Mechanisms associated with tolerance to flooding during germination and early seedling growth in rice (Oryza sativa)

Background and Aims

Flooding slows seed germination, imposes fatalities and delays seedling establishment in direct-seeded rice. This study describes responses of contrasting rice genotypes subjected to flooding or low oxygen stress during germination and discusses the basis of tolerance shown by certain cultivars.

Methods

In one set of experiments, dry seeds were sown in soil and either watered normally or flooded with 10 cm of water. Seedling survival and shoot and root growth were assessed and seed portions of germinating seedlings were assayed for soluble sugars and starch concentrations. The whole germinating seedlings were assayed for amylase and peroxidase activities and for ethylene production. Activities of enzymes associated with anaerobic respiration were examined and gene expression was analysed separately with seeds germinating under different amounts of dissolved oxygen in dilute agar.

Key Results

Flooding during germination reduced survival but to a lesser extent in tolerant genotypes. Starch concentration in germinating seeds decreased while sugar concentration increased under flooding, but more so in tolerant genotypes. Amylase activity correlated positively with elongation (r = 0·85 for shoot and 0·83 for root length) and with plant survival (r = 0·92). Tolerant genotypes had higher amylase activity and higher RAmy3D gene expression. Ethylene was not detected in seeds within 2 d after sowing, but increased thereafter, with a greater increase in tolerant genotypes starting 3 d after sowing. Peroxidase activity was higher in germinating seeds of sensitive genotypes and correlated negatively with survival.

Conclusions

Under low oxygen stress, tolerant genotypes germinate, grow faster and more seedlings survive. They maintain their ability to use stored starch reserves through higher amylase activity and anaerobic respiration, have higher rates of ethylene production and lower peroxidase activity as germinating seeds and as seedlings. Relevance of these traits to tolerance of flooding during germination and early growth is discussed.Key words: Amylase, anoxia, crop establishment, direct-seeded rice, ethylene, flooding, germination, hypoxia, Oryza sativa     

Latitudinal variation in seeds characteristics of Acer platanoides and A. pseudoplatanus

Climate change will likely affect population dynamics of numerous plant species by modifying several aspects of the life cycle. Because plant regeneration from seeds may be particularly vulnerable, here we assess the possible effects of climate change on seed characteristics and present an integrated analysis of seven seed traits (nutrient concentrations, samara mass, seed mass, wing length, seed viability, germination percentage, and seedling biomass) of Acer platanoides and A. pseudoplatanus seeds collected along a wide latitudinal gradient from Italy to Norway. Seed traits were analyzed in relation to the environmental conditions experienced by the mother trees along the latitudinal gradient. We found that seed traits of A. platanoides were more influenced by the climatic conditions than those of A. pseudoplatanus. Additionally, seed viability, germination percentage, and seedling biomass of A. platanoides were strongly related to the seed mass and nutrient concentration. While A. platanoides seeds were more influenced by the environmental conditions (generally negatively affected by rising temperatures), compared to A. pseudoplatanus, A. platanoides still showed higher germination percentage and seedling biomass than A. pseudoplatanus. Thus, further research on subsequent life-history stages of both species is needed. The variation in seed quality observed along the climatic gradient highlights the importance of studying the possible impact of climate change on seed production and species demography.     

Amylolytic activity and carbohydrate levels in relation to coleoptile anoxic elongation in Oryza sativa genotypes

Among starchy seeds, rice has the unique capacity to germinate successfully under complete anaerobiosis. In this conditions, starch degradation is supported by a complete set of starch-degrading enzymes that are absent or inactive in cereals except rice. A characterization of carbohydrate metabolism and starch-degrading enzyme activity across twenty-nine genotypes of Oryza sativa L. is presented here. The zymogram of amylolytic activities present in rice embryos and endosperms under anaerobic conditions seven days after sowing (DAS) revealed marked differences among cultivars. Coleoptile elongation was positively correlated with total amylolytic activities and α-amylase activity in embryos, and negatively correlated with α-amylase activity in endosperm. Moreover, carbohydrate content in embryos was found to be positively correlated with total amylolytic activities under anaerobic conditions, while a negative relationship was recorded in the endosperm. Carbohydrate status in rice seedlings has a primary importance in sustaining coleoptile elongation towards the surface. The relationship between carbohydrate level in embryo and anoxic germination, as well as with total amylolytic activities present in rice embryo under anaerobic condition 7 DAS, is consistent with the role of sugar metabolism to support rice germination under oxygen-deprived environment.     

