A Composite Analysis of Flowering Time Regulation in Lettuce

Pollinator-assisted plant phenotyping, selection, and breeding for crop resilience to abiotic stresses

Food security is threatened by climate change, with heat and drought being the main stresses affecting crop physiology and ecosystem services, such as plant-pollinator interactions. We hypothesize that tracking and ranking pollinators' preferences for flowers under environmental pressure could be used as a marker of plant quality for agricultural breeding to increase crop stress tolerance. Despite increasing relevance of flowers as the most stress sensitive organs, phenotyping platforms aim at identifying traits of resilience by assessing the plant physiological status through remote sensing-assisted vegetative indexes, but find strong bottlenecks in quantifying flower traits and in accurate genotype-to-phenotype prediction. However, as the transport of photoassimilates from leaves (sources) to flowers (sinks) is reduced in low-resilient plants, flowers are better indicators than leaves of plant well-being. Indeed, the chemical composition and amount of pollen and nectar that flowers produce, which ultimately serve as food resources for pollinators, change in response to environmental cues. Therefore, pollinators' preferences could be used as a measure of functional source-to-sink relationships for breeding decisions. To achieve this challenging goal, we propose to develop a pollinator-assisted phenotyping and selection platform for automated quantification of Genotype × Environment × Pollinator interactions through an insect geo-positioning system. Pollinator-assisted selection can be validated by metabolic, transcriptomic, and ionomic traits, and mapping of candidate genes, linking floral and leaf traits, pollinator preferences, plant resilience, and crop productivity. This radical new approach can change the current paradigm of plant phenotyping and find new paths for crop redomestication and breeding assisted by ecological decisions.

Low frequency of the wild-type freezing-tolerance LsCBF7 allele among lettuce population suggests a negative selection during domestication and breeding

KEY MESSAGE

Sustainable winter production in lettuce requires freezing tolerant varieties. This study identified a wild-type allele of LsCBF7 that could contribute to freezing tolerance improvement in lettuce. Lettuce is one of the most consumed vegetables globally. While ideally grown in 13-21 °C, its cultivation extends into winter in milder climates. However, occasional freezing temperatures can significantly reduce yields. Therefore, the development of freezing-tolerant lettuce varieties has become a long-term goal of lettuce breeding programs. Despite its significance, our understanding of freezing tolerance in lettuce remains limited. Plants have evolved a coping mechanism against freezing, known as cold acclimation, whereby they can increase freezing tolerance when pre-exposed to low nonfreezing temperatures. The CBF pathway is well-known for its central role in cold acclimation. Previously, we identified 14 CBF genes in lettuce and discovered that one of them, LsCBF7, had a loss-of-function mutation. In this study, we uncovered that accessions from colder regions carried the wild-type allele of LsCBF7 and this allele likely contributed to increased freezing tolerance, with 14% of the lettuce population carrying this allele. Interestingly, in wild lettuce (L. serriola) that is considered a progenitor of cultivated lettuce, this wild-type allele was much more common, with a frequency of 90%. This finding suggests that this wild-type allele may have undergone negative selection during the domestication or breeding of lettuce. Our data strongly indicate that this allele could be linked to early bolting, an undesirable trait in lettuce, which may have driven the negative selection. While this wild-type allele shows promise for improving freezing tolerance in lettuce, it is crucial to decouple it from the early bolting trait to fully harness its potential in lettuce breeding.

Genome-Wide Identification and Characterization of the GRAS Gene Family in Lettuce Revealed That Silencing LsGRAS13 Delayed Bolting

