Priming the pump: translational dynamics from seed to seedling transition under priming treatment

Seed germination is considered a developmental transition phase, as it is part of the process in which metabolically quiescent dry seeds grow into vigorous seedlings through metabolic reactivation after water uptake, accompanied by dramatic changes in the associated gene expression networks (Silva et al., 2016). Seed priming is a pre-sowing technique used in crop production to improve germination performance, resulting in more vigorous and stress-tolerant seedlings (Pagano et al., 2023). This technique stimulates the transition from seeds to seedlings by initiating controlled seed hydration and then drying before germination. The trade-off is that primed seeds generally age and lose viability more rapidly than unprimed seeds (Fabrissin et al., 2021). An article recently published in New Phytologist, by Gran et al. (2025; doi: 10.1111/nph.70098) ‘Unravelling the dynamics of seed stored mRNAs during seed priming’, provides novel insights into the translational changes underlying the transition from seeds to seedlings through priming treatment.

‘How can we capture dynamic changes in gene expression induced by seed priming? has been a major challenge in elucidating the molecular mechanisms underlying priming-related traits.’

In seed plants, seeds function as dispersal units for the next generation, incorporating various survival strategies. The desiccation tolerance of seeds enables their survival in a quiescent state under extreme water-deficient conditions. Subsequently, under environmentally favourable conditions, seed germination and seedling establishment occur, accompanied by a complete loss of desiccation tolerance after germination (Sano & Verdier, 2024). Another key environmental adaptation mechanism is seed dormancy, which prevents vivipary and, following seed dispersal, delays and staggers germination over time. The depth of dormancy responses to seasonal environmental changes determines the optimal timing of germination for maximizing seedling survival and growth (Sano & Marion-Poll, 2021). Following dispersal from the mother plant, seeds are subjected to fluctuations in temperature and hydration–dehydration cycles, which alter the embryo's lifespan and could influence seed persistence in the soil seed bank under natural environmental conditions (Long et al., 2015). A pre-sowing treatment, ‘seed priming’, mimics such environmental fluctuations. In agriculture, rapid and uniform germination is essential. Priming reduces dormancy and promotes germination through controlled hydration, followed by re-drying before desiccation tolerance is lost. This process enables storage and distribution of primed seeds before sowing. However, a phenomenon known as ‘overpriming’ causes seed death after re-drying, and primed seeds more generally have a reduced storage life, leading to financial losses (Fabrissin et al., 2021). Seed priming thus involves a complex balance between effects on germination, dormancy, desiccation tolerance, longevity, and stress resistance. As food demand rises and climate change threatens crop production, a deeper understanding of the molecular mechanisms underlying priming is urgently needed.

In terms of understanding mechanisms of gene expression during germination, a landmark discovery was made in the 1960s when de novo protein synthesis was observed in germinating cotton seeds, even while transcription was inhibited during imbibition (Dure & Waters, 1965). This finding demonstrated that protein synthesis in the early phase of germination uses pre-existing mRNA templates stored in mature dry seeds. During seed maturation, > 10 000 mRNAs accumulate as ‘stored mRNAs,’ some of which are also referred to as ‘long-lived mRNAs’ because they remain translatable for extended periods, even under stressful conditions. However, it is unlikely that all of these stored mRNAs are translated to proteins specifically during germination. Many may be involved in housekeeping activities within cells or persist from seed developmental processes. Stored mRNAs could be considered a backup for late seed maturation, reflecting the mother plant's history and enabling the seed to adapt to environmental fluctuations while regulating germination appropriately (Sano et al., 2020). Distinguishing stored mRNAs from newly transcribed mRNAs in the seed transcriptome during the early phase of imbibition is inherently technically challenging, and comparative transcriptome analyses of germination have faced methodological limitations. This complexity extends to seed priming, where gene expression changes related to germination, dormancy, desiccation tolerance, and stress response are superimposed. Thus, the question of ‘How can we capture dynamic changes in gene expression induced by seed priming?’ has been a major challenge in elucidating the molecular mechanisms underlying priming-related traits.