Global 5-methylcytosine alterations in DNA during ageing of Quercus robur seeds

Background and Aims Epigenetic regulation plays an important role in the management of plant growth, development and response to stress factors, and several reports have indicated that DNA methylation plays a critical role in seed development and viability. This study examines changes in 5-methylcytosine (m⁵C) levels in the DNA of seeds during ageing, a process that has important implications for plant conservation and agriculture.Methods Changes in the global level of m⁵C were measured in mature seeds of oak, Quercus robur. The extent of DNA methylation was measured using a protocol based on two-dimensional thin-layer chromatography. Viability of seeds was determined by germination and seedling emergence tests.Key Results An ageing-related decrease in total m⁵C during storage of recalcitrant seeds was highly and significantly correlated with a decrease in seed viability, as reflected by a reduction in germination (r = 0·8880) and seedling emergence (r = 0·8269).Conclusions The decrease in viability during ageing of Q. robur seeds is highly correlated with a global decline in the amount of m⁵C in genomic DNA, and it is possible that this may represent a typical response to ageing and senescence in recalcitrant seeds. Potential mechanisms that drive changes in genomic DNA methylation during ageing are discussed, together with their implications for seed viability.     

Effects of primary seed dormancy on lifetime fitness of Arabidopsis thaliana in the field

Background and AimsSeed dormancy determines the environmental niche of plants in seasonal environments, and has consequences for plant performance that potentially go far beyond the seed and seedling stages. In this study, we examined the cascading effects of seed dormancy on the expression of subsequent life-history traits and fitness in the annual herb Arabidopsis thaliana.MethodsWe planted seeds of >200 recombinant inbred lines (RILs) derived from a cross between two locally adapted populations (Italy and Sweden), and both parental genotypes at the native site of the Swedish population in three consecutive years. We quantified the relationship between primary seed dormancy and the expression of subsequent life-history traits and fitness in the RIL population with path analysis. To examine the effects of differences in dormancy on the relative fitness of the two parental genotypes, we planted dormant seeds during the seed dispersal period and non-dormant seeds during the germination period of the local population.Key ResultsIn the RIL population, strong primary dormancy was associated with high seedling survival, but with low adult survival and fecundity, and path analysis indicated that this could be explained by effects on germination timing, rosette size and flowering start. The relationship between primary seed dormancy and germination proportion varied among years, and this was associated with differences in seasonal changes in soil moisture. The planting of dormant and non-dormant seeds indicated that the lower primary dormancy of the local Swedish genotype contributed to its higher germination proportion in two years and to its higher fecundity in one year.ConclusionsOur results show that seed dormancy affects trait expression and fitness components across the life cycle, and suggest that among-year variation in the incidence of drought during the germination period should be considered when predicting the consequences of climatic change for population growth and evolution.     

Detection of QTLs for genotype × environment interactions in tomato seeds and seedlings

Seed quality and seedling establishment are the most important factors affecting successful crop development. They depend on the genetic background and are acquired during seed maturation and therefor, affected by the maternal environment under which the seeds develop. There is little knowledge about the genetic and environmental factors that affect seed quality and seedling establishment. The aim of this study is to identify the loci and possible molecular mechanisms involved in acquisition of seed quality and how these are controlled by adverse maternal conditions. For this, we used a tomato recombinant inbred line (RIL) population consisting of 100 lines which were grown under two different nutritional environmental conditions, high phosphate and low nitrate. Most of the seed germination traits such as maximum germination percentage (G_max), germination rate (t₅₀) and uniformity (U₈₄₁₆) showed ample variation between genotypes and under different germination conditions. This phenotypic variation leads to identification of quantitative trait loci (QTLs) which were dependent on genetic factors, but also on the interaction with the maternal environment (QTL × E). Further studies of these QTLs may ultimately help to predict the effect of different maternal environmental conditions on seed quality and seedling establishment which will be very useful to improve the production of high-performance seeds.     

