Seed-maturation Temperature Research Articles

The maternal environment interacts with genetic variation in regulating seed dormancy in Swedish Arabidopsis thaliana.

Seed dormancy is a complex adaptive trait that controls the timing of seed germination, one of the major fitness components in many plant species. Despite being highly heritable, seed dormancy is extremely plastic and influenced by a wide range of environmental cues. Here, using a set of 92 Arabidopsis thaliana lines from Sweden, we investigate the effect of seed maturation temperature on dormancy variation at the population level. The response to temperature differs dramatically between lines, demonstrating that genotype and the maternal environment interact in controlling the trait. By performing a genome-wide association study (GWAS), we identified several candidate genes that could presumably account for this plasticity, two of which are involved in the photoinduction of germination. Altogether, our results provide insight into both the molecular mechanisms and the evolution of dormancy plasticity, and can serve to improve our understanding of environmentally dependent life-history transitions.

Canalization of Seasonal Phenology in the Presence of Developmental Variation: Seed Dormancy Cycling in an Annual Weed.

Variation in the developmental timing in one life stage may ramify within and across generations to disrupt optimal phenology of other life stages. By focusing on a common mechanism of developmental arrest in plants-seed dormancy-we investigated how variation in flowering time influenced seed germination behavior and identified potential processes that can lead to canalized germination behavior despite variation in reproductive timing. We quantified effects of reproductive timing on dormancy cycling by experimentally manipulating the temperature during seed maturation and the seasonal timing of seed dispersal/burial, and by assessing temperature-dependent germination of un-earthed seeds over a seasonal cycle. We found that reproductive timing, via both seed-maturation temperature and the timing of dispersal, strongly influenced germination behavior in the weeks immediately following seed burial. However, buried seeds subsequently canalized their germination behavior, after losing primary dormancy and experiencing natural temperature and moisture conditions in the field. After the complete loss of primary dormancy, germination behavior was similar across seed-maturation and dispersal treatments, even when secondary dormancy was induced. Maternal effects themselves may contribute to the canalization of germination: first, by inducing stronger dormancy in autumn-matured seeds, and second by modifying the responses of those seeds to their ambient environment. Genotypes differed in dormancy cycling, with functional alleles of known dormancy genes necessary for the suppression of germination at warm temperatures in autumn through spring across multiple years. Loss of function of dormancy genes abolished almost all dormancy cycling. In summary, effects of reproductive phenology on dormancy cycling of buried seeds were apparent only as long as seeds retained primary dormancy, and a combination of genetically imposed seed dormancy, maternally induced seed dormancy, and secondary dormancy can mitigate variation in germination behavior imposed by variation in reproductive phenology.

Maternal temperature effects on dormancy influence germination responses to water availability in Arabidopsis thaliana

With climate change, germination cuing to water availability is expected to be especially important for seedling survival. Here, we examined germination responses to low water potential and tested whether dormancy status mediates these responses. We considered both genetically based dormancy (genotypes with allelic variation in dormancy genes) as well as dormancy imposed by the environment (low seed-maturation temperature or short duration of dry afterripening). We examined (a) germination capacity at low water potential, (b) germination acceleration in response to pre-incubation at low water potential, and (c) secondary dormancy induction by low water potential. We found that both environmentally imposed dormancy and genetically based dormancy influenced germination responses to low water potential. Specifically, dormancy established via introgression of a strong dormancy allele and dormancy induced by low seed-maturation temperatures both reduced the ability to germinate at low water potential. Pre-incubation at low water potential accelerated germination, but the rate differed between both dormancy-inducing environments and among dormancy genotypes. Prolonged incubation at low water potential induced secondary dormancy, and this effect was greater in fresher (more dormant) seeds and in seeds that were matured at low temperature (a dormancy-inducing treatment). Although genotypes also varied in secondary dormancy induction, their level of primary dormancy did not predict their induction into secondary dormancy. Environmentally induced dormancy also influenced the expression of genetic differences in germination responses to low water potential. Thus environmentally determined dormancy influences not only germination responses to low water potential but also their evolutionary potential.

