Genetic variation for lettuce seed thermoinhibition is associated with temperature-sensitive expression of abscisic Acid, gibberellin, and ethylene biosynthesis, metabolism, and response genes

Advance in the Thermoinhibition of Lettuce (Lactuca sativa L.) Seed Germination

Thermoinhibition refers to the inability of seeds to germinate when inhibited by high temperatures, but when environmental conditions return to normal, the seeds are able to germinate rapidly again, which is different from thermodormancy. Meanwhile, with global warming, the effect of the thermoinhibition phenomenon on the yield and quality of crops in agricultural production is becoming common. Lettuce, as a horticultural crop sensitive to high temperature, is particularly susceptible to the effects of thermoinhibition, resulting in yield reduction. Therefore, it is crucial to elucidate the intrinsic mechanism of action of thermoinhibition in lettuce seeds. This review mainly outlines several factors affecting thermoinhibition of lettuce seed germination, including endosperm hardening, alteration of endogenous or exogenous phytohormone concentrations, action of photosensitizing pigments, production and inhibition of metabolites, maternal effects, genetic expression, and other physical and chemical factors. Finally, we also discuss the challenges and potential of lettuce seed germination thermoinhibition research. The purpose of this study is to provide theoretical support for future research on lettuce seed germination thermoinhibition, and with the aim of revealing the mechanisms and effects behind lettuce seed thermoinhibition. This will enable the identification of more methods to alleviate seed thermoinhibition or the development of superior heat-tolerant lettuce seeds.

The MKK3 MAPK cascade integrates temperature and after-ripening signals to modulate seed germination

The timing of seed germination is controlled by the combination of internal dormancy and external factors. Temperature is a major environmental factor for seed germination. The permissive temperature range for germination is narrow in dormant seeds and expands during after-ripening (AR) (dormancy release). Quantitative trait loci analyses of preharvest sprouting in cereals have revealed that MKK3, a mitogen-activated protein kinase (MAPK) cascade protein, is a negative regulator of grain dormancy. Here, we show that the MAPKKK19/20-MKK3-MPK1/2/7/14 cascade modulates the germination temperature range in Arabidopsis seeds by elevating the germinability of the seeds at sub- and supraoptimal temperatures. The expression of MAPKKK19 and MAPKKK20 is induced around optimal temperature for germination in after-ripened seeds but repressed in dormant seeds. MPK7 activation depends on the expression levels of MAPKKK19/20, with expression occurring under conditions permissive for germination. Abscisic acid (ABA) and gibberellin (GA) are two major phytohormones which are involved in germination control. Activation of the MKK3 cascade represses ABA biosynthesis enzyme gene expression and induces expression of ABA catabolic enzyme and GA biosynthesis enzyme genes, resulting in expansion of the germinable temperature range. Our data demonstrate that the MKK3 cascade integrates temperature and AR signals to phytohormone metabolism and seed germination.

Computing the effects of temperature and osmotic stress on the seed germination of Helianthus annuus L. by using a mathematical model

An extremely important oil crop in the world, Helianthus annuus L. is one of the world's most significant members of the Asteraceae family. The rate and extent of seed germination and agronomic features are consistently affecting  by temperature (T) and changes in water potential (ψ). A broad hydrothermal time model with T and ψ components could explain sunflower responses over suboptimal T and ψ. A lab experiment was performed using the HTT model to discover both T and ψ and their interactive effects on sunflower germination and also to figure  out the cardinal Ts values. The sunflower seeds were germinated at temperatures (15 °C, 20 °C, 25 °C and 30 °C); each Ts had five constant ψs of 0, 0.3, 0.6, 0.9, and 1.2 MPa via PEG 6000 as osmotic stress inducer. The results revealed that highest germination index was found in seed grown at 20 °C in distilled water (0 MPa) and the lowest at 30 °C with osmotic stress of (- 1.2 MPa). The highest value of germination rate index was found in seed grown at 20 °C in distilled water (0 MPa) and the lowest at 15 °C with an osmotic stress of (- 1.2 MPa). In conclusion, water potential, temperature, and their interactions have a considerable impact on seed germination rate, and other metrics (GI, SVI-I, GRI, GE, SVI-II, and MGT). Seeds sown  at 20 °C with zero water potential showed high germination metrics such as GE, GP, GRI, and T50%. The maximum value to TTsub noted at 30 °C in - 0.9 MPa osmotic stress and the minimum value was calculated at 15 °C in - 1.2 MPa osmotic stress. The result of TTsupra recorded highest at 15 °C in  controlled group (0 MPa). Moreover, θH was  highest at 30 °C in controlled condition (0 MPa) and minimum value was observed at  20 °C under - 1.2 MPa osmotic stress. The value of θHTT were  maximum at  30 °C in controlled group (0 MPa) and minimum value was  recorded at 15 °C under - 1.2 MPa osmotic potential. The base, optimum and ceiling temperatures for sunflower germination metrics in this experiment were noted  6.8, 20 and 30 °C respectively.

Genetic, Epigenetic, and Environmental Control of Seed Dormancy and Germination

Seed germination is controlled by a combination of the seed dormancy level and environmental conditions such as light, temperature, moisture, and nitrate levels. Seed dormancy is programed genetically, but it is also sensitive to maternal environmental conditions before and after anthesis. Recent developments in molecular genetics and bioinformatics have greatly enhanced our understanding of the molecular mechanisms of seed dormancy and germination in model plants and economically important crop species. This chapter focuses on temperature as an environmental factor and discusses the genetic and epigenetic mechanisms of dormancy and germination.

The role of ethylene in plant temperature stress response

Temperature influences the seasonal growth and geographical distribution of plants. Heat or cold stress occur when temperatures exceed or fall below the physiological optimum ranges, resulting in detrimental and irreversible damage to plant growth, development, and yield. Ethylene is a gaseous phytohormone with an important role in plant development and multiple stress responses. Recent studies have shown that, in many plant species, both heat and cold stress affect ethylene biosynthesis and signaling pathways. In this review, we summarize recent advances in understanding the role of ethylene in plant temperature stress responses and its crosstalk with other phytohormones. We also discuss potential strategies and knowledge gaps that need to be adopted and filled to develop temperature stress-tolerant crops by optimizing ethylene response.

Comprehensive metabolomic and lipidomic alterations in response to heat stress during seed germination and seedling growth of Arabidopsis

Temperature affects seed germination and seedling growth, which is a critical and complex stage in plant life cycle. However, comprehensive metabolic basis on temperature implicating seed germination and seedling growth remains less known. Here, we applied the high-throughput untargeted metabolomic and advanced shotgun lipidomic approaches to profile the Arabidopsis 182 metabolites and 149 lipids under moderate (22°C, 28°C) and extreme high (34°C, 40°C) temperatures. Our results showed that a typical feature of the metabolism related to organic acids/derivates and amines was obviously enriched at the moderate temperature, which was implicated in many cellular responses towards tricarboxylic acid cycle (TCA), carbohydrates and amino acids metabolism, peptide biosynthesis, phenylpropanoid biosynthesis and indole 3-acetate (IAA) biosynthetic pathway. Whereas, under extreme high temperatures, there was no seed germination, but 148 out of total 182 metabolites were highly enriched, involving in the galactose metabolism, fatty acid degradation, tryptophan/phenylalanine metabolism, and shikimic acid-mediated pathways especially including alkaloids metabolism and glucosinolate/flavone/flavonol biosynthesis. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) also exhibited the gradually increased tendency from moderate temperatures to extreme high temperatures; whereas phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylglycerol (PG), monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) were contrary to decrease. Another typical feature of the distinguished metabolites between 22°C and 28°C, the TCA, disaccharides, nucleotides, polypeptides, SQDG and the biosynthesis of fatty acids and glucobrassicin-mediated IAA were obviously decreased at 28°C, while amino acids, trisaccharides, PE, PC, PA, PS, MGDG, DGDG and diacylglycerol (DAG) preferred to enrich at 28°C, which characterized the alteration of metabolites and lipids during fast seedling growth. Taking together, our results provided the comprehensive metabolites phenotyping, revealed the characteristics of metabolites necessary for seed germination and/or seedling growth under different temperatures, and provided insights into the different metabolic regulation of metabolites and lipid homeostasis for seed germination and seedling growth.

