Introduction
In temperate zones plants use environmental temperature as a key signal to synchronize their life history with the seasons. Thus temperature is an important regulator of a number of developmental processes, including seed dormancy and germination. For annual plants chilling or alternating day and night temperatures are often strong germination-promoting cues, while daily maximum temperature during key windows of seed maturation is important for determining the level of primary dormancy. For secondary dormant seeds lying buried in the soil seed bank, temperature, together with soil moisture content, is the predominant signal that allows dormancy to cycle on a seasonal basis (Bewley and Black, Reference Bewley and Black1994).
Climate change is predicted to be most apparent in the northernmost latitudes, where significant warming has already taken place compared to standard measures of 20th-century mean temperatures (IPCC, 2007). Because dormancy and germination are temperature-dependent processes, it is possible that climate change will alter annual plant life history and, over time, select new life-history variants from existing populations (Kover et al., Reference Kover, Rowntree, Scarcelli, Savriama, Eldridge and Schaal2009). One study has even concluded that in Europe, over 30% of all herbaceous plant species may be driven to extinction primarily through an effect of climate warming on dormancy cycling in seeds and buds (Thuiller et al., Reference Thuiller, Lavorel, Araujo, Sykes and Prentice2005). However, predicting the effects of a changing climate on natural populations is extraordinarily challenging, as existing and new variation may provide material for deriving populations capable of thriving in the modified conditions. Thus we need to understand the genetic basis of the link between temperature and plant life history, the key processes underlying the control of key life-history traits, and how these might evolve. This is a big challenge, even in a model species such as Arabidopsis thaliana, which is an excellent model system for the study of life-history evolution as well as the genetic basis of plant traits (Metcalf and Mitchell-Olds, Reference Metcalf and Mitchell-Olds2009). Such information can also be expected to inform future plant breeding initiatives, for instance to breed crops with new combinations of traits better adapted to growing in future climate scenarios.
Temperature during seed maturation determines primary dormancy depth
During seed maturation, environmental stimuli such as temperature and photoperiod are important determinants of many characteristics of seeds, which in turn can potentially affect the developmental stages of the plant that follow. Effects of such stimuli on seed characteristics such as seed size and weight have been noted since the early 1950s. Chenopodium polyspermum L. seeds from mother plants grown in long days have lower germination frequencies and thicker seed coats in comparison to seeds from mother plants grown in short days (Pourrat and Jacques, Reference Pourrat and Jacques1975). Maternal temperature also influences the germination of seeds from the mother plant and, in almost every reported case, higher temperatures during seed maturation correlate with increased germination (Fenner, Reference Fenner1991). One interesting and often overlooked feature of this phenomenon is that the temperature applied to vegetative tissues pre-anthesis has been shown to influence dormancy of seed subsequently produced. Two clear examples have been shown using tobacco (Thomas and Raper, Reference Thomas and Raper1975) and wild oats (Sawhney et al., Reference Sawhney, Quick and Hsiao1985), and we show here that the same phenomenon can be observed with Arabidopsis thaliana (Fig. 1). Therefore plants can pass a memory of previous temperatures to their offspring. Our understanding of how the maternal environmental temperature leads to changes in dormancy is still poor. More recently, work on Arabidopsis and rice has begun to demonstrate the genetics important for the influence of the maternal experience of temperature on primary seed dormancy (Gu et al., Reference Gu, Kianian and Foley2006; Schmuths et al., Reference Schmuths, Bachmann, Weber, Horres and Hoffmann2006; Donohue et al., Reference Donohue, Heschel, Butler, Barua, Sharrock, Whitelam and Chiang2008; Chiang et al., Reference Chiang, Barua, Kramer, Amasino and Donohue2009; Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011).
Figure 1 Environmental temperature before and after anthesis of the mother plant influences Arabidopsis seed dormancy. Plants were grown to anthesis of the first flower at either 15°C or 22°C, and then swapped to the other temperature. Seed dormancy was compared to control plants maintained for the whole life cycle at either 15°C or 25°C. Data represent the mean germination of five independent seed lots per treatment.