Multiple paths to similar germination behavior in Arabidopsis thaliana

Germination timing influences plant fitness, and its sensitivity to temperature may cause it to change as climate shifts. These changes are likely to be complex because temperatures that occur during seed maturation and temperatures that occur post-dispersal interact to define germination timing. We used the model organism Arabidopsis thaliana to determine how flowering time (which defines seed-maturation temperature) and post-dispersal temperature influence germination and the expression of genetic variation for germination. Germination responses to temperature (germination envelopes) changed as seeds aged, or after-ripened, and these germination trajectories depended on seed-maturation temperature and genotype. Different combinations of genotype, seed-maturation temperature, and after-ripening produced similar germination envelopes. Likewise, different genotypes and seed-maturation temperatures combined to produce similar germination trajectories. Differences between genotypes were most likely to be observed at high and low germination temperatures. The germination behavior of some genotypes responds weakly to maternal temperature but others are highly plastic. We hypothesize that weak dormancy induction could synchronize germination of seeds dispersed at different times. By contrast, we hypothesize that strongly responsive genotypes may spread offspring germination over several possible germination windows. Considering germination responses to temperature is important for predicting phenology expression and evolution in future climates.

Secondary dormancy dynamics depends on primary dormancy status inArabidopsis thaliana

AbstractSeed dormancy can prevent germination under unfavourable conditions that reduce the chances of seedling survival. Freshly harvested seeds often have strong primary dormancy that depends on the temperature experienced by the maternal plant and which is gradually released through afterripening. However, seeds can be induced into secondary dormancy if they experience conditions or cues of future unfavourable conditions. Whether this secondary dormancy induction is influenced by seed-maturation conditions and primary dormancy has not been explored in depth. In this study, we examined secondary dormancy induction in seeds ofArabidopsis thalianamatured under different temperatures and with different levels of afterripening. We found that low water potential and a range of temperatures, from 8°C to 35°C, induced secondary dormancy. Secondary dormancy induction was affected by the state of primary dormancy of the seeds. Specifically, afterripening had a non-monotonic effect on the ability to be induced into secondary dormancy by stratification; first increasing in sensitivity as afterripening proceeded, then declining in sensitivity after 5 months of afterripening, finally increasing again by 18 months of afterripening. Seed-maturation temperature sometimes had effects that were independent of expressed primary dormancy, such that seeds that had matured at low temperature, but which had comparable germination proportions as seeds matured at warmer temperatures, were more easily induced into secondary dormancy. Because seed-maturation temperature is a cue of when seeds were matured and dispersed, these results suggest that the interaction of seed-maturation temperature, afterripening and post-dispersal conditions all combine to regulate the time of year of seed germination.

DOG1-imposed dormancy mediates germination responses to temperature cues

Seed dormancy and environment-dependent germination requirements interact to determine the timing of germination in natural environments. This study tested the contribution of the dormancy gene Delay Of Germination 1 (DOG1) to primary and secondary dormancy induction in response to environmental cues, and evaluated how DOG1-mediated dormancy influenced germination responses to different temperature cues. We verified that DOG1 is involved in the induction of primary dormancy in response to cool seed-maturation temperature experienced by maternal plants, and we found that it is also involved in secondary dormancy in response to warm and prolonged cold stratification experienced by seeds during imbibition. DOG1-imposed dormancy can also mediate germination responses to environmental conditions, including cold stratification and germination temperatures experienced by imbibing seeds. Specifically, germination responsiveness to temperature cues is most apparent when seeds exhibit an intermediate degree of dormancy. However, DOG1 itself does not seem to directly regulate the response to cold stratification nor does it determine the function of temperature-dependent germination, since DOG1 mutants were capable of exhibiting increased germination after cold stratification as well as temperature-dependent germination. Instead, DOG1 has major effects on germination behavior primarily by exposing or masking underlying environmental sensitivity, and thereby strongly influences how environmentally responsive germination can be, and when during a season, it is likely to exhibit environmental sensitivity.

