Overexpression of a Chimeric Gene, OsDST-SRDX, Improved Salt Tolerance of Perennial Ryegrass

Design and Characterization of a Transcriptional Repression Toolkit for Plants

Regulation of gene expression is essential for all life. Tools to manipulate the gene expression level have therefore proven to be very valuable in efforts to engineer biological systems. However, there are few well-characterized genetic parts that reduce gene expression in plants, commonly known as transcriptional repressors. We characterized the repression activity of a library consisting of repression motifs from approximately 25% of the members of the largest known family of repressors. Combining sequence information with our trans-regulatory function data, we next generated a library of synthetic transcriptional repression motifs with function predicted in advance. After characterizing our synthetic library, we demonstrated not only that many of our synthetic constructs were functional as repressors but also that our advance predictions of repression strength were better than random guesses. Finally, we assessed the functionality of known transcriptional repression motifs from a wide range of eukaryotes. Our study represents the largest plant repressor motif library experimentally characterized to date, providing unique opportunities for tuning transcription in plants.

Dissection of the IDA promoter identifies WRKY transcription factors as abscission regulators in Arabidopsis

Plants shed organs such as leaves, petals, or fruits through the process of abscission. Monitoring cues such as age, resource availability, and biotic and abiotic stresses allow plants to abscise organs in a timely manner. How these signals are integrated into the molecular pathways that drive abscission is largely unknown. The INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene is one of the main drivers of floral organ abscission in Arabidopsis and is known to transcriptionally respond to most abscission-regulating cues. By interrogating the IDA promoter in silico and in vitro, we identified transcription factors that could potentially modulate IDA expression. We probed the importance of ERF- and WRKY-binding sites for IDA expression during floral organ abscission, with WRKYs being of special relevance to mediate IDA up-regulation in response to biotic stress in tissues destined for separation. We further characterized WRKY57 as a positive regulator of IDA and IDA-like gene expression in abscission zones. Our findings highlight the promise of promoter element-targeted approaches to modulate the responsiveness of the IDA signaling pathway to harness controlled abscission timing for improved crop productivity.

Epichloë fungal endophyte interactions in perennial ryegrass (Lolium perenne L.) modified to accumulate foliar lipids for increased energy density

BACKGROUND

Commercial cultivars of perennial ryegrass infected with selected Epichloë fungal endophytes are highly desirable in certain pastures as the resulting mutualistic association has the capacity to confer agronomic benefits (such as invertebrate pest deterrence) largely due to fungal produced secondary metabolites (e.g., alkaloids). In this study, we investigated T2 segregating populations derived from two independent transformation events expressing diacylglycerol acyltransferase (DGAT) and cysteine oleosin (CO) genes designed to increase foliar lipid and biomass accumulation. These populations were either infected with Epichloë festucae var. lolii strain AR1 or Epichloë sp. LpTG-3 strain AR37 to examine relationships between the introduced trait and the endophytic association. Here we report on experiments designed to investigate if expression of the DGAT + CO trait in foliar tissues of perennial ryegrass could negatively impact the grass-endophyte association and vice versa. Both endophyte and plant characters were measured under controlled environment and field conditions.

RESULTS

Expected relative increases in total fatty acids of 17-58% accrued as a result of DGAT + CO expression with no significant difference between the endophyte-infected and non-infected progeny. Hyphal growth in association with DGAT + CO expression appeared normal when compared to control plants in a growth chamber. There was no significant difference in mycelial biomass for both strains AR1 and AR37, however, Epichloë-derived alkaloid concentrations were significantly lower on some occasions in the DGAT + CO plants compared to the corresponding null-segregant progenies, although these remained within the reported range for bioactivity.

CONCLUSIONS

These results suggest that the mutualistic association formed between perennial ryegrass and selected Epichloë strains does not influence expression of the host DGAT + CO technology, but that endophyte performance may be reduced under some circumstances. Further investigation will now be required to determine the preferred genetic backgrounds for introgression of the DGAT + CO trait in combination with selected endophyte strains, as grass host genetics is a major determinant to the success of the grass-endophyte association in this species.

