Abstract
BackgroundDevelopmental cues to start meiosis occur late in plants. Ameiotic1 (Am1) encodes a plant-specific nuclear protein (AM1) required for meiotic entry and progression through early prophase I. Pollen mother cells (PMCs) remain mitotic in most am1 mutants including am1-489, while am1-praI permits meiotic entry but PMCs arrest at the leptotene/zygotene (L/Z) transition, defining the roles of AM1 protein in two distinct steps of meiosis. To gain more insights into the roles of AM1 in the transcriptional pre-meiotic and meiotic programs, we report here an in depth analysis of gene expression alterations in carefully staged anthers at 1 mm (meiotic entry) and 1.5 mm (L/Z) caused by each of these am1 alleles.Results1.0 mm and 1.5 mm anthers of am1-489 and am1-praI were profiled in comparison to fertile siblings on Agilent® 4 × 44 K microarrays. Both am1-489 and am1-praI anthers are cytologically normal at 1.0 mm and show moderate transcriptome alterations. At the 1.5-mm stage both mutants are aberrant cytologically, and show more drastic transcriptome changes. There are substantially more absolute On/Off and twice as many differentially expressed genes (sterile versus fertile) in am1-489 than in am1-praI. At 1.5 mm a total of 4,418 genes are up- or down-regulated in either am1-489 or am1-praI anthers. These are predominantly stage-specific transcripts. Many putative meiosis-related genes were found among them including a small subset of allele-specific, mis-regulated genes specific to the PMCs. Nearly 60% of transcriptome changes in the set of transcripts mis-regulated in both mutants (N = 530) are enriched in PMCs, and only 1% are enriched in the tapetal cell transcriptome. All array data reported herein will be deposited and accessible at MaizeGDB http://www.maizegdb.org/.ConclusionsOur analysis of anther transcriptome modulations by two distinct am1 alleles, am1-489 and am1-praI, redefines the role of AM1 as a modulator of expression of a subset of meiotic genes, important for meiotic progression and provided stage-specific insights into the genetic networks associated with meiotic entry and early prophase I progression.
Highlights
Developmental cues to start meiosis occur late in plants
Archesporial cells proliferate mitotically for several days (Figure 1b) pause for 3 days as they mature into Pollen mother cells (PMCs) and functional meiocytes; concomitantly, the maize anthers grow from 1 mm to 1.5 mm (Figures 1c and 1j)
For all am1 alleles except am1-praI, the PMCs remain mitotic rather than switching to meiosis, suggesting that both repression of mitosis and activation or maintenance of meiotic programs could be required to enter meiosis and that AM1 contributes to these processes
Summary
Developmental cues to start meiosis occur late in plants. Ameiotic (Am1) encodes a plant-specific nuclear protein (AM1) required for meiotic entry and progression through early prophase I. Mutants defective in the reproductive cells often have no or few defects in the soma and vice versa; once specified, the pre-meiotic cells can proliferate mitotically and differentiate for meiosis successfully even when the surrounding somatic tissue is abnormal [2]. L2-d cells that do not differentiate as archesporial cells form 3 somatic layers, each of a single cell type, that encircle the pre-meiotic population (Figure 1c). It is presently unclear how L2-d cells are programmed to be pre-meiotic within anthers or how this identity is retained during the mitotic divisions prior to PMC maturation. Archesporial cell divisions are asynchronous, maize meiosis is highly synchronized, likely facilitated by plasmodesmatal (cytoplasmic) connections among the PMCs
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