Functional Analysis of Maize RAD51 in Meiosis and Double-Strand Break Repair

Abstract

In Saccharomyces cerevisiae, Rad51p plays a central role in homologous recombination and the repair of double-strand breaks (DSBs). Double mutants of the two Zea mays L. (maize) rad51 homologs are viable and develop well under normal conditions, but are male sterile and have substantially reduced seed set. Light microscopic analyses of male meiosis in these plants reveal reduced homologous pairing, synapsis of nonhomologous chromosomes, reduced bivalents at diakinesis, numerous chromosome breaks at anaphase I, and that .33% of quartets carry cells that either lack an organized nucleolus or have two nucleoli. This indicates that RAD51 is required for efficient chromosome pairing and its absence results in nonhomologous pairing and synapsis. These phenotypes differ from those of an Arabidopsis rad51 mutant that exhibits completely disrupted chromosome pairing and synapsis during meiosis. Unexpectedly, surviving female gametes produced by maize rad51 double mutants are euploid and exhibit near-normal rates of meiotic crossovers. The finding that maize rad51 double mutant embryos are extremely susceptible to radiation-induced DSBs demonstrates a conserved role for RAD51 in the repair of mitotic DSBs in plants, vertebrates, and yeast.

Role of RAD51 in the Repair ofMuDR-Induced Double-Strand Breaks in Maize (Zea maysL.)

“…This model predicts that in the absence of HR, Mu-insertion alleles will give rise to high rates of germinal deletions caused by alternative, error-prone DSBs repair pathways. To test this model, the rates at which germinal deletions arose from a1-m5216 in stocks with or without RAD51 (Li et al 2007), which is central to HR, were compared via two crossing strategies (crosses 4-7, materials and methods).…”

“…The maize genome contains only two rad51 genes (Franklin et al 1999;Li et al 2007). Deletion derivatives (rad51A1-54F11d1 and rad51A2-98E7d4) of both of these genes were isolated as described (Li et al 2007).…”

“…Deletion derivatives (rad51A1-54F11d1 and rad51A2-98E7d4) of both of these genes were isolated as described (Li et al 2007). Plants that are homozygous for both of these mutant alleles are referred to as RAD51 À ; all other genotypes with functional RAD51 are referred to as RAD51…”

“…RAD51, the RecA homolog in eukaryotes, plays a central role in the HR pathway (Baumann and West 1998;Thacker 1999), including that of maize (Franklin et al 1999;Li et al 2007). We have compared the behavior of a MuDR transposon in RAD51 1 and RAD51 À maize plants.…”

Pairing in plants: Import is important

“…The NBS1 protein regulates MRE11 and attracts other proteins to the double-strand break. RAD51 and DMC1 associate with the single-stranded overhangs and facilitate the invasion of the complementary sequences on the homologous chromosome (12,13). The breaks are repaired to produce cross-overs between homologous pairs or gene conversions.…”