Lettuce is susceptible to high-temperature stress during cultivation, leading to bolting and affecting yield. Plant-specific transcription factors, known as GRAS proteins, play a crucial role in regulating plant growth, development, and abiotic stress responses. In this study, the entire lettuce LsGRAS gene family was identified. The results show that 59 LsGRAS genes are unevenly distributed across the nine chromosomes. Additionally, all LsGRAS proteins showed 100% nuclear localization based on the predicted subcellular localization and were phylogenetically classified into nine conserved subfamilies. To investigate the expression profiles of these genes in lettuce, we analyzed the transcription levels of all 59 LsGRAS genes in the publicly available RNA-seq data under the high-temperature treatment conducted in the presence of exogenous melatonin. The findings indicate that the transcript levels of the LsGRAS13 gene were higher on days 6, 9, 15, 18, and 27 under the high-temperature (35/30 °C) treatment with melatonin than on the same treatment days without melatonin. The functional studies demonstrate that silencing LsGRAS13 accelerated bolting in lettuce. Furthermore, the paraffin sectioning results showed that flower bud differentiation in LsGRAS13-silenced plants occurred significantly faster than in control plants. In this study, the LsGRAS genes were annotated and analyzed, and the expression pattern of the LsGRAS gene following melatonin treatment under high-temperature conditions was explored. This exploration provides valuable information and identifies candidate genes associated with the response mechanism of lettuce plants high-temperature stress.

Impact of Sclerotinia sclerotiorum Infection on Lettuce (Lactuca sativa L.) Survival and Phenolics Content-A Case Study in a Horticulture Farm in Poland

Sclerotinia sclerotiorum is a cause of a prevalent and destructive disease that attacks many horticultural food crops, such as lettuce. This soil-borne necrotrophic fungal pathogen causes significant economic losses in worldwide lettuce production annually. Furthermore, current methods utilized for management and combatting the disease, such as biocontrol, are insufficient. In this study, three cultivars of lettuce (one Crispy and two Leafy cultivars of red and green lettuce) were grown in central Poland (Lodz Voivodeship), a widely known Polish horticultural region. In the summer and early autumn, lettuce cultivars were grown in control and S. sclerotiorum-infected fields. The lettuce cultivars (Templin, Lollo Rossa, and Lollo Bionda) differed phenotypically and in terms of the survival of the fungal infection. The Crispy iceberg Templin was the most susceptible to S. sclerotiorum infection compared to the other cultivars during both vegetation seasons. The total content of phenolic compounds, flavonoids, and anthocyanins varied among cultivars and fluctuated during infection. Moreover, phenolic content was affected by vegetation season with alterable environmental factors such as air temperature, humidity, soil temperature, and pH. The most increased levels of phenolics, both flavonoids and anthocyanins in infected plants, were observed in the Leafy red Lollo Rossa cultivar in both crops. However, the highest survival/resistance to the fungus was noticed for Lollo Rossa in the summer crop and Lollo Bionda in the autumn crop.

An overview of floral regulatory genes in annual and perennial plants

The floral initiation in angiosperms is a complex process influenced by endogenous and exogenous signals. With this approach, we aim to provide a comprehensive review to integrate this complex floral regulatory process and summarize the regulatory genes and their functions in annuals and perennials. Seven primary paths leading to flowering have been discovered in Arabidopsis under several growth condition that include; photoperiod, ambient temperature, vernalization, gibberellins, autonomous, aging and carbohydrates. These pathways involve a series of interlinked signaling pathways that respond to both internal and external signals, such as light, temperature, hormones, and developmental cues, to coordinate the expression of genes that are involved in flower development. Among them, the photoperiodic pathway was the most important and conserved as some of the fundamental loci and mechanisms are shared even by closely related plant species. The activation of floral regulatory genes such as FLC, FT, LFY, and SOC1 that determine floral meristem identity and the transition to the flowering stage result from the merging of these pathways. Recent studies confirmed that alternative splicing, antisense RNA and epigenetic modification play crucial roles by regulating the expression of genes related to blooming. In this review, we documented recent progress in the floral transition time in annuals and perennials, with emphasis on the specific regulatory mechanisms along with the application of various molecular approaches including overexpression studies, RNA interference and Virus-induced flowering. Furthermore, the similarities and differences between annual and perennial flowering will aid significant contributions to the field by elucidating the mechanisms of perennial plant development and floral initiation regulation.

