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Infectivity studies with highly infectious RNA inocula generated by ribozyme cleavage were used to compare the biological properties of three apparently nonviable mutants of potato spindle tuber viroid (PSTVd). One of these mutants (PSTVd-P) contains three nucleotide substitutions in the left terminal loop, and mechanical inoculation of tomato seedlings with RNA transcripts at levels equivalent to 10 3 -10 5 times the ID 50 for PSTVd-Intermediate failed to result in systemic infection. Viable progeny containing a spontaneous C G change at position 4 could, however, be recovered from transgenic Nicotiana benthamiana plants that constitutively expressed PSTVd-P RNA. The initial mutations in

Anthers from three sets of wheat (Triticum aestivum L. em. Thell) F1's and their reciprocal crosses, made between parental lines differing greatly in their ability to produce microspore derived callus, were cultured on the Chinese potato medium so that we could 1) more clearly define the role of nuclear or cytoplasmic factors within T. aestivum in transferring the ability to undergo in vitro androgenesis, and 2) to briefly review the gametic representation and disease screening potential of the resulting polyhaploid wheat plants. The microspore derived calli values from F1's were slightly less than the midparental value. Statistical analysis indicated that the ability of each F1 to produce callus either did not significantly differ from that of the respective parental line having the highest androgenic yield or it exceeded its respective parental line having the lowest yield. No differences were noted between the members of each pair of reciprocal crosses. The results indicate that the transfer of in vitro androgenic ability to F1 hybrids is not dependent upon the maternal cytoplasm source. Polyhaploid plants, carrying the Pm 3 a powdery mildew resistance gene, expressed resistance to culture 4 a of powdery mildew.

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