The distribution of microspore-expressed genes among functional categories was similar to that of rosette leaves and all genes on the ATH1 microarray (Fig. 5). Among rare exceptions were genes involved in signaling and stress-responses (~ 1% under-represented) and genes involved in protein synthesis and transport (~ 1% over-representation). More striking differences were found when the distribution of expressed genes was represented according to their relative expression levels. This led to a dramatic expansion in the contribution of genes involved in protein synthesis from 5% to 14%. This highlights the importance of the protein synthesis program initiated early during male gametophyte development.
With some exceptions (e.g., some transcription factors and receptor proteins), we can expect many gametophytic genes with important functions to be relatively highly expressed. The contribution of such genes is more easily visualized when only mRNAs forming the high-abundance class5 are considered (2197 genes). Within the high abundance class, genes involved in protein synthesis are again the most numerous (15% of all 2197 genes) and the most highly expressed (25% of overall signal intensity). The increase of other over-represented functional categories (metabolism, protein fate) was less dramatic with only a 1-3% increase in gene number and signal intensity. A different figure was obtained when only genes specific to the male game-tophyte were analysed (423 genes). Highly expressed, but mostly constitutive genes involved in protein synthesis disappeared (only 2% genes and signal) and were replaced by newly emerging gene categories of cell wall metabolism (massive increase to 6% gene number and 12% signal) and transport (7% genes, 11% signal). Both of these categories were even more dominant among highly expressed genes, together comprising nearly one third of the overall expression signal.
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