First published online February 24, 2009; 10.1105/tpc.109.210212
The Plant Cell 21:365
Functions of DNA Polymerase 
Nancy A. Eckardt
Senior Features Editor
neckardt{at}aspb.org
Eukaryotes contain multiple DNA polymerases which ensure the accurate and efficient replication of the genome as well as protection and repair from endogenous and environmental DNA damaging agents. In most eukaryotes, the main replicative enzymes are DNA polymerases  ,  , and  (reviewed in Garcia-Diaz and Bebenek, 2007 ). In yeast, DNA polymerase (DNA pol ) initiates synthesis on the lagging strand. The bulk of chain elongation proceeds via the activities of DNA pol and , which appear to function principally on the lagging and leading strands, respectively (Pursell et al., 2007). Some functions of the DNA polymerases likely are conserved across eukaryotes, but the functions of the plant enzymes have not been well characterized.
DNA polymerase (Pol ) in yeast consists of a single catalytic and three regulatory subunits, and this structure appears to be conserved in other organisms. There are two genes encoding the Pol catalytic subunit in Arabidopsis, Pol2a and Pol2b, and single genes encoding each of the regulatory subunits. The catalytic subunit Pol2A and the regulatory subunit DPB2 in Arabidopsis are known to be essential for early embryogenesis (Ronceret et al., 2005). Pol2a appears to encode the main catalytic subunit active during embryogenesis and in meristems, whereas Pol2b is expressed mainly under stress conditions and null mutations produce no noticeable phenotype. Arabidopsis mutants with partial loss of function of Pol2a (homozygous weak alleles) have longer cell cycles, larger cells, and delayed development of embryos, emphasizing the critical role of this subunit in proper cell cycle progression and patterning in the embryo (Jenik et al., 2005).
Now, Yin et al. (pages 386–402) have identified a nonlethal mutation of Pol2a in Arabidopsis that causes an ABA overly sensitive phenotype, suggesting a link between Pol activity and ABA signaling. In addition, the mutant showed a dramatic 60-fold increase in the frequency of somatic homologous recombination (HR; see figure
), enhanced sensitivity to DNA-damaging agents, activation of silenced loci, and early flowering, indicating a potential role for Pol in HR and the maintenance of epigenetic states.
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The Arabidopsis Pol2a mutant allele abo4-1 increases frequency of HR. Seedlings were transformed with a marker for HR (1415) that produces visible spots on leaves. (Adapted from Figure 4 of Yin et al. [2009].)
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The authors show that the new allele of Pol2a, called abo4-1, releases transcriptional gene silencing at tested loci without affecting DNA methylation. They further show that an early flowering phenotype is associated with reduced expression of FLC and increased expression of FT concurrent with altered histone H3 modifications at these loci. The results suggest a role for Pol in ABA-dependent effects on genome stability, epigenetic states of chromatin, and chromatin-mediated gene expression in plants.
The link to HR is intriguing, although further studies are needed to show if Pol2a plays a direct or indirect role. It was shown that mutations in Pol2a lead to lengthening of the S-phase of DNA replication (Jenik et al., 2005; Yin et al., 2009), and Yin et al. suggest that this leads to increased formation of double-strand breaks, activation of cell cycle checkpoints, and the induction of HR. This work significantly extends our knowledge of DNA Pol activity in plants and suggests exciting new avenues for future research.
Footnotes
www.plantcell.org/cgi/doi/10.1105/tpc.109.210212
REFERENCES
Garcia-Diaz, M., and Bebenek, K. (2007). Multiple functions of DNA polymerases. Crit. Rev. Plant Sci. 26: 105–122.[CrossRef][Medline] Jenik, P.D., Jurkuta, R.E., and Barton, M.K. (2005). Interactions between the cell cycle and embryonic patterning in Arabidopsis uncovered by a mutation in DNA polymerase epsilon. Plant Cell 17: 3362–3377.[Abstract/Free Full Text] Pursell, Z.F., Isoz, I., Lundstrom, E.B., Johansson, E., and Kunkel, T.A. (2007). Yeast DNA polymerase epsilon participates in leading-strand DNA replication. Science 317: 127–130.[Abstract/Free Full Text] Ronceret, A., Guilleminot, J., Lincker, F., Gadea-Vacas, J., Delorme, V., Bechtold, N., Pelletier, G., Delseny, M., Chaboute, M.E., and Devic, M. (2005). Genetic analysis of two Arabidopsis DNA polymerase epsilon subunits during early embryogenesis. Plant J. 44: 223–236.[CrossRef][Web of Science][Medline] Yin, H., Zhang, X., Liu, J., Wang, Y., He, J., Yang, T., Hong, X., Yang, Q., and Gong, Z. (2009). Epigenetic regulation, somatic homologous recombination, and abscisic acid signaling are influenced by DNA polymerase mutation in Arabidopsis. Plant Cell 21: 386–402.[Abstract/Free Full Text]
Related articles in Plant Cell:
- Epigenetic Regulation, Somatic Homologous Recombination, and Abscisic Acid Signaling Are Influenced by DNA Polymerase
 Mutation in Arabidopsis
- Haibo Yin, Xia Zhang, Jun Liu, Youqun Wang, Junna He, Tao Yang, Xuhui Hong, Qing Yang, and Zhizhong Gong
Plant Cell 2009 21: 386-402.
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