Abstract
Studies of the anther transcriptome on non-model plants without a known genome are surprisingly scarce. RNA-Seq and digital gene expression (DGE) profiling provides a comprehensive approach to identify candidate genes contributing to developmental processes in non-model species. Here we built a transcriptome library of developing anthers of Hamelia patens and analyzed DGE profiles from each stage to identify genes that regulate tapetum and pollen development. In total 7,720 putative differentially expressed genes across four anther stages were identified. The number of putative stage-specific genes was: 776 at microspore mother cell stage, 807 at tetrad stage, 322 at uninucleate microspore stage, and the highest number (1,864) at bicellular pollen stage. GO enrichment analysis revealed 243 differentially expressed and 108 stage-specific genes that are potentially related to tapetum development, sporopollenin synthesis, and pollen wall. The number of expressed genes, their function and expression profiles were all significantly correlated with anther developmental processes. Overall comparisons of anther and pollen transcriptomes with those of rice and Arabidopsis together with the expression profiles of homologs of known anther-expressed genes, revealed conserved patterns and also divergence. The divergence may reflect taxon-specific differences in gene expression, the use RNA-seq as a more sensitive methodology, variation in tissue composition and sampling strategies. Given the lack of genomic sequence, this study succeeded in assigning putative identity to a significant proportion of anther-expressed genes and genes relevant to tapetum and pollen development in H. patens. The anther transcriptome revealed a molecular distinction between developmental stages, serving as a resource to unravel the functions of genes involved in anther development in H. patens and informing the analysis of other members of the Rubiaceae.
Highlights
Pollen grains are the microgametophytes of seed plants that produce the male gametes required for sexual reproduction (Borg and Twell, 2011)
Ultrastructural changes involved in anther development have been described for many angiosperms, especially for model species including Arabidopsis and rice (Sanders et al, 1999; Zhang and Wilson, 2009; Zhang et al, 2011), there are differences in the resolution and detail of the stages described (Owen and Makaroff, 1995)
Anther and pollen development of H. patens was shown to be typical of many angiosperms, with some features common in the Rubiaceae family
Summary
Pollen grains are the microgametophytes of seed plants that produce the male gametes required for sexual reproduction (Borg and Twell, 2011). The diversity and evolution of palynological characters were documented across the angiosperms based on the most comprehensive classification APG III (2009) Many characters, such as exine structure and aperture features, have indicated key evolutionary transitions in pollen morphology and have proven to be informative at different taxonomic levels (Lu et al, 2015; Wortley et al, 2015). Further comparative analysis of anther and pollen transcriptome profiles for diverse angiosperms would complement understanding the conservation of the molecular mechanisms underlying pollen development (Wilson and Zhang, 2009; Rutley and Twell, 2015). The cysteine protease CEP1, a key executor involved in tapetal programmed cell death, regulates pollen development in Arabidopsis.
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