In Arabidopsis, low temperature during seed maturation leads to high primary dormancy levels, whereas warm temperatures lead to lower dormancy. This effect can be observed across a wide range of genetic backgrounds, with only strongly dormant ecotypes such as CVI showing insensitivity to this effect (Schmuths et al., Reference Schmuths, Bachmann, Weber, Horres and Hoffmann2006; Penfield and Springthorpe, Reference Penfield and Springthorpe2011). Lower seed maturation temperatures are likely to be experienced in the wild, either in winter annuals setting seed early in the spring, or in late-setting summer annuals whose reproduction lasts into autumn. In the latter case low seed maturation temperatures are often essential for seeds to make the necessary transition into secondary dormancy, required for overwintering in the soil seed bank (Penfield and Springthorpe, Reference Penfield and Springthorpe2011). This is because in many ecotypes where low seed maturation temperatures are not experienced, the prolonged cold and dark incubation of seeds required to shift Arabidopsis seeds into secondary dormancy (Finch-Savage et al., Reference Finch-Savage, Cadman, Toorop, Lynn and Hilhorst2007) causes germination of seeds even in the absence of light, suggesting that these seeds are committed to germinating in the same growing season as they are set (Penfield and Springthorpe, Reference Penfield and Springthorpe2011). For seeds set in warmer times of the year, germination conditions may be more favourable for immediate resumption of the life cycle, or further genetic dormancy-inducing mechanisms may confer an after-ripening requirement that delays germination until a set of dormancy-breaking conditions have been fulfilled. The complex mechanism regulating the coupling of dormancy level to temperature is only just starting to be unravelled.
The relationship between primary seed dormancy and seed maturation temperature is also critically important for seed quality in cereal crops. In addition to rainfall, environmental temperature is an important determinant of the frequency of pre-harvest germination and pre-harvest sprouting, both of which damage grain quality in a range of cereals, including barley, wheat and sorghum (Cochrane, Reference Cochrane1993; Rodriguez et al., Reference Rodríguez, Margineda, González-Martín, Insausti and Benech-Arnold2001). Importantly, Rodriguez et al. (Reference Rodríguez, Margineda, González-Martín, Insausti and Benech-Arnold2001) were able to localize the period of barley seed maturation in which dormancy was maximally sensitive to temperature to a short window, using a simple thermal time model. This study also revealed that during this window eventual germination correlated with daily mean temperature, rather than the maximum or minimum. Elucidation of such details is necessary to predict the performance of crops species in new environments.
Temperature regulation of primary seed dormancy shares gene networks with other developmental pathways, such as flowering. FLOWERING LOCUS C (FLC) negatively regulates a number of genes that promote flowering, and confers a vernalization requirement on winter annuals (recently reviewed in Kim et al., Reference Kim, Doyle, Sung and Amasino2009). Epigenetic regulation represses FLC following vernalization through chromatin remodelling, thus allowing the expression of SUPPRESSOR OF OVER-EXPRESSION OF CONSTANS 1 (SOC1) and FLOWERING LOCUS T (FT) in spring. FLC has been shown to have a maternal role in the regulation of dormancy, but this regulation is apparent only when seeds are matured at low temperatures and then incubated at cool imbibition temperatures (Chiang et al., Reference Chiang, Barua, Kramer, Amasino and Donohue2009). Under these conditions near isogenic lines (NILs) containing strong alleles of FLC display higher germination when imbibed at 10°C in comparison to the wild type, but this phenotype is extremely weak at higher germination temperatures (Chiang et al., Reference Chiang, Barua, Kramer, Amasino and Donohue2009). This increase in germination is correlated with an increase in the abscisic acid (ABA) catabolic gene CYP707A2, and the gibberellic acid (GA)-biosynthesis gene GIBBERELLIN 2-OXIDASE 1 (GA2ox1) expression during imbibition, suggesting that FLC can regulate metabolism associated with hormone balance. Surprisingly, expression of DELAY OF GERMINATION 1 (DOG1), a gene identified as a quantitative trait locus which is involved in regulation of seed dormancy (Bentsink et al., Reference Bentsink, Jowett, Hanhart and Koornneef2006 l Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011) also appears to be up-regulated in the high FLC expressing line and this is usually characteristic of seeds displaying high dormancy levels.
