Genomic expression profiling of mature soybean (Glycine max) pollen

Abstract

Background: Pollen, the male partner in the reproduction of flowering plants, comprises either two or three cells at maturity. The current knowledge of the pollen transcriptome is limited to the model plant systems Arabidopsis thaliana and Oryza sativa which have tri-cellular pollen grains at maturity. Comparative studies on pollen of other genera, particularly crop plants, are needed to understand the pollen gene networks that are subject to functional and evolutionary conservation. In this study, we used the Affymetrix Soybean GeneChip to perform transcriptional profiling on mature bi-cellular soybean pollen.

Results: Compared to the sporophyte transcriptome, the soybean pollen transcriptome revealed a restricted and unique repertoire of genes, with a significantly greater proportion of specifically expressed genes than is found in the sporophyte tissue. Comparative analysis shows that, among the 37,500 soybean transcripts addressed in this study, 10,299 transcripts (27.46%) are expressed in pollen. Of the pollen-expressed sequences, about 9,489 (92.13%) are also expressed in sporophytic tissues, and 810 (7.87%) are selectively expressed in pollen. Overall, the soybean pollen transcriptome shows an enrichment of transcription factors (mostly zinc finger family proteins), signal recognition receptors, transporters, heat shock-related proteins and members of the ubiquitin proteasome proteolytic pathway.

Conclusion: This is the first report of a soybean pollen transcriptional profile. These data extend our current knowledge regarding regulatory pathways that govern the gene regulation and development of pollen. A comparison between transcription factors up-regulated in soybean and those in Arabidopsis revealed some divergence in the numbers and kinds of regulatory proteins expressed in both species.

Figure 1

MA plot comparing the transcript profile of pollen against sporophytic tissues (stems, roots and leaves tissues) or shoot apical meristems (SAM; Haerizadeh et al, unpublished) against stems, roots and leaves tissues (this study).

Figure 2

Functional categorization of up- and down-regulated transcripts in the soybean mature pollen in comparison to sporophytic tissues. Red or Green bar denotes up-or down-regulated categories, respectively

Figure 3

Phylogenetic relationship of virtually translated GmaAffx.56241.1.S1 and GmaAffx.86574.1.S1 with heat shock factors (At-HSF) from Arabidopsis thaliana. The phylogenetic tree is constructed using CLUSTAL W (version 1.83) and the results displayed as NJ-tree with branch length. Protein sequences of At-HSFs were retrieved from TAIR website and the predicted protein sequence for GmaAffx.56241.1.S1 or GmaAffx.86574.1.S1 from PHYTOZOME .

Figure 4

Photomicrograph of isolated pollen under light microscopy (left) and fluorescein diacetate viability screen in epifluorescence microscopy (right).