Two papers with quite different objectives established protocols that proved pivotal for future work on the role of gibberellins in seed germination. In their paper published in 1967, Russell Jones and Joseph Varner (Planta 72: 155-161) developed a bioassay based on induction of α-amylase activity in barley embryo-less half-seeds that was specific for bioactive gibberellins. The induction of α-amylase in the aleurone of barley and other cereals was to become the experimental system of choice to study gibberellin signalling. However, despite much progress in identifying the molecular events linking gibberellin action and α-amylase gene expression, in many cases their role in the process is still unclear. In 1987, Steven Groot and Cees Karssen (Planta 171:525-531) showed that germination of tomato seeds was limited by the ability of the radicle to penetrate the surrounding layers, with the endosperm forming the major barrier. They used a modified needle attached to a tensiometer to measure the force required to break through the endosperm. While in wild-type seeds, a factor from the embryo, assumed to be gibberellin, promoted breakdown of the endosperm, gibberellin-deficient seeds required an external supply of the hormone to weaken the endosperm or for it to be mechanically disrupted for germination to occur. The paradigm of seed germination being physically restricted by surrounding layers and the role of gibberellin in weakening these tissues has been confirmed in many eudicot species. Gibberellin signalling induces the production of cell-wall loosening enzymes in the micropylar endosperm adjacent to the radicle, but it is unclear whether or not this is a direct response. In both eudicot and monocot systems, there is still much to learn about the role of gibberellin signalling in germination.

• Keywords
• Aleurone, Bioassays, Endosperm, Germination, Gibberellin action, α-amylase,
• MeSH
• alpha-Amylases * metabolism   genetics   MeSH
• Endosperm * physiology   metabolism   MeSH
• Gibberellins * metabolism   MeSH
• Hordeum enzymology   genetics   MeSH
• Germination physiology   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Growth Regulators metabolism   MeSH
• Seeds * physiology   enzymology   MeSH
• Plant Dormancy * physiology   MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• alpha-Amylases * MeSH
• Gibberellins * MeSH
• Plant Growth Regulators MeSH

Plants in habitats with unpredictable conditions often have diversified bet-hedging strategies that ensure fitness over a wider range of variable environmental factors. A striking example is the diaspore (seed and fruit) heteromorphism that evolved to maximize species survival in Aethionema arabicum (Brassicaceae) in which external and endogenous triggers allow the production of two distinct diaspores on the same plant. Using this dimorphic diaspore model, we identified contrasting molecular, biophysical, and ecophysiological mechanisms in the germination responses to different temperatures of the mucilaginous seeds (M+ seed morphs), the dispersed indehiscent fruits (IND fruit morphs), and the bare non-mucilaginous M- seeds obtained by pericarp (fruit coat) removal from IND fruits. Large-scale comparative transcriptome and hormone analyses of M+ seeds, IND fruits, and M- seeds provided comprehensive datasets for their distinct thermal responses. Morph-specific differences in co-expressed gene modules in seeds, as well as in seed and pericarp hormone contents, identified a role of the IND pericarp in imposing coat dormancy by generating hypoxia affecting abscisic acid (ABA) sensitivity. This involved expression of morph-specific transcription factors, hypoxia response, and cell wall remodeling genes, as well as altered ABA metabolism, transport, and signaling. Parental temperature affected ABA contents and ABA-related gene expression and altered IND pericarp biomechanical properties. Elucidating the molecular framework underlying the diaspore heteromorphism can provide insight into developmental responses to globally changing temperatures.

• MeSH
• Brassicaceae * genetics   physiology   metabolism   MeSH
• Germination * genetics   physiology   MeSH
• Abscisic Acid metabolism   MeSH
• Fruit * genetics   physiology   growth & development   metabolism   MeSH
• Gene Expression Regulation, Plant * MeSH
• Plant Growth Regulators metabolism   MeSH
• Seeds * genetics   physiology   growth & development   metabolism   MeSH
• Temperature * MeSH
• Transcriptome genetics   MeSH
• Plant Dormancy genetics   physiology   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Abscisic Acid MeSH
• Plant Growth Regulators MeSH