Differences in the Effects of Dehusking during Formation of Seeds on the Germination of Seeds of Indica and Japonica Rice (Oryza sativaL.)

Differences in germination of intact and dehusked seeds of two japonica and two indica cultivars of rice (Oryza sativaL.) were examined during the development and maturation of seeds both under high-temperature (30/23°C) and low-temperature (20/13°C) regimes and a 14-h photoperiod in growth cabinets. As described previously for seeds grown in the paddy fields, germination of freshly harvested japonica rice seeds that developed and matured in growth cabinets was inhibited by dehusking. We observed a roughly consistent triphasic pattern with respect to the germination of intact and dehusked seeds during the development and maturation of seeds of indica and japonica rice. The triphasic pattern consisted of: (a) an initial phase, during which germination was stimulated by dehusking both in indica and japonica rice; (b) a second phase during which almost no dehusked seeds of indica or japonica rice germinated; and (c) a third phase during which intrinsic differences between indica and japonica rice were observed, with dehusking stimulating germination of indica rice but inhibiting that of japonica. Temperature regimes did not affect this triphasic pattern, but the time from the day of anthesis to mass maturity was affected by temperature regimes, and the effect was more pronounced in indica than in japonica rice.     

The Effects of Kinetin and Gibberellin on the Germination of Dehusked Seeds of Indica and Japonica Rice (Oryza sativaL.) under Anaerobic and Aerobic Conditions

The effects of kinetin and gibberellin were examined under anaerobicconditions (0% oxygen) and aerobic conditions (20% oxygen) onthe germination of dehusked seeds of indica and japonica ricecultivars that had been harvested at different times duringthe formation of seeds. Surjamkhi was used as a representativeof deep dormant indica cultivars and Assam IV as a less dormantindica cultivar. Sasanishiki was used as the japonica rice cultivar.Both phytohormones were applied at a concentration of 10^-3Mwhichproved to have the greatest stimulatory effect in preliminarywork at concentrations of 10^-3–10^-5M. Under aerobic conditions,inhibition of germination by dehusking of Sasanishiki seedsthat had been harvested either 30 or 60 d after anthesis wasovercome by kinetin and all seeds germinated. Complete germinationinduced by kinetin under aerobic conditions was also achievedwith the dehusked seeds of the indica rice cultivar Assam IVthat had been harvested on two occasions and of Surjamkhi thathad been harvested 28 d after anthesis. In contrast, germinationof dehusked japonica seeds stimulated by anaerobiosis was inhibitedby kinetin. The stimulatory effects of gibberellin on the germinationof indica and japonica rice seeds were observed under aerobicand anaerobic conditions. Under anaerobic conditions, the responsesof dehusked indica and japonica rice seeds to kinetin and gibberellindiffered, being negative with kinetin and positive with gibberellin.Under aerobic conditions, the stimulatory effects of kinetinon germination of dehusked seeds were greater than those ofgibberellin. Thus, treatment with kinetin appears to be usefulfor breaking the considerable dormancy commonly observed inthe dehusked seeds of indica rice. Mechanisms are proposed toexplain the stimulatory effects of these phytohormones on thegermination of dehusked seeds of indica and japonica rice underaerobic and anaerobic conditions. Rice; Oryza sativaL.; seed germination; dehusking treatment; gibberellin; indica; japonica; kinetin; oxygen; dormancy; germination inhibition; seed formation     

