Plant cell suspension cultures sustain long-term replication of potato spindle tuber viroid

Cell suspension cultures were established from tomato plants (Lycopersicum esculentum cv. "Rutgers") infected with either a severe (TPS cell line) or a mild (TPM cell line) strain of potato spindle tuber viroid (PSTV) and from uninfected plants (TH cell line). Based on measurements of packed cell volume, the TPS line exhibited a slower growth rate than the TH line, and reached lower levels of total cell volume during the stationary phase. The TPM line was intermediate between the other two. All the lines were highly polyploid (over 72 chromosomes). PSTV was detected consistently by electrophoretic analysis in both the TPS and TPM lines after more than 11/2 years of subculturing. Newly synthesized PSTV in actively dividing cells was detectable after 1 hr of [3H]uridine labeling (ca. 0.1% of the radioactivity incorporated into total soluble RNA) and its proportion increased in subsequent samplings up to a steady state level of 0.24-0.28 or 0.31-0.39% for TPM and TPS, respectively. Analysis on two-dimensional gels of proteins synthesized in the TH and TPS cell lines following incorporation of radiolabeled amino acids detected neither quantitative nor qualitative changes resulting from maintenance of viroid in the cell line.