Spatiotemporally dynamic, cell-type–dependent premeiotic and meiotic phasiRNAs in maize anthers

Abstract

Maize 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 lowcopy, 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. phasiRNAs | anther development | piRNAs | tapetum | tasiRNAs D iverse small RNAs exist in male reproductive cells of animals and plants. In animals, PIWI proteins, a subfamily of the ARGONAUTEs, and their interacting piRNAs are required for spermatogenesis; mutants defective for the PIWI-encoding genes fail to produce mature sperm (1-3). Whereas most Drosophila piRNAs are repeat-derived and function to silence transposable elements (TEs) (4, 5), mammalian piRNAs predominantly map to unique intergenic regions and have unclear but essential roles during gonad development. Based on their expression timing, different sizes, and distinctive PIWI partners, mammalian piRNAs are further classified as prepachytene or pachytene (6). Given the continuum of developmental stages in the testes, the prepachytene class is characteristic of gonads in which no cells have reached pachytene, whereas the pachytene-associated small RNAs are characteristic of gonads in which the most advanced germ line cells have reached this meiotic stage and all prior stages are also present in the more immature zone of the gonad.In flowering plants, the anther is equivalent to the mammalian testes in that it consists of multiple somatic cell types required to support the premeiotic, meiotic, and postmeiotic haploid cells. In contrast to the continuum of mammalian gonads, however, an entire anther progres...

RNAi in Plants: An Argonaute-Centered View

“…AGO18b is predominantly expressed in the tapetal and meiotic cells , which matches the distribution and timing of 24-nucleotide meiotic phasiRNAs (Zhai et al, 2015a). This implies that AGO18b might be an effector protein for the 24-nucleotide phasiRNAs.…”

MS23, a master basic helix-loop-helix factor, regulates the specification and development of tapetum in maize

“…4B). By RNA-seq, Ms23 transcript was detectable in anthers before Stage 4 at 200 and 400 μm (Zhai et al, 2015), and the levels rose steadily, peaking around Stage 10, after which it gradually decreased.…”

On plant defense signaling networks and early land plant evolution

“…miR482/2118 is a direct regulator of resistance to a diverse range of pathogens in dicots [9,98,101-103]. In monocots miR482/2118 is expressed in reproductive tissue and may function in its development [104]. Given the expression patterns of miR482/2118 in P. abies , with some members of this family solely expressed in reproductive organs [100], a broader function in the regulation of both reproductive organ development and disease resistance seems to be the more ancient mechanism.…”