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-AB. These results are consistent with PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19639295 our cytological analysis and indicate that telomere protection is intact in Peretinoin site plants lacking PARPs. Alternatively, if PARPs promote telomere 7 PARP Function at Plant Telomeres To investigate whether PARPs act in concert with telomerase to promote telomere stability, TF-PCR was conducted with 3-ABtreated wild type and G4 tert seedlings. A background level of TFPCR products was observed with one of the wild type seedling samples, while abundant products were evident in two G4 tert mutants. One of the 3-AB-treated G4 tert samples displayed telomere fusions, but their abundance was comparable to untreated G4 tert. Moreover, two other 3-AB-treated G4 tert seedling samples showed no evidence of telomere fusion at all. This result is likely to reflect the stochastic onset of telomere dysfunction among individual tert mutant seedlings. End-toend chromosome fusions are detected when telomeres reach a critical length of about 1 kb. At least a subset of telomeres in these plants are above this threshold. We cannot rule out the possibility that PARPs promote some degree of chromosome end-joining reactions in Arabidopsis. Recently, A. thaliana PARP1 and PARP2 were shown to be involved in microhomology-mediated end-joining, a function similar to human PARPs in the alternative pathway for NHEJ. Although, multiple mechanisms have been implicated in the fusion of dysfunctional telomeres, the canonical NHEJ pathway is intact in this setting and thus it is unlikely that PARPs play a significant role in telomere fusion. PARPs do not Make a Significant Contribution to Telomere Length Maintenance in Arabidopsis Our negative results for TF-PCR do not preclude the possibility that telomeres were modestly shortened in PARP mutants. To test this possibility, we monitored bulk telomere length PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19640586 in parp1 and parp2 single mutants and several double mutants using terminal protection in a subset of cells, such as in the root meristem, the phenotype could have been diluted out in our assays, which used whole seedlings or flowers. PARP Function at Plant Telomeres restriction fragment analysis. Telomeres in these plants, including the wild type control, were within the typical size range for the Col-0 ecotype of A. thaliana, 25 kb. We detected some variability in telomere length between plants doubly deficient for PARP1 and PARP2. While four of the double mutants we analyzed had telomere lengths similar to wild type and the parp1 and parp2 single mutants, the telomere tracts for one individual double mutant plant were shorter. To obtain another measure of telomere length, we used Primer Extension Telomere Length Amplification to examine the telomere length distribution on four different chromosome arms. As with TRF analysis, PETRA showed that telomeres in single parp1 or parp2 mutants did not differ perceptively from wild type. The same result was obtained for double mutants as well as 3-AB-treated plants. Finally, to determine if PARPs contribute to telomere length maintenance in the absence of TERT, PETRA was conducted on G4 tert mutants in the presence or absence of 3-AB. Telomeres in tert seedlings treated with 3-AB showed no significant size difference from the corresponding DMSO controls. Thus, while we cannot rule out a role for PARPs in modulating telomere length in A. thaliana, our findings argue that any contribution is likely to be very modest. Discussion Our investigation of Arabidopsis PARPs adds to the growing body of res