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 lowdormancy 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.environmental signaling | environmental sensing | seed ecology | soil seed bank S eeds are the mobile phase of the plant's life cycle; vegetative development is suspended as seeds transport the plant's genetic complement through space and time. This is achieved by a seed remaining dormant, potentially for many years/decades in the soil, until conditions occur that are suitable for the resulting plant to survive, be competitive, and reproduce. Field observations by seed ecologists show that seeds act as environmental sensors and adjust their depth of dormancy in response to a range of signals (1). Some signals (e.g., soil temperature and moisture) are related to slow seasonal change that indicates when a suitable time of year and climate space exists (temporal window). These signals are integrated over time to alter the depth of dormancy and therefore the sensitivity to a second set of signals (e.g., light, nitrate, alternating temperatures). This second set of signals indicates in a more immediate way that conditions are suitable to terminate dormancy and induce the completion of germination (spatial window: appropriate soil depth, temperature, and moisture and lack of competing plants). If the correct spatial window does not occur, the temporal window will close for another year.Despite the obvious...

Seeds use environmental cues to sense the seasons and their surroundings to initiate the life cycle of the plant. The dormancy cycling underlying this process is extensively described, but the molecular mechanism is largely unknown. To address this we selected a range of representative genes from published array experiments in the laboratory, and investigated their expression patterns in seeds of Arabidopsis ecotypes with contrasting life cycles over an annual dormancy cycle in the field. We show how mechanisms identified in the laboratory are coordinated in response to the soil environment to determine the dormancy cycles that result in winter and summer annual phenotypes. Our results are consistent with a seed-specific response to seasonal temperature patterns (temporal sensing) involving the gene DELAY OF GERMINATION 1 (DOG1) that indicates the correct season, and concurrent temporally driven co-opted mechanisms that sense spatial signals, i.e. nitrate, via CBL-INTERACTING PROTEIN KINASE 23 (CIPK23) phosphorylation of the NITRATE TRANSPORTER 1 (NRT1.1), and light, via PHYTOCHROME A (PHYA). In both ecotypes studied, when all three genes have low expression there is enhanced GIBBERELLIN 3 BETA-HYDROXYLASE 1 (GA3ox1) expression, exhumed seeds have the potential to germinate in the laboratory, and the initiation of seedling emergence occurs following soil disturbance (exposure to light) in the field. Unlike DOG1, the expression of MOTHER of FLOWERING TIME (MFT) has an opposite thermal response in seeds of the two ecotypes, indicating a role in determining their different dormancy cycling phenotypes.

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Seed dormancy cycling plays a crucial role in the lifecycle timing of many plants. Little is known of how the seeds respond to the soil seed bank environment following dispersal in spring into the short-term seed bank before seedling emergence in autumn.Seeds of the winter annual Arabidopsis ecotype Cvi were buried in field soils in spring and recovered monthly until autumn and their molecular eco-physiological responses were recorded.DOG1 expression is initially low and then increases as dormancy increases. MFT expression is negatively correlated with germination potential. Abscisic acid (ABA) and gibberellin (GA) signalling responds rapidly following burial and adjusts to the seasonal change in soil temperature. Collectively these changes align germination potential with the optimum climate space for seedling emergence.Seeds naturally dispersed to the soil in spring enter a shallow dormancy cycle dominated by spatial sensing that adjusts germination potential to the maximum when soil environment is most favourable for germination and seedling emergence upon soil disturbance. This behaviour differs subtly from that of seeds overwintered in the soil seed bank to spread the period of potential germination in the seed population (existing seed bank and newly dispersed). As soil temperature declines in autumn, deep dormancy is re-imposed as seeds become part of the persistent seed bank.

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