Epigenetic silencing and unstable inheritance of MuDR activity monitored at four bz2-mu alleles in maize (Zea mays L.)

Abstract

Maize MuDR/Mu elements are one of the most active Class II transposons and are widely used for transposon tagging for gene cloning. The autonomous MuDR encodes a transposase, while diverse non-autonomous elements share similarity to MuDR only within their ~215 bp terminal inverted repeats (TIRs). Four independent Mu-induced mutable alleles of the anthocyanin pigment pathway Bronze2 (Bz2) locus have been sequenced; bz2-mu1, bz2-mu2, and bz2-mu3 contain Mu1 element insertions while bz2-mu4 contains a MuDR insertion. Somatic excision activity can be monitored for each allele as a purple spotted phenotype on the otherwise beige epidermal layer of the kernel. To study epigenetic silencing of Mu elements, we investigated inheritance of somatic transposition of these four reporter alleles, and using samples from leaves just below the ear and the tassel, DNA methylation status and mudrA expression were quantified through three consecutive generations. Percentages of spotted kernels when crossed into bz2 tester as pollen parent were lower than those when crossed with bz2 tester as ear parent in all bz2-mu alleles. The propensity for silencing in kernels with different frequencies of spotting was investigated. In the inactive lines, both MuDR and Mu1 elements were de novo methylated. The transposition frequency was negatively correlated with the level of Mu methylation and positively correlated with the level of mudrA transcript. The most reliable indicator of incipient silencing was a decrease in mudrA transcript levels in the leaf below the tassel, and this transcriptional silencing could precede methylation of Mu elements.

Mu killer-Mediated and Spontaneous Silencing of Zea mays Mutator Family Transposable Elements Define Distinctive Paths of Epigenetic Inactivation

“…The mutator system can silence spontaneously, and there is a very wide variation in this frequency and in the percentage of spotted kernels during progressive spontaneous silencing (Takumi and Walbot, 2007). Therefore, interpreting loss of the somatic kernel phenotype is problematic.…”

“…Although some individuals switch abruptly from an active to inactive status, in many cases loss of Mu element insertion and excision occur over a span of time. Silencing is progressive, both within the sequential organs of a developing plant (Martienssen et al, 1990) and over several generations (Takumi and Walbot, 2007). The trigger for silencing other than the correlation with high MuDR/Mu copy number has been difficult to define, although there are clearly separable steps for imposition and maintenance of silencing (Woodhouse et al, 2006).…”

Mutator transposon activity reprograms the transcriptomes and proteomes of developing maize anthers

“…Anthers have four locules each comprised of four concentric cell layers that surround the developing meiotic cells (Figure 2a). Using size as a guide for staging, mitotic (1.0 mm), pre‐meiotic (1.5 mm) and prophase‐I meiotic (2.0 mm) anthers (Figure 2b) were dissected from a Mutator active line and an epigenetically silenced (inactive) sister line (Takumi and Walbot, 2007) confirmed by RT‐PCR (Figure 1e). Transcriptome profiling experiments were performed on a 4 × 44K platform, with 42 034 60‐mer probes to 39 174 unique maize genes, representing about 80% of the current gene estimate based on a near‐complete genome (Walbot, 2008), and enriched for anther‐expressed genes (Ma et al.…”

Mutator and MULE transposons

“…In contrast to default methylation of non-autonomous elements and deletion derivatives, methylation of otherwise active MuDR elements is invariably associated with transcriptional gene silencing (Martienssen and Baron, 1994;Slotkin et al, 2003;Takumi and Walbot, 2007). This process is often progressive (Brown et al, 1994) and occurs preferentially in the tassel (Walbot, 1986).…”

“…This spontaneous silencing occurs preferentially in the tassel. In general however, once MuDR silencing begins in a given plant or lineage rarely reverses itself, and it is associated with the loss of MuDR expression (Hershberger et al, 1991;Takumi and Walbot, 2007).…”