The hy2-1 mutant has reduced levels of all five phytochromes (Chory et al., Reference Chory, Peto, Ashbaugh, Saganich, Pratt and Ausubel1989) and, like the wild type, shows high dormancy levels when matured at 10°C. However, cold stratification and after-ripening have no dormancy-breaking effects in this background, whereas approximately 80% germination can be obtained following cold stratification of seed matured at warmer temperatures (Donohue et al., Reference Donohue, Heschel, Butler, Barua, Sharrock, Whitelam and Chiang2008). This suggests that primary dormancy levels are higher in hy2-1 and that phytochromes not only respond to temperature during imbibition, but also to the maternal environment. Since the low-temperature-induced dormancy in hy2-1 is not broken by any stratification treatment nor after-ripening, it seems that lack of phytochrome can induce a state in which seeds are not responsive to dormancy-breaking signals. Interestingly, Donohue et al. (Reference Donohue, Heschel, Butler, Barua, Sharrock, Whitelam and Chiang2008) show that low-temperature induced dormancy is only alleviated by a period of warm stratification followed by cold stratification. This warm/cold stratification did not promote germination in phyB and phyD mutants and, following after-ripening, this stratification regime does not break dormancy. Recently, we have shown that seed maturation temperature controls the level of both phyB and phyE transcripts in dry seeds, suggesting that temperature can modify the light requirement for germination by impacting directly on phytochrome levels (Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011).
Dormancy and germination are both regulated by a fine balance of ABA and GA signalling, and shifts in the ratio of ABA to GA can cause different dormancy states. Levels of ABA are considerably higher in dry seeds matured at 10°C in comparison to 20°C. GA levels show the opposite, whereby levels are lower in the low-temperature matured seeds (Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011). This change was coupled with the increased expression of two GIBBERELLIN 2-OXIDASE 6 isoforms by lower temperatures, GA2ox2 and GA2ox6. Expression of CYP707A2 is also down-regulated in seeds matured at low temperature (Chiang et al., Reference Chiang, Barua, Kramer, Amasino and Donohue2009; Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011), whereas NCED4, which is involved in ABA biosynthesis, is up-regulated. This suggests that metabolism of both ABA and GA may be key to the mechanism by which temperature regulates primary dormancy, and that the levels of ABA and GA in mature seeds correlate strongly with dormancy depth. Low temperature during seed maturation causes an up-regulation in DOG1 (Bentsink et al., Reference Bentsink, Jowett, Hanhart and Koornneef2006; Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011). A summary of our understanding of the genetics of the influence of maternal and zygotic seed maturation temperature on seed dormancy is shown in Fig. 2.
Figure 2 The genetic control of primary dormancy by environmental temperature experienced by the mother plant, and during seed maturation. Maternal temperature is sensed by pathways known to be important in the control of the timing of the floral transition (Chiang et al., Reference Chiang, Barua, Kramer, Amasino and Donohue2009), but it is unclear which tissues are important. During seed maturation, temperature influences the gibberellic acid (GA) content of the dry seed, and the level of DOG1 and phytochrome expression (Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011). (A colour version of this figure can be found online at http://journals.cambridge.org/ssr).
Chilling and dormancy control in the imbibed seed
Chilling after imbibition is a widely conserved dormancy-breaking stimulus, and chilling requirements can vary enormously between species. An interesting conundrum that is not understood is how low temperature promotes dormancy during seed maturation but promotes germination during imbibition. Thus there must be a mechanism that ameliorates the dormancy-promoting effect of chilling in imbibed seed, and a second which attenuates the dormancy-breaking effect of chilling during seed maturation. It is often assumed that chilling can have both dormancy-inducing and dormancy-breaking effects simultaneously (Batlla and Benech-Arnold, Reference Batlla and Benech-Arnold2009). Similarly, extended chilling times can induce secondary dormancy, especially in Arabidopsis (Finch-Savage et al., Reference Finch-Savage, Cadman, Toorop, Lynn and Hilhorst2007; Penfield and Springthorpe, Reference Penfield and Springthorpe2011), and this requires that low temperature again induces dormancy. Germination promotion by chilling during imbibition is dependent on expression of GA3ox1, involved in later steps of the GA-biosynthetic pathway, which is upregulated at 4°C in comparison to 22°C and leads to an increase in bioactive GA levels (Yamauchi et al., Reference Yamauchi, Ogawa, Kuwahara, Hanada, Kamiya and Yamaguchi2004). This may be a clue to the mechanism, as in vegetative tissues of many species, low temperatures have been consistently shown to slow growth by inducing reductions in GA content (Tonkinson et al., Reference Tonkinson, Lyndon, Arnold and Lenton1997; Stavang et al., Reference