Transplantation and subsequent behavior of mitochondria in cells of Phytophthora

A new variant produced by Rhizoctonia solani AG1-IC isolate CH-1 with a new type of nuclei

BACKGROUND

Isolate CH-1 of Rhizoctonia solani Kühn was commonly used in our studies of the pathogenicity and genetics of this pathogen. During the preparation of homokaryons through protoplast regeneration and tuft formation, a defective homokaryon was detected and a new variant was obtained.

RESULTS

When tuft formation was used to identify the karyotic nature of single protoplast regenerants (SPRs) of Rhizoctonia solani AG1-IC isolate CH-1, one homokaryon type designated as A type and the parental heterokaryon designated as AB type were obtained. The homokaryon B type was not found. Various approaches were used to obtain SPRs, including from fast or slow growing protoplast regenerants, and from regenerants of protoplasts released from mycelia grown in different nutrient broths or at different temperatures. Without exception, all these SPRs were either homokaryon A or heterokaryon AB. Moreover, the SPRs obtained from different generations of SPRs, and from different generations of SPRs treated with lytic enzymes 3 to 4 times also were invariably either homokaryon A or heterokaryon AB. When single hyphal isolates were obtained from the tuft resulting from the pairing between homokaryon A and heterokaryon AB, only the heterokaryon and a variant were obtained. The variant did not form tuft when paired with parental heterokaryon AB or homokaryon A. Its protoplast regenerants gave rise to heterokaryon AB, homokaryon A and the variant, indicating that it is a new kind of heterokaryon.

CONCLUSION

Inability to obtain homokaryon B despite numerous attempts suggests that the B type nuclei are probably defective and are dependent on A type nuclei for their multiplication. This is the first report of a heterokaryotic R. solani strain carrying a defective type of nuclei. A new variant which is a new kind of heterokaryon was obtained from the tuft resulting from the paring between the homokaryon A and the parental heterokaryon AB.

Factors affecting protoplast formation by Rhizoctonia solani

Novozym 234 was the most frequently used enzyme for production of Rhizoctonia solani protoplasts. Since manufacture of this enzyme was discontinued in the late 1990s, a new procedure was developed by testing lytic enzymes from Sigma and by examining factors affecting protoplast formation. The combination of 20 mg/mL Driselase and 10mg/mL lysing enzyme was effective in releasing protoplasts from R. solani. The optimal condition for enzyme treatment of mycelium was incubation at 37 degrees C for 15 min followed by 34 degrees C for 105 min. The amount of protoplasts produced was positively correlated with growth rate and negatively correlated with mycelial density. Under favorable conditions, R. solani mycelia released 1.68 x 10(6) protoplasts/mL that is comparable with that produced with Novozym 234. Among various media tested, the best solid medium for protoplast regeneration was 1% V-8 juice agar, while the best liquid medium was 10% potato dextrose broth.

Evidence for mitochondrial gene control of mating types in Phytophthora

When protoplasts carrying metalaxyl-resistant (Mr) nuclei from the A1 isolate of Phytophthora parasitica were fused with protoplasts carrying chloroneb-resistant (Cnr) nuclei from the A2 isolate of the same species, fusion products carrying Mr nuclei were either the A2 or A1A2 type, while those carrying Cnr nuclei were the A1, A2, or A1A2 type. Fusion products carrying Mr and Cnr nuclei also behaved as the A1, A2, or A1A2 type. The result refutes the hypothesis that mating types in Phytophthora are controlled by nuclear genes. When nuclei from the A1 isolate of P. parasitica were fused with protoplasts from the A2 isolate of the same species and vice versa, all of the nuclear hybrids expressed the mating type characteristics of the protoplast parent. The same was true when the nuclei from the A1 isolate of P. parasitica were fused with the protoplasts from the A0 isolate of Phytophthora capsici and vice versa. These results confirm the observation that mating type genes are not located in the nuclei and suggest the presence of mating type genes in the cytoplasms of the recipient protoplasts. When mitochondria from the A1 isolate of P. parasitica were fused with protoplasts from the A2 isolate of the same species, the mating type of three out of five regenerated protoplasts was changed to the A1 type. The result demonstrated the decisive effect of mitochondrial donor sexuality on mating type characteristics of mitochondrial hybrids and suggested the presence of mating type genes in mitochondria. All of the mitochondrial hybrids resulting from the transfer of mitochondria from the A0 isolate of P. capsici into protoplasts from the A1 isolate of P. parasitica were all of the A0 type. The result supports the hypothesis of the presence of mating type genes in mitochondria in Phytophthora.