Transcriptome analysis reveals season-specific RbcS gene expression profiles in diploid Lolium perenne L

Perennial ryegrass (Lolium perenne L.) is a major grass species used for forage and turf throughout the world, and gains by conventional breeding have reached a plateau. Perennial ryegrass is an outcrossing, self-incompatible diploid (2n = 2x = 14) with a relatively large genome (4067 Mbp/diploid genome; Evans, G.M., Rees, H., Snell, C.L. and Sun, S. (1972). The relation between nuclear DNA amount and the duration of the mitotic cycle. Chrom. Today, 3, 24–31). Using tissues sourced from active pastures during the peak of the autumn, winter, spring and summer seasons, we analysed the ryegrass transcriptome employing a Serial Analysis of Gene Expression (SAGE™) protocol, with the dual goals of understanding the seasonal changes in perennial ryegrass gene expression and enhancing our ability to select genes for genetic manipulation. A total of 159 002 14-mer SAGE™ tags was sequenced and mapped to the perennial ryegrass DNA database, comprising methyl-filtered (GeneThresher®) and expressed sequence tag (EST) sequences. The analysis of 14 559 unique SAGE™ tags, which were present more than once in our SAGE™ library, revealed 964, 1331, 346 and 131 exclusive transcripts to autumn, winter, spring and summer, respectively. Intriguingly, our analysis of the SAGE™ tags revealed season-specific expression profiles for the small subunit of ribulose-1,5-bisphosphate carboxylase (Rubisco), LprbcS. The transcript level for LprbcS was highest in spring, and then decreased gradually between summer and winter. Five different copies of LprbcS were revealed in ryegrass, with one possibly producing splice variant transcripts. Two highly expressed LprbcS genes were reported, one of which was not active in autumn. Another LprbcS gene showed an inverse expression profile to the autumn inactive LprbcS in a manner to compensate the expression level. Keywords: paddock samples, pasture, perennial ryegrass, Serial Analysis of Gene Expression (SAGE™), season specific expression, monocot Ryegrass tissue Perennial ryegrass (Lolium perenne L.) cv. Bronsyn was used throughout this study. Field-grown samples, comprising mainly viable leaves and pseudostems, were collected at midday from livestock-active monocultural paddocks at Dexcel, Hamilton, New Zealand during the peak of each season, and frozen immediately in liquid nitrogen. Samples were transported in dry-ice and stored at −80 oC until use. During spring and summer, care was taken not to include any floral stems in the tissue sample. Grass samples were collected from pregrazed (15–60 days post-grazing) and post-grazed (1 day post-grazing) ryegrass swards, which were visibly healthy and free of any pests. Construction of SAGE™ libraries RNA was extracted using TRIZOL® reagent (Invitrogen, Carlsbad,CA, USA). For each SAGE™ library, 100 ug of pre- and post-grazed sample-derived total RNA was employed, and the libraries were created using an I-SAGE™ or I-SAGE™Long Kit (Invitrogen) according to the manufacturer’s protocol. Pre- and post-grazed SAGE™ libraries were sequenced at the Australian Genome Research Facility (Brisbane, Australia), and the tags were extracted using SAGE2000 software and combined to produce the transcript profile for the season.