Structure and Maintenance of Chromosome Ends in Plants

et us open this chapter by answering two basic questions: which features distinguish theterminal parts of plant chromosomes from their internal regions, and what is specialabout plant chromosome ends with respect to those in other eukaryotes?Like the rest of the chromosome, its terminal regions consist of DNA complexed withvarious proteins. The fact that a vast majority or maybe all genome functions occur at thedynamic supramolecular nucleoprotein complex structures generally termed chromatin isoften ignored and particular processes are simply attributed to particular DNA sequences orproteins. Current progress in telomere biology gives a good example of the necessity of acomplex view covering at least several main players in the telomere field: DNA sequences oftelomeres and of adjacent subtelomeric regions are associated in vivo with histones and variousnon-histone and telomere-specific proteins. Further, the most common tool for telomeremaintenance, telomerase, functions as a ribonucleoprotein complex. The remarkable structuralflexibility of telomeres shown mainly by in vitro and in situ experiments, the potential to adoptvarious kinds of local structures formed by 1 to 4 DNA strands stabilized by specific proteins,would suggest a higher degree of structural polymorphism of these apparently monotonous“non-coding” DNA domains. Of course it remains questionable whether one could not find asimilar level of structural polymorphism in internal chromosome loci if they would be exposedto comparably focused research. Nevertheless, representing the natural physical ends ofchromosomes which have to be recognized, treated and maintained in a way different fromchromosome breaks, telomeres can be regarded as true specific chromosome functional units.Compared to animal chromosomes, repetitive sequences commonly constitute a considerablylarger fraction of plant chromosomes, suggesting that even chromosome regions formedpredominantly by repeated DNA sequences like centromeres, telomeres and subtelomeres, couldbe more expanded on plant chromosomes. Further, contrary to the situation in most otherhigher eukaryotes, no true germline is set aside in plants in early embryogenesis, and bothvegetative and generative parts are derived from meristems during growth and differentiation,implying that any change in the nuclear genome of meristem cells that has occurred duringplant life, including telomere shortening, can be transmitted to sexual progeny.