Stavang, Lindgård, Erntsen, Lid, Moe and Olsen2005; Achard et al., Reference Achard, Gong, Cheminant, Alioua, Hedden and Genschik2008). In Arabidopsis, this reduction in GA content has been shown to be mediated by cold-induced expression of three APETALA2-domain transcription factors known as C-REPEAT BINDING FACTORS (CBFS; Stockinger et al., Reference Stockinger, Gilmour and Thomashow1997; Achard et al., Reference Achard, Gong, Cheminant, Alioua, Hedden and Genschik2008), genes also important in the development of freezing tolerance. These slow growth by promoting the transcription of GA2ox3, which leads to inactivation of GAs. Interestingly, in imbibed seeds no up-regulation of CBF expression is detected in response to low temperature, showing that a mechanism exists to prevent the temperature-regulation of CBF transcription in seeds (Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011). This must be important because expression of high GA2ox3 levels in seed in response to chilling would not be expected to lead to germination and, indeed, CBF overexpression causes a germination inhibition that can be overcome by exogenous GA (Kendall et al., Reference Kendall, Hellwege, Marriot, Whalley, Graham and Penfield2011). In contrast, absence of CBF leads to a reduction in DOG1 and GA2ox6 and to low dormancy expression in dry seeds exposed to low temperature during seed maturation, suggesting that CBF action in maturing seeds can contribute to dormancy control by low maturation temperatures.
Interestingly, the role of phytochromes during seed imbibition appears to be temperature dependent. PHYE contributes to germination at low temperatures, whereas PHYA is important for germination at warm temperatures. PHYB, on the other hand, is important for germination over a range of temperatures (Heschel et al., Reference Heschel, Selby, Butler, Whitelam, Sharrock and Donohue2007). This therefore suggests that the phytochromes are not only important for responding to light signals but are able to exert influence over temperature signal transduction. Light and temperature also regulate the transcription factor SPATULA (SPT), a member of the PHYTOCHROME INTERACTING FACTOR sub-family of bHLH proteins. SPT can act as a positive or negative regulator of germination depending on the ecotype background (Penfield et al., Reference Penfield, Josse, Kannangara, Gilday, Halliday and Graham2005; Gan et al., Reference Gan, Josse, Penfield, Gilday, Halliday and Graham2007). In Ler, the standard Arabidopsis laboratory accession Landsberg erecta, SPT is a germination promoter and the spt-2 mutation blocks chilling-responsive germination and the induction of GA3ox expression by cold. Chilling stabilizes the SPT protein in seedlings, where it also acts to repress growth (Sidaway-Lee et al., Reference Sidaway-Lee, Josse, Brown, Gan, Halliday, Graham and Penfield2010), showing that temperature can directly impact the SPT protein. Thus chilling may promote germination in part through stabilization of the SPT protein. SPT is also expressed during seed development, and so may be important for establishing the chilling-responsiveness of primary dormancy, but less important in the imbibed seed. SPT can also interact with DELLA proteins, linking temperature signalling to hormone response pathways. Elucidation of the targets of SPT will help uncover its precise mode of action.
Thermoinhibition of germination
Dormancy is often related to the control of the permissive temperature range within which germination can occur, with more dormant seeds germinating only after the experience of a narrower window of germination conditions. However, high temperatures can suppress the germination of seeds with little or no dormancy in a phenomenon known as thermoinhibition. Expression of the ABA biosynthesis genes NCED2, NCED5 and NCED9 is higher in seeds imbibed at 34°C in comparison to 22°C, thus suggesting that they contribute to enhanced ABA biosynthesis at high temperature (Toh et al., Reference Toh, Imamura, Watanabe, Nakabayashi, Okamoto, Jikumaru, Hanada, Aso, Ishiyama, Tamura, Iuchi, Kobayashi, Yamaguchi, Kamiya, Nambara and Kawakami2008). A significant reduction in expression of the ABA catabolic genes CYP707A1, CYP707A2 and CYP707A3 was also observed at 34°C. In addition to regulation of ABA in response to high temperature, GA also plays an important role. Levels of both GA1 and GA4 are reduced in seeds imbibed at high temperature and this is due to a reduction in expression of GA biosynthesis genes. This suppression requires ABA since aba2-2 mutants display increased expression of these genes at high temperature. Mutants isolated from a screen for high temperature resistant germination included a new abi3 allele, abi3-14, and transparent testa (tt) mutants (Tamura et al., Reference Tamura, Yoshida, Tanaka, Sasaki, Bando, Toh, Lepiniec and Kawakami2006). The isolation of mutants affected in dormancy control in genetic screens for thermoinhibition insensitivity suggests that this effect is an extreme form of dormancy. The role of ABA synthesis in the thermoinhibition of germination appears to be conserved across species, as a key quantitative trait locus (QTL) for high temperature germination in lettuce also corresponds to an NCED orthologue (Argyris et al., Reference Argyris, Truco, Ochoa, McHale, Dahal, Van Deynze, Michelmore and Bradford2011).