Ectopic Expression of PvHMA2.1 Enhances Cadmium Tolerance in Arabidopsis thaliana

Cadmium (Cd) in soil inhibits plant growth and development and even harms human health through food chain transmission. Switchgrass (Panicum virgatum L.), a perennial C4 biofuel crop, is considered an ideal plant for phytoremediation due to its high efficiency in removing Cd and other heavy metals from contaminated soil. The key to understanding the mechanisms of switchgrass Cd tolerance is to identify the genes involved in Cd transport. Heavy-metal ATPases (HMAs) play pivotal roles in heavy metal transport, including Cd, in Arabidopsis thaliana and Oryza sativa, but little is known about the functions of their orthologs in switchgrass. Therefore, we identified 22 HMAs in switchgrass, which were distributed on 12 chromosomes and divided into 4 groups using a phylogenetic analysis. Then, we focused on PvHMA2.1, which is one of the orthologs of the rice Cd transporter OsHMA2. We found that PvHMA2.1 was widely expressed in roots, internodes, leaves, spikelets, and inflorescences, and was significantly induced in the shoots of switchgrass under Cd treatment. Moreover, PvHMA2.1 was found to have seven transmembrane domains and localized at the cell plasma membrane, indicating that it is a potential transporter. The ectopic expression of PvHMA2.1 alleviated the reduction in primary root length and the loss of fresh weight of Arabidopsis seedlings under Cd treatment, suggesting that PvHMA2.1 enhanced Cd tolerance in Arabidopsis. The higher levels of relative water content and chlorophyll content of the transgenic lines under Cd treatment reflected that PvHMA2.1 maintained water retention capacity and alleviated photosynthesis inhibition under Cd stress in Arabidopsis. The roots of the PvHMA2.1 ectopically expressed lines accumulated less Cd compared to the WT, while no significant differences were found in the Cd contents of the shoots between the transgenic lines and the WT under Cd treatment, suggesting that PvHMA2.1 reduced Cd absorption from the environment through the roots in Arabidopsis. Taken together, our results showed that PvHMA2.1 enhanced Cd tolerance in Arabidopsis, providing a promising target that could be engineered in switchgrass to repair Cd-contaminated soil.

Callus Induction from Diverse Explants and Genotypes Enables Robust Transformation of Perennial Ryegrass (Lolium perenne L.)

Genetic transformation of perennial ryegrass (Lolium perenne L.) is critical for fundamental and translational research in this important grass species. It often relies on Agrobacterium-mediated transformation of callus tissue. However, callus induction is restricted to a few genotypes that respond well to tissue culture. Here, we report callus induction from different perennial ryegrass genotypes and explants, such as shoot tips, seeds, and anthers, which were transformed with several plasmids for functional genomics. β-glucuronidase (GUS) histochemical staining showed the LmdsRNAbp promoter sequence was active in stigmas, spikelets, anthers, and leaves. We also transformed calli with plasmids allowing gene silencing and gene knock-out using RNA interference and CRISPR/Cas9, respectively, for which genotypic and phenotypic investigations are ongoing. Using 19 different constructs, 262 transgenic events were regenerated. Moreover, the protocol regenerated a doubled haploid transgenic event from anther-derived calli. This work provides a proof-of-concept method for expanding the range of genotypes amenable to transformation, thus, serving research and breeding initiatives to improve this important grass crop for forage and recreation.

Insights into the Response of Perennial Ryegrass to Abiotic Stress: Underlying Survival Strategies and Adaptation Mechanisms

Perennial ryegrass (Lolium perenne L.) is an important turfgrass and gramineous forage widely grown in temperate regions around the world. However, its perennial nature leads to the inevitable exposure of perennial ryegrass to various environmental stresses on a seasonal basis and from year to year. Like other plants, perennial ryegrass has evolved sophisticated mechanisms to make appropriate adjustments in growth and development in order to adapt to the stress environment at both the physiological and molecular levels. A thorough understanding of the mechanisms of perennial ryegrass response to abiotic stresses is crucial for obtaining superior stress-tolerant varieties through molecular breeding. Over the past decades, studies of perennial ryegrass at the molecular and genetic levels have revealed a lot of useful information to understand the mechanisms of perennial ryegrass adaptation to an adverse environment. Unfortunately, molecular mechanisms by which perennial ryegrass adapts to abiotic stresses have not been reviewed thus far. In this review, we summarize the recent works on the genetic and molecular mechanisms of perennial ryegrass response to the major abiotic stresses (i.e., drought, salinity, and extreme temperatures) and discuss new directions for future studies. Such knowledge will provide valuable information for molecular breeding in perennial ryegrass to improve stress resistance and promote the sustainability of agriculture and the environment.

