Disruption of the telomerase catalytic subunit gene from Arabidopsis inactivates telomerase and leads to a slow loss of telomeric DNA

Telomerase is an essential enzyme that maintains telomeres on eukaryotic ch romosomes. In mammals, telomerase is required for the lifelong proliferativ e capacity of normal regenerative and reproductive tissues and for sustaine d growth in a dedifferentiated state. Although the importance of telomeres was first elucidated in plants 60 years ago, little is known about the role of telomeres and telomerase in plant growth and development. Here we repor t the cloning and characterization of the Arabidopsis telomerase reverse tr anscriptase (TERT) gene, AtTERT, AtTERT is predicted to encode a highly bas ic protein of 131 kDa that harbors the reverse transcriptase and telomerase -specific motifs common to all known TERT proteins, AtTERT mRNA is 10-20 ti mes more abundant in callus, which has high levels of telomerase activity, versus leaves, which contain no detectable telomerase. Plants homozygous fo r a transfer DNA insertion into the AtTERT gene lack telomerase activity, c onfirming the identity and function of this gene. Because telomeres in wild -type Arabidopsis are short, the discovery that telomerase-null plants are viable for at least two generations was unexpected. In the absence of telom erase, telomeres decline by approximately 500 bp per generation, a rate 10 times slower than seen in telomerase-deficient mice. This gradual loss of t elomeric DNA may reflect a reduced rate of nucleotide depletion per round o f DNA replication, or the requirement for fewer cell divisions per organism al generation. Nevertheless, progressive telomere shortening in the mutants , however slow, ultimately should be lethal.