Rice (Oryza sativa) has the rare ability to germinate and elongate a coleoptile under oxygen-deficient conditions, which include both hypoxia and anoxia. It has previously been shown that Alcohol Dehydrogenase 1 (ADH1) is required for cell division and cell elongation in the coleoptile of submerged rice seedlings by means of studies using a rice ADH1-deficient mutant, reduced adh activity (rad).
A recent paper in Annals of Botany aims to understand how low ADH1 in rice affects carbohydrate metabolism in the embryo and endosperm, and lactate and alanine synthesis in the embryo during germination and subsequent coleoptile growth in submerged seedlings.
Even in a submerged environment containing substantial amounts of dissolved oxygen, a reduction in ADH (as brought about by an ADH1 mutation) reduces seedling viability, changes the balance between the end-products of glycolysis and decreases sugar concentrations in the endosperm and embryo. Exogenous sugar did not improve the growth or survival of the ADH1 mutant, indicating that sugar processing in the embryo was probably the limiting factor. However, how low ADH activity affects the endosperm deserves further experimental attention. The endosperm is well suited for investigations of sugar production and transport because of its simple composition and metabolism.
GPT2,a glucose 6-phosphate/phosphate translocator, plays an important role in environmental sensing in mature leaves of Arabidopsis thaliana, and its expression has also been detected in arabidopsisseeds and seedlings. Dyson et al.study wild-type A. thaliana and a gpt2 T-DNA insertion knockout line, and find that plants lacking GPT2 expression are delayed in seedling establishment, specifically in the process of cotyledon greening (rather than germination). This phenotype cannot be rescued by glucose in the growth medium, with greening being hypersensitive to glucose. Germination itself is, however, hyposensitive to glucose in the gpt2 mutant. They conclude that endogenous sugar signals function in controlling germination and the transition from heterotrophic to autotrophic growth, and that the partitioning of glucose 6-phosphate, or related metabolites, between the cytosol and the plastid modulates these developmental responses.
Seed responses to light can control the timing of germination in the field, impacting seedling survival, as well as growth and fitness in subsequent life stages. Seeds that require light for germination are usually small. Milberg et al. (2000) suggested that a light response and seed mass coevolved as an adaptation to ensure germination of small-seeded species only when close to the soil surface. On the other hand, a phylogenetic component of light-promoted germination – regardless of seed size – has been suggested.
Temperature is a major factor modulating seed responses to light: a seed may require light to germinate at a given temperature but not at other temperatures. For some species, temperature fluctuations can fully or partially substitute for the light requirement. The amplitude of soil temperature fluctuations is highest on or close to the surface of bare soil and in vegetation gaps.
Phytochromes are well known to mediate light-promoted germination; they are also known to increase the amount of bioactive gibberellins in seeds. Exogenously applied gibberellins promote germination of photorequiring seeds in darkness. Conversely, nitrates which are naturally occurring in the soil, can also substitute for the light requirement in some cases.
A recent paper in Annals of Botany aims to determine the effect of light on germination for 131 taxa in the Campanulaceae. The authors tested germination in light and darkness at constant and alternating temperatures; associated the response to light with seed mass and alternating vs. constant temperatures; and examined whether gibberellic acid and nitrate can substitute for the light requirement. They find that the influence of light on germination was much stronger in smaller than in larger seeded species; thus germination is prevented when seeds are buried deep in the soil. Larger seeded species can germinate in deeper soil depths in the presence of fluctuating temperatures.
Koutsovoulou, K., Daws, M.I., & Thanos, C.A. Campanulaceae: a family with small seeds that require light for germination. (2014) Annals of Botany, 113(1), 135-143.
The Campanulaceae is a large cosmopolitan family, but is understudied in terms of germination, and seed biology in general. Small seed mass (usually in the range 10–200 µg) is a noteworthy trait of the family, and having small seeds is commonly associated with a light requirement. Thus, the purpose of this study was to investigate the effect of light on germination in 131 taxa of the Campanulaceae family, from all five continents of its distribution.
For all taxa, seed germination was tested in light (8 or 12 h photoperiod) and continuous darkness under constant and alternating temperatures. For four taxa, the effect of light on germination was examined over a wide range of temperatures on a thermogradient plate, and the possible substitution of the light requirement by gibberellic acid and nitrate was examined in ten taxa.
For all 131 taxa, seed germination was higher in light than in darkness for every temperature tested. Across species, the light requirement decreased significantly with increasing seed mass. For larger seeded species, germination in the dark reached higher levels under alternating than under constant temperatures. Gibberellic acid promoted germination in darkness whereas nitrates partially substituted for a light requirement only in species showing some dark germination.