Analysis of Gene Expression Changes in Plants Grown in Salty Soil in Response to Inoculation with Halophilic Bacteria

Soil salinity is an increasing problem facing agriculture in many parts of the world. Climate change and irrigation practices have led to decreased yields of some farmland due to increased salt levels in the soil. Plants that have tolerance to salt are thus needed to feed the world's population. One approach addressing this problem is genetic engineering to introduce genes encoding salinity, but this approach has limitations. Another fairly new approach is the isolation and development of salt-tolerant (halophilic) plant-associated bacteria. These bacteria are used as inoculants to stimulate plant growth. Several reports are now available, demonstrating how the use of halophilic inoculants enhance plant growth in salty soil. However, the mechanisms for this growth stimulation are as yet not clear. Enhanced growth in response to bacterial inoculation is expected to be associated with changes in plant gene expression. In this review, we discuss the current literature and approaches for analyzing altered plant gene expression in response to inoculation with halophilic bacteria. Additionally, challenges and limitations to current approaches are analyzed. A further understanding of the molecular mechanisms involved in enhanced plant growth when inoculated with salt-tolerant bacteria will significantly improve agriculture in areas affected by saline soils.

ADAP is a possible negative regulator of glucosinolate biosynthesis in Arabidopsis thaliana based on clustering and gene expression analyses

Glucosinolates (GSLs) are plant secondary metabolites consisting of sulfur and nitrogen, commonly found in Brassicaceae crops, such as Arabidopsis thaliana. These compounds are known for their roles in plant defense mechanisms against pests and pathogens. 'Guilt-by-association' (GBA) approach predicts genes encoding proteins with similar function tend to share gene expression pattern generated from high throughput sequencing data. Recent studies have successfully identified GSL genes using GBA approach, followed by targeted verification of gene expression and metabolite data. Therefore, a GSL co-expression network was constructed using known GSL genes obtained from our in-house database, SuCComBase. DPClusO was used to identify subnetworks of the GSL co-expression network followed by Fisher's exact test leading to the discovery of a potential gene that encodes the ARIA-interacting double AP2-domain protein (ADAP) transcription factor (TF). Further functional analysis was performed using an effective gene silencing system known as CRES-T. By applying CRES-T, ADAP TF gene was fused to a plant-specific EAR-motif repressor domain (SRDX), which suppresses the expression of ADAP target genes. In this study, ADAP was proposed as a negative regulator in aliphatic GSL biosynthesis due to the over-expression of downstream aliphatic GSL genes (UGT74C1 and IPMI1) in ADAP-SRDX line. The significant over-expression of ADAP gene in the ADAP-SRDX line also suggests the behavior of the TF that negatively affects the expression of UGT74C1 and IPMI1 via a feedback mechanism in A. thaliana.

Role of a complex of two proteins in alleviating sodium ion stress in an economic crop

An economically valuable woody plant species tree bean (Cajanus cajan (L.) Millsp.) is predominantly cultivated in tropical and subtropical areas and is regarded as an important food legume (or pulse) crop that is facing serious sodium ion stress. NAM (N-acetyl-5-methoxytryptamine) has been implicated in abiotic and biotic stress tolerance in plants. However, the role of NAM in sodium ion stress tolerance has not been determined. In this study, the effect of NAM was investigated in the economically valuable woody plant species, challenged with stress at 40 mM sodium ion for 3 days. NAM-treated plants (200 μM) had significantly higher fresh weight, average root length, significantly reduced cell size, increased cell number, and increased cytoskeleton filaments in single cells. The expression pattern of one of 10 Tree bean Dynamic Balance Movement Related Protein (TbDMP), TbDMP was consistent with the sodium ion-stress alleviation by NAM. Using TbDMP as bait, Dynamic Balance Movement Related Kinase Protein (TbDBK) was determined to interact with TbDMP by screening the tree bean root cDNA library in yeast. Biochemical experiments showed that NAM enhanced the interaction between the two proteins which promoted resist sodium ion stress resistance. This study provides evidence of a pathway through which the skeleton participates in NAM signaling.

Isolation of gene conferring salt tolerance from halophilic bacteria of Lunsu, Himachal Pradesh, India

Background

Halophiles offer an attractive source of genes conferring salt tolerance. Halobacillus trueperi SS1 strain of Lunsu, Himachal Pradesh, India, a strict halophile, was exploited to isolate and clone the genes for salt tolerance. The genomic library of BamH1 digest of H. trueperi SS1 was constructed in pUC19, and recombinants were screened for salt tolerance on an LB medium containing ampicillin (100 μg/ml) and NaCl (0 to 1.5 M).