Development and application of Single Primer Enrichment Technology (SPET) SNP assay for population genomics analysis and candidate gene discovery in lettuce

Single primer enrichment technology (SPET) is a novel high-throughput genotyping method based on short-read sequencing of specific genomic regions harboring polymorphisms. SPET provides an efficient and reproducible method for genotyping target loci, overcoming the limits associated with other reduced representation library sequencing methods that are based on a random sampling of genomic loci. The possibility to sequence regions surrounding a target SNP allows the discovery of thousands of closely linked, novel SNPs. In this work, we report the design and application of the first SPET panel in lettuce, consisting of 41,547 probes spanning the whole genome and designed to target both coding (~96%) and intergenic (~4%) regions. A total of 81,531 SNPs were surveyed in 160 lettuce accessions originating from a total of 10 countries in Europe, America, and Asia and representing 10 horticultural types. Model ancestry population structure clearly separated the cultivated accessions (Lactuca sativa) from accessions of its presumed wild progenitor (L. serriola), revealing a total of six genetic subgroups that reflected a differentiation based on cultivar typology. Phylogenetic relationships and principal component analysis revealed a clustering of butterhead types and a general differentiation between germplasm originating from Western and Eastern Europe. To determine the potentiality of SPET for gene discovery, we performed genome-wide association analysis for main agricultural traits in L. sativa using six models (GLM naive, MLM, MLMM, CMLM, FarmCPU, and BLINK) to compare their strength and power for association detection. Robust associations were detected for seed color on chromosome 7 at 50 Mbp. Colocalization of association signals was found for outer leaf color and leaf anthocyanin content on chromosome 9 at 152 Mbp and on chromosome 5 at 86 Mbp. The association for bolting time was detected with the GLM, BLINK, and FarmCPU models on chromosome 7 at 164 Mbp. Associations were detected in chromosomal regions previously reported to harbor candidate genes for these traits, thus confirming the effectiveness of SPET for GWAS. Our findings illustrated the strength of SPET for discovering thousands of variable sites toward the dissection of the genomic diversity of germplasm collections, thus allowing a better characterization of lettuce collections.

Genome-wide screening and characterization of long noncoding RNAs involved in flowering/bolting of Lactuca sativa

BACKGROUND

Lettuce (Lactuca sativa L.) is considered the most important vegetable in the leafy vegetable group. However, bolting affects quality, gives it a bitter taste, and as a result makes it inedible. Bolting is an event induced by the coordinated effects of various environmental factors and endogenous genetic components. Although bolting/flowering responsive genes have been identified in most sensitive and non-sensitive species, non-coding RNA molecules like long non-coding RNAs (lncRNAs) have not been investigated in lettuce. Hence, in this study, potential long non-coding RNAs that regulate flowering /bolting were investigated in two lettuce strains S24 (resistant strain) and S39 (susceptible strain) in different flowering times to better understand the regulation of lettuce bolting mechanism. For this purpose, we used two RNA-seq datasets to discover the lncRNA transcriptome profile during the transition from vegetative to reproductive phase.

RESULTS

For identifying unannotated transcripts in these datasets, a 7-step pipeline was employed to filter out these transcripts and terminate with 293 novel lncRNAs predicted by PLncPRO and CREMA. These transcripts were then utilized to predict cis and trans flowering-associated targets and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Computational predictions of target gene function showed the involvement of putative flowering-related genes and enrichment of the floral regulators FLC, CO, FT, and SOC1 in both datasets. Finally, 17 and 18 lncRNAs were proposed as competing endogenous target mimics (eTMs) for novel and known lncRNA miRNAs, respectively.

CONCLUSION

Overall, this study provides new insights into lncRNAs that control the flowering time of plants known for bolting, such as lettuce, and opens new windows for further study.

