Transcriptomes and Proteomes Define Gene Expression Progression in Pre-meiotic Maize Anthers

Abstract

Plants lack a germ line; consequently, during reproduction adult somatic cells within flowers must switch from mitotic proliferation to meiosis. In maize (Zea mays L.) anthers, hypoxic conditions in the developing tassel trigger pre-meiotic competence in the column of pluripotent progenitor cells in the center of anther lobes, and within 24 hr these newly specified germinal cells have patterned their surrounding neighbors to differentiate as the first somatic niche cells. Transcriptomes were analyzed by microarray hybridization in carefully staged whole anthers during initial specification events, after the separation of germinal and somatic lineages, during the subsequent rapid mitotic proliferation phase, and during final pre-meiotic germinal and somatic cell differentiation. Maize anthers exhibit a highly complex transcriptome constituting nearly three-quarters of annotated maize genes, and expression patterns are dynamic. Laser microdissection was applied to begin assigning transcripts to tissue and cell types and for comparison to transcriptomes of mutants defective in cell fate specification. Whole anther proteomes were analyzed at three developmental stages by mass spectrometric peptide sequencing using size-fractionated proteins to evaluate the timing of protein accumulation relative to transcript abundance. New insights include early and sustained expression of meiosis-associated genes (77.5% of well-annotated meiosis genes are constitutively active in 0.15 mm anthers), an extremely large change in transcript abundances and types a few days before meiosis (including a class of 1340 transcripts absent specifically at 0.4 mm), and the relative disparity between transcript abundance and protein abundance at any one developmental stage (based on 1303 protein-to-transcript comparisons).

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

“…S6) (30). Some AGO members are specifically expressed in flowers (29,30) and are further enriched in either somatic or germinal cells of anthers (19,31,32). Presumably, this AGO expansion reflects a functional diversification of plant small RNAs for roles specific to anther developmental stages and cell types.…”

“…The binding partner of meiotic phasiRNAs has not yet been reported. Based on transcriptional analysis of lasermicrodissected cell types (19,32), plus the RNA-seq and microarray profiling of different anther stages (Dataset S6), we found that the expression profile of maize AGO18b matches the expression timing of meiotic phasiRNAs. Both AGO18b transcripts and proteins are enriched in the tapetal and meiotic cells (29).…”

“…Anthers lack an organizing center, in contrast to the meristem regions of shoots and roots. Meristems organize a continuum of developmental stages displaced from the stem cell population, whereas anthers "self-organize" tissue layers and the entire organ progresses through development as one unit with high fidelity and temporal regularity (15,18,32). The potential movement of phasiRNAs from the site of biogenesis to neighboring cell layers (Fig.…”

The Genetics of Sex: Exploring Differences

“…They posit that these fundamental differences in meiotic processes evolved to accommodate hemizygosity of the X chromosome. Zhang et al (2014) examined the transcriptomes and proteomes during the differentiation of maize anthers, a model system well suited for this study because of the relative ease of collecting tissues at different developmental stages. In angiosperms, the germline arises from somatic flower cells through a switch from mitotic proliferation to meiosis.…”

“…Two other articles examine differences in the evolution of sex chromosomes: Blackmon and Demuth (2014) look at the evolutionary turnover of sex chromosomes in beetles and Kirkpatrick and Guerrero (2014) use the recombining region of sex chromosomes to measure the strength of sexually antagonistic selection. Finally, two studies focus on gametogenesis-sex differences in meiosis in Caenorhabditis elegans (Checchi et al 2014) and the specification and development of germinal cells in maize (Zhang et al 2014).…”