Promoter-pervasive transcription causes RNA polymerase II pausing to boost DOG1 expression in response to salt

Eukaryotic genomes are pervasively transcribed by RNA polymerase II. Yet, the molecular and biological implications of such a phenomenon are still largely puzzling. Here, we describe noncoding RNA transcription upstream of the Arabidopsis thaliana DOG1 gene, which governs salt stress responses and is a key regulator of seed dormancy. We find that expression of the DOG1 gene is induced by salt stress, thereby causing a delay in seed germination. We uncover extensive transcriptional activity on the promoter of the DOG1 gene, which produces a variety of lncRNAs. These lncRNAs, named PUPPIES, are co-directionally transcribed and extend into the DOG1 coding region. We show that PUPPIES RNAs respond to salt stress and boost DOG1 expression, resulting in delayed germination. This positive role of pervasive PUPPIES transcription on DOG1 gene expression is associated with augmented pausing of RNA polymerase II, slower transcription and higher transcriptional burst size. These findings highlight the positive role of upstream co-directional transcription in controlling transcriptional dynamics of downstream genes.

Cold-induced secondary dormancy and its regulatory mechanisms in Beta vulgaris

The dynamic behaviour of seeds in soil seed banks depends on their ability to act as sophisticated environmental sensors to adjust their sensitivity thresholds for germination by dormancy mechanisms. Here we show that prolonged incubation of sugar beet fruits at low temperature (chilling at 5°C, generally known to release seed dormancy of many species) can induce secondary nondeep physiological dormancy of an apparently nondormant crop species. The physiological and biophysical mechanisms underpinning this cold-induced secondary dormancy include the chilling-induced accumulation of abscisic acid in the seeds, a reduction in the embryo growth potential and a block in weakening of the endosperm covering the embryonic root. Transcriptome analysis revealed distinct gene expression patterns in the different temperature regimes and upon secondary dormancy induction and maintenance. The chilling caused reduced expression of cell wall remodelling protein genes required for embryo cell elongation growth and endosperm weakening, as well as increased expression of seed maturation genes, such as for late embryogenesis abundant proteins. A model integrating the hormonal signalling and master regulator expression with the temperature-control of seed dormancy and maturation programmes is proposed. The revealed mechanisms of the cold-induced secondary dormancy are important for climate-smart agriculture and food security.

Effect of differences in light source environment on transcriptome of leaf lettuce (Lactuca sativa L.) to optimize cultivation conditions

When used in closed-type plant factories, light-emitting diode (LED) illumination systems have the particular advantages of low heat emission and high luminous efficiency. The effects of illumination quality and intensity on the growth and morphogenesis of many plant species have been examined, but improvements are needed to optimize the illumination systems for better plant products with lower resource investments. In particular, new strategies are needed to reduce the wastage of plant products related to leaf senescence, and to better control the ingredients and appearance of leafy vegetables. Although the quality of light is often altered to change the characteristics of plant products, the transcriptional status underlying the physiological responses of plants to light has not been established. Herein, we performed a comprehensive gene expression analysis using RNA-sequencing to determine how red, blue, and red/blue LEDs and fluorescent light sources affect transcriptome involved in the leaf aging of leaf lettuce. The RNA-sequencing profiling revealed clear differences in the transcriptome between young and old leaves. Red LED light caused large variation between the two age classes, while a pure or mixed blue LED light spectrum induced fewer transcriptome differences between young and old leaves. Collectively, the expression levels of genes that showed homology with those of other model organisms provide a detailed physiological overview, incorporating such characteristics as the senescence, nutrient deficiency, and anthocyanin synthesis of the leaf lettuce plants. Our findings suggest that transcriptome profiles of leaf lettuce grown under different light sources provide helpful information to achieve better growth conditions for marketable and efficient green-vegetable production, with improved wastage control and efficient nutrient inputs.

Enhancement in Seed Priming-Induced Starch Degradation of Rice Seed Under Chilling Stress via GA-Mediated α-Amylase Expression

Chilling stress is the major abiotic stress that severely limited the seedling establishment of direct-seeded rice in temperate and sub-tropical rice production regions. While seed priming is an efficient pre-sowing seed treatment in enhancing crop establishment under abiotic stress. Our previous research has identified two seed priming treatments, selenium priming (Se) and salicylic priming (SA) that effectively improved the seed germination and seedling growth of rice under chilling stress. To further explore how seed priming enhance the starch degradation of rice seeds under chilling stress, the present study evaluated the effects of Se and SA priming on germination and seedling growth, α-amylase activity, total soluble sugar content, hormone content and associated gene relative expression under chilling stress. The results showed that both Se and SA priming significantly increased the seed germination and seedling growth attributes, and enhanced the starch degradation ability by increasing α-amylase activity and total soluble sugar content under chilling stress. Meanwhile, seed priming increased the transcription level of OsRamy1A, OsRamy3B that regulated by GA, and increased the transcription level of OsRamy3E that regulated by sugar signals. Furthermore, seed priming significantly improved the GA₃ contents in rice seeds by up-regulating the expression of OsGA3ox1 and OsGA20ox1, and decreased the ABA content and the expression of OsNCED1, indicating that the improved starch degradation ability in primed rice seeds under chilling stress might be attributed to the increased GA₃ and decreased ABA levels in primed rice seeds, which induced the expression of GA-mediated α-amylase. However, studies to explore how seed priming mediate hormonal metabolism and the expression of OsRamy3E are desperately needed.

Dynamics of seed dormancy and germination at high temperature stress is affected by priming and phytohormones in rapeseed (Brassica napus L.)

High temperature stress (HTS) imposes secondary dormancy (SD) also known as thermo-dormancy in many seeds. Priming by soil moisture however, may improve germination though under HTS it may compromise seed longevity. Knowledge of how HTS and priming affect dormancy status/viability loss of a particular crop seed species is essential in agriculture. Accordingly, control non-primed and hydro-primed seeds from Dk-xpower and Traper rapeseed cultivars with low and high potential for SD induction, respectively, were compared for germination behavior, response to GA and some phytohormone effectors under HTS. HTS reduced germination in non-primed Dk-xpower and Traper seeds mainly through the induction of thermo-inhibition/death and thermo-dormancy, respectively. Under HTS, GA₃ application reduced thermo-dormancy in favor of thermo-inhibition only in Traper but the GA inhibitor paclobutrazol intensified thermo-dormancy in both cultivars. The ABA inhibitor, fluridone also reduced thermo-dormancy in favor of thermo-inhibition only in Traper. Thus, under HTS, GA biosynthesis is determinant in seed thermo-dormancy/thermo-inhibition dynamics. Hydropriming improved germination under HTS through reduced thermo-inhibition/death (Dk-xpower) and thermo-dormancy (Traper). Here, GA₃ application increased death and compromised germination mainly in Dk-xpower. Paclubutrazol application however, increased thermo-dormancy by compromising thermo-inhibition/death in Traper. Overall, hydro-priming weakened seed phytohormonal germination responses. Controlled deterioration resulted in decreased longevity of hydro-primed seeds but induced SD in non-primed Traper seeds. Thus, down-regulation of GA biosynthesis may control differential induction of SD in rapeseed seeds under HTS while hydro-priming stimulates seed germination possibly through overcoming limitations in GA biosynthesis. The agricultural importance of these findings at the ecosystem scale is discussed.

Morpho-Physiological Traits and Functional Markers Based Molecular Dissection of Heat-Tolerance in Urdbean

Urdbean (Vigna mungo L. Hepper) is one of the important pulse crops. Its cultivation is not so popular during summer seasons because this crop is unable to withstand excessive heat stress beside lack of humidity in the atmosphere. Therefore, a panel of 97 urdbean diverse genotypes was assessed for yield under stress and non-stress conditions with an aim to identify heat tolerant genotypes. This study identified 8 highly heat tolerant and 35 highly heat sensitive genotypes based on heat susceptibility index. Further, physiological and biochemical traits-based characterization of a group of six highly heat sensitive and seven highly heat tolerant urdbean genotypes showed genotypic variability for leaf nitrogen balance index (NBI), chlorophyll (SPAD), epidermal flavnols, and anthocyanin contents under 42/25°C max/min temperature. Our results showed higher membrane stability index among heat tolerant genotypes compared to sensitive genotypes. Significant differences among genotypes for ETR at different levels of PAR irradiances and PAR × genotypes interactions indicated high photosynthetic ability of a few genotypes under heat stress. Further, the most highly sensitive genotype PKGU-1 showed a decrease in different fluorescence parameters indicating distortion of PS II. Consequently, reduction in the quantum yield of PS II was observed in a sensitive one as compared to a tolerant genotype. Fluorescence kinetics showed the delayed and fast quenching of Fm in highly heat sensitive (PKGU 1) and tolerant (UPU 85-86) genotypes, respectively. Moreover, tolerant genotype (UPU 85-86) had high antioxidant activities explaining their role for scavenging superoxide radicals (ROS) protecting delicate membranes from oxidative damage. Molecular characterization further pinpointed genetic differences between heat tolerant (UPU 85-86) and heat sensitive genotypes (PKGU 1). These findings will contribute to the breeding toward the development of heat tolerant cultivars in urdbean.

