Abstract
Telomeres are specialized nucleoprotein complexes that are essential for preserving chromosome integrity in eukaryotic cells. Several potential telomere binding proteins have recently been identified in higher plants, but nothing is known about their in vivo functions. We previously identified NgTRF1 as a double-stranded telomeric repeat binding factor in tobacco (Nicotiana tabacum) and here show that the binding of NgTRF1 to telomeric repeats inhibits telomerase-mediated telomere extension. To determine whether NgTRF1 is involved in telomere length regulation, we established transgenic tobacco BY-2 cell lines that overexpress or suppress NgTRF1. Pulsed-field gel electrophoresis showed that 35S::NgTRF1 cells exhibited significantly shortened telomeres (45 to 10 kb), whereas 35S::antisense-NgTRF1 cells contained longer telomeres (80 to 25 kb) compared with wild-type and 35S::GUS control cells (65 to 15 kb), indicating that telomere length inversely correlates with the amount of functional NgTRF1 in BY-2 cells. 35S::NgTRF1 cells with shorter telomeres displayed a progressive reduction in cell viability and stopped dividing after 25 to 40 successive rounds of 12-d batch subculture, in sharp contrast with control cells, which have an unlimited capacity for division. Internucleosomal DNA fragmentation, mitochondrial release of cytochrome c, and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling positive nuclei were detected in 35S::NgTRF1 cells during prolonged subculture, indicating that enhanced cell death was attributable to an apoptosis-like mechanism. 35S::antisense-NgTRF1 cells containing low levels of NgTRF1 also exhibited a progressive decrease in cell viability and apoptotic cell death, but less so than did 35S::NgTRF1 cells, suggesting that the level of NgTRF1 is critically associated with cell viability. Taken together, these data indicate that perturbation of NgTRF1 expression results in changes in telomere length and stability, which in turn causes apoptotic cell death in transgenic BY-2 cells. These results are discussed in light of the suggestion that NgTRF1 is involved in the mechanism by which telomere length and stability are maintained. We further suggest that the structural stability of telomeres, in addition to length maintenance, is essential for their function and for the immortality of BY-2 cells.