Gran et al. were the first to apply polysome profiling to seeds during priming treatment using the model plant Arabidopsis thaliana. Polysome profiling is a method for genome-wide analysis of mRNA translation based on the concept that to be translated, mRNAs associate with multiple ribosomes to form polysomes (Wu et al., 2024). Most stored mRNAs accumulate in dry seeds as monosomes (mRNAs bound to a single ribosome), while polysomes (mRNAs with multiple ribosomes) accumulate upon seed imbibition (Bai et al., 2020). Gran et al.'s polysome profiling data show that primed seeds contain more polysomes compared to unprimed seeds, suggesting that mRNAs in primed seeds are poised in a translation-ready state, facilitating faster translation. This observation may explain why primed seeds germinate more quickly than unprimed seeds. Interestingly, the data also revealed a pattern of monosome peak changes during priming. After imbibition, monosome levels initially decreased, but in primed seeds that had subsequently been dried, the monosome peak was restored and then surpassed that of unprimed seeds. Previous studies hypothesized that the monosome state may be crucial for ensuring the survival of stored mRNAs in dry seeds (Bai et al., 2020). In a study on seed dormancy, nondormant seeds had more free, inactive monosomes than dormant seeds after 24 h of imbibition, suggesting that storing ribosomes in an unproductive state in nondormant seeds could serve as a reserve for translation initiation after germination (Basbouss-Serhal et al., 2015). Thus, although the physiological role of monosomes in seeds remains debated, Gran et al.'s polysome profiling results highlight a unique facet of translatome dynamics triggered by priming. Gran et al. also conducted ribosome-nascent chain complex-Seq followed by Gene Ontology (GO) analysis to gain further insights into the translation of transcripts during priming. Ribosome-nascent chain complex-Seq is a method for detecting actively translating mRNAs by isolating ribosome-nascent chain complexes, enabling the characterization of translation dynamics and regulatory mechanisms at the transcriptome level. The results suggested that priming has a dual effect on the biological pathways of seeds: it enhances translational activity while potentially reducing stress tolerance. Notably, the inactivation of stress response-related pathways could help explain the shortened seed lifespan observed after priming treatment, highlighting the suitability of this approach for uncovering potentially complex molecular mechanisms underlying the trade-offs associated with priming. Furthermore, Gran et al. performed functional network analysis based on their translatome data during priming and discovered that ARGININE-GLYCINE–GLYCINE RNA BINDING PROTEIN B serves as a hub within the network. RNA-binding proteins (RBPs) bind to and dissociate from their RNA partners in a dynamic manner, accompanying virtually every stage of RNA processing and function (Mateos & Staiger, 2023). RBPs that function in the translation of the stored mRNAs or newly transcribed mRNAs after imbibition, or in the stabilization of the transcriptome following dry-back during priming, may play a crucial role in determining post-priming traits. The rggb mutant seeds exhibited significantly higher dormancy levels than the wild-type, as well as slower germination in both primed and unprimed conditions. Although germination-related phenotypes altered in the rggb mutant were not specific to priming, Gran et al.'s results shed new light on the potential impact of RBPs on priming-related traits.

Seed priming is one of the representative techniques that improve germination and seedling stress tolerance in a ‘nongenetic manner’, distinguishing it from trait improvements through breeding programmes and genetic engineering (Srivastava et al., 2021). Various priming protocols, including hydro-, osmo-, hormo-, chemo-, and bio-priming, have been developed and tailored to plant genotype and seed lot, offering solutions to agricultural and ecosystem challenges (Pagano et al., 2023). Combining seed priming with genetic approaches effectively controls germination traits. For instance, pre-harvest sprouting can be minimized by using genetically strongly-dormant cultivars, while priming can subsequently alleviate dormancy of post-harvest seeds and promote germination. However, the unique features and applications of priming compared to genetic approaches may explain why research specifically focused on this technique using quantitative genetics and reverse genetics remains limited, leaving the genes influencing priming largely unknown. By contrast, priming has been the target of numerous omics analyses aimed at better characterizing the underlying molecular mechanisms, leading to the uncovering of the key biological pathways involved in the priming response, such as respiration, cell cycle regulation, DNA repair, plant hormone signalling, reactive oxygen species scavenging systems, amino acid and energy metabolism, reserve mobilization, and membrane integrity (Corbineau et al., 2023; Pagano et al., 2023). The analytical framework employed by Gran et al. is one of the first to explore the translatome and RBPs as a strategic entry point in the omics analyses of priming. Applying this pioneering approach to other plant species and various priming techniques in the future is likely to both provide critical insights into post-transcriptional gene regulation within previously elucidated biological pathways, and facilitate the identification of the elusive key genes directly involved in priming effects.