Alternative transcripts from Sorghum bicolor GAMYB and their posible involvement in dormancy in sorghum grains.

RODRIGUEZ V, CANTORO R, MUT P, AUGE G, BENECH-ARNOLD R.

York, Inglaterra

Workshop; 3rd Workshop on the Molecular Aspects of Seed Dormancy and Germination.; 2010

International Society for Seed Science

Alternative transcripts of SbGAMYB and their posible involvement in dormancy in sorghum grains Transcription factor GAMYB has been widely studied in cereal species [1, 2, 3, 5]. It is known that GAMYB regulates positively the expression of GA response genes involved in seed germination like ALPHA-AMYLASE among many others, by directly binding to the GARE element in their promoters. Also, expression of TaGAMYB has been shown to be down-regulated through protein kinase PKABA1 in wheat [1], which links ABA and GA signalling in a negative interaction. We analyzed the transcript levels of a GAMYB orthologue in embryos of imbibed dormant sorghum (Sorghum bicolor (L) moench) grains from two inbred lines with contrasting sprouting behaviour (IS9530, resistant, and RedlandB2, susceptible). Expression of SbGAMYB detected by RT-QPCR was induced over 6-fold after imbibition in dormant grains from IS9530 (that did not germinate) while no induction was observed in less dormant RedlandB2 grains, which also exhibited higher levels of active GA1 and GA4. These results (which are in clear contradiction with what is known about this component of GA signalling in cereal seeds) could be explained by the existence of several transcripts arising from alternative splicing for this gene. Several products were obtained with primers located on the 5´and 3´ regions of the mRNA, and one of them was recently cloned and sequenced and confirmed to be an alternative transcript for SbGAMYB. Theoretical translation of this sorghum transcript indicates that the protein has a functional DNA binding domain, but a 508 bp region from exon 3 is eliminated resulting in a truncated protein lacking the conserved Box2 and Box3 domains, and possibly impairing transactivation [5]. Other authors [1] have shown that an artificially truncated version of GAMYB is able to inhibit normal GAMYB function in transient assays; this suggests that SbGAMYB alternative transcripts may have negative effects on GA signalling, and that the relative contribution of these transcripts may be regulated during the expression of dormancy and germination. In addition, this alternative sorghum transcript also lacks a conserved sequence recognized by miRNA, which is necessary to target degradation of GAMYB transcripts in many tissues [6]. To further explore the physiological meaning of these alternative transcripts in sorghum and a possible role in dormancy, we are sequencing other transcripts, with the aim to quantify their expression individually in RT-QPCR assays. Ongoing experiments include expression analysis of individual transcripts under different hormone treatments and dormancy conditions.