Proteomic approach to address low seed germination in <Emphasis Type="Italic">Cyclobalnopsis gilva</Emphasis>

Seeds play essential roles in plant life cycle and germination is a complex process which is associated with different phases of water imbibition. Upon imbibition, seeds begin utilization of storage substances coupled with metabolic activity and biosynthesis of new proteins. Regeneration of organelles and emergence of radicals lead to the establishment of seedlings. All these activities are regulated in coordinated manners. Translation is the requirement of germination of seeds via involvements of several proteins like beta-amylase, starch phosphorylase. Some important proteins involved in seed germination are discussed in this review. In the past decade, several proteomic studies regarding seed germination of various species such as rice, Arabidopsis have been conducted. We face A paucity of proteomic data with respect to woody plants e.g. Fagus, Pheonix etc. With particular reference to Cyclobalnopsis gilva, a woody plant having low seed germination rate, no proteomic studies have been conducted. The review aims to reveal the complex seed germination mechanisms from woody and herbaceous plants that will help in understanding different seed germination phases and the involved proteins in C. gilva.     

Frugivory and the effects of ingestion by bats on the seed germination of three pioneering plants

The dispersion and seedling establishment of pioneering plants can be favoured by the presence of frugivorous bats because the bats usually improve seed germination after ingestion. Although seed germinability is known to vary greatly after ingestion by different bats, the relative contribution of each bat species to seed germination within plant communities is poorly understood. In this study, we first determined the fauna of frugivorous bats in a semideciduous seasonal forest remnant in southern Brazil and subsequently identified the plant species of the seeds passed through their guts. Second, the germination performance (i.e., germination percentage and speed) of the seeds of three pioneering plants (Piper aduncum, Piper hispidinervum and Solanum granuloso-leprosum) ingested by the most abundant bats was compared with that of the non-ingested seeds (seeds collected from fruits). Additionally, the effects on seed germination of different bat species were compared. During one year, five species of frugivorous bats were caught, and the seeds of eleven identifiable plant species (not counting those of undetermined species) were found in their faeces. We found that the germination performance of the seeds of Piper species was significantly enhanced after ingestion by bats, whereas S. granuloso-leprosum seeds had neutral or reduced germinability when seeds in faeces were compared with pulp-removed seeds. Our results revealed that the bat species that were captured exerted different effects upon seed germination; such a disparity is expected to result in different rates of early establishment of these pioneer plants in tropical forests, most likely affecting forest composition and structure, particularly during the initial stages of succession.     

Ethylene antagonizes the inhibition of germination in Arabidopsis induced by salinity by modulating the concentration of hydrogen peroxide

The capacity of plants to achieve successful germination and early seedling establishment under high salinity is crucial for tolerance of plants to salt. The gaseous hormone ethylene has been implicated in modulating salt tolerance, but the detailed role of how ethylene modulates the response of early seedling establishment to salt is unclear. To better understand the role of the ethylene signal transduction pathway during germination and seedling establishment, an ethylene insensitive mutation (ein2-5) and an ethylene sensitive mutation (ctr1-1) of Arabidopsis were analyzed under saline conditions and compared with the wild type plant (Col-0) as control. High salinity (>100?mM NaCl) inhibited and delayed germination. These effects were more severe in the ethylene insensitive mutants (ein2-5) and less severe in the constitutive ethylene sensitive plants (ctr1-1) compared with Col-0 plants. Addition of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or inhibitors of ethylene action implied that ethylene was essential for early seedling establishment under normal and saline conditions. Salt stress increased the endogenous concentration of hydrogen peroxide (H₂O₂) in germinating seeds and ACC reduced its concentration. Our results suggest that ethylene promotes germination under salinity by modulating the endogenous concentration of H₂O₂ in germinating seeds. These findings demonstrate that ethylene is involved in regulating germination as an initiator of the process rather than consequence, and that ethylene promotes germination by modulating the endogenous concentration of H₂O₂ in germinating seeds under salinity.     