Understanding chilling responses in Arabidopsis seeds and their contribution to life history

Winter chilling is of central importance in the phenology of temperate annual and perennial plants. Chilling accelerates flowering through the process of vernalization and breaks both bud and seed dormancy, permitting the onset of growth in the spring. The quantitative effects of chilling in floral promotion in winter annual Arabidopsis accessions are well-documented, but very little is known about the basic physiology underlying summer annual responses to winter chilling, which acts on seeds within the soil seed bank. Here, we analyse the response of wild accessions to extended chilling in seeds, and explore the interaction between seed-maturation temperature and chilling responses. We show that two weeks of chilling induces secondary dormancy, and that this time period is not dependent on seed-maturation temperature. In addition, we found that seeds for most accessions set under simulated summer conditions in the laboratory are unable to overwinter in the soil seed bank, as they germinate without light during extended chilling treatments. This shows that these seeds are committed to re-establishment in the same growing season. Understanding how winter chilling affects the timing of Arabidopsis phenology will enable us to explore the genetics behind adaptation to changing climates, and inform rational approaches to breeding crops with improved performance under new climate scenarios and develop a systems ecology of Arabidopsis.

DOG1 expression is predicted by the seed‐maturation environment and contributes to geographical variation in germination in Arabidopsis thaliana

Seasonal germination timing of Arabidopsis thaliana strongly influences overall life history expression and is the target of intense natural selection. This seasonal germination timing depends strongly on the interaction between genetics and seasonal environments both before and after seed dispersal. DELAY OF GERMINATION 1 (DOG1) is the first gene that has been identified to be associated with natural variation in primary dormancy in A.thaliana. Here, we report interaccession variation in DOG1 expression and document that DOG1 expression is associated with seed-maturation temperature effects on germination; DOG1 expression increased when seeds were matured at low temperature, and this increased expression was associated with increased dormancy of those seeds. Variation in DOG1 expression suggests a geographical structure such that southern accessions, which are more dormant, tend to initiate DOG1 expression earlier during seed maturation and achieved higher expression levels at the end of silique development than did northern accessions. Although elimination of the synthesis of phytohormone abscisic acid (ABA) results in the elimination of maternal temperature effects on dormancy, DOG1 expression predicted dormancy better than expression of genes involved in ABA metabolism.

Phytochromes differentially regulate seed germination responses to light quality and temperature cues during seed maturation

The ratio of red to far-red light (R : FR) experienced by seeds during maturation affects germination, but the genetic regulation of this effect is poorly understood. In Arabidopsis thaliana, responses to R : FR are governed by five phytochrome photoreceptors, PHYA-PHYE. PHYA, PHYB and PHYE mediate germination, but their roles in germination response to the seed maturation environment are largely unknown. Seeds of A. thaliana phytochrome mutants and natural accessions were matured in a factorial combination of cold (16 degrees C) and warm (24 degrees C) temperatures and high (R : FR = 1) and low (R : FR = 0.6) R : FR environments, resembling sunlight and foliar shade, respectively. Germination was observed in resulting seeds. All five phytochromes mediated germination responses to seed maturation temperature and/or R : FR environment. PHYA suppressed germination in seeds matured under cold temperature, and PHYB promoted germination under the same conditions. PHYD and PHYE promoted germination of seeds matured under warm temperature, but this effect diminished when seeds matured under reduced R : FR. The A. thaliana natural accessions exhibited interesting variation in germination responses to the experimental conditions. Our results suggest that the role of individual PHY loci in regulating plant responses to R : FR varies depending on temperature and provide novel insights into the genetic basis of maternal effects.

