Tolerance of dividing cells to replication stress in UVB-irradiated Arabidopsis roots: requirements for DNA translesion polymerases eta and zeta.

In tissues of multicellular organisms, DNA lesions that block replication can disrupt division of the transiently amplifying (TA) cells and stem cells that drive growth. To study how tissue growth is maintained despite DNA damage, stem cells and other cell types must be clearly identifiable. In plants, root growth depends directly on cell divisions in the root meristem. In Arabidopsis thaliana, cell identities in root meristems are unambiguously defined by position relative to the quiescent center and are readily visualized by microscopy. We evaluated roles of two DNA translesion polymerases, AtPoleta (Eta) and AtPolzeta (Zeta), in resistance of dividing root cells to a model genotoxin, UVB-radiation. The major UV photoproducts in DNA, cyclobutane pyrimidine dimers (CPDs), were induced to roughly 0.03CPD/kb by a threshold dose (0.28 kJ m(-2)) that minimally affected wild-type roots. In roots lacking AtPoleta and/or AtPolzeta, this dose inhibited cell division and tissue growth and specifically killed stem cells; severities of all three phenotypes increased in the order eta-<zeta-<eta-zeta-. One to 2 days after CPDs had disappeared from eta-zeta- roots, TA cell pools were depleted and there were novel cell divisions in the quiescent center. This delayed "secondary" response to genotoxic stress may reflect changes in the balance of proliferation and differentiation signals. In eta-zeta- roots, death of stem cells was substantial even in the absence of irradiation. The lethality of Polzeta ablation in mice had confined most previous analyses of Polzeta (and concomitant Poleta) function to unicellular (yeast) and chicken-cell culture models, so these studies illustrate the advantages afforded by the Arabidopsis-root model system in studies of growth and development of multicellular tissues.