Scatter Hoarding of Seeds Confers Survival Advantages and Disadvantages to Large-Seeded Tropical Plants at Different Life Stages

Scatter hoarding of seeds by animals contributes significantly to forest-level processes, including plant recruitment and forest community composition. However, the potential positive and negative effects of caching on seed survival, germination success, and seedling survival have rarely been assessed through experimental studies. Here, I tested the hypothesis that seed burial mimicking caches made by scatter hoarding Central American agoutis (Dasyprocta punctate) enhances seed survival, germination, and growth by protecting seeds from seed predators and providing favorable microhabitats for germination. In a series of experiments, I used simulated agouti seed caches to assess how hoarding affects seed predation by ground-dwelling invertebrates and vertebrates for four plant species. I tracked germination and seedling growth of intact and beetle-infested seeds and, using exclosures, monitored the effects of mammals on seedling survival through time. All experiments were conducted over three years in a lowland wet forest in Costa Rica. The majority of hoarded palm seeds escaped predation by both invertebrates and vertebrates while exposed seeds suffered high levels of infestation and removal. Hoarding had no effect on infestation rates of D. panamensis, but burial negatively affected germination success by preventing endocarp dehiscence. Non-infested palm seeds had higher germination success and produced larger seedlings than infested seeds. Seedlings of A. alatum and I. deltoidea suffered high mortality by seed-eating mammals. Hoarding protected most seeds from predators and enhanced germination success (except for D. panamensis) and seedling growth, although mammals killed many seedlings of two plant species; all seedling deaths were due to seed removal from the plant base. Using experimental caches, this study shows that scatter hoarding is beneficial to most seeds and may positively affect plant propagation in tropical forests, although tradeoffs in seed survival do exist.     

On the role of a Lipid-Transfer Protein. Arabidopsis ltp3 mutant is compromised in germination and seedling growth.

Plant Lipid-Transfer Proteins (LTPs) exhibit the ability to reversibly bind/transport lipids in vitro. LTPs have been involved in diverse physiological processes but conclusive evidence on their role has only been presented for a few members, none of them related to seed physiology. Arabidopsis seeds rely on storage oil breakdown to supply carbon skeletons and energy for seedling growth. Here, Arabidopsis ltp3 mutant was analyzed for its ability to germinate and for seedling establishment. Ltp3 showed delayed germination and reduced germination frequency. Seedling growth appeared reduced in the mutant but this growth restriction was rescued by the addition of an exogenous carbon supply, suggesting a defective oil mobilization. Lipid breakdown analysis during seedling growth revealed a differential profile in the mutant compared to the wild type. The involvement of LTP3 in germination and seedling growth and its relationship with the lipid transfer ability of this protein is discussed.     

The Effect of Arsenate and Other Inhibitors on Early Events during the Germination of Lettuce Seeds (Lactuca sativa L.)

The effect of arsenate, arsenite, 2,4-dinitrophenol, and anaerobiosis on early events in seed germination was investigated using both intact and punched seeds of lettuce (Lactuca sativa L.). It was found that punching the seed removes penetration barriers to the entrance of inhibitors without an undue loss of germination or light responses. The kinetics of the action of germination inhibitors were established by 2-hour pulse experiments. Arsenate and 2, 4-dinitrophenol have very different kinetics. The inhibition of germination in punched seeds by arsenate given in conjunction with phosphate compared with the lack of inhibition of arsenate plus phosphate on the growing seedling, suggest a distinct metabolic change in the germinating embryo at some time between the onset of germination and subsequent seedling growth.     