Toward the Design of Potato Tolerant to Abiotic Stress

Potato is a major global crop that has an important role to play in food security, reducing poverty and improving human nutrition. Productivity in potato however is limited in many environments by its sensitivity to abiotic stresses such as elevated temperature, drought, frost, and salinity. In this chapter we focus on the effects of elevated temperature on potato yields as high temperature is the most important uncontrollable factor affecting growth and yield of potato. We describe some of the physiological impacts of elevated temperature and review recent findings about response mechanisms. We describe genetic approaches that could be used to identify allelic variants of genes that may be useful to breed for increased climate resilience, an approach that could be deployed with recent advances in potato breeding.

Temperature regulation of plant hormone signaling during stress and development

Global climate change has broad-ranging impacts on the natural environment and human civilization. Increasing average temperatures along with more frequent heat waves collectively have negative effects on cultivated crops in agricultural sectors and wild species in natural ecosystems. These aberrantly hot temperatures, together with cold stress, represent major abiotic stresses to plants. Molecular and physiological responses to high and low temperatures are intricately linked to the regulation of important plant hormones. In this review, we shall highlight our current understanding of how changing temperatures regulate plant hormone pathways during immunity, stress responses and development. This article will present an overview of known temperature-sensitive or temperature-reinforced molecular hubs in hormone biosynthesis, homeostasis, signaling and downstream responses. These include recent advances on temperature regulation at the genomic, transcriptional, post-transcriptional and post-translational levels - directly linking some plant hormone pathways to known thermosensing mechanisms. Where applicable, diverse plant species and various temperature ranges will be presented, along with emerging principles and themes. It is anticipated that a grand unifying synthesis of current and future fundamental outlooks on how fluctuating temperatures regulate important plant hormone signaling pathways can be leveraged towards forward-thinking solutions to develop climate-smart crops amidst our dynamically changing world.

The genetic basis of water-use efficiency and yield in lettuce

BACKGROUND

Water supply limits agricultural productivity of many crops including lettuce. Identifying cultivars within crop species that can maintain productivity with reduced water supply is a significant challenge, but central to developing resilient crops for future water-limited climates. We investigated traits known to be related to water-use efficiency (WUE) and yield in lettuce, a globally important leafy salad crop, in a recombinant inbred line (RIL) lettuce mapping population, produced from a cross between the cultivated Lactuca sativa L. cv. Salinas and its wild progenitor L. serriola L.

RESULTS

Wild and cultivated lettuce differed in their WUE and we observed transgressive segregation in yield and water-use traits in the RILs. Quantitative trait loci (QTL) analysis identified genomic regions controlling these traits under well-watered and droughted conditions. QTL were detected for carbon isotope discrimination, transpiration, stomatal conductance, leaf temperature and yield, controlling 4-23 % of the phenotypic variation. A QTL hotspot was identified on chromosome 8 that controlled carbon isotope discrimination, stomatal conductance and yield under drought. Several promising candidate genes in this region were associated with WUE, including aquaporins, late embryogenesis abundant proteins, an abscisic acid-responsive element binding protein and glutathione S-transferases involved in redox homeostasis following drought stress were also identified.

CONCLUSIONS

For the first time, we have characterised the genetic basis of WUE of lettuce, a commercially important and water demanding crop. We have identified promising candidate genomic regions determining WUE and yield under well-watered and water-limiting conditions, providing important pre-breeding data for future lettuce selection and breeding where water productivity will be a key target.

ABA Metabolism and Homeostasis in Seed Dormancy and Germination

Abscisic acid (ABA) is a key hormone that promotes dormancy during seed development on the mother plant and after seed dispersal participates in the control of dormancy release and germination in response to environmental signals. The modulation of ABA endogenous levels is largely achieved by fine-tuning, in the different seed tissues, hormone synthesis by cleavage of carotenoid precursors and inactivation by 8'-hydroxylation. In this review, we provide an overview of the current knowledge on ABA metabolism in developing and germinating seeds; notably, how environmental signals such as light, temperature and nitrate control seed dormancy through the adjustment of hormone levels. A number of regulatory factors have been recently identified which functional relationships with major transcription factors, such as ABA INSENSITIVE3 (ABI3), ABI4 and ABI5, have an essential role in the control of seed ABA levels. The increasing importance of epigenetic mechanisms in the regulation of ABA metabolism gene expression is also described. In the last section, we give an overview of natural variations of ABA metabolism genes and their effects on seed germination, which could be useful both in future studies to better understand the regulation of ABA metabolism and to identify candidates as breeding materials for improving germination properties.

Divergent Selection for Seed Ability to Germinate at Extreme Temperatures in Perennial Ryegrass (Lolium perenne L.)

Various adaptive mechanisms can ensure that seedlings are established at the most favourable time and place. These mechanisms include seed dormancy i.e., incapacity to germinate in any environment without a specific environmental trigger and inhibition i.e., incapacity to germinate in an unfavourable environment (water availability, temperature: thermoinhibition and light). The objective of this research was to study in the temperate range for germination of forage and turf grass species perennial ryegrass, if the thermal requirements for germination are under genetic controlled and could be selectively bred. Two divergent selections of three cycles were realized on a natural population: one to select for the capacity to germinate at 10°C vs. the impossibility to germinate at 10°C, and one to select for the capacity to germinate at 32°C vs. the impossibility to germinate at 32°C. Seeds of all the lots obtained from the two divergent selections were then germinated at constant temperatures from 5 to 35°C to evaluate their germination ability. Concerning the positive selection, the first cycle of positive selection at 10°C was highly efficient with a very strong increase in the germination percentage. However, afterward no selection effect was observed during the next two cycles of positive selection. By contrast, the positive selection at 32°C was efficient during all cycles with a linear increase of the percentage of germination at 32°C. Concerning the negative selection, we observed only a large positive effect of the first cycle of selection at 10°C. These findings demonstrate that seed thermoinhibition at 10 and 32°C observed in a natural population of perennial ryegrass has a genetic basis and a single recessive gene seems to be involved at 10°C.

Greenhouse evaluation of branching, leaf yield and biochemical compositions of Stevia rebaudiana Bertoni to decapitation and foliar application of abscisic acid and fluridone

Stevia is a herbaceous plant containing non-sugar sweeteners that could be regarded as a successor to sugar for diabetics. However, there are some problems with the cultivation of stevia, especially in the greenhouse, including unbranching and low sweetening agents. To overcome this issue, an experiment was designed to increase not only the branching and leaf production but also the sweetness. Therefore, a novel strategy using abscisic acid (0, 50, 100, 150 μM), its inhibitor, i.e. fluridone (0, 50, 100, 150 μM) and decapitation of plant apical meristems was applied. Results showed that when stevia was decapitated, dormant buds responded to the application of abscisic acid and fluridone. Under these conditions, axillary buds were developed to branches. As well leaf, total dry weight, soluble sugars and steviol glycosides (SGs) were significantly increased. In addition, the interaction of abscisic acid (50 and 100 μM) and fluridone (50 μM) had the highest positive effects on plant growth and steviol glycosides production rather than their sole applications. Results also indicated that decapitation removed a terminal dominance over a limited period of time and the terminal dominance was re-established with the growth of the terminal branches, whereas the influence of 50 µM fluridone on stevia was long term and the number of shoots was greater. Since stevia is a costly sweetener, the results of this study could be used in greenhouses, where the cultivation of stevia seems to be reasonable in terms of economic aspects.

Innovative breeding technologies in lettuce for improved post-harvest quality

Societal awareness of healthy eating is increasing alongside the market for processed bagged salads, which remain as one of the strongest growing food sectors internationally, including most recently from indoor growing systems. Lettuce represents a significant proportion of this ready-to-eat salad market. However, such products typically have a short shelf life, with decay of post-harvest quality occurring through complex biochemical and physiological changes in leaves and resulting in spoilage, food waste and risks to health. We review the functional and quantitative genetic understanding of lettuce post-harvest quality, revealing that few findings have translated into improved cultivar development. We identify (i) phytonutrient status (for enhanced antioxidant and vitamin status, aroma and flavour) (ii) leaf biophysical, cell wall and water relations traits (for longer shelf life) (iii) leaf surface traits (for enhanced food safety and reduced spoilage) and (iv) chlorophyll, other pigments and developmental senescence traits (for appearance and colour), as key targets for future post-harvest breeding. Lettuce is well-placed for rapid future exploitation to address postharvest quality traits with extensive genomic resources including the recent release of the lettuce genome and the development of innovative breeding technologies. Although technologies such as CRISPR/Cas genome editing are paving the way for accelerated crop improvement, other equally important resources available for lettuce include extensive germplasm collections, bi-parental mapping and wide populations with genotyping for genomic selection strategies and extensive multiomic datasets for candidate gene discovery. We discuss current progress towards post-harvest quality breeding for lettuce and how such resources may be utilised for future crop improvement.