Structural Changes in Lettuce Seed During Germination at High Temperature Altered by Genotype, Seed Maturation Temperature, and Seed Priming

To investigate thermotolerance in seeds of lettuce (Lactuca sativa L.), primed, nonprimed, or seeds matured at 20/10 and 30/20 °C (day/night on a 12-h photoperiod) were imbibed at 36 °C for various periods and then dissected. Structural changes in seed coverings in front of the radicle tip were observed during germination at high temperature. Thermotolerant genotypes, ‘Everglades’ and PI 251245, were compared with a thermosensitive cultivar, ‘Dark Green Boston’. In all seeds that germinated, regardless of seed maturation temperature or priming, a crack appeared on one side of the cap tissue (constriction of the endosperm membrane near the basal end of the seed) at the micropylar region and the endosperm separated from the integument in front of the radicle tip. Additional changes took place during imbibition in these seeds; the protein bodies in the vacuoles enlarged and gradually depleted, large empty vacuoles formed, the cytoplasm condensed, the endosperm shrank, the endosperm cell wall dissolved and ruptured, and then the radicle elongated toward this ruptured area. The findings suggested that the endosperm layer presented mechanical resistance to germination in seeds that could not germinate at 36 °C. Weakening of this layer was a prerequisite to radicle protrusion at high temperature. Seeds of ‘Dark Green Boston’, ‘Everglades’, and PI 251245 matured at 30/20 °C had greater thermotolerance than those matured at 20/10 °C. Results of the anatomical study indicated that the endosperm cell walls in front of the radicle of seeds matured at 30/20 °C were more readily disrupted and ruptured during imbibition than seeds matured at 20/10 °C, suggesting a reason why these seeds could germinate quickly at supraoptimal temperatures. Similar endosperm structural alterations also were observed in primed seeds. Priming led to rapid and uniform germination, circumventing the inhibitory effects of high temperatures. From anatomical studies conducted to identify and characterize thermotolerance in lettuce seed germination, we observed that genotype, seed maturation temperature, or seed priming had the ability to reduce physical resistance of the endosperm by weakening the cell wall and by depleting stored reserves leading to cell collapse.

Diversification of phytochrome contributions to germination as a function of seed‐maturation environment

Environmental conditions during seed maturation influence germination, but the genetic basis of maternal environmental effects on germination is virtually unknown. Using single and multiple mutants of phytochromes, it is shown here that different phytochromes contributed to germination differently, depending on seed-maturation conditions. Arabidopsis thaliana wild-type seeds that were matured under cool temperatures were intensely dormant compared with seeds matured at warmer temperature, and this dormancy was broken only after warm seed-stratification followed by cold seed-stratification. The warm-cold stratification broke dormancy in fresh seeds but not in dry after-ripened seeds. Functional PHYB and PHYD were necessary to break cool-induced dormancy, which indicates a previously unknown and ecologically important function for PHYD. Disruption of PHYA in combination with PHYD (but not PHYB) restored germination to near wild-type levels, indicating that PHYA contributes to the maintenance of cool-induced dormancy on a phyD background. Effects of seed-maturation temperature were much stronger than effects of seed-maturation photoperiod. PHYB contributed to germination somewhat more strongly in seeds matured under short days, whereas PHYD contributed to germination somewhat more strongly in seeds matured under long days. The variable contributions of different phytochromes to germination as a function of seed-maturation conditions reveal further functional diversification of the phytochromes during the process of germination. This study identifies among the first genes to be associated with maternal environmental effects on germination.

(207) Sensitivity of Two Lettuce Genotypes to Abscisic Acid during Germination

In many lettuce (Lactuca sativa L.) cultivars, temperatures above 30 °C can inhibit germination completely. Lettuce seeds imbibed at supraoptimal temperature for 72 hours or more will not germinate even when the temperature is returned to the optimal range. Sung et al. (1998) reported that thermosensitive `Dark Green Boston' and thermotolerant `Everglades' responded to temperature at seed maturation by being able to show greatly enhanced germination at 36 °C when seeds were matured at 30/20 °C. Abscisic acid (ABA) plays an important role relative to both dormancy and germination of many seeds and may contribute to lettuce seed thermodormancy. Therefore, sensitivity of `Everglades' and `Dark Green Boston' to ABA of seeds maturated at 30/20 °C and 20/10 °C in light and dark was determined. Seeds were germinated at 20 °C and 36 °C in light and dark. All seeds of `Dark Green Boston'and `Everglades' matured at 30/20 °C and 20/10 °C germinated similarly regardless of maturation temperature and light/dark condition at 20 °C. At 36 °C in dark, both genotypes matured at 30/20 °C germinated more than those matured at 20/10 °C. `Dark Green Boston' genotypes were more sensitive to ABA (0.5, 1, 10, 50, 100 μM) during germination at 36°C in dark. When ABA was added, seeds matured at 30/20 °C germinated better than those matured at 20/10 °C, regardless of genotype. At 36 °C in light, 40% of both genotypes were inhibited at 0.5 μM ABA. At 36 °C in dark, germination of all seeds but `Everglades' matured at 30/20 °C were completely inhibited at all ABA concentrations. Thus, seed maturation temperature has an influence on lettuce seed germination at high temperature and sensitivity to ABA.