A quantitative trait locus,qSE3, promotes seed germination and seedling establishment under salinity stress in rice

Seed germination is a complex trait determined by both quantitative trait loci (QTLs) and environmental factors and also their interactions. In this study, we mapped one major QTLqSE3 for seed germination and seedling establishment under salinity stress in rice. To understand the molecular basis of this QTL, we isolated qSE3 by map‐based cloning and found that it encodes a K⁺ transporter gene, OsHAK21. The expression of qSE3 was significantly upregulated by salinity stress in germinating seeds. Physiological analysis suggested that qSE3 significantly increased K⁺ and Na⁺ uptake in germinating seeds under salinity stress, resulting in increased abscisic acid (ABA) biosynthesis and activated ABA signaling responses. Furthermore, qSE3 significantly decreased the H₂O₂ level in germinating seeds under salinity stress. All of these seed physiological changes modulated by qSE3 might contribute to seed germination and seedling establishment under salinity stress. Based on analysis of single‐nucleotide polymorphism data of rice accessions, we identified a HAP3 haplotype of qSE3 that was positively correlated with seed germination under salinity stress. This study provides important insights into the roles of qSE3 in seed germination and seedling establishment under salinity stress and facilitates the practical use of qSE3 in rice breeding.     

Application of hydrotime model to predict early vigour of rapeseed (<Emphasis Type="Italic">Brassica napus</Emphasis> L.) under abiotic stresses

Rapeseed (Brassica napus L.) is important for edible oil production in semi-arid areas. Abiotic stresses are threatening rapeseed production in such areas. This study was conducted to find tolerant genotypes of rapeseed and to determine which traits of crop establishment is related to abiotic stress tolerance. Hydrotime model parameters were determined in a laboratory germination test, and seedling emergence and growth were evaluated in pot experiments under control, drought, salinity, deep sowing, low and high temperatures for 19 rapeseed genotypes. Results indicated that the predicted germination time courses at the various water potentials generally fitted well with the observed germination data. The estimated values of θ _H, ψ_b(50), and σψ_b differed significantly across genotypes. Seedling emergence and growth differed significantly under each environmental condition. PCA showed that genotypes of Hayola 401 and line 285 were the most tolerant to abiotic stresses during crop establishment and seedling growth. The first PC explained 40% of variations, and a correlation was observed between PC1 and ψ_b(50). Correlations among hydrotime model parameters and early seed vigour variables indicated that ψ_b(50) negatively correlated with seedling emergence percentage and rate (day^?1) under all abiotic stresses. It shows that genotypes with more negative values of ψ_b(50) have more seedling emergence percentage and a larger seedling emergence rate (days^?1) under a wide range of environmental conditions. Thus, it can be concluded that, to identify tolerant genotypes of rapesee to abiotic stresses, ψ_b(50) is a good trait and that breeders can focus on reducing ψ_b(50) to increase tolerance of abiotic stresses.     

Effect of cold stratification on seed germination in <Emphasis Type="Italic">Solidago</Emphasis> × <Emphasis Type="Italic">niederederi</Emphasis> (<Emphasis Type="Italic">Asteraceae</Emphasis>) and its parental species

In this study, we investigated the influence of cold stratification on seed germination in S. × niederederi, a hybrid between the North American S. canadensis and the European S. virgaurea, using fruit samples collected in 2016 in Poland. We aimed to test the hypothesis that the low temperature exposure decreases the final percentage and speed of seed germination in the hybrid and its parental species. For each species, sets of 100 achenes in three replications were mixed with dry sand and stored in Petri dishes in darkness for 12 weeks, at ?18 °C and?+?4 °C, and?+?25 °C. The seeds were incubated for 21 d at room temperature (+25 °C), under the 12 h photoperiod (630 lx). We showed a lack of significant differences in: (i) the final percentage of germinated seeds of studied species stored at the same conditions, (ii) the final percentage of germinated seeds between the applied stratification conditions in the hybrid and its parental species, and (iii) the mean values of Timson’s index, mean germination time, and coefficient of velocity of germination between the stratification conditions in each species. The statistically significant inter-specific differences in the mean germination time parameter after the +25 °C treatment suggest that the seeds of S. × niederederi are able to germinate faster than the seeds of its parental species. However, to improve our knowledge of naturalization and invasion abilities of S. × niederederi by sexual reproduction, the seed germination and seedling survival of the hybrid should be tested in the field.