AGAMOUS-LIKE67 Cooperates with the Histone Mark Reader EBS to Modulate Seed Germination under High Temperature

Seed germination is a vital developmental process that is tightly controlled by environmental signals, ensuring germination under favorable conditions. High temperature (HT) suppresses seed germination. This process, known as thermoinhibition, is achieved by activating abscisic acid and inhibiting gibberellic acid biosynthesis. The zinc-finger protein SOMNUS (SOM) participates in thermoinhibition of seed germination by altering gibberellic acid/abscisic acid metabolism, but the underlying regulatory mechanism is poorly understood. In this study, we report that SOM binds to its own promoter and activates its own expression in Arabidopsis (Arabidopsis thaliana) and identify the MADS-box transcription factor AGAMOUS-LIKE67 (AGL67) as a critical player in SOM function, based on its ability to recognize CArG-boxes within the SOM promoter and mediate the trans-activation of SOM under HTs. In addition, AGL67 recruits the histone mark reader EARLY BOLTING IN SHORT DAY (EBS), which recognizes H3K4me3 at SOM chromatin. In response to HTs, AGL67 and EBS are highly enriched around the SOM promoter. The AGL67-EBS complex is also necessary for histone H4K5 acetylation, which activates SOM expression, ultimately inhibiting seed germination. Taken together, our results reveal an essential mechanism in which AGL67 cooperates with the histone mark reader EBS, which bridges the process of H3K4me3 recognition with H4K5 acetylation, thereby epigenetically activating SOM expression to suppress seed germination under HT stress.

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.

Seed germination of Bidens subalternans DC. exposed to different environmental factors

Bidens subalternans DC. is a weed found in several tropical countries such as Brazil. Large number of produced seeds and easy dispersion favor the colonization of agricultural fields by this species. To know the factors that affect the germination of B. subalternans can help to understand its ecology, permitting to develop control strategies. Laboratory experiments were carried out to evaluate how the temperature, photoperiod, burial depth, water deficit, and salt stress affect the seed germination of B. subalternans. The means of the treatments of each experiment were shown in scatter plots with the bars indicating the least significant difference (LSD, p≤0.05). The results showed a germination percentage above 77% for a wide alternating temperature (15/20 C to 30/35 C night/day). The highest germination and uniformity occurred at 25/30°C night/day. Only 11% of the seeds germinated at a temperature of 35/40°C night/day. The deeper burial of seeds reduced their germination. Only 17% of the seeds germinated in darkness conditions. However, in constant light and 12 hours of light/dark conditions the germination percentage was over 96%, confirming the light dependence of the B. subalternans during germination. In constant light and 12 hours of light/dark, the germination was over 96%. B. subalternans seeds showed sensitivity to water and salt stress, and their germination was inhibited under a water potential of -0.4 MPa and 100.09 mM, respectively. The sensitivity of B. subalternans seeds to high temperatures, water stress, and salt stress explains the high frequency of this weed in south-central Brazil. The light and sowing depth showed that burial of seeds by mechanical control is a strategy to reduce the high infestation of B. subalternans.

Plant hormonal changes and differential expression profiling reveal seed dormancy removal process in double dormant plant-herbaceous peony

Herbaceous peony (Paeonia lactiflora Pall.) is a popular ornamental and medicinal plant. Taking approximately six to seven months, the seeds germination under natural conditions experiences dual dormancies, which seriously affects horticultural cultivation. Few studies have been conducted on exploring both biological and molecular mechanism that regulates dormancy removal process in hypocotyls double dormant plants. Here, we first measured ABA and GA₃ content changes at four key dormancy break stages, and then performed transcriptomic analyses to identify the differentially expressed genes (DEGs) using RNA-seq. We subsequently carried out Quantitative real-time PCR (qRT-PCR) to validate RNA-seq data. ABA content decreased during the whole dormancy removal process and GA₃ content exhibited decreasing slightly and then increasing trend. RNA sequencing de novo assembly generated a total of 99,577 unigenes. 20,344 unigenes were differentially expressed in the whole dormancy release process. The qPCR results of 54 selected unigenes were consistent with the FPKM values obtained from RNA-seq. Our results summarize a valuable collection of gene expression profiles characterizing the dormancy release process. The DEGs are candidates for functional analyses of genes affecting the dormancy release, which is a precious resource for the on-going physiological and molecular investigation of seeds dormancy removal in other perennial plants.

Distinct Metabolome Changes during Seed Germination of Lettuce (Lactuca sativa L.) in Response to Thermal Stress as Revealed by Untargeted Metabolomics Analysis

Temperature strongly influences lettuce (Lactuca sativa L.) seed germination. Different lettuce genotypes respond differently to higher temperatures or thermal stress. In this study, we evaluated the germination performance of 304 lettuce accessions incubated at three temperature settings, 21 °C, 28 °C and 35 °C, respectively, for 40 h. At 21 °C, seeds of all 304 accessions germinated with very well an average germination percentage of 87.72%; at 28 °C, the average germination percentage dropped to 42.84% and at 35 °C, the germination decreased to 1.01%. Then, we investigated changes in metabolic profiles of lettuce seed response to thermal stress using an untargeted metabolomics approach. Results suggested that seeds of thermal-sensitive and thermal-tolerant cultivars employed different metabolic strategies in response to thermal stress during germination. Thermal-sensitive buds accumulated more significant amounts of organic acids, amino acids, sugars, sterols, phenolic compounds and terpenoids compared to thermal-tolerant buds at 21 °C. Thermal-tolerant lettuce cultivar accumulated higher concentrations of amino acids, organic acids, sugars, sesquiterpene lactones, sterols, and fatty acids derivatives during the germination at 35 °C compared to germinated at 21 °C. This investigation paves the way to link the metabolomics to other external and internal factors affecting lettuce seed germination under thermal stress.

The hydrogen sulfide signal enhances seed germination tolerance to high temperatures by retaining nuclear COP1 for HY5 degradation

Seed germination is a critical stage during the initiation of the plant lifecycle and is strongly affected by endogenous phytohormones and environmental stress. High temperature (HT) upregulates endogenous abscisic acid (ABA) to suppress seed germination, and ABA-INSENSITIVE 5 (ABI5) is the key positive regulator in the ABA signal-mediated modulation of seed germination. In plants, hydrogen sulfide (H₂S) is a small gas messenger that participates in multiple physiological processes, but its role in seed germination thermotolerance has not been thoroughly elucidated to date. In this study, we found that H₂S enhanced the seed germination rate under HT. Moreover, HT accelerates the efflux of the E3 ligase CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) from the nucleus to the cytoplasm, which results in increased nuclear accumulation of ELONG HYPCOTYL 5 (HY5) to activate the expression of ABI5 and thereby suppress seed germination. However, the H₂S signal reversed the HT effect, as characterized by increased COP1 in the nucleus, which resulted in increased degradation of HY5 and reduced expression of ABI5 and thereby enhanced the seed germination thermotolerance. Thus, our findings reveal a novel role for the H₂S signal in the modulation of seed germination thermotolerance through the nucleocytoplasmic partitioning of COP1 and the downstream HY5 and ABI5 pathways.

Aethionema arabicum: a novel model plant to study the light control of seed germination

The timing of seed germination is crucial for seed plants and is coordinated by internal and external cues, reflecting adaptations to different habitats. Physiological and molecular studies with lettuce and Arabidopsis thaliana have documented a strict requirement for light to initiate germination and identified many receptors, signaling cascades, and hormonal control elements. In contrast, seed germination in several other plants is inhibited by light, but the molecular basis of this alternative response is unknown. We describe Aethionema arabicum (Brassicaceae) as a suitable model plant to investigate the mechanism of germination inhibition by light, as this species has accessions with natural variation between light-sensitive and light-neutral responses. Inhibition of germination occurs in red, blue, or far-red light and increases with light intensity and duration. Gibberellins and abscisic acid are involved in the control of germination, as in Arabidopsis, but transcriptome comparisons of light- and dark-exposed A. arabicum seeds revealed that, upon light exposure, the expression of genes for key regulators undergo converse changes, resulting in antipodal hormone regulation. These findings illustrate that similar modular components of a pathway in light-inhibited, light-neutral, and light-requiring germination among the Brassicaceae have been assembled in the course of evolution to produce divergent pathways, likely as adaptive traits.