Integrated analysis of miRNAome transcriptome and degradome reveals miRNA-target modules governing floral florescence development and senescence across early- and late-flowering genotypes in tree peony

As a candidate national flower of China, tree peony has extremely high ornamental, medicinal and oil value. However, the short florescence and rarity of early-flowering and late-flowering varieties restrict further improvement of the economic value of tree peony. Specific miRNAs and their target genes engaged in tree peony floral florescence, development and senescence remain unknown. This report presents the integrated analysis of the miRNAome, transcriptome and degradome of tree peony petals collected from blooming, initial flowering, full blooming and decay stages in early-flowering variety Paeonia ostii 'Fengdan', an early-flowering mutant line of Paeonia ostii 'Fengdan' and late-flowering variety Paeonia suffruticosa 'Lianhe'. Transcriptome analysis revealed a transcript ('psu.G.00014095') which was annotated as a xyloglucan endotransglycosylase/hydrolase precursor XTH-25 and found to be differentially expressed across flower developmental stages in Paeonia ostii 'Fengdan' and Paeonia suffruticosa 'Lianhe'. The miRNA-mRNA modules were presented significant enrichment in various pathways such as plant hormone signal transduction, indole alkaloid biosynthesis, arachidonic acid metabolism, folate biosynthesis, fatty acid elongation, and the MAPK signaling pathway. Multiple miRNA-mRNA-TF modules demonstrated the potential functions of MYB-related, bHLH, Trihelix, NAC, GRAS and HD-ZIP TF families in floral florescence, development, and senescence of tree peony. Comparative spatio-temporal expression investigation of eight floral-favored miRNA-target modules suggested that transcript 'psu.T.00024044' and microRNA mtr-miR166g-5p are involved in the floral florescence, development and senescence associated agronomic traits of tree peony. The results might accelerate the understanding of the potential regulation mechanism in regards to floral florescence, development and abscission, and supply guidance for tree peony breeding of varieties with later and longer florescence characteristics.

LsARF3 mediates thermally induced bolting through promoting the expression of LsCO in lettuce (Lactuca sativa L.)

Lettuce (Lactuca sativa L.) is a leafy vegetable whose edible organs usually are leaf or stems, and thus high-temperature induced bolting followed by flower initiation is an undesirable trait in lettuce production. However, the molecular mechanism that controls lettuce bolting and flowering upon thermal treatments is largely unknown. Here, we identified a Lettuce auxin response factor 3 (LsARF3), the expression of which was enhanced by heat and auxin treatments. Interestingly, LsARF3 is preferentially expressed in stem apex, suggesting it might be associated with lettuce bolting. Transgenic lettuce overexpressing LsARF3 displayed early bolting and flowering, whereas knockout of LsARF3 dramatically delayed bolting and flowering in lettuce under normal or high temperature conditions. Furthermore, Exogenous application of IAA failed to rescue the late-bolting and -flowering phenotype of lsarf3 mutants. Several floral integrator genes including LsCO, LsFT, and LsLFY were co-expressed with LsARF3 in the overexpression and knockout lettuce plants. Yeast one-hybrid (Y1H) experiments suggested that LsARF3 could physically interact with the LsCO promoter, which was further confirmed by a dual luciferase assay in tobacco leaves. The results indicated that LsARF3 might directly modulate the expression of LsCO in lettuce. Therefore, these results demonstrate that LsARF3 could promote lettuce bolting in response to the high temperature by directly or indirectly activating the expression of floral genes such as LsCO, which provides new insights into lettuce bolting in the context of ARFs signaling and heat response.

LsMYB15 Regulates Bolting in Leaf Lettuce (Lactuca sativa L.) Under High-Temperature Stress

High temperature is one of the primary environmental stress factors affecting the bolting of leaf lettuce. To determine the potential role of melatonin in regulating high-temperature induced bolting in leaf lettuce (Lactuca sativa L.), we conducted melatonin treatment of the bolting-sensitive cultivar "S39." The results showed that 100 μmol L^-1 melatonin treatment significantly promoted growth, and melatonin treatment delayed high-temperature-induced bolting in lettuce. RNA-seq analysis revealed that the differentially expressed genes (DEGs) involved in "plant hormone signal transduction" and "phenylpropanoid biosynthesis" were significantly enriched during high-temperature and melatonin treatment. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis suggested that the expression patterns of abscisic acid (ABA)-related genes positively correlated with stem length during leaf lettuce development. Furthermore, weighted gene co-expression network analysis (WGCNA) demonstrated that MYB15 may play an important role in melatonin-induced resistance to high temperatures. Silencing the LsMYB15 gene in leaf lettuce resulted in early bolting, and exogenous melatonin delayed early bolting in leaf lettuce at high temperatures. Our study provides valuable data for future studies of leaf lettuce quality.