Germination promotion by alternating temperatures
This is still a very poorly understood physiological process and there is only limited understanding of how and why alternating temperatures appear to be critical for germination in many dormant species. A favoured theory is that vegetation cover can reduce the daily amplitude of temperature oscillations and that the alternating temperature response is an adaptation to germination after canopy removal (Toole et al., Reference Toole, Hendricks, Borthwick and Toole1956). An interesting early demonstration of the importance of alternating temperatures was by Stotzky and Cox (1962), who realized that germination of Musa balbisiana seeds was strongly inhibited in their greenhouse in winter, when it was artificially heated to maintain temperature. Mechanisms touted as candidates were the need for sequential destruction of inhibitor and synthesis of activators, or the regulation of some aspect of metabolism, or light signalling (Toole et al., Reference Toole, Hendricks, Borthwick and Toole1956). The adaptive significance of the alternating temperature response was probed in an important multi-species study of germination responses on seeds from various ecosystems (Thompson et al., Reference Thompson, Grime and Mason1977). This showed that seeds from habitats which experience seasonal flooding have clear requirements for alternating temperatures, and also confirmed that canopy removal increases the daily temperature amplitude. Thus the requirement for large daily temperature amplitudes may be an adaptation to time germination to a seasonal dry spell in wetland areas. Alternating temperatures have been shown to be important for the germination of many species, and even promote increased germination in dormant Arabidopsis seeds (Ali-Rachedi et al., Reference Ali-Rachedi, Bouinot, Wagner, Bonnet, Sotta, Grappin and Jullien2004) and one complete temperature cycle is sufficient for many species.
Germination promotion by alternating temperatures may be a phenomenon related to thermoperiodism (Went, Reference Went1944). Thermoperiodic growth responses have been observed in many species and are characterized as a growth promotion by oscillating day and night temperatures, compared to a constant warm temperature alone. Thermoperiodism acts, at least in part, through the regulation of GA levels (Stavang et al., Reference Stavang, Lindgård, Erntsen, Lid, Moe and Olsen2005). In Arabidopsis, disruption of circadian rhythms has been shown to attenuate germination promotion by alternating temperatures, suggesting that entrainment of the circadian clock to temperature signals may be important for germination, or that the circadian clock might be necessary for the measurement of temperature differentials (Penfield and Hall, Reference Penfield and Hall2009). Because of the importance of alternating temperatures as a dormancy-breaking cue, further work is required to understand the mechanism underlying the effect.
Conclusions
Seeds use environmental temperature in a variety of ways to co-ordinate germination timing, but the mechanisms of temperature signal transduction have only recently begun to be understood. Because temperature influences a wide variety of seed traits, a better understanding of the molecular mechanisms underlying temperature signalling is essential to breed crops giving consistently high seed quality in a variety of seed maturation environments. Our best hope for this is to combine knowledge of the genetic basis of key traits gleaned from model systems to identify candidate targets for selection during breeding. A second important consideration is the genotype of the parent plants, and understanding how and when environmental signals are influencing traits of interest. Finally, because of pervasive pleiotropy in the control of seasonal events in plants (Chiang et al., Reference Chiang, Barua, Kramer, Amasino and Donohue2009; Kover et al., Reference Kover, Rowntree, Scarcelli, Savriama, Eldridge and Schaal2009) it is critical to understand the overlap between the control of seed traits and other crop characters vital to performance.