Re-localization of hormone effectors is associated with dormancy alleviation by temperature and after-ripening in sunflower seeds

Temperature is the primary factor that affects seed dormancy and germination. However, the molecular mechanism that underlies its effect on dormancy alleviation remained largely unknown. In this study, we investigate hormone involvement in temperature induced germination as compared to that caused by after-ripening. Dormant (D) sunflower seeds cannot germinate at 10 °C but fully germinate at 20 °C. After-ripened seeds become non-dormant (ND), i.e. able to germinate at 10 °C. Pharmacological experiments showed the importance of abscisic acid (ABA), gibberellins (GAs) and ethylene in temperature- and after-ripening-induced germination of sunflower seeds. Hormone quantification showed that after-ripening is mediated by a decline in both ABA content and sensitivity while ABA content is increased in D seeds treated at 10 or 20 °C, suggesting that ABA decrease is not a prerequisite for temperature induced dormancy alleviation. GAs and ethylene contents were in accordance with germination potential of the three conditions (GA₁ was higher in D 20 °C and ND 10 °C than in D 10 °C). Transcripts analysis showed that the major change concerns ABA and GAs metabolism genes, while ABA signalling gene expression was significantly unchanged. Moreover, another level of hormonal regulation at the subcellular localization has been revealed by immunocytolocalization study. Indeed, ABA, protein Abscisic acid-Insensitive 5 (ABI5), involved in ABA-regulated gene expression and DELLA protein RGL2, a repressor of the gibberellins signalling pathway, localized mainly in the nucleus in non-germinating seeds while they localized in the cytosol in germinating seeds. Furthermore, ACC-oxidase (ACO) protein, the key ethylene biosynthesis enzyme, was detected in the meristem only in germinating seeds. Our results reveal the importance of hormone actors trafficking in the cell and their regulation in specialized tissue such as the meristem in dormancy alleviation and germination.

Characterization of and genetic variation for tomato seed thermo-inhibition and thermo-dormancy

BACKGROUND

Exposing imbibed seeds to high temperatures may lead to either thermo-inhibition of germination or thermo-dormancy responses. In thermo-inhibition, seed germination is inhibited but quickly resumed when temperatures are lowered. Upon prolonged exposure to elevated temperatures, thermo-dormancy may be induced and seeds are not able to germinate even at optimal temperatures. In order to explore underlying physiological and molecular aspects of thermo-induced secondary dormancy, we have investigated the physiological responses of tomato seeds to elevated temperatures and the molecular mechanisms that could explain the performance of tomato seeds at elevated temperature.

RESULTS

In order to investigate how tomato seeds respond to high temperature we used two distinct tomato accessions: Solanum lycopersicum (cv. Moneymaker) (MM) and Solanum pimpinellifolium accession CGN14498 (PI). MM seeds did not germinate under high temperature conditions while seeds of PI reached a maximum germination of 80%. Despite the high germination percentage of PI, germinated seeds did not produce healthy seedling at 37 °C. By using a candidate gene approach we have tested if similar molecular pathways (abscisic acid (ABA) and gibberellic acid (GA)) present in lettuce and Arabidopsis, are regulating thermo-inhibition and thermo-dormancy responses in tomato. We showed that the ABA biosynthesis pathway genes NCED1 and NCED9 were upregulated whereas two of the GA-biosynthesis regulators (GA3ox1 and GA20ox1) were downregulated in tomato thermo-dormant seeds at elevated temperature. To identify novel regulators of tomato seed performance under high temperature, we screened a Recombinant Inbred Line (RIL) population derived from a cross between the two tomato accessions MM and PI for thermo-inhibition and dormancy induction. Several QTLs were detected, particularly for thermo-dormancy, which may be caused by new regulators of thermo-inhibition and thermo-dormancy in tomato.

CONCLUSIONS

None of the genes studied in this research were co-locating with the detected QTLs. The new QTLs discovered in this study will therefore be useful to further elucidate the molecular mechanisms underlying the responses of tomato seeds to high temperature and eventually lead to identification of the causal genes regulating these responses.

Evolution and diversification of the plant gibberellin receptor GID1

The plant gibberellin receptor GID1 shows sequence similarity to carboxylesterase, suggesting that it is derived from an enzyme. However, how GID1 evolved and was modified is unclear. We identified two amino acids that are essential for GID1 activity, and we found that adjustment of these residues caused GID1 to recognize novel GAs carrying 13-OH as active GAs and to strictly refuse inactive GAs. Phylogenetic analysis of 169 GID1s revealed seven subtypes, and the B-type in core eudicots showed unique characteristics. In fact, certain B-type GID1s showed a higher nonsynonymous-to-synonymous divergence ratio in the region determining GA affinity. Such B-type GID1s with higher affinity were preferentially expressed in the roots in some core eudicot plants and conferred adaptive growth under stress.

The plant gibberellin (GA) receptor GID1 shows sequence similarity to carboxylesterase (CXE). Here, we report the molecular evolution of GID1 from establishment to functionally diverse forms in eudicots. By introducing 18 mutagenized rice GID1s into a rice gid1 null mutant, we identified the amino acids crucial for GID1 activity in planta. We focused on two amino acids facing the C2/C3 positions of ent-gibberellane, not shared by lycophytes and euphyllophytes, and found that adjustment of these residues resulted in increased GID1 affinity toward GA₄, new acceptance of GA₁ and GA₃ carrying C13-OH as bioactive ligands, and elimination of inactive GAs. These residues rendered the GA perception system more sophisticated. We conducted phylogenetic analysis of 169 GID1s from 66 plant species and found that, unlike other taxa, nearly all eudicots contain two types of GID1, named A- and B-type. Certain B-type GID1s showed a unique evolutionary characteristic of significantly higher nonsynonymous-to-synonymous divergence in the region determining GA₄ affinity. Furthermore, these B-type GID1s were preferentially expressed in the roots of Arabidopsis, soybean, and lettuce and might be involved in root elongation without shoot elongation for adaptive growth under low-temperature stress. Based on these observations, we discuss the establishment and adaption of GID1s during plant evolution.

Gene expression patterns regulating the seed metabolism in relation to deterioration/ageing of primed mung bean (Vigna radiata L.) seeds

We are proposing mechanisms to account for the loss of viability (seed deterioration/ageing) and enhancement in seed quality (post-storage priming treatment). In order to understand the regulatory mechanism of these traits, we conducted controlled deterioration (CD) test for up to 8 d using primed mung bean seeds and examined how CD effects the expression of many genes, regulating the seed metabolism in relation to CD and priming. Germination declined progressively with increased duration of CD, and the priming treatment completely/partially reversed the inhibition depending on the duration of CD. The loss of germination capacity by CD was accompanied by a reduction in total RNA content and RNA integrity, indicating that RNA quantity and quality impacts seed longevity. Expression analysis revealed that biosynthesis genes of GA, ethylene, ABA and ROS-scavenging enzymes were differentially affected in response to duration of CD and priming, suggesting coordinately regulated mechanisms for controlling the germination capacity of seeds by modifying the permeability characteristics of biological membranes and activities of different enzymes. ABA genes were highly expressed when germination was delayed and inhibited by CD. Whereas, GA and ethylene genes were more highly expressed when germination was enhanced and permitted by priming under similar conditions. GSTI, a well characterized enzyme family involved in stress tolerance, was expressed in primed seeds over the period of CD, suggesting an additional protection against deterioration. The results are discussed in light of understanding the mechanisms underlying longevity/priming which are important issues economically and ecologically.

Physiological and molecular characterisation for high temperature stress in Lens culinaris

In the present study, 11 lentil (Lens culinaris Medik) genotypes including heat tolerant and heat sensitive genotypes identified after a screening of 334 accessions of lentil for traits imparting heat tolerance, were characterised based on physiological traits and molecular markers. Results showed a higher reduction in pollen viability among sensitive genotypes (up to 52.3%) compared with tolerant genotypes (up to 32.4%) at 43°C. Higher photosynthetic electron transport rate was observed among heat tolerant genotypes and two heat tolerant lentil genotypes, IG 4258 (0.43) and IG 3330 (0.38) were having highest Fv/Fm values. However, membrane stability was significantly higher in only one heat tolerant genotype, ILL 10712, indicating that different mechanisms are involved to control heat tolerance in lentil. The molecular characterisation of lentil genotypes with 70 polymorphic SSR and genic markers resulted into distinct clusters in accordance with their heat stress tolerance. A functional marker ISM11257 (intron spanning marker) amplifying an allele of 205bp in size was present only among heat tolerant genotypes, and could be further used in a breeding program to identify heat tolerant lentil genotypes. The findings of this study will contribute to the development of heat tolerant lentil cultivars.

