Component of the CST complex, a complex that binds to single-stranded DNA and is required to protect telomeres from DNA degradation. The CST complex binds single-stranded DNA with high affinity in a sequence-independent manner, while isolated subunits bind DNA with low affinity by themselves (PubMed:19854131). Associates with enzymatically active telomerase (PubMed:25329641). Component of the CST complex, composed of CTC1, TEN1 and STN1. Interacts with POT1A (PubMed:25329641). Severe telomere deprotection accompanied by a rapid onset of developmental defects and sterility. The large majority of plants have grossly distorted floral phyllotaxy with an irregular branching pattern and fasciated (thick and broad) main and lateral stems and siliques. Although most mutants produce an influorescence bolt, this structure is highly variable in size, ranging from very short to wild-type. Flowers and siliques are often fused, and seed yield is typically reduced to 10% of wild-type. The germination efficiency of the few seeds that could be recovered was extremely low. Telomeric and subtelomeric tracts are dramatically eroded, and chromosome ends exhibit increased G overhangs, recombination, and end-to-end fusions. Belongs to the CTC1 family.