A light requirement for germination, observed in virtually all taxa examined, constitutes a collective characteristic of the family. It is postulated that smaller seeded taxa might germinate only on the soil surface or at shallow depths, while larger seeded species might additionally germinate when buried in the soil if cued to do so by fluctuating temperatures.
Environmental and genetic effects on a cline in seed dormancy
Climate may determine changes in seed dormancy in the short and the long term, shaping plant responses to global change. Fernández-Pascual et al. investigate germination in Centaurium somedanum, a narrow endemic species, using seeds collected from different wild populations along a local altitudinal gradient and seeds of a subsequent generation produced in a common garden. They find a local dormancy cline that is related to climatic differences between sites and to population genetic composition. This cline is further affected by the weather conditions during seed maturation, which influence the receptiveness to dormancy-breaking factors. The presence of intraspecific variation at such a local scale highlights the great potential of physiological dormancy to adapt to environmental changes.
Seedlings are more sensitive to severe environmental conditions than both adult plants and seeds of the same species, making germination a risky one-way transition in the plant life cycle. Consequently, strong selection pressures act on germination responses, resulting in a range of strategies to time germination appropriately to places or times that are suitable for seedling survival and onward growth. A range of environmental conditions tell seeds whether or not they are in a place suitable for germination, such as light conditions, smoke after burning and moisture. Timing strategies for germination are often cued by temperature and cold stratification. It is often argued that life under severe and unfavourable climatic conditions will select for increased environmental tolerance in local populations.
Calluna is the keystone species of Europe’s heathland systems and occurs throughout a broad geographical and climatic range, being found along Europe’s western coast from the Strait of Gibraltar to northern Norway, from sea level into the alpine zone (Pyrenees, Alps, Scottish Highlands and Scandinavian Mountains) and even in continental Western Russia. A new study in Annals of Botany investigates germination behaviour along climatic gradients in heather, Calluna vulgaris. The finding of a conditional cold-avoidance strategy for Calluna germination together with previous records from Scotland, France and Spain support a theory of gradual replacement of cold as the main hazard for seedlings as we move south in Europe by first competition and then, further south, possibly drought, that explains varying germination patterns in relation to temperature. Our main results suggest that Calluna in Northern Europe generally avoids hazards imposed by cold climates by cueing germination towards the relatively warm frost-free late spring to early summer season. In populations from less adverse climates, the species’ cold-avoidance strategy seems to be weakened in favour of earlier germination, which would allow the species to address other limitations of, for example, light and space as a consequence of higher competition under warmer climates.
Remember the millennium? Maybe you’re too young. (Or possibly you just went to a better party than I did.) Aside from wasting silly amounts of money building stupid domes, one of the better ideas that quite a few people had was the creation of millennium seedbanks and as way of ensuring the prosperity (and full bellies) of future generations by preserving the germplasm of plant species in long term storage. And for flowering plants, that means seeds. But what if, when we really need them at some point in the future, the seeds don’t grow?
Seeds stored for prolonged periods are subjected to severe oxidative damage, caused by the progressive accumulation of reactive oxygen species (ROS) and that loss of seed viability and reduced germination represent the undesired consequences of ageing. Significant factors in seed longevity are the level of DNA damage and the DNA repair response, the amount of non-enzymatic antioxidants and activity of ROS-scavenging enzymes. In order to preserve the high seed viability at the pre-emergence step, both the DNA repair functions and the overall antioxidant activities must be kept at an appropriate level in the embryo. Different DNA repair pathways are activated during the early phase of seed imbibition. The ability to carry out ROS scavenging, expressed as the seed antioxidant potential, is a critical requirement to withstand stress and improve germination. The cell antioxidant systems prevent ROS attack but when ROS production exceeds the capacity of the antioxidant machinery, oxidative injury takes place.
Factors such as temperature and humidity are positively correlated with seed ageing and they must be strictly controlled during seed manipulation for long-term conservation in seed banks. To date, germination tests represent the most reliable method to assess seed viability, although it is a time-consuming and labour-intensive operation. Novel low-cost and equally reliable methods are required, which might speed up the seed viability analysis. Molecular and biochemical markers of seed ageing might be used for these purposes. A deeper understanding of the complex network of molecular events which control seed longevity is, however, required in order to select appropriate markers providing information on deterioration and germination potential of seed stocks collected for bank storage.
A new paper in Annals of Botany investigates reliable markers of seed deterioration. The response to DNA damage induced by artificial ageing was compared in seeds of Silene vulgaris and S. acaulis inhabiting low- and high-altitude locations of Northern Italy. Previous investigations have demonstrated that these species differ in seed longevity, making them useful candidates to assess novel markers of seed deterioration. An in-depth investigation which included ROS accumulation profiles, antioxidant capacity and telomere length was carried out, focusing mainly on dry seeds and seeds subjected to rehydration. A positive impact of the reported results could be envisaged within a relatively short time, since specific suggestions can be derived for improving the rehydration protocol of seeds from a high-altitude location.