Engineering heat tolerance in potato by temperature-dependent expression of a specific allele of HEAT-SHOCK COGNATE 70

For many commercial potato cultivars, tuber yield is optimal at average daytime temperatures in the range of 14-22 °C. Further rises in ambient temperature can reduce or completely inhibit potato tuber production, with damaging consequences for both producer and consumer. The aim of this study was to use a genetic screen based on a model tuberization assay to identify quantitative trait loci (QTL) associated with enhanced tuber yield. A candidate gene encoding HSc70 was identified within one of the three QTL intervals associated with elevated yield in a Phureja-Tuberosum hybrid diploid potato population (06H1). A particular HSc70 allelic variant was linked to elevated yield in the 06H1 progeny. Expression of this allelic variant was much higher than other alleles, particularly on exposure to moderately elevated temperature. Transient expression of this allele in Nicotiana benthamiana resulted in significantly enhanced tolerance to elevated temperature. An TA repeat element was present in the promoter of this allele, but not in other HSc70 alleles identified in the population. Expression of the HSc70 allelic variant under its native promoter in the potato cultivar Desiree resulted in enhanced HSc70 expression at elevated temperature. This was reflected in greater tolerance to heat stress as determined by improved yield under moderately elevated temperature in a model nodal cutting tuberization system and in plants grown from stem cuttings. Our results identify HSc70 expression level as a significant factor influencing yield stability under moderately elevated temperature and identify specific allelic variants of HSc70 for the induction of thermotolerance via conventional introgression or molecular breeding approaches.

Abscisic acid regulates seed germination of Vellozia species in response to temperature

The relationship between the phytohormones, gibberellin (GA) and abscisic acid (ABA) and light and temperature on seed germination is still not well understood. We aimed to investigate the role of the ABA and GA on seed germination of Vellozia caruncularis, V. intermedia and V. alutacea in response to light/dark conditions on different temperature. Seeds were incubated in GA (GA₃ or GA₄ ) or ABA and their respective biosynthesis inhibitors (paclobutrazol - PAC, and fluridone - FLU) solutions at two contrasting temperatures (25 and 40 °C). Furthermore, endogenous concentrations of active GAs and those of ABA were measured in seeds of V. intermedia and V. alutacea during imbibition/germination. Exogenous ABA inhibited the germination of Vellozia species under all conditions tested. GA, FLU and FLU + GA₃ stimulated germination in the dark at 25 °C (GA₄ being more effective than GA₃ ). PAC reduced seed germination in V. caruncularis and V. alutacea, but did not affect germination of V. intermedia at 40 °C either under light or dark conditions. During imbibition in the dark, levels of active GAs decreased in the seeds of V. intermedia, but were not altered in those of V. alutacea. Incubation at 40 °C decreased ABA levels during imbibition in both V. caruncularis and V. alutacea. We conclude that the seeds of Vellozia species studied here require light or high temperature to germinate and ABA has a major role in the regulation of Vellozia seed germination in response to light and temperature.

Transcriptome-Wide Identification and Prediction of miRNAs and Their Targets in Paris polyphylla var. yunnanensis by High-Throughput Sequencing Analysis

Long dormancy period of seeds limits the large-scale artificial cultivation of the scarce Paris polyphylla var. yunnanensis, an important traditional Chinese medicine. Characterizing miRNAs and their targets is crucial to understanding the role of miRNAs during seed dormancy in this species. Considering the limited genome information of this species, we first sequenced and assembled the transcriptome data of dormant seeds and their seed coats as the reference genome. A total of 146,671 unigenes with an average length of 923 bp were identified and showed functional diversity based on different annotation methods. Two small RNA libraries from respective seeds and seed coats were sequenced and the combining data indicates that 263 conserved miRNAs belonging to at least 83 families and 768 novel miRNAs in 1174 transcripts were found. The annotations of the predicted putative targets of miRNAs suggest that these miRNAs were mainly involved in the cell, metabolism and genetic information processing by direct and indirect regulation patterns in dormant seeds of P. polyphylla var. yunnanensis. Therefore, we provide the first known miRNA profiles and their targets, which will assist with further study of the molecular mechanism of seed dormancy in P. polyphylla var. yunnanensis.

Promotion of Germination Using Hydroxamic Acid Inhibitors of 9-cis-Epoxycarotenoid Dioxygenase

Abscisic acid (ABA) inhibits seed germination and the regulation of ABA biosynthesis has a role in maintenance of seed dormancy. The key rate-limiting step in ABA biosynthesis is catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). Two hydroxamic acid inhibitors of carotenoid cleavage dioxygenase (CCD), D4 and D7, previously found to inhibit CCD and NCED in vitro, are shown to have the novel property of decreasing mean germination time of tomato (Solanum lycopersicum L.) seeds constitutively overexpressing LeNCED1. Post-germination, D4 exhibited no negative effects on tomato seedling growth in terms of height, dry weight, and fresh weight. Tobacco (Nicotiana tabacum L.) seeds containing a tetracycline-inducible LeNCED1 transgene were used to show that germination could be negatively and positively controlled through the chemical induction of gene expression and the chemical inhibition of the NCED protein: application of tetracycline increased mean germination time and delayed hypocotyl emergence in a similar manner to that observed when exogenous ABA was applied and this was reversed by D4 when NCED expression was induced at intermediate levels. D4 also improved germination in lettuce (Lactuca sativa L.) seeds under thermoinhibitory temperatures and in tomato seeds imbibed in high osmolarity solutions of polyethylene glycol. D4 reduced ABA and dihydrophaseic acid accumulation in tomato seeds overexpressing LeNCED1 and reduced ABA accumulation in wild type tomato seeds imbibed on polyethylene glycol. The evidence supports a mode of action of D4 through NCED inhibition, and this molecule provides a lead compound for the design of NCED inhibitors with greater specificity and potency.

Rapid identification of lettuce seed germination mutants by bulked segregant analysis and whole genome sequencing

Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when imbibed at warm temperatures. Chemical mutagenesis was employed to develop lettuce lines that exhibit germination thermotolerance. Two independent thermotolerant lettuce seed mutant lines, TG01 and TG10, were generated through ethyl methanesulfonate mutagenesis. Genetic and physiological analyses indicated that these two mutations were allelic and recessive. To identify the causal gene(s), we applied bulked segregant analysis by whole genome sequencing. For each mutant, bulked DNA samples of segregating thermotolerant (mutant) seeds were sequenced and analyzed for homozygous single-nucleotide polymorphisms. Two independent candidate mutations were identified at different physical positions in the zeaxanthin epoxidase gene (ABSCISIC ACID DEFICIENT 1/ZEAXANTHIN EPOXIDASE, or ABA1/ZEP) in TG01 and TG10. The mutation in TG01 caused an amino acid replacement, whereas the mutation in TG10 resulted in alternative mRNA splicing. Endogenous abscisic acid contents were reduced in both mutants, and expression of the ABA1 gene from wild-type lettuce under its own promoter fully complemented the TG01 mutant. Conventional genetic mapping confirmed that the causal mutations were located near the ZEP/ABA1 gene, but the bulked segregant whole genome sequencing approach more efficiently identified the specific gene responsible for the phenotype.

α-Xylosidase plays essential roles in xyloglucan remodelling, maintenance of cell wall integrity, and seed germination in Arabidopsis thaliana

Regulation and maintenance of cell wall physical properties are crucial for plant growth and environmental response. In the germination process, hypocotyl cell expansion and endosperm weakening are prerequisites for dicot seeds to complete germination. We have identified the Arabidopsis mutant thermoinhibition-resistant germination 1 (trg1), which has reduced seed dormancy and insensitivity to unfavourable conditions for germination owing to a loss-of-function mutation of TRG1/XYL1, which encodes an α-xylosidase. Compared to those of wild type, the elongating stem of trg1 showed significantly lower viscoelasticity, and the fruit epidermal cells were longitudinally shorter and horizontally enlarged. Actively growing tissues of trg1 over-accumulated free xyloglucan oligosaccharides (XGOs), and the seed cell wall had xyloglucan with a greatly reduced molecular weight. These observations suggest that XGOs reduce xyloglucan size by serving as an acceptor in transglycosylation and eventually enhancing cell wall loosening. TRG1/XYL1 gene expression was abundant in growing wild-type organs and tissues but relatively low in cells at most actively elongating part of the tissues, suggesting that α-xylosidase contributes to maintaining the mechanical integrity of the primary cell wall in the growing and pre-growing tissues. In germinating seeds of trg1, expression of genes encoding specific abscisic acid and gibberellin metabolism enzymes was altered in accordance with the aberrant germination phenotype. Thus, cell wall integrity could affect seed germination not only directly through the physical properties of the cell wall but also indirectly through the regulation of hormone gene expression.

