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Abstract Background Cotton fibers provide a powerful model for studying cell differentiation and elongation. Each cotton fiber is a singular and elongated cell derived from epidermal-layer cells of a cotton seed. Efforts to understand this dramatic developmental shift have been impeded by the difficulty of separation between fiber and epidermal cells. Results Here we employed laser-capture microdissection (LCM) to separate these cell types. RNA-seq analysis revealed transitional differences between fiber and epidermal-layer cells at 0 or 2 days post anthesis. Specifically, down-regulation of putative cell cycle genes was coupled with upregulation of ribosome biosynthesis and translation-related genes, which may suggest their respective roles in fiber cell initiation. Indeed, the amount of fibers in cultured ovules was increased by cell cycle progression inhibitor, Roscovitine, and decreased by ribosome biosynthesis inhibitor, Rbin-1. Moreover, subfunctionalization of homoeologs was pervasive in fiber and epidermal cells, with expression bias towards 10% more D than A homoeologs of cell cycle related genes and 40–50% more D than A homoeologs of ribosomal protein subunit genes. Key cell cycle regulators were predicted to be epialleles in allotetraploid cotton. MYB-transcription factor genes displayed expression divergence between fibers and ovules. Notably, many phytohormone-related genes were upregulated in ovules and down-regulated in fibers, suggesting spatial-temporal effects on fiber cell development. Conclusions Fiber cell initiation is accompanied by cell cycle arrest coupled with active ribosome biosynthesis, spatial-temporal regulation of phytohormones and MYB transcription factors, and homoeolog expression bias of cell cycle and ribosome biosynthesis genes. These valuable genomic resources and molecular insights will help develop breeding and biotechnological tools to improve cotton fiber production. 

Arabidopsisseed development involves maternal small interfering RNAs (siRNAs) that induce RNA-directed DNA methylation (RdDM) through theNRPD1-mediated pathway. To investigate their biological functions, we characterized siRNAs in the endosperm and seed coat that were separated by laser-capture microdissection (LCM) in reciprocal genetic crosses with annrpd1mutant. We also monitored the spatial-temporal activity of theNRPD1-mediated pathway on seed development using the AGO4:GFP::AGO4 (promoter:GFP::protein) reporter and promoter:GUS sensors of siRNA-mediated silencing. From these approaches, we identified four distinct groups of siRNA loci dependent on or independent of the maternalNRPD1allele in the endosperm or seed coat. A group of maternally expressedNRPD1-siRNA loci targets endosperm-preferred genes, including those encoding AGAMOUS-LIKE (AGL) transcription factors. Using translational promoter:AGL::GUS constructs as sensors, we demonstrate that spatial and temporal expression patterns of these genes in the endosperm are regulated by theNRPD1-mediated pathway irrespective of complete silencing (AGL91) or incomplete silencing (AGL40) of these target genes. Moreover, altered expression of these siRNA-targeted genes affects seed size. We propose that the corresponding maternal siRNAs could account for parent-of-origin effects on the endosperm in interploidy and hybrid crosses. These analyses reconcile previous studies on siRNAs and imprinted gene expression during seed development.

 

Polyploidy is an evolutionary innovation for many animals and all flowering plants, but its impact on selection and domestication remains elusive. Here we analyze genome evolution and diversification for all five allopolyploid cotton species, including economically important Upland and Pima cottons. Although these polyploid genomes are conserved in gene content and synteny, they have diversified by subgenomic transposon exchanges that equilibrate genome size, evolutionary rate heterogeneities and positive selection between homoeologs within and among lineages. These differential evolutionary trajectories are accompanied by gene-family diversification and homoeolog expression divergence among polyploid lineages. Selection and domestication drive parallel gene expression similarities in fibers of two cultivated cottons, involving coexpression networks andN⁶-methyladenosine RNA modifications. Furthermore, polyploidy induces recombination suppression, which correlates with altered epigenetic landscapes and can be overcome by wild introgression. These genomic insights will empower efforts to manipulate genetic recombination and modify epigenetic landscapes and target genes for crop improvement.