Molecular Determinants of in vitro Plant Regeneration: Prospects for Enhanced Manipulation of Lettuce (Lactuca sativa L.)

In vitro plant regeneration involves dedifferentiation and molecular reprogramming of cells in order to regenerate whole organs. Plant regeneration can occur via two pathways, de novo organogenesis and somatic embryogenesis. Both pathways involve intricate molecular mechanisms and crosstalk between auxin and cytokinin signaling. Molecular determinants of both pathways have been studied in detail in model species, but little is known about the molecular mechanisms controlling de novo shoot organogenesis in lettuce. This review provides a synopsis of our current knowledge on molecular determinants of de novo organogenesis and somatic embryogenesis with an emphasis on the former as well as provides insights into applying this information for enhanced in vitro regeneration in non-model species such as lettuce (Lactuca sativa L.).

Application of exogenous auxin and gibberellin regulates the bolting of lettuce (Lactuca sativa L.)

Plant bolting is regulated and controlled by various internal and external factors. We aimed to provide an improved method for breeding to determine whether there is a synergism between hormones and to explore the regulatory effect of plant hormones on the bolting of leaf lettuce. Lettuce plants were sprayed with exogenous auxin and gibberellin separately or in combination. The specific bolting period was determined by the change in stem length and cytological observation. The dynamic changes in endogenous hormones and genes closely related to bolting were analyzed. Treatment with gibberellin alone and the combined application of auxin and gibberellin induced bolting on the fourth day, and treatment with auxin alone resulted in bolting on the eighth day. In the early bolting stage, the auxin contents in the stems of the treatment groups, especially the combined gibberellin and auxin group, were higher than those of the control group. After the application of exogenous auxin and gibberellin, we found that the expression of the ARF8 and GID1 genes was upregulated. Based on the results of our study, combined treatment with exogenous gibberellin and auxin was the best method to promote the bolting of leaf lettuce, and the ARF8 and GID1 genes are closely related to this process.

Molecular Mechanisms of Early Flowering in Tomatoes Induced by Manganese Ferrite (MnFe2O4) Nanomaterials

Nanomaterials (NMs) have demonstrated enormous potential to improve agricultural production. Ten mg L^-1 of customized manganese ferrite (MnFe₂O₄) NMs was selected as the optimal dose based on its outstanding effects on promoting tomato flowering and production. After the foliar application before flowering, MnFe₂O₄ NMs increased the leaf chlorophyll content by 20 percent, and significantly upregulated the expressions of ferredoxin, PsaA, and PsbA in leaves, likely by serving as an electron donor, leading to a significant increase in photosynthesis efficiency by 13.3%. Long distance transport of sucrose was then confirmed by the upregulation of sucrose transporter SUT1 and SUT2 in NM-treated leaves and meristems. The genes associated with gibberellin biosynthesis, including GA20ox2, GA20ox3, and SIGAST, and a flowering induction gene SFT, were also significantly upregulated. Importantly, the flowering time was 13 days earlier by MnFe₂O₄ NMs over the control. At the reproductive stage, MnFe₂O₄ NMs increased pollen activity and ovule size, leading to the significant increase in fruit number per plant, single fruit weight, and fruit weight per plant by 50%, 30%, and 75%, respectively. Metabolically, a significant increase of glucose-6-phosphate, phenylalanine, rutin, and ascorbic acid (vitamin C), as well as a significant decrease of tomatine and methionine, demonstrates an increased nutritional value of the tomato fruits. A verified companion field experiment showed an increase of 84.1% in total tomato production with the MnFe₂O₄ NM amendment. These findings provide support for the early flowering and yield improvement in nano-enabled agricultural systems.