Breeding for plant heat tolerance at vegetative and reproductive stages

KEY MESSAGE

Thermotolerant crop research. Global warming has become a serious worldwide threat. High temperature is a major environmental factor limiting crop productivity. Current adaptations to high temperature via alterations to technical and management systems are insufficient to sustain yield. For this reason, breeding for heat-tolerant crops is in high demand. This review provides an overview of the effects of high temperature on plant physiology, fertility and crop yield and discusses the strategies for breeding heat-tolerant cultivars. Generating thermotolerant crops seems to be a challenging task as heat sensitivity is highly variable across developmental stages and processes. In response to heat, plants trigger a cascade of events, switching on numerous genes. Although breeding has made substantial advances in developing heat-tolerant lines, the genetic basis and diversity of heat tolerance in plants remain largely unknown. The development of new varieties is expensive and time-consuming, and knowledge of heat tolerance mechanisms would aid the design of strategies to screen germplasm for heat tolerance traits. However, gains in heat tolerance are limited by the often narrow genetic diversity. Exploration and use of wild relatives and landraces in breeding can increase useful genetic diversity in current crops. Due to the complex nature of plant heat tolerance and its immediate global concern, it is essential to face this breeding challenge in a multidisciplinary holistic approach involving governmental agencies, private companies and academic institutions.

DELAY OF GERMINATION1 (DOG1) regulates both seed dormancy and flowering time through microRNA pathways

Seed germination and flowering, two critical developmental transitions in plant life cycles, are coordinately regulated by genetic and environmental factors to match plant establishment and reproduction to seasonal cues. The DELAY OF GERMINATION1 (DOG1) gene is involved in regulating seed dormancy in response to temperature and has also been associated genetically with pleiotropic flowering phenotypes across diverse Arabidopsis thaliana accessions and locations. Here we show that DOG1 can regulate seed dormancy and flowering times in lettuce (Lactuca sativa, Ls) and Arabidopsis through an influence on levels of microRNAs (miRNAs) miR156 and miR172. In lettuce, suppression of LsDOG1 expression enabled seed germination at high temperature and promoted early flowering in association with reduced miR156 and increased miR172 levels. In Arabidopsis, higher miR156 levels resulting from overexpression of the MIR156 gene enhanced seed dormancy and delayed flowering. These phenotypic effects, as well as conversion of MIR156 transcripts to miR156, were compromised in DOG1 loss-of-function mutant plants, especially in seeds. Overexpression of MIR172 reduced seed dormancy and promoted early flowering in Arabidopsis, and the effect on flowering required functional DOG1 Transcript levels of several genes associated with miRNA processing were consistently lower in dry seeds of Arabidopsis and lettuce when DOG1 was mutated or its expression was reduced; in contrast, transcript levels of these genes were elevated in a DOG1 gain-of-function mutant. Our results reveal a previously unknown linkage between two critical developmental phase transitions in the plant life cycle through a DOG1-miR156-miR172 interaction.

Molecular mechanisms underlying the entrance in secondary dormancy of Arabidopsis seeds

As seasons change, dormant seeds cycle through dormant states until the environmental conditions are favourable for seedling establishment. Dormancy cycle is widespread in the plant kingdom allowing the seeds to display primary and secondary dormancy. Several reports in the last decade have focused on understanding the molecular mechanisms of primary dormancy, but our knowledge regarding secondary dormancy is limited. Here, we studied secondary dormancy induced in Arabidopsis thaliana by incubating seeds at 25 °C in darkness for 4 d. By physiological, pharmacological, expression and genetics approaches, we demonstrate that (1) the entrance in secondary dormancy involves changes in the content and sensitivity to GA, but the content and sensitivity to ABA do not change, albeit ABA is required; (2) RGL2 promotes the entrance in secondary dormancy through ABI5 action; and (3) multivariate analysis with 18 geographical and environmental parameters of accession collection place suggests that temperature is an important variable influencing the induction of secondary dormancy in nature.

Genetic Variation for Thermotolerance in Lettuce Seed Germination Is Associated with Temperature-Sensitive Regulation of ETHYLENE RESPONSE FACTOR1 (ERF1)

Seeds of most lettuce (Lactuca sativa) cultivars are susceptible to thermoinhibition, or failure to germinate at temperatures above approximately 28°C, creating problems for crop establishment in the field. Identifying genes controlling thermoinhibition would enable the development of cultivars lacking this trait and, therefore, being less sensitive to high temperatures during planting. Seeds of a primitive accession (PI251246) of lettuce exhibited high-temperature germination capacity up to 33°C. Screening a recombinant inbred line population developed from PI215246 and cv Salinas identified a major quantitative trait locus (Htg9.1) from PI251246 associated with the high-temperature germination phenotype. Further genetic analyses discovered a tight linkage of the Htg9.1 phenotype with a specific DNA marker (NM4182) located on a single genomic sequence scaffold. Expression analyses of the 44 genes encoded in this genomic region revealed that only a homolog of Arabidopsis (Arabidopsis thaliana) ETHYLENE RESPONSE FACTOR1 (termed LsERF1) was differentially expressed between PI251246 and cv Salinas seeds imbibed at high temperature (30°C). LsERF1 belongs to a large family of transcription factors associated with the ethylene-signaling pathway. Physiological assays of ethylene synthesis, response, and action in parental and near-isogenic Htg9.1 genotypes strongly implicate LsERF1 as the gene responsible for the Htg9.1 phenotype, consistent with the established role for ethylene in germination thermotolerance of Compositae seeds. Expression analyses of genes associated with the abscisic acid and gibberellin biosynthetic pathways and results of biosynthetic inhibitor and hormone response experiments also support the hypothesis that differential regulation of LsERF1 expression in PI251246 seeds elevates their upper temperature limit for germination through interactions among pathways regulated by these hormones. Our results support a model in which LsERF1 acts through the promotion of gibberellin biosynthesis to counter the inhibitory effects of abscisic acid and, therefore, promote germination at high temperatures.

De novo assembly and characterization of germinating lettuce seed transcriptome using Illumina paired-end sequencing

At supraoptimal temperature, germination of lettuce (Lactuca sativa L.) seeds exhibits a typical germination thermoinhibition, which can be alleviated by sodium nitroprusside (SNP) in a nitric oxide-dependent manner. However, the molecular mechanism of seed germination thermoinhibition and its alleviation by SNP are poorly understood. In the present study, the lettuce seeds imbibed at optimal temperature in water or at supraoptimal temperature with or without 100 μM SNP for different periods of time were used as experimental materials, the total RNA was extracted and sequenced, we gained 147,271,347 raw reads using Illumina paired-end sequencing technique and assembled the transcriptome of germinating lettuce seeds. A total of 51,792 unigenes with a mean length of 849 nucleotides were obtained. Of these unigenes, a total of 29,542 unigenes were annotated by sequence similarity searching in four databases, NCBI non-redundant protein database, SwissProt protein database, euKaryotic Ortholog Groups database, and NCBI nucleotide database. Among the annotated unigenes, 22,276 unigenes were assigned to Gene Ontology database. When all the annotated unigenes were searched against the Kyoto Encyclopedia of Genes and Genomes Pathway database, a total of 8,810 unigenes were mapped to 5 main categories including 260 pathways. We first obtained a lot of unigenes encoding proteins involved in abscisic acid (ABA) signaling in lettuce, including 11 ABA receptors, 94 protein phosphatase 2Cs and 16 sucrose non-fermenting 1-related protein kinases. These results will help us to better understand the molecular mechanism of seed germination, thermoinhibition of seed germination and its alleviation by SNP.

A proteomic analysis of rice seed germination as affected by high temperature and ABA treatment

Seed germination is a critical phase in the plant life cycle, but the specific events associated with seed germination are still not fully understood. In this study, we used two-dimensional gel electrophoresis followed by mass spectrometry to investigate the changes in the proteome during imbibition of Oryza sativa seeds at optimal temperature with or without abscisic acid (ABA) and high temperature (germination thermoinhibition) to further identify and quantify key proteins required for seed germination. A total of 121 protein spots showed a significant change in abundance (1.5-fold increase/decrease) during germination under all conditions. Among these proteins, we found seven proteins specifically associated with seed germination including glycosyl hydrolases family 38 protein, granule-bound starch synthase 1, Os03g0842900 (putative steroleosin-B), N-carbamoylputrescine amidase, spermidine synthase 1, tubulin α-1 chain and glutelin type-A; and a total of 20 imbibition response proteins involved in energy metabolism, cell growth, cell defense and storage proteins. High temperature inhibited seed germination by decreasing the abundance of proteins involved in methionine metabolism, amino acid biosynthesis, energy metabolism, reserve degradation, protein folding and stress responses. ABA treatment inhibited germination and decreased the abundance of proteins associated with methionine metabolism, energy production and cell division. Our results show that changes in many biological processes including energy metabolism, protein synthesis and cell defense and rescue occurred as a result of all treatments, while enzymes involved in methionine metabolism and weakening of cell wall specifically accumulated when the seeds germinated at the optimal temperature.