High Maturation Temperature of Lettuce Seeds during Development Increased Ethylene Production and Germination at Elevated Temperatures

Ethylene synthesis and sensitivity, and their relation to germination at supraoptimal temperatures, were investigated in lettuce (Lactuca sativa L.) seeds matured at 30/20 °C [12-h day/night, high temperature matured (HTM)] or 20/10 °C [12-h day/night, low temperature matured (LTM)]. HTM seeds of both thermosensitive `Dark Green Boston' (DGB) and thermotolerant `Everglades' (EVE) had greater germination at a supraoptimal temperature (36 °C), in both light or dark, than LTM seeds of DGB and EVE. HTM seeds of DGB and EVE produced more ethylene during germination than LTM seeds, regardless of imbibition conditions. The ethylene action inhibitor, silver thiosulfate, led to reduced germination in both cultivars. The ethylene precursor, 1-aminocyclopropane-1-carboxylic acid at 10 mm increased germination of both cultivars at supraoptimal temperatures, whereas germination of HTM seeds was greater than that of LTM seeds. No differences in ethylene perception were detected between HTM and LTM germinating seeds using a triple response bioassay. This study demonstrated that at least one method through which seed maturation temperature influences lettuce germination is by affecting ethylene production.

Effects of Seed Maturation Temperature on Seed Yield Characteristics and Subsequent Generations of Lupin

Phenology and morphology of white lupin (Lupinus albus L.) varied in plants derived from different seed lots of the same cultivar, presumably because thermosensitive genotypes are affected by thermal environment during early development. This study was conducted to determine whether the maternal thermal environment of developing seeds influenced subsequent plant development. We grew white lupin plants cv. Ultra, at the mainstem first flower stage, in controlled growth chambers at 13 or 28C temperature under a 14‐h photoperiod. At harvest maturity, yield components were quantified. The two resultant seed populations were planted in the greenhouse, and at the mainstem first flower stage, one‐half of each group of plants was placed in growth chambers at 13 or 28°C. This regimen was carried out through three generations. Seed dry weight per plant and weight per seed were greater for plants matured at 13°C than at 28°C. All plants produced from maternal seed that had matured at 28°C developed at a slower rate than those at 13°C. Floral events in these plants lagged behind by as much as 15 d. No differences were observed in mainstem vegetative node number. We concluded that phenotypic variability among seedlots of the same genotype can be caused by thermal environmental conditions occurring as embryos develop. These results show that thermal environment during seed production can affect subsequent crop performance, which can impact cultivar production and consistency among years.