The Genetic and Hormonal Inducers of Continuous Flowering in Orchids: An Emerging View

Orchids are the flowers of magnetic beauty. Vivid and attractive flowers with magnificent shapes make them the king of the floriculture industry. However, the long-awaited flowering is a drawback to their market success, and therefore, flowering time regulation is the key to studies about orchid flower development. Although there are some rare orchids with a continuous flowering pattern, the molecular regulatory mechanisms are yet to be elucidated to find applicable solutions to other orchid species. Multiple regulatory pathways, such as photoperiod, vernalization, circadian clock, temperature and hormonal pathways are thought to signalize flower timing using a group of floral integrators. This mini review, thus, organizes the current knowledge of floral time regulators to suggest future perspectives on the continuous flowering mechanism that may help to plan functional studies to induce flowering revolution in precious orchid species.

Enhancing crop diversity for food security in the face of climate uncertainty

Global agriculture is dominated by a handful of species that currently supply a huge proportion of our food and feed. It additionally faces the massive challenge of providing food for 10 billion people by 2050, despite increasing environmental deterioration. One way to better plan production in the face of current and continuing climate change is to better understand how our domestication of these crops included their adaptation to environments that were highly distinct from those of their centre of origin. There are many prominent examples of this, including the development of temperate Zea mays (maize) and the alteration of day-length requirements in Solanum tuberosum (potato). Despite the pre-eminence of some 15 crops, more than 50 000 species are edible, with 7000 of these considered semi-cultivated. Opportunities afforded by next-generation sequencing technologies alongside other methods, including metabolomics and high-throughput phenotyping, are starting to contribute to a better characterization of a handful of these species. Moreover, the first examples of de novo domestication have appeared, whereby key target genes are modified in a wild species in order to confer predictable traits of agronomic value. Here, we review the scale of the challenge, drawing extensively on the characterization of past agriculture to suggest informed strategies upon which the breeding of future climate-resilient crops can be based.

Genetic insights into the regulatory pathways for continuous flowering in a unique orchid Arundina graminifolia

BACKGROUND

Manipulation of flowering time and frequency of blooming is key to enhancing the ornamental value of orchids. Arundina graminifolia is a unique orchid that flowers year round, although the molecular basis of this flowering pattern remains poorly understood.

RESULTS

We compared the A. graminifolia transcriptome across tissue types and floral developmental stages to elucidate important genetic regulators of flowering and hormones. Clustering analyses identified modules specific to floral transition and floral morphogenesis, providing a set of candidate regulators for the floral initiation and timing. Among candidate floral homeotic genes, the expression of two FT genes was positively correlated with flower development. Assessment of the endogenous hormone levels and qRT-PCR analysis of 32 pathway-responsive genes supported a role for the regulatory networks in floral bud control in A. graminifolia. Moreover, WGCNA showed that flowering control can be delineated by modules of coexpressed genes; especially, MEgreen presented group of genes specific to flowering.

CONCLUSIONS

Candidate gene selection coupled with hormonal regulators brings a robust source to understand the intricate molecular regulation of flowering in precious orchids.

Mapping and identification of genetic loci affecting earliness of bolting and flowering in lettuce