DNA profiling, telomere analysis and antioxidant properties as tools for monitoring ex situ seed longevity. (2013) Annals of Botany 111 (5): 987-998. doi: 10.1093/aob/mct058
The germination test currently represents the most used method to assess seed viability in germplasm banks, despite the difficulties caused by the occurrence of seed dormancy. Furthermore, seed longevity can vary considerably across species and populations from different environments, and studies related to the eco-physiological processes underlying such variations are still limited in their depth. The aim of the present work was the identification of reliable molecular markers that might help in monitoring seed deterioration. Dry seeds were subjected to artificial ageing and collected at different time points for molecular/biochemical analyses. DNA damage was measured using the RAPD (random amplified polymorphic DNA) approach while the seed antioxidant profile was obtained using both the DPPH (1,1-diphenyl, 2-picrylhydrazyl) assay and the Folin–Ciocalteu reagent method. Electron paramagnetic resonance (EPR) provided profiles of free radicals. Quantitative real-time polymerase chain reaction (QRT-PCR) was used to assess the expression profiles of the antioxidant genes MT2 (type 2 metallothionein) and SOD (superoxide dismutase). A modified QRT-PCR protocol was used to determine telomere length. The RAPD profiles highlighted different capacities of the two Silene species to overcome DNA damage induced by artificial ageing. The antioxidant profiles of dry and rehydrated seeds revealed that the high-altitude taxon Silene acaulis was characterized by a lower antioxidant specific activity. Significant upregulation of the MT2 and SOD genes was observed only in the rehydrated seeds of the low-altitude species. Rehydration resulted in telomere lengthening in both Silene species. Different seed viability markers have been selected for plant species showing inherent variation of seed longevity. RAPD analysis, quantification of redox activity of non-enzymatic antioxidant compounds and gene expression profiling provide deeper insights to study seed viability during storage. Telomere lengthening is a promising tool to discriminate between short- and long-lived species.
Post-shedding seed development in Galanthus and Narcissus
Seeds of the moist temperate woodland species Galanthus nivalis and Narcissus pseudonarcissus, which disperse during spring or early summer, germinate poorly in laboratory tests. Newton et al.study seed characteristics and find a distinct typology of seed development matching their ecological niche, whereby seeds are dispersed close to the end of seed filling without substantial developmental arrest or significant maturation drying. Seeds are comparatively immature with limited desiccation tolerance at shedding, with substantial embryo elongation occurring slowly post-shedding, consistent with the idea that seed development continues naturally on woodland floors or in soil seed banks under the tree canopy after seed dispersal.
Ecological significance of seed recalcitrance in Quercus
Several widespread tree species of temperate forests produce recalcitrant (desiccation-sensitive) seeds, but the ecological significance of this is largely unknown. Using Quercus ilex (holm oak) woodlands in France as a model system, Joët et al.study relationships between winter climate and the water status and viability of seeds in the spring. They find that percentage germination and normal seedling development are tightly linked to the water content of seeds after the winter period, indicating that in situ desiccation is a major cause of mortality: cumulative rainfall and maximum temperatures during winter dramatically influence the water status and viability of seeds. They conclude that seed desiccation sensitivity is a key functional trait that may influence the success of recruitment in temperate recalcitrant seed species, particularly within the context of future climate change.
Germination responses to smoke-water, KAR1 and glyceronitrile
Some plants live in environments where fire is a frequent enemy to survival. Unless you’re big enough to outgrow the risk of fire with tough bark like Sequoia, the obvious answer is to lie dormant in the soil until the fire has passed, helpfully leaving lots of nice nutrients available for germinating seeds. But if you’re buried in the soil, you need to know when the time for germination comes before you get out-competed by species which are quicker off the mark.
In 1990, de Lange and Boucher reported the landmark discovery that aerosol smoke and aqueous smoke-water could promote the germination of Audouinia capitata, a rare South African species. Subsequently, smoke has been shown to promote the germination of many other species, many of which were previously difficult to germinate. Smoke-stimulated germination has extensive implications for horticulture, weed control, conservation and habitat restoration. Much effort has focused on determining the chemical(s) in smoke responsible for stimulating germination.