Proteomic analysis of lettuce seed germination and thermoinhibition by sampling of individual seeds at germination and removal of storage proteins by polyethylene glycol fractionation

Germination and thermoinhibition in lettuce (Lactuca sativa 'Jianyexianfeng No. 1') seeds were investigated by a proteomic comparison among dry seeds, germinated seeds at 15°C, at 15°C after imbibition at 25°C for 48 h, or at 25°C in KNO3 (all sampled individually at germination), and ungerminated seeds at 25°C, a thermoinhibitory temperature. Before two-dimensional gel electrophoresis analysis, storage proteins (greater than 50% of total extractable protein) were removed by polyethylene glycol precipitation, which significantly improved the detection of less abundant proteins on two-dimensional gels. A total of 108 protein spots were identified to change more than 2-fold (P<0.05) in abundance in at least one germination treatment. Nineteen proteins increasing and one protein decreasing in abundance during germination had higher abundance in germinated 15°C, 15°C after imbibition at 25°C for 48 h, and 25°C in KNO3 seeds than in ungerminated 25°C seeds. Gene expression of 12 of those proteins correlated well with the protein accumulation. Methionine metabolism, ethylene production, lipid mobilization, cell elongation, and detoxification of aldehydes were revealed to be potentially related to lettuce seed germination and thermoinhibition. Accumulation of three proteins and expression of five genes participating in the mevalonate (MVA) pathway of isoprenoid biosynthesis correlated positively with seed germinability. Inhibition of this pathway by lovastatin delayed seed germination and increased the sensitivity of germination to abscisic acid. MVA pathway-derived products, cytokinins, partially reversed the lovastatin inhibition of germination and released seed thermoinhibition at 25°C. We conclude that the MVA pathway for isoprenoid biosynthesis is involved in lettuce seed germination and thermoinhibition.

Ethylene, a key factor in the regulation of seed dormancy

Ethylene is an important component of the gaseous environment, and regulates numerous plant developmental processes including seed germination and seedling establishment. Dormancy, the inability to germinate in apparently favorable conditions, has been demonstrated to be regulated by the hormonal balance between abscisic acid (ABA) and gibberellins (GAs). Ethylene plays a key role in dormancy release in numerous species, the effective concentrations allowing the germination of dormant seeds ranging between 0.1 and 200 μL L(-1). Studies using inhibitors of ethylene biosynthesis or of ethylene action and analysis of mutant lines altered in genes involved in the ethylene signaling pathway (etr1, ein2, ain1, etr1, and erf1) demonstrate the involvement of ethylene in the regulation of germination and dormancy. Ethylene counteracts ABA effects through a regulation of ABA metabolism and signaling pathways. Moreover, ethylene insensitive mutants in Arabidopsis are more sensitive to ABA and the seeds are more dormant. Numerous data also show an interaction between ABA, GAs and ethylene metabolism and signaling pathways. It has been increasingly demonstrated that reactive oxygen species (ROS) may play a significant role in the regulation of seed germination interacting with hormonal signaling pathways. In the present review the responsiveness of seeds to ethylene will be described, and the key role of ethylene in the regulation of seed dormancy via a crosstalk between hormones and other signals will be discussed.

Selection of candidate reference genes for real-time PCR studies in lettuce under abiotic stresses

The process of selection and validation of reference genes is the first step in studies of gene expression by real-time quantitative polymerase chain reaction (RT-qPCR). The genome of lettuce, the most popular leaf vegetable cultivated worldwide, has recently been sequenced; therefore, suitable reference genes for reliable results in RT-qPCR analyses are required. In the present study, 17 candidate reference genes were selected, and their expression stability in lettuce leaves under drought, salt, heavy metal, and UV-C irradiation conditions and under the application of abscisic acid (ABA) was evaluated using geNorm and NormFinder software. The candidate reference genes included protein-coding traditional and novel reference genes and microRNAs (miRNAs). The results indicate that the expression stability is dependent on the experimental conditions. The novel protein-coding reference genes were more suitable than the traditional reference genes under drought, UV-C irradiation, and heavy metal conditions and under the application of ABA. Only under salinity conditions were the traditional protein-coding reference genes more stable than the novel genes. In addition, the miRNAs, mainly MIR169, MIR171/170 and MIR172, were stably expressed under the abiotic stresses evaluated, representing a suitable alternative approach for gene expression data normalization. The expression of phenylalanine ammonia lyase (PAL) and 4-hydroxyphenylpyruvate dioxygenase (HPPD) was used to further confirm the validated protein-coding reference genes, and the expression of MIR172 and MIR398 was used to confirm the validated miRNA genes, showing that the use of an inappropriate reference gene induces erroneous results. This work is the first survey of the stability of reference genes in lettuce and provides guidelines to obtain more accurate RT-qPCR results in lettuce studies.

Amplification of ABA biosynthesis and signaling through a positive feedback mechanism in seeds

Abscisic acid is an essential hormone for seed dormancy. Our previous study using the plant gene switch system, a chemically induced gene expression system, demonstrated that induction of 9-cis-epoxycarotenoid dioxygenase (NCED), a rate-limiting ABA biosynthesis gene, was sufficient to suppress germination in imbibed Arabidopsis seeds. Here, we report development of an efficient experimental system that causes amplification of NCED expression during seed maturation. The system was created with a Triticum aestivum promoter containing ABA responsive elements (ABREs) and a Sorghum bicolor NCED to cause ABA-stimulated ABA biosynthesis and signaling, through a positive feedback mechanism. The chimeric gene pABRE:NCED enhanced NCED and ABF (ABRE-binding factor) expression in Arabidopsis Columbia-0 seeds, which caused 9- to 73-fold increases in ABA levels. The pABRE:NCED seeds exhibited unusually deep dormancy which lasted for more than 3 months. Interestingly, the amplified ABA pathways also caused enhanced expression of Arabidopsis NCED5, revealing the presence of positive feedback in the native system. These results demonstrated the robustness of positive feedback mechanisms and the significance of NCED expression, or single metabolic change, during seed maturation. The pABRE:NCED system provides an excellent experimental system producing dormant and non-dormant seeds of the same maternal origin, which differ only in zygotic ABA. The pABRE:NCED seeds contain a GFP marker which enables seed sorting between transgenic and null segregants and are ideal for comparative analysis. In addition to its utility in basic research, the system can also be applied to prevention of pre-harvest sprouting during crop production, and therefore contributes to translational biology.

Effect of Aluminum Treatment on Proteomes of Radicles of Seeds Derived from Al-Treated Tomato Plants

Aluminum (Al) toxicity is a major constraint to plant growth and crop yield in acid soils. Tomato cultivars are especially susceptible to excessive Al³⁺ accumulated in the root zone. In this study, tomato plants were grown in a hydroponic culture system supplemented with 50 µM AlK(SO₄)₂. Seeds harvested from Al-treated plants contained a significantly higher Al content than those grown in the control hydroponic solution. In this study, these Al-enriched tomato seeds (harvested from Al-treated tomato plants) were germinated in 50 µM AlK(SO₄)₂ solution in a homopiperazine-1,4-bis(2-ethanesulfonic acid) buffer (pH 4.0), and the control solution which contained the buffer only. Proteomes of radicles were analyzed quantitatively by mass spectrometry employing isobaric tags for relative and absolute quantitation (iTRAQ^®). The proteins identified were assigned to molecular functional groups and cellular metabolic pathways using MapMan. Among the proteins whose abundance levels changed significantly were: a number of transcription factors; proteins regulating gene silencing and programmed cell death; proteins in primary and secondary signaling pathways, including phytohormone signaling and proteins for enhancing tolerance to abiotic and biotic stress. Among the metabolic pathways, enzymes in glycolysis and fermentation and sucrolytic pathways were repressed. Secondary metabolic pathways including the mevalonate pathway and lignin biosynthesis were induced. Biological reactions in mitochondria seem to be induced due to an increase in the abundance level of mitochondrial ribosomes and enzymes in the TCA cycle, electron transport chains and ATP synthesis.