research. The leaf disk test was not destructive, so seedlings and rare germplasm could be evaluated. Among the section Zantedeschia, Z. aethiopica appeared resistant to moderately resistant, while Z. odorata appeared susceptible. Only some cultivars of the section Aestivae ('Neroli', 'Coral Sunset' and 'Hazel Marie') appeared significantly less susceptible than the very susceptible reference 'Florex Gold'. Partial resistance was observed in Z. albomaculata subsp. macrocarpa and in Z. rehmannii . To determine the genetic basis of the partial resistance, hybrids were developed of species that have different resistance levels: Z. albomaculata (susceptible to partially resistant), Z. rehmannii (partially resistant), Z. elliotiana (susceptible) and Z. pentlandii (very susceptible). The families of these interspecific crosses (F 1 -hybrids) segregated for leaf colour (chlorophyll content) and vigour, dependent on the parents and the crossing direction. This segregation was not Mendelian, but depended on the crossing direction. Descendents of Z. rehmannii as mother were green (with a chlorophyll content that was similar as selfings of the parents) and vigorous. Descendents of Z. elliotiana as mother, however, were mainly pale green and these were less vigorous in some cases, depending on the father. The cause of these reciprocal differences was determined to evaluate the effect to the level of resistance. Plastome-specific CAPS-markers (Cleaved Amplified Polymorphic Sequence) were used to determine that there existed at leat three plastomes that differed in the level of compatibility to the genomes of the different species of section Aestivae . The segregation in leaf colour (chlorophyll content) within families of the same crossing direction originated from plastids that had inherited via pollen. This paternal influence in plastid inheritance was observed in 24 intespecific families of Z. rehmannii , Z. albomaculata , Z. elliotiana and Z. pentlandii. Thus was concluded that biparental ininheritance of plastids is common common within interspecific crossings among section Aestivae . The plastome of Z. rehmannii was the most compatible to the other genomes. Plants with not fully compatible plastome-genome combinations had decreased vigour and level of resistance. Genetic analyses were therefore performed only on plants that did not suffer from plastome-genome incompatibility. The level of resistance of F 1 -hybrids was correlated to the level of resistance of the parents en transgression was observed in seedlings of Z. rehmannii and Z. albomaculata . This relation indicates a genetic basis for resistance with complementary or additive genes with great potential for resistance breeding.