Karrikinolide (KAR1, or 3-methyl-2H-furo[2,3-c]pyran-2-one if you prefer) was once considered by some to be the sole chemical responsible for all smoke-stimulated germination. Tersonia cyathiflora, an Australian fire ephemeral with an obligate requirement for smoke to germinate, is unresponsive to known smoke chemicals such as KAR1. Most related species are fire ephemerals that germinate predominantly after fire, often in large numbers, and live for only a few years, thereafter persisting as seeds in the soil seed-bank until a subsequent fire. In addition to T. cyathiflora, other fire ephemerals in the Gyrostemonaceae, including Gyrostemon racemiger and G. ramulosus, germinate in response to smoke following a period of burial. However, the response of Gyrostemon seeds to KAR1 had not yet been tested previously. Germination stimulation of T. cyathiflora by plant-derived smoke-water, but not KAR1, suggests that there may be other chemical(s) in smoke-water that promote the germination of certain species. Glyceronitrile (2,3-dihydroxypropanenitrile) was recently isolated from smoke-water and stimulates the germination of a number of species, including various Anigozanthos spp. that are also unresponsive to KAR1. This chemical contains nitrogen in addition to carbon, hydrogen and oxygen, and is proposed to operate through the release of cyanide.
New research published in Annals of Botany tested whether seeds of Gyrostemon racemiger and G. ramulosus respond to plant-derived smoke-water and/or KAR1; and, secondly, whether seeds of G. racemiger were stimulated to germinate by glyceronitrile or nitrogen oxides. The results show that these species respond to smoke-water but not to KAR1 or to glyceronitrile. However, making smoke-water by burning cellulose alone showed that germination is stimulated by a compound composed only of carbon, hydrogen and oxygen alone. Whether the smoke-stimulated germination response is an ancient trait or has arisen from convergent evolution has been debated. Both smoke and KAR1 responsiveness are phylogenetically widespread which gives some support to this being an ancient trait.
Comparison of germination responses of Anigozanthos flavidus (Haemodoraceae), Gyrostemon racemiger and Gyrostemon ramulosus (Gyrostemonaceae) to smoke-water and the smoke-derived compounds karrikinolide (KAR1) and glyceronitrile. (2013) Annals of Botany 111(3): 489-497. Abstract
A major germination-promoting chemical in smoke-water is 3-methyl-2H-furo[2,3-c]pyran-2-one (karrikinolide, KAR1). However, not all species that germinate in response to smoke-water are responsive to KAR1, such as Tersonia cyathiflora (Gyrostemonaceae). In this study, a test was made of whether two Gyrostemon species (Gyrostemonaceae) that have previously been shown to respond to smoke-water, respond to KAR1. If not, then the smoke-derived chemical that stimulates germination of these species is currently unknown. Recently, glyceronitrile was isolated from smoke-water and promoted the germination of certain Anigozanthos species (Haemodoraceae). Whether this chemical promotes Gyrostemon racemiger germination is also examined. Furthermore, an investigation was carried out into whether these species germinate in response to smoke-water derived from burning cellulose alone. Gyrostemon racemiger and G. ramulosus seeds were buried after collection and retrieved in autumn the following year when dormancy was alleviated and seeds had become responsive to smoke-water. Anigozanthos flavidus seeds were after-ripened at 35 °C to alleviate dormancy. Gyrostemon and Anigozanthos seeds were then tested with ‘Seed Starter’ smoke-water, KAR1, glyceronitrile and cellulose-derived smoke-water. Although Gyrostemon racemiger, G. ramulosus and A. flavidus were all stimulated to germinate by ‘Seed Starter’ smoke-water, none of these species responded to KAR1. Gyrostemon racemiger germination was not promoted by glyceronitrile. This is in contrast to A. flavidus, where glyceronitrile, at concentrations of 1–500 µM, promoted germination, although seedling growth was inhibited at ≥400 µM. Maximum A. flavidus germination occurred at glyceronitrile concentrations of 25–300 µM. Some Gyrostemon germination was promoted by cellulose-derived smoke-water. KAR1 and glyceronitrile, chemicals in smoke-water that are known to stimulate germination in other species, did not promote the germination of G. racemiger. This suggests that other chemical(s) which promote germination are present in smoke, and may be derived from burning cellulose alone.
At this point, it’s time to reward the plant scientists “of a certain age” who will have been humming this all the way through reading this post:
Sun/shade conditions and seedling recruitment in a cactus
Early life-history stages of cacti can benefit from the facilitative effects of nurse plants that reduce solar radiation and water stress. Miranda-Jácome et al. conduct a reciprocal transplant experiment, coupled with the artificial manipulation of sun/shade conditions, to test for the effects of local adaptation on germination, seedling survival and growth of the columnar cactus Pilosocereus leucocephalus. They find that significant local adaptation is mainly detected under full sunlight conditions, indicating that sun/shade acts as a selective agent in water-limited environments. Facilitation provided by nurse plants in these environments can attenuate the patterns of local adaptation among plants benefiting from nurse plant effects.