Results

One recombinant clone named as salt-tolerant clone (STC) conferred salt tolerance to host Escherichia coli/DH5α, which showed growth in the LB medium supplemented with ampicillin and 1.2 M NaCl. Restriction digestion and PCR analysis revealed the presence of an insert of approximately 2000 bp in the STC. DNA sequencing of the 2-kb insert on both strands yielded a sequence of 2301 nucleotides. Protein BLAST analysis of 2301-bp sequence of H. trueperi SS1 present in STC showed 97% identity to multidrug transport ATP binding/permease protein of Halobacillus karajensis. The insert contained in STC was subcloned into pGEX4T2 vector, and the recombinant clone STC/pGEX4T2 conferred salt tolerance to the bacterial host E. coli.

Conclusions

The present study led to the isolation of salt tolerance gene encoding a putative multidrug transport ATP binding/permease protein from H. trueperi SS1. The salt tolerance gene can be subcloned for transferring salt tolerance traits into agricultural crop plants for cultivation in saline and coastal lands.

Genome-wide investigation of the NAC transcript factor family in perennial ryegrass (Lolium perenne L.) and expression analysis under various abiotic stressor

NAC is one of the largest family of plant-specific transcription factors, and it plays important roles in plant development and stress responses. The study identified 72 LpNACs genes from the perennial ryegrass genome database. Gene length, MW and pI of NAC family transcription factors varied, but the gene structure and motifs were relatively conserved in bioinformatics analysis. Phylogenetic analyses of perennial ryegrass, rice and Arabidopsis were performed to study the evolutionary and functional relationships in various species. The expression of LpNAC genes that respond to various abiotic stresses including high salinity, ABA, high temperature, polyethylene glycol (PEG) and heavy metal was comprehensively analyzed. The present study provides a basic understanding of the NAC gene family in perennial ryegrass for further abiotic stress studies and improvements in breeding.

Heterologous expression of a chimeric gene, OsDST-SRDX, enhanced salt tolerance of transgenic switchgrass (Panicum virgatum L.)

Overexpression of OsDST-SRDX chimeric gene in switchgrass promotes plant growth and improves the salt tolerance of transgenic switchgrass by improving its antioxidative ability. Switchgrass (Panicum virgatum L.) is a forage and model feedstock plant. To avoid competing with crops in arable land utilization, improving salt tolerance of switchgrass is required to use marginal saline land for switchgrass production. To improve salt tolerance of switchgrass, a chimeric DROUGHT AND SALT TOLERANCE (DST) gene OsDST-SRDX was constructed using the Chimeric REpressor gene-Silencing Technology (CRES-T), and introduced into switchgrass genome by Agrobacterium-mediated transformation. Compared to wild-type (WT) plants, OsDST-SRDX transgenic (TG) switchgrass plants showed wider leaves and thicker stems. They performed better under salt stress, had higher relative leaf water content, lower electrolyte leakage and lower malondialdehyde (MDA) content, and accumulated less Na⁺ and more K⁺ than WT controls. The transgenic plants had also higher activities of antioxidant enzymes associated with suppressed expressing of genes in H₂O₂ homeostasis, including glutathione S-transferase (GST2, GST6), cytochrome P450, peroxidase 24 precursor, and induced expressing of CAT and SOD under salt stress to eliminate excess H₂O₂. Our results indicate that overexpression of the chimeric gene OsDST-SRDX improves salt tolerance of switchgrass, a C₄ biofuel crop.

Melatonin Application Improves Salt Tolerance of Alfalfa (Medicago sativa L.) by Enhancing Antioxidant Capacity

Alfalfa (Medicago sativa L.) is an important and widely cultivated forage grass. The productivity and forage quality of alfalfa are severely affected by salt stress. Melatonin is a bioactive molecule with versatile physiological functions and plays important roles in response to various biotic and abiotic stresses. Melatonin has been proven efficient in improving alfalfa drought and waterlogging tolerance in recent studies. In our reports, we applied melatonin exogenously to explore the effects of melatonin on alfalfa growth and salt resistance. The results demonstrated that melatonin application promoted alfalfa seed germination and seedling growth, and reduced oxidative damage under salt stress. Further application research found that melatonin alleviated salt injury in alfalfa plants under salt stress. The electrolyte leakage, malondialdehyde (MDA) content and H₂O₂ content were significantly reduced, and the activities of catalase (CAT), peroxidase (POD), and Cu/Zn superoxide dismutase (Cu/Zn-SOD) were increased with melatonin pretreatment compared to control plants under salt stress with the upregulation of genes related to melatonin and antioxidant enzymes biosynthesis. Melatonin was also involved in reducing Na⁺ accumulation in alfalfa plants. Our study indicates that melatonin plays a primary role as an antioxidant in scavenging H₂O₂ and enhancing activities of antioxidant enzymes to improve the salt tolerance of alfalfa plants.

