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The frequent mutations recovered recently from the pollen of select maize lines resulted from the meiotic mobilization of specific low-copy number long-terminal repeat (LTR) retrotransposons, which differ among lines. Mutations that arise at male meiosis produce kernels with concordant mutant phenotypes in both endosperm and embryo because the two sperms that participate in double fertilization are genetically identical. Those are in a majority. However, a small minority of kernels with a mutant endosperm carry a nonconcordant normal embryo, pointing to a postmeiotic or microgametophytic origin. In this study, we have identified the basis for those nonconcordant mutations. We find that all are produced by transposition of a defective LTR retrotransposon that we have termeddRemp(defective retroelement mobile in pollen). This element has several unique properties. Unlike the mutagenic LTR retrotransposons identified previously,dRempis present in hundreds of copies in all sequenced lines. It seems to transpose only at the second pollen mitosis because alldRempinsertion mutants are nonconcordant yet recoverable in either the endosperm or the embryo. Although it does not move in most lines,dRempis highly mobile in the Corn Belt inbred M14, identified earlier by breeders as being highly unstable. Lastly, it can be recovered in an array of structures, ranging from solo LTRs to tandemdRemprepeats containing several internal LTRs, suggestive of extensive recombination during retrotransposition. These results shed further light on the spontaneous mutation process and on the possible basis for inbred instability in maize.

 

Maintaining sufficient water transport during flowering is essential for proper organ growth, fertilization, and yield. Water deficits that coincide with flowering result in leaf wilting, necrosis, tassel browning, and sterility, a stress condition known as “tassel blasting.” We identified a mutant, necrotic upper tips1 ( nut1 ), that mimics tassel blasting and drought stress and reveals the genetic mechanisms underlying these processes. The nut1 phenotype is evident only after the floral transition, and the mutants have difficulty moving water as shown by dye uptake and movement assays. These defects are correlated with reduced protoxylem vessel thickness that indirectly affects metaxylem cell wall integrity and function in the mutant. nut1 is caused by an Ac transposon insertion into the coding region of a unique NAC transcription factor within the VND clade of Arabidopsis . NUT1 localizes to the developing protoxylem of root, stem, and leaf sheath, but not metaxylem, and its expression is induced by flowering. NUT1 downstream target genes function in cell wall biosynthesis, apoptosis, and maintenance of xylem cell wall thickness and strength. These results show that maintaining protoxylem vessel integrity during periods of high water movement requires the expression of specialized, dynamically regulated transcription factors within the vasculature. 

While studying spontaneous mutations at the maizebronze(bz) locus, we made the unexpected discovery that specific low-copy number retrotransposons are mobile in the pollen of some maize lines, but not of others. We conducted large-scale genetic experiments to isolate newbzmutations from severalBzstocks and recovered spontaneous stable mutations only in the pollen parent in reciprocal crosses. Most of the new stablebzmutations resulted from either insertions of low-copy number long terminal repeat (LTR) retrotransposons or deletions, the same two classes of mutations that predominated in a collection of spontaneouswxmutations [Wessler S (1997)The Mutants of Maize, pp 385–386]. Similar mutations were recovered at the closely linkedshlocus. These events occurred with a frequency of 2–4 × 10⁻⁵in two lines derived from W22 and in 4Co63, but not at all in B73 or Mo17, two inbreds widely represented in Corn Belt hybrids. Surprisingly, the mutagenic LTR retrotransposons differed in the active lines, suggesting differences in the autonomous element make-up of the lines studied. Some active retrotransposons, likeHopscotch,Magellan, andBs2, aBs1variant, were described previously; others, likeFotoandFocouin 4Co63, were not. By high-throughput sequencing of retrotransposon junctions, we established that retrotranposition ofHopscotch,Magellan, andBs2occurs genome-wide in the pollen of active lines, but not in the female germline or in somatic tissues. We discuss here the implications of these results, which shed light on the source, frequency, and nature of spontaneous mutations in maize.

 

The unusual eukaryoticHelitrontransposons can readily capture host sequences and are, thus, evolutionarily important. They are presumed to amplify by rolling‐circle replication (RCR) because some elements encode predicted proteins homologous toRCRprokaryotic transposases. In support of this replication mechanism, it was recently shown that transposition of a batHelitrongenerates covalently closed circular intermediates. Another strong prediction is thatRCRshould generate tandemHelitronconcatemers, yet almost allHelitronsidentified to date occur as solo elements in the genome. To investigate alternative modes ofHelitronorganization in present‐day genomes, we have applied the novel computational tool HelitronScanner to 27 plant genomes and have uncovered numerous tandem arrays of partially decayed, truncatedHelitronsin all of them. Strikingly, most of theseHelitrontandem arrays are interspersed with other repeats in centromeres. Many of these arrays have multipleHelitron5′ ends, but a single 3′ end. The number of repeats in any one array can range from a handful to several hundreds. We propose here anRCRmodel that conforms to the presentHelitronlandscape of plant genomes. Our study provides strong evidence that plantHelitronsamplify byRCRand that the tandemly arrayed replication products accumulate mostly in centromeres.