Seed Developmental Temperature Regulation of Thermotolerance in Lettuce

Lettuce (Lactuca sativa L.) seeds can fail to germinate at temperatures above 24 °C. The degree of thermotolerance is thought to be at least partly related to the environment under which the seed developed. In order to study the effects of temperature during seed development on subsequent germination, various lettuce genotypes were screened for their ability to germinate at temperatures ranging from 20 to 38 °C. Seeds of the selected genotypes `Dark Green Boston' and `Valmaine' (thermosensitive), `Floricos 83', `Everglades', and PI 251245 (thermotolerant) were produced at 20/10, 25/15, 30/20, and 35/25 °C day/night temperature regimes in plant growth chambers. Seeds were germinated on a thermogradient bar from 24 to 36 °C under 12 h light/dark cycles. As germination temperature increased, the number of seeds that failed to germinate increased. Above 27 °C, seeds matured at 20/10 or 25/15 °C exhibited a lower percent germination than seeds that matured at 30/20 or 35/25 °C. Seeds of `Dark Green Boston' and `Everglades' that matured at 30/20 °C exhibited improved thermotolerance over those that matured at lower temperatures. Seeds of `Valmaine' produced at 20/10 °C exhibited 40% germination at 30 °C, but seeds that matured at higher temperatures exhibited over 95% germination. Germination of `Valmaine' at temperatures above 30 °C was not affected by seed maturation temperature. The upper temperature limit for germination of lettuce seed could thus be modified by manipulating the temperature during seed production. The potential thermotolerance of seed thereby increased, wherein thermosensitive genotypes became thermotolerant and thermotolerant genotypes (e.g., PI251245) germinated fully at 36 °C. This information is useful for improving lettuce seed germination during periods of high soil temperature, and can be used to study the biology of thermotolerance in lettuce.

Structural Changes in Lettuce Seed during Germination Altered by Genotype, Seed Maturation Temperature, and Priming

Thermotolerance in lettuce seed at high temperature was investigated using primed and nonprimed seed or seeds matured at 20/10°C and 30/20°C. During seed germination at 36°C, the structural changes of the seed coverings in front of the radicle tip were observed in an anatomical study. In all seeds during imbibition, regardless of seed maturation temperature or priming, a crack appeared on one side of the cap tissue and the endosperm separated from the integument in front of the radicle tip. Additional changes took place during imbibition: the protein bodies in the vacuoles enlarged and were gradually depleted, large empty vacuoles formed, the cytoplasm condensed, the endosperm shrank, the endosperm cell wall dissolved and ruptured, then the radicle elongated toward this ruptured area. The findings suggested that the papery endosperm layer presented mechanical resistance to lettuce seed germination and the weakening of this layer was a prerequisite to radicle protrusion at high temperature. Seeds of `Dark Green Boston', `Everglades', and PI 251245 matured at 30/20°C had greater thermotolerance than those matured at 20/10°C. Results of the anatomical study indicated that the endosperm cell walls in front of the radicle of seeds matured at 30/20°C were more easily disrupted and ruptured during early imbibition than seeds matured at 20/10°C, suggesting that these seeds could germinate quickly at supra-optimal temperatures. From anatomical studies conducted to identify and characterize thermotolerance in lettuce seed germination, it was observed that genotype thermotolerance had the ability to reduce physical resistance of the endosperm by weakening the cell wall and by depleting stored reserves.

Soluble Carbohydrates in White Lupin Seeds Matured at 13 and 28°C

Maturation of white lupin seed (Lupinus albus L. cv. Ultra) at 28°C results in later flowering and reduced seed yield in plants grown from these seed compared with plants grown from seed matured at 13°. The objectives of this study were to determine the changes in seed mass and soluble carbohydrates of white lupin seed as affected by seed maturation temperature. Fourteen soluble carbohydrates were identified and quantified by high resolution gas chromatography of the trimethylsilylimidazole‐derivatization products. Reducing sugars were not detected, and sucrose was 10 to 15% of total soluble carbohydrates in the axis and 12 to 20% in the cotyledons. Seventy to 80% of the total soluble carbohydrates were raffinose, stachyose, and verbascose. In addition to the raffinose series oligosaccharides, four series of galactosyl cyclitols were present including the galactinol series, galactopinitol A series, galactopinitol B series, and fagopyritol B1 series. Seed matured at 28°C accumulated only 53 to 70% as much dry matter as seeds matured at 13<C. Only minor changes in the raffinose series oligosaccharides were observed. Pinitol and the galactose‐containing pinitols were more than doubled by seed maturation at 28°C, but collectively, these compounds are <10% of the total soluble carbohydrates. We conclude that the effect of seed maturation temperature on the composition and concentration of soluble carbohydrates is minimal.