KEY MESSAGE

Photoperiod and temperature conditions elicit different genetic regulation over lettuce bolting and flowering. This study identifies environment-specific QTLs and putative genes and provides information for genetic marker assay. Bolting, defined as stem elongation, marks the plant life cycle transition from vegetative to reproductive stage. Lettuce is grown for its leaf rosettes, and premature bolting may reduce crop quality resulting in economic losses. The transition to reproductive stage is a complex process that involves many genetic and environmental factors. In this study, the effects of photoperiod and ambient temperature on bolting and flowering regulation were studied by utilizing a lettuce mapping population to identify quantitative trait loci (QTL) and by gene expression analyses of genotypes with contrasting phenotypes. A recombinant inbred line (RIL) population, derived from a cross between PI 251246 (early bolting) and cv. Salinas (late bolting), was grown in four combinations of short (8 h) and long (16 h) days and low (20 °C) and high (35 °C) temperature. QTL models revealed both genetic (G) and environmental (E) effects, and GxE interactions. A major QTL for bolting and flowering time was found on chromosome 7 (qFLT7.2), and two candidate genes were identified by fine mapping, homology, and gene expression studies. In short days and high temperature conditions, qFLT7.2 had no effect on plant development, while several small-effect loci on chromosomes 2, 3, 6, 8, and 9 were associated with bolting and flowering. Of these, the QTL on chromosome 2, qBFr2.1, co-located with the Flowering Locus T (LsFT) gene. Polymorphisms between parent genotypes in the promotor region may explain identified gene expression differences and were used to design a genetic marker which may be used to identify the late bolting trait.

Quantitative Trait Loci and Candidate Genes Associated with Photoperiod Sensitivity in Lettuce (Lactuca spp.)

KEY MESSAGE

A population of lettuce that segregated for photoperiod sensitivity was planted under long-day and short-day conditions. Genetic mapping revealed two distinct sets of QTLs controlling daylength-independent and photoperiod-sensitive flowering time. The molecular mechanism of flowering time regulation in lettuce is of interest to both geneticists and breeders because of the extensive impact of this trait on agricultural production. Lettuce is a facultative long-day plant which changes in flowering time in response to photoperiod. Variations exist in both flowering time and the degree of photoperiod sensitivity among accessions of wild (Lactuca serriola) and cultivated (L. sativa) lettuce. An F6 population of 236 recombinant inbred lines (RILs) was previously developed from a cross between a late-flowering, photoperiod-sensitive L. serriola accession and an early-flowering, photoperiod-insensitive L. sativa accession. This population was planted under long-day (LD) and short-day (SD) conditions in a total of four field and screenhouse trials; the developmental phenotype was scored weekly in each trial. Using genotyping-by-sequencing (GBS) data of the RILs, quantitative trait loci (QTL) mapping revealed five flowering time QTLs that together explained more than 20% of the variation in flowering time under LD conditions. Using two independent statistical models to extract the photoperiod sensitivity phenotype from the LD and SD flowering time data, we identified an additional five QTLs that together explained more than 30% of the variation in photoperiod sensitivity in the population. Orthology and sequence analysis of genes within the nine QTLs revealed potential functional equivalents in the lettuce genome to the key regulators of flowering time and photoperiodism, FD and CONSTANS, respectively, in Arabidopsis.

QTL Analysis of Stem Elongation and Flowering Time in Lettuce Using Genotyping-by-Sequencing

Lettuce plants tend to undergo floral initiation by elongation of flower stalks (bolting) under high-temperature and long-day conditions, which is a serious problem for summer lettuce production. Our objective was to generate a high-density genetic map using SNPs obtained from genotyping-by-sequencing (GBS) analysis of F5 recombinant inbred lines (RILs) and to map QTLs involved in stem growth and flowering time in lettuce. A set of 127 intra-specific RIL mapping populations derived from a cross between two varieties, green and red leaf lettuce, were used to identify QTLs related to the number of days from sowing to bolting (DTB), to flowering of the first flower (DTF), to seed-setting of the first flower (DTS), and the total number of leaves (LN), plant height (PH), and total number of branches of main inflorescence (BN) for two consecutive years. Of the 15 QTLs detected, one that controls DTB, DTF, DTS, LN, and PH detected on LG 7, and another QTL that controls DTF, DTS, and PH detected on LG 1. Analysis of the genomic sequence corresponding to the QTL detected on LG 7 led to the identification of 22 putative candidate genes. A consistent QTL related to bolting and flowering time, and corresponding candidate genes has been reported. This study will be valuable in revealing the genetic basis of stem growth and flowering time in lettuce.