The transition from germinating seeds to emerging seedlings is one of the most vulnerable plant life cycle stages. Heteromorphic diaspores (seed and fruit dispersal units) are an adaptive bet-hedging strategy to cope with spatiotemporally variable environments. While the roles and mechanisms of seedling traits have been studied in monomorphic species, which produce one type of diaspore, very little is known about seedlings in heteromorphic species. Using the dimorphic diaspore model Aethionema arabicum (Brassicaceae), we identified contrasting mechanisms in the germination responses to different temperatures of the mucilaginous seeds (M+ seed morphs), the dispersed indehiscent fruits (IND fruit morphs), and the bare non-mucilaginous M- seeds obtained from IND fruits by pericarp (fruit coat) removal. What follows the completion of germination is the pre-emergence seedling growth phase, which we investigated by comparative growth assays of early seedlings derived from the M+ seeds, bare M- seeds, and IND fruits. The dimorphic seedlings derived from M+ and M- seeds did not differ in their responses to ambient temperature and water potential. The phenotype of seedlings derived from IND fruits differed in that they had bent hypocotyls and their shoot and root growth was slower, but the biomechanical hypocotyl properties of 15-day-old seedlings did not differ between seedlings derived from germinated M+ seeds, M- seeds, or IND fruits. Comparison of the transcriptomes of the natural dimorphic diaspores, M+ seeds and IND fruits, identified 2,682 differentially expressed genes (DEGs) during late germination. During the subsequent 3 days of seedling pre-emergence growth, the number of DEGs was reduced 10-fold to 277 root DEGs and 16-fold to 164 shoot DEGs. Among the DEGs in early seedlings were hormonal regulators, in particular for auxin, ethylene, and gibberellins. Furthermore, DEGs were identified for water and ion transporters, nitrate transporter and assimilation enzymes, and cell wall remodeling protein genes encoding enzymes targeting xyloglucan and pectin. We conclude that the transcriptomes of seedlings derived from the dimorphic diaspores, M+ seeds and IND fruits, undergo transcriptional resetting during the post-germination pre-emergence growth transition phase from germinated diaspores to growing seedlings.

• Keywords
• bet-hedging strategy, diaspore dimorphism, fruit and seed heteromorphism, pericarp-imposed dormancy, pre-emergence growth, seed seedling transition, seedling stress resilience, transcriptome resetting,
• Publication type
• Journal Article MeSH

Dormancy and heteromorphism are innate seed properties that control germination timing through adaptation to the prevailing environment. The degree of variation in dormancy depth within a seed population differs considerably depending on the genotype and maternal environment. Dormancy is therefore a key trait of annual weeds to time seedling emergence across seasons. Seed heteromorphism, the production of distinct seed morphs (in color, mass or other morphological characteristics) on the same individual plant, is considered to be a bet-hedging strategy in unpredictable environments. Heteromorphic species evolved independently in several plant families and the distinct seed morphs provide an additional degree of variation. Here we conducted a comparative morphological and molecular analysis of the dimorphic seeds (black and brown) of the Amaranthaceae weed Chenopodium album. Freshly harvested black and brown seeds differed in their dormancy and germination responses to ambient temperature. The black seed morph of seedlot #1 was dormant and 2/3rd of the seed population had non-deep physiological dormancy which was released by after-ripening (AR) or gibberellin (GA) treatment. The deeper dormancy of the remaining 1/3rd non-germinating seeds required in addition ethylene and nitrate for its release. The black seeds of seedlot #2 and the brown seed morphs of both seedlots were non-dormant with 2/3rd of the seeds germinating in the fresh mature state. The dimorphic seeds and seedlots differed in testa (outer seed coat) thickness in that thick testas of black seeds of seedlot #1 conferred coat-imposed dormancy. The dimorphic seeds and seedlots differed in their abscisic acid (ABA) and GA contents in the dry state and during imbibition in that GA biosynthesis was highest in brown seeds and ABA degradation was faster in seedlot #2. Chenopodium genes for GA and ABA metabolism were identified and their distinct transcript expression patterns were quantified in dry and imbibed C. album seeds. Phylogenetic analyses of the Amaranthaceae sequences revealed a high proportion of expanded gene families within the Chenopodium genus. The identified hormonal, molecular and morphological mechanisms and dormancy variation of the dimorphic seeds of C. album and other Amaranthaceae are compared and discussed as adaptations to variable and stressful environments.

• Keywords
• abscisic acid, coat-imposed dormancy, gibberellins, hormone metabolism, seed coat properties, seed heteromorphism, thermal time modelling, weed seed bank,
• Publication type
• Journal Article MeSH

Developing innovative agri-technologies is essential for the sustainable intensification of global food production. Seed dormancy is an adaptive trait which defines the environmental conditions in which the seed is able to germinate. Dormancy release requires sensing and integration of multiple environmental signals, a complex process which may be mimicked by seed treatment technologies. Here, we reveal molecular mechanisms by which non-thermal (cold) atmospheric gas plasma-activated water (GPAW) releases the physiological seed dormancy of Arabidopsis thaliana. GPAW triggered dormancy release by synergistic interaction between plasma-generated reactive chemical species (NO3-, H2O2, ·NO, and ·OH) and multiple signalling pathways targeting gibberellin and abscisic acid (ABA) metabolism and the expression of downstream cell wall-remodelling genes. Direct chemical action of GPAW on cell walls resulted in premature biomechanical endosperm weakening. The germination responses of dormancy signalling (nlp8, prt6, and dog1) and ABA metabolism (cyp707a2) mutants varied with GPAW composition. GPAW removes seed dormancy blocks by triggering multiple molecular signalling pathways combined with direct chemical tissue weakening to permit seed germination. Gas plasma technologies therefore improve seed quality by mimicking permissive environments in which sensing and integration of multiple signals lead to dormancy release and germination.

