Spatiotemporally dynamic, cell-type–dependent premeiotic and meiotic phasiRNAs in maize anthers
Saturday, 2015/03/14 | 07:04:31
|
SignificanceBy RNA profiling of 10 stages of maize anthers plus mature pollen, we found two distinct classes of phased small-interfering RNAs (phasiRNAs): 21-nt premeiotic phasiRNAs, after germinal and somatic cell specification, and 24-nt meiotic phasiRNAs coordinately accumulated during meiosis and persist into pollen. Sequencing of RNA from five male-sterile, anther developmental mutants—ocl4, mac1, ms23, msca1, and ameiotic1—demonstrated the involvement of specific somatic layers. Premeiotic phasiRNAs require a functional epidermis, whereas meiotic phasiRNAs require a normal tapetum. Mammalian germ cells express “prepachytene” or “pachytene” PIWI-interacting RNAs (piRNAs). Whereas differences in biogenesis indicate independent origins, grass phasiRNAs and mammalian piRNAs share developmental timing, a lack of obvious targets, and an impact on male fertility, suggesting a possible evolutionary convergence. AbstractMaize anthers, the male reproductive floral organs, express two classes of phased small-interfering RNAs (phasiRNAs). PhasiRNA precursors are transcribed by RNA polymerase II and map to low-copy, intergenic regions similar to PIWI-interacting RNAs (piRNAs) in mammalian testis. From 10 sequential cohorts of staged maize anthers plus mature pollen we find that 21-nt phased siRNAs from 463 loci appear abruptly after germinal and initial somatic cell fate specification and then diminish, whereas 24-nt phasiRNAs from 176 loci coordinately accumulate during meiosis and persist as anther somatic cells mature and haploid gametophytes differentiate into pollen. Male-sterile ocl4 anthers defective in epidermal signaling lack 21-nt phasiRNAs. Male-sterile mutants with subepidermal defects—mac1 (excess meiocytes), ms23 (defective pretapetal cells), and msca1 (no normal soma or meiocytes)—lack 24-nt phasiRNAs. ameiotic1 mutants (normal soma, no meiosis) accumulate both 21-nt and 24-nt phasiRNAs, ruling out meiotic cells as a source or regulator of phasiRNA biogenesis. By in situ hybridization, miR2118 triggers of 21-nt phasiRNA biogenesis localize to epidermis; however, 21-PHAS precursors and 21-nt phasiRNAs are abundant subepidermally. The miR2275 trigger, 24-PHAS precursors, and 24-nt phasiRNAs all accumulate preferentially in tapetum and meiocytes. Therefore, each phasiRNA type exhibits independent spatiotemporal regulation with 21-nt premeiotic phasiRNAs dependent on epidermal and 24-nt meiotic phasiRNAs dependent on tapetal cell differentiation. Maize phasiRNAs and mammalian piRNAs illustrate putative convergent evolution of small RNAs in male reproduction.
See: http://www.pnas.org/content/112/10/3146.abstract.html?etoc PNAS March 10, 2015 vol. 112 no. 10 3146-3151
Fig. 2. The 21-nt premeiotic and 24-nt meiotic phasiRNAs are developmentally regulated. (A) Anthers at 10 different lengths (developmental stages) were analyzed plus pollen. Above, a schematic of cell patterns in a single lobe of the anther; cell types in colors shown in key (Right). (B) Heat maps depicting the abundances of 21-nt premeiotic phasiRNAs from 463 loci (Left) and 24-nt meiotic phasiRNAs from 176 loci (Right) at each stage. Hierarchical clustering was based on similarity of expression pattern. (Left) Solid bubbles represent the total abundance of phasiRNAs; blue pie charts represent the proportion of 21-nt phasiRNAs from all 21-nt sRNAs at each stage; striped blue bubbles represent miR2118 abundances (trigger of premeiotic phasiRNA). (Right) The solid and striped orange bubbles represent 24-nt phasiRNA and miR2275 abundances, respectively; orange pie charts represent 24-nt phasiRNAs from all 24-nt sRNAs. Controls (Far Right): green represents TAS3-derived ta-siRNAs, and pink represents all TE-associated siRNAs mapped to the first 100 Mb of maize chromosome 1 (as a proxy for the whole genome). (C) Quantification of 21-PHAS and 24-PHAS precursor transcripts by RNA-seq. |
![]() ![]() ![]() |
[ Other News ]___________________________________________________
|