The soil‐borne obligate pathogen Plasmodiophora brassicae causes clubroot disease in species of Brassicaceae, including Arabidopsis thaliana. The host–pathogen interaction was studied with respect to the age of the plant at the time point of inoculation and to different infection pressures in order to establish a standardization of infection parameters and evaluation of disease extent for A. thaliana lines. Spore number per root weight, root and shoot weight of inoculated and non‐inoculated plants as well as infection rate and disease index (DI) were analysed and correlated. The disease extent of different lines was comparable as measured by the relation of root weight of inoculated and non‐inoculated plants (Ri/Rni index) and the DI. Most of the 71 screened A. thaliana lines turned out to be susceptible. However, the mutant lines tu8, tu3, det1‐1, and rhd3‐1 showed a certain degree of tolerance under specific culture conditions. The reactions of rhd3‐1 indicate that hypertrophy is a prerequisite for maturation of the pathogen. The reactions of the tu3 and tu8 mutants indicate a role of indole glucosinolates and indole‐3‐acetonitrile/IAA in development of clubroot disease.

Mesophyll protoplasts of Lycopersicon esculentum Mill. var. cerasiforme (Dunal) Alef, mutant yellow green 6, Rick and protoplasts of a liquid callus culture of the dihaploid strain HH258 of Solanum tuberosum L. were prepared and many fusion products were visible after the protoplasts were incubated together first in the presence of polyethylene glycol and then with a high Ca 2 + ion concentration. The protoplasts were transferred to a rich medium and the resultant calli were cultured. Some calli regenerated normal green shoots which were transferred to soil or grafted onto a tomato stock. The subunit polypeptide pattern of ribulose 1,5-bisphosphate carboxylase prepared from leaf material of four regenerated plants was analyzed by isoelectric focusing. The ribulose bisphosphate carboxylase enzyme oligomer in the four plants contained the small subunit products resulting from the expression of both tomato and potato nuclear genes proving these plants to be somatic hybrids between tomato and potato. In three of the four plants the large subunit polypeptides and hence the functional chloroplast DNA were from tomato whereas in the fourth the large subunit and therefore the chloroplast DNA was derived from potato. The plant material was insufficient to establish the chromosome numbers precisely, however counts close to 50 which is near to the expected 48 were obtained for three of the hybrids whereas in the fourth a number close to 72 was observed. In the absence of a selection system against the potato parent, the analysis of ribulose bisphosphate carboxylase provides a convenient marker to demonstrate the hybrid nature of the plants.

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