As in other cultivated species, dormancy can be seen as a problem in cereal production, either due to its short duration or to its long persistence. Indeed, cereal crops lacking enough dormancy at harvest can be exposed to pre-harvest sprouting damage, while a long-lasting dormancy can interfere with processes that rely on rapid germination, such as malting or the emergence of a uniform crop. Because the ancestors of cereal species evolved under very diverse environments worldwide, different mechanisms have arisen as a way of sensing an appropriate germination environment (a crucial factor for winter or summer annuals such as cereals). In addition, different species (and even different varieties within the same species) display diverse grain morphology, allowing some structures to impose dormancy in some cereals but not in others. As in seeds from many other species, the antagonism between the plant hormones abscisic acid and gibberellins is instrumental in cereal grains for the inception, expression, release and re-induction of dormancy. However, the way in which this antagonism operates is different for the various species and involves different molecular steps as regulatory sites. Environmental signals (i.e. temperature, light quality and quantity, oxygen levels) can modulate this hormonal control of dormancy differently, depending on the species. The practical implications of knowledge accumulated in this field are discussed.

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Several genes involved in ABA signalling are regulated differently in imbibed caryopses from two sorghum lines with contrasting pre-harvest sprouting response before - but not after - physiological maturity. A role for ABI5 in the expression of dormancy during grain development is discussed.

Grain sorghum [Sorghum bicolor (L) moench] exhibits intraspecific variability for the rate of dormancy release and pre-harvest sprouting behavior. Two inbred lines with contrasting sprouting response were compared: IS9530 (resistant) and RedlandB2 (susceptible). Precocious dormancy release in RedlandB2 is related to an early loss of embryo sensitivity to ABA and higher levels of gibberellins in imbibed grains as compared with IS9530. With the aim of identifying potential regulatory sites for gibberellin metabolism involved in the expression of dormancy in immature grains of both lines, we carried out a time course analysis of transcript levels of putative gibberellin metabolism genes and hormone content (GA(1), GA(4), GA(8) and GA(34)). A lower embryonic GA(4) level in dormant IS9530 was related to a sharp and transient induction of two SbGA2-oxidase (inactivation) genes. In contrast, these genes were not induced in less dormant RedlandB2, while expression of two SbGA20-oxidase (synthesis) genes increased together with active GA(4) levels before radicle protrusion. Embryonic levels of GA(4) and its catabolite GA(34) correlated negatively. Thus, in addition to the process of gibberellin synthesis, inactivation is also important in regulating GA(4) levels in immature grains. A negative regulation by gibberellins was observed for SbGA20ox2, SbGA2ox1 and SbGA2ox3 and also for SbGID1 encoding a gibberellin receptor. We propose that the coordinated regulation at the transcriptional level of several gibberellin metabolism genes identified in this work affects the balance between gibberellin synthesis and inactivation processes, controlling active GA(4) levels during the expression of dormancy in maturing sorghum grains.

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.

Sunflower (Helianthus annuus L.) achenes often display pericarp‐imposed dormancy, which is long‐lasting and causes serious problems to crop production and the seed industry. For this study we assessed an extensively used sunflower inbred line that has this type of dormancy. Our goals were (i) to determine the effect of pericarp on germination and to evaluate its impact on crop field emergence, (ii) to provide insight into the physiological basis of pericarp‐imposed dormancy by determining the effects of abscisic acid (ABA) accumulation in the embryo and the embryo sensitivity to ABA during incubation at different temperatures, (iii) to assess the effect of oxidant agents and other compounds on dormancy termination, and (iv) to evaluate the feasibility of using oxidants to remove dormancy at an industrial scale. Incubation at high temperatures (i.e., 25 to 30°C) allowed the expression of dormancy, which was imposed by the pericarp and was accompanied by an increase in embryo sensitivity to ABA, but not in ABA concentration. Treated achenes with sodium hypochlorite, or their incubation in presence of an ethylene precursor or gibberellins overcame dormancy. ABA concentration decreased during incubation when treated with sodium hypochlorite. Application of sodium hypochlorite on a commercial seed lot (i.e., washing with 3 and 7%, after additional chemicals used by the industry were applied) resulted in higher germination compared with dormant non‐treated controls. Field trials showed that pericarp‐imposed dormancy reduced crop emergence in the inbred line tested herein. However, treating achenes with sodium hypochlorite using described industrial procedures improved field emergence.

Summary• High incubation temperatures (i.e. 30 ° C) stimulate the germination of dormant sorghum grains. To test the hormonal nature of this response, experiments were carried out with two varieties with contrasting dormancy at harvest: Redland B2 (low dormancy, high germination percentages attained under a wide incubation thermal range) and IS 9530 (high dormancy, high germination percentages attained only at 30 ° C).• Redland B2 grains with reduced GA content (paclobutrazol-treated) reached high germination temperatures ( c. 100%) only when incubated at 30 ° C. By contrast, IS 9530 grains with reduced ABA content (fluridone-treated) reached 100% germination at 30, 25, 20 and 15 ° C.• Incubation temperatures did not alter embryo responsiveness to ABA, nor did it modify the pattern of changes in embryo ABA content throughout incubation. Low GA 3 concentrations (0.1 µ  ) were required to totally overcome the inhibition imposed by ABA in embryos incubated at 30 ° C; by contrast, even the highest GA 3 concentrations used (1000 µ  ) were not able to revert ABA inhibition in embryos incubated at 15 ° C.• These results show the hormonal nature of the stimulatory effect of high incubation temperatures, and suggest that this effect is mediated by an increase in tissue responsiveness to GAs.

Pre-harvest sprouting (PHS) in Sorghum bicolor is one of the main constrains for its production in the central region of Argentina, as grain maturation often coincides with rainy or high environmental humidity conditions. The obtention of more dormant genotypes with higher PHS resistance has always been a desirable trait for breeders but the typical quantitative nature of seed dormancy makes its manipulation difficult through classical breeding. Dissecting this quantitative variability into quantitative trait loci (QTL) is a main concern especially in cereal species. In this work, a sorghum segregating population including 190 families was genotyped with microsatellite markers and the SbABI5 candidate gene. A genetic map encompassing 96 markers and a total length of 1331 cM was built. Seed dormancy was phenotyped in F 3 and F 4 panicles in two contrasting Argentinean environments (Castelar and Manfredi). Six seed dormancy QTL for mature grains were identified (qGI-1, qGI-3, qGI-4, qGI-6, qGI-7 and qGI-9) with the aid of QTL Cartographer and QTLNetwork, three of them (qGI-3, qGI-7 and qGI-9) being co-localised by both approaches. No epistasis was detected for the identified QTL but QTL-byenvironment interaction was significant for qGI-7 and qGI-9. Interestingly, seed dormancy candidate genes Ruth A. Heinz and Roberto L. Benech-Arnold have contributed equally to this study.Electronic supplementary material The online version of this article (