Two Faces of One Seed: Hormonal Regulation of Dormancy and Germination

Seed dormancy is an important component of plant fitness that causes a delay of germination until the arrival of a favourable growth season. Dormancy is a complex trait that is determined by genetic factors with a substantial environmental influence. Several of the tissues comprising a seed contribute to its final dormancy level. The roles of the plant hormones abscisic acid and gibberellin in the regulation of dormancy and germination have long been recognized. The last decade saw the identification of several additional factors that influence dormancy including dormancy-specific genes, chromatin factors and non-enzymatic processes. This review gives an overview of our present understanding of the mechanisms that control seed dormancy at the molecular level, with an emphasis on new insights. The various regulators that are involved in the induction and release of dormancy, the influence of environmental factors and the conservation of seed dormancy mechanisms between plant species are discussed. Finally, expected future directions in seed dormancy research are considered.

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Molecular mechanisms of seed dormancy

We developed and validated a robust marker toolkit for high-throughput and cost-effective screening of a large number of functional genes in wheat. Functional markers (FMs) are the most valuable markers for crop breeding programs, and high-throughput genotyping for FMs could provide an excellent opportunity to effectively practice marker-assisted selection while breeding cultivars. Here we developed and validated kompetitive allele-specific PCR (KASP) assays for genes that underpin economically important traits in bread wheat including adaptability, grain yield, quality, and biotic and abiotic stress resistances. In total, 70 KASP assays either developed in this study or obtained from public databases were validated for reliability in application. The validation of KASP assays were conducted by (a) comparing the assays with available gel-based PCR markers on 23 diverse wheat accessions, (b) validation of the derived allelic information using phenotypes of a panel comprised of 300 diverse cultivars from China and 13 other countries, and (c) additional testing, where possible, of the assays in four segregating populations. All KASP assays being reported were significantly associated with the relevant phenotypes in the cultivars panel and bi-parental populations, thus revealing potential application in wheat breeding programs. The results revealed 45 times superiority of the KASP assays in speed than gel-based PCR markers. KASP has recently emerged as single-plex high-throughput genotyping technology; this is the first report on high-throughput screening of a large number of functional genes in a major crop. Such assays could greatly accelerate the characterization of crossing parents and advanced lines for marker-assisted selection and can complement the inflexible, high-density SNP arrays. Our results offer a robust and reliable molecular marker toolkit that can contribute towards maximizing genetic gains in wheat breeding programs.

Development and validation of KASP assays for genes underpinning key economic traits in bread wheat.

Seeds respond to environmental signals, tuning their dormancy cycles to the seasons and thereby determining the optimum time for plant establishment. The molecular regulation of dormancy cycling is unknown, but an extensive range of mechanisms have been identified in laboratory experiments. Using a targeted investigation of gene expression over the dormancy cycle of Arabidopsis seeds in the field, we investigated how these mechanisms are seasonally coordinated. Depth of dormancy and gene expression patterns were correlated with seasonal changes in soil temperature. The results were consistent with abscisic acid (ABA) signaling linked to deep dormancy in winter being repressed in spring concurrent with enhanced DELLA repression of germination as depth of dormancy decreased. Dormancy increased during winter as soil temperature declined and expression of ABA synthesis (NCED6) and gibberellic acid (GA) catabolism (GA2ox2) genes increased. This was linked to an increase in endogenous ABA that plateaus, but dormancy and DOG1 and MFT expression continued to increase. The expression of SNF1-related protein kinases, SnrK 2.1 and 2.4, also increased consistent with enhanced ABA signaling and sensitivity being modulated by seasonal soil temperature. Dormancy then declined in spring and summer. Endogenous ABA decreased along with positive ABA signaling as expression of ABI2, ABI4, and ABA catabolism (CYP707A2) and GA synthesis (GA3ox1) genes increased. However, during the low-dormancy phase in the summer, expression of transcripts for the germination repressors RGA and RGL2 increased. Unlike deep winter dormancy, this represson can be removed on exposure to light, enabling the completion of germination at the correct time of year.

https://www.pnas.org/content/pnas/108/50/20236.full.pdf

Dormancy cycling in Arabidopsis seeds is controlled by seasonally distinct hormone-signaling pathways

The FLOWERING LOCUS T/TERMINAL FLOWER 1 Gene Family: Functional Evolution and Molecular Mechanisms

Seed dormancy is an adaptive mechanism and an important agronomic trait. Temperature during seed development strongly affects seed dormancy in wheat (Triticum aestivum) with lower temperatures producing higher levels of seed dormancy. To identify genes important for seed dormancy, we used a wheat microarray to analyze gene expression in embryos from mature seeds grown at lower and higher temperatures. We found that a wheat homolog of MOTHER OF FT AND TFL1 (MFT) was upregulated after physiological maturity in dormant seeds grown at the lower temperature. In situ hybridization analysis indicated that MFT was exclusively expressed in the scutellum and coleorhiza. Mapping analysis showed that MFT on chromosome 3A (MFT-3A) colocalized with the seed dormancy quantitative trait locus (QTL) QPhs.ocs-3A.1. MFT-3A expression levels in a dormant cultivar used for the detection of the QTL were higher after physiological maturity; this increased expression correlated with a single nucleotide polymorphism in the promoter region. In a complementation analysis, high levels of MFT expression were correlated with a low germination index in T1 seeds. Furthermore, precocious germination of isolated immature embryos was suppressed by transient introduction of MFT driven by the maize (Zea mays) ubiquitin promoter. Taken together, these results suggest that MFT plays an important role in the regulation of germination in wheat.

A Wheat Homolog of MOTHER OF FT AND TFL1 Acts in the Regulation of Germination

ABSTRACT Frost injury is a concern in early-sown spring wheat (Triticum aestivum) in the Tokai region of Japan. To assess the efficacy of ground rolling as a technique for avoiding frost injury, we investigated its effects on the apical development and yield of ‘Ayahikari’ spring wheat. In both 2018–19 and 2019–20, rolling delayed spikelet initiation by 4–5 days, the start of internode elongation by 5–12 days, and jointing by 3–9 days, but did not affect the apical development rate, which depended on the thermal time. Ground rolling prevented frost injury in 2018–19 but not in 2019–20, probably because the warmer-than-average winter promoted earlier jointing, and so plants were exposed to later low temperatures in February. These results suggest that ground rolling is an effective method for avoiding frost injury in spring wheat during cold winters. GRAPHICAL ABSTRACT

https://doi.org/10.1080/1343943x.2022.2136097

Kou Nakazono

Papers

A Wheat Homolog of MOTHER OF FT AND TFL1 Acts in the Regulation of Germination

Ground rolling delays apical development and reduces frost injury in early-sown spring wheat