Abstract
The precise adjustment of the timing of dormancy release according to final grain usage is still a challenge for many cereal crops. Grain sorghum [Sorghum bicolor (L.) Moench] shows wide intraspecific variability in dormancy level and susceptibility to pre-harvest sprouting (PHS). Both embryo sensitivity to abscisic acid (ABA) and gibberellin (GA) metabolism play an important role in the expression of dormancy of the developing sorghum grain. In previous works, it was shown that, simultaneously with a greater embryo sensitivity to ABA and higher expression of SbABA-INSENSITIVE 4 (SbABI4) and SbABA-INSENSITIVE 5 (SbABI5), dormant grains accumulate less active GA4 due to a more active GA catabolism. In this work, it is demonstrated that the ABA signalling components SbABI4 and SbABI5 interact in vitro with a fragment of the SbGA 2-OXIDASE 3 (SbGA2ox3) promoter containing an ABA-responsive complex (ABRC). Both transcription factors were able to bind the promoter, although not simultaneously, suggesting that they might compete for the same cis-acting regulatory sequences. A biological role for these interactions in the expression of dormancy of sorghum grains is proposed: either SbABI4 and/or SbABI5 activate transcription of the SbGA2ox3 gene in vivo and promote SbGA2ox3 protein accumulation; this would result in active degradation of GA4, thus preventing germination of dormant grains. A comparative analysis of the 5′-regulatory region of GA2oxs from both monocots and dicots is also presented; conservation of the ABRC in closely related GA2oxs from Brachypodium distachyon and rice suggest that these species might share the same regulatory mechanism as proposed for grain sorghum.
Highlights
Seed dormancy is considered as the failure of an intact viable seed to complete germination under conditions that are otherwise favourable for non-dormant seeds (Bewley, 1997)
Among the abscisic acid (ABA) response group, an ABRE, a coupling element 1 (CE1), and a DRE motif were found to Sorghum ABI4 and ABI5 bind the GA2ox3 promoter | Page 5 of 15
Be located at positions –194, –218, and –225 bp upstream of the TATA-box, respectively. This particular succession of regulatory elements has been previously reported by other authors to be present in the promoters of ABA-regulated genes (Himmelbach et al, 2003; Shen et al, 2004) and could be considered as an ‘ABA response complex’ (ABRC) on the SbGA2ox3 promoter
Summary
Seed dormancy is considered as the failure of an intact viable seed to complete germination under conditions that are otherwise favourable for non-dormant seeds (Bewley, 1997). The process of domestication has pushed towards a fast and uniform germination, followed by rapid seedling establishment and high crop yields (Finkelstein et al, 2008). This has been achieved through the selection of genotypes with a briefer dormancy. Low dormancy levels before grain harvest can lead to preharvest sprouting (PHS) if adequate temperature and humid
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