• Keywords
• Arabidopsis thaliana, Abscisic acid metabolism, endosperm weakening, gas plasma-activated water, nitrogen signalling, non-thermal atmospheric gas plasma technology, plant hormone signalling, reactive oxygen species, seed dormancy,
• MeSH
• Arabidopsis * metabolism   MeSH
• Germination physiology   MeSH
• Abscisic Acid metabolism   MeSH
• Hydrogen Peroxide metabolism   MeSH
• Arabidopsis Proteins * metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Seeds metabolism   MeSH
• Technology MeSH
• Plant Dormancy physiology   MeSH
• Water metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• DOG1 protein, Arabidopsis MeSH Browser
• Abscisic Acid MeSH
• Hydrogen Peroxide MeSH
• Arabidopsis Proteins * MeSH
• Water MeSH

Molecular responses of plants to natural phytotoxins comprise more general and compound-specific mechanisms. How phytotoxic chalcones and other flavonoids inhibit seedling growth was widely studied, but how they interfere with seed germination is largely unknown. The dihydrochalcone and putative allelochemical myrigalone A (MyA) inhibits seed germination and seedling growth. Transcriptome (RNAseq) and hormone analyses of Lepidium sativum seed responses to MyA were compared to other bioactive and inactive compounds. MyA treatment of imbibed seeds triggered the phased induction of a detoxification programme, altered gibberellin, cis-(+)-12-oxophytodienoic acid and jasmonate metabolism, and affected the expression of hormone transporter genes. The MyA-mediated inhibition involved interference with the antioxidant system, oxidative signalling, aquaporins and water uptake, but not uncoupling of oxidative phosphorylation or p-hydroxyphenylpyruvate dioxygenase expression/activity. MyA specifically affected the expression of auxin-related signalling genes, and various transporter genes, including for auxin transport (PIN7, ABCG37, ABCG4, WAT1). Responses to auxin-specific inhibitors further supported the conclusion that MyA interferes with auxin homeostasis during seed germination. Comparative analysis of MyA and other phytotoxins revealed differences in the specific regulatory mechanisms and auxin transporter genes targeted to interfere with auxin homestasis. We conclude that MyA exerts its phytotoxic activity by multiple auxin-dependent and independent molecular mechanisms.

• Keywords
• ATP-binding cassette (ABC) transporter, PIN auxin efflux carrier, WRKY transcription factors, allelochemical and allelopathy, aquaporin-mediated water transport, auxin transport and homeostasis, cis-(+)-12-oxophytodienoic acid (OPDA) reductase, gibberellin metabolism, phytotoxin detoxification programme, seed germination,
• MeSH
• Chalcones MeSH
• Homeostasis MeSH
• Hormones metabolism   MeSH
• Germination * genetics   MeSH
• Indoleacetic Acids metabolism   MeSH
• Lepidium sativum * metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Growth Regulators metabolism   pharmacology   MeSH
• Seeds genetics   MeSH
• Seedlings metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chalcones MeSH
• Hormones MeSH
• Indoleacetic Acids MeSH
• myrigalone A MeSH Browser
• Plant Growth Regulators MeSH

The dynamic behaviour of seeds in soil seed banks depends on their ability to act as sophisticated environmental sensors to adjust their sensitivity thresholds for germination by dormancy mechanisms. Here we show that prolonged incubation of sugar beet fruits at low temperature (chilling at 5°C, generally known to release seed dormancy of many species) can induce secondary nondeep physiological dormancy of an apparently nondormant crop species. The physiological and biophysical mechanisms underpinning this cold-induced secondary dormancy include the chilling-induced accumulation of abscisic acid in the seeds, a reduction in the embryo growth potential and a block in weakening of the endosperm covering the embryonic root. Transcriptome analysis revealed distinct gene expression patterns in the different temperature regimes and upon secondary dormancy induction and maintenance. The chilling caused reduced expression of cell wall remodelling protein genes required for embryo cell elongation growth and endosperm weakening, as well as increased expression of seed maturation genes, such as for late embryogenesis abundant proteins. A model integrating the hormonal signalling and master regulator expression with the temperature-control of seed dormancy and maturation programmes is proposed. The revealed mechanisms of the cold-induced secondary dormancy are important for climate-smart agriculture and food security.

• Keywords
• coat dormancy, cold-induced dormancy, embryo growth potential, endosperm weakening, germination temperature, secondary dormancy, seed transcriptomes, sugar beet,
• MeSH
• Beta vulgaris * genetics   MeSH
• Germination physiology   MeSH
• Abscisic Acid metabolism   MeSH
• Seeds physiology   MeSH
• Plant Dormancy genetics   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Abscisic Acid MeSH