Comparative transcript analyses of the ovule, microspore, and mature pollen in Brassica napus

Transcriptome data for plant reproductive organs/cells currently is very limited as compared to sporophytic tissues. Here, we constructed cDNA libraries and obtained ESTs for Brassica napus pollen (4,864 ESTs), microspores (i.e., early stage pollen development; 6,539 ESTs) and ovules (10,468 ESTs). Clustering and assembly of the 21,871 ESTs yielded a total of 10,782 unigenes, with 3,362 contigs and 7,420 singletons. The pollen transcriptome contained high levels of polygalacturonases and pectinesterases, which are involved in cell wall synthesis and expansion, and very few transcription factors or transcripts related to protein synthesis. The set of genes expressed in mature pollen showed little overlap with genes expressed in ovules or in microspores, suggesting in the latter case that a marked differentiation had occurred from the early microspore stages through to pollen development. Remarkably, the microspores and ovules exhibited a high number of co-expressed genes (N = 1,283) and very similar EST functional profiles, including high transcript numbers for transcriptional and translational processing genes, protein modification genes and unannotated genes. In addition, examination of expression values for genes co-expressed among microspores and ovules revealed a highly statistically significant correlation among these two tissues (R = 0.360, P = 1.2 x 10(-40)) as well as a lack of differentially expressed genes. Overall, the results provide new insights into the transcriptional profile of rarely studied ovules, the transcript changes during pollen development, transcriptional regulation of pollen tube growth and germination, and describe the parallels in the transcript populations of microspore and ovules which could have implications for understanding the molecular foundation of microspore totipotency in B. napus.