Safety Assessment of Genetically Modified Feed: Is There Any Difference From Food?

Food security is one of major concerns for the growing global population. Modern agricultural biotechnologies, such as genetic modification, are a possible solution through enabling an increase of production, more efficient use of natural resources, and reduced environmental impacts. However, new crop varieties with altered genetic materials may be subjected to safety assessments to fulfil the regulatory requirements, prior to marketing. The aim of the assessment is to evaluate the impact of products from the new crop variety on human, animal, and the environmental health. Although, many studies on the risk assessment of genetically modified (GM) food have been published, little consideration to GM feedstuff has been given, despite that between 70 to 90% of all GM crops and their biomass are used as animal feed. In addition, in some GM plants such as forages that are only used for animal feeds, the assessment of the genetic modification may be of relevance only to livestock feeding. In this article, the regulatory framework of GM crops intended for animal feed is reviewed using the available information on GM food as the baseline. Although, the majority of techniques used for the safety assessment of GM food can be used in GM feed, many plant parts used for livestock feeding are inedible to humans. Therefore, the concentration of novel proteins in different plant tissues and level of exposure to GM feedstuff in the diet of target animals should be considered. A further development of specific methodologies for the assessment of GM crops intended for animal consumption is required, in order to provide a more accurate and standardized assessment to the GM feed safety.

MiR319 mediated salt tolerance by ethylene

Salinity-induced accumulation of certain microRNAs accompanied by gaseous phytohormone ethylene production has been recognized as a mechanism of plant salt tolerance. MicroRNA319 (miR319) has been characterized as an important player in abiotic stress resistance in some C3 plants, such as Arabidopsis thaliana and rice. However, its role in the dedicated biomass plant switchgrass (Panicum virgatum L.), a C4 plant, has not been reported. Here, we show crosstalk between miR319 and ethylene (ET) for increasing salt tolerance. By overexpressing Osa-MIR319b and a target mimicry form of miR319 (MIM319), we showed that miR319 positively regulated ET synthesis and salt tolerance in switchgrass. By experimental treatments, we demonstrated that ET-mediated salt tolerance in switchgrass was dose-dependent, and miR319 regulated the switchgrass salt response by fine-tuning ET synthesis. Further experiments showed that the repression of a miR319 target, PvPCF5, in switchgrass also led to enhanced ethylene accumulation and salt tolerance in transgenic plants. Genome-wide transcriptome analysis demonstrated that overexpression of miR319 (OE-miR319) down-regulated the expression of key genes in the methionine (Met) cycle but promoted the expression of genes in ethylene synthesis. The results enrich our understanding of the synergistic effects of the miR319-PvPCF5 module and ethylene synthesis in the salt tolerance of switchgrass, a C4 bioenergy plant.

Genetic redundancy of senescence-associated transcription factors in Arabidopsis

Leaf senescence is a genetically programmed process that constitutes the last stage of leaf development, and involves massive changes in gene expression. As a result of the intensive efforts that have been made to elucidate the molecular genetic mechanisms underlying leaf senescence, 184 genes that alter leaf senescence phenotypes when mutated or overexpressed have been identified in Arabidopsis thaliana over the past two decades. Concurrently, experimental evidence on functional redundancy within senescence-associated genes (SAGs) has increased. In this review, we focus on transcription factors that play regulatory roles in Arabidopsis leaf senescence, and describe the relationships among gene duplication, gene expression level, and senescence phenotypes. Previous findings and our re-analysis demonstrate the widespread existence of duplicate SAG pairs and a correlation between gene expression levels in duplicate genes and senescence-related phenotypic severity of the corresponding mutants. We also highlight effective and powerful tools that are available for functional analyses of redundant SAGs. We propose that the study of duplicate SAG pairs offers a unique opportunity to understand the regulation of leaf senescence and can guide the investigation of the functions of redundant SAGs via reverse genetic approaches.

Enhanced Cold Tolerance and Tillering in Switchgrass (Panicum virgatum L.) by Heterologous Expression of Osa-miR393a

The microRNA393 (miR393) family is one of the conserved miRNA families in the plant kingdom. MiR393 was reported to regulate rice tillering and abiotic stress resistance positively through an auxin signaling pathway. However, little is known about the function of miR393 in switchgrass (Panicum virgatum L.), an important bioenergy C4 grass plant. We tested the expression level of miR393 and its four putative target genes (PvAFB1, PvAFB2, PvAFB3 and PvTIR1) in switchgrass, and found that these genes all responded to cold stress and exogenous 1-naphthaleneacetic acid (NAA) treatment. To investigate the function of miR393 in switchgrass, we enhanced miR393 expression by introducing an Osa-miR393a gene into switchgrass. The results showed that cold tolerance of the transgenic T0 and T1 generation plants was highly improved. Cold tolerance-related genes PvCOR47, PvICE1 and PvRAV1 were negatively regulated by exogenous NAA, and the expression of these genes was significantly higher in transgenic plants than in wild-type plants. The transgenic T1 seedlings were more tolerant to exogenous NAA treatment, accumulating less H2O2 after cold treatments. It was also observed that the miR393/target module regulates cold tolerance responses in Arabidopsis. In addition, transgenic plants overexpressing miR393 had significantly more tillers and higher biomass yield per plant in greenhouse and field tests. Forage quality analyses revealed that the soluble sugar contents of transgenic plants were increased markedly. Overall, the results suggested that overexpression of miR393 improved cold tolerance and tillering of switchgrass through regulation of auxin signaling transduction.

Tying the knot: occurrence and possible significance of gene fusions in plant metabolism and beyond

Gene fusions have recently attracted attention especially in the field of plant specialized metabolism. The occurrence of a gene fusion, in which originally separate gene products are combined into a single polypeptide, often corresponds to the functional association of individual components within a single metabolic pathway. Examples include gene fusions implicated in benzylisoquinoline alkaloid (BIA), terpenoid, and amino acid biosynthetic pathways, in which distinct domains within a fusion catalyze consecutive, yet independent reactions. Both genomic and transcriptional mechanisms result in the fusion of gene products, which can include partial or complete domain repeats and extensive domain shuffling as evident in the BIA biosynthetic enzyme norcoclaurine synthase. Artificial gene fusions are commonly deployed in attempts to engineer new or improved pathways in plants or microorganisms, based on the premise that fusions are advantageous. However, a survey of functionally characterized fusions in microbial systems shows that the functional impact of fused gene products is not straightforward. For example, whereas enzyme fusions might facilitate the metabolic channeling of unstable intermediates, this channeling can also occur between tightly associated independent enzymes. The frequent occurrence of both fused and unfused enzymes in plant and microbial metabolism adds additional complexity, in terms of both pathway functionality and evolution.

Cold acclimation alters DNA methylation patterns and confers tolerance to heat and increases growth rate in Brassica rapa

Epigenetic modifications are implicated in plant adaptations to abiotic stresses. Exposure of plants to one stress can induce resistance to other stresses, a process termed cross-adaptation, which is not well understood. In this study, we aimed to unravel the epigenetic basis of elevated heat-tolerance in cold-acclimated Brassica rapa by conducting a genome-wide DNA methylation analysis of leaves from control (CK) and cold-acclimated (CA) plants. We found that both methylation and demethylation occurred during cold acclimation. Two significantly altered pathways, malate dehydrogenase activity and carbon fixation, and 1562 differentially methylated genes, including BramMDH1, BraKAT2, BraSHM4, and Bra4CL2, were identified in CA plants. Genetic validation and treatment of B. rapa with 5-aza-2-deoxycytidine (Aza) suggested that promoter demethylation of four candidate genes increased their transcriptional activities. Physiological analysis suggested that elevated heat-tolerance and high growth rate were closely related to increases in organic acids and photosynthesis, respectively. Functional analyses demonstrated that the candidate gene BramMDH1 (mMDH: mitochondrial malate dehydrogenase) directly enhances organic acids and photosynthesis to increase heat-tolerance and growth rate in Arabidopsis. However, Aza-treated B. rapa, which also has elevated BramMDH1 levels, did not exhibit enhanced heat-tolerance. We therefore suggest that DNA demethylation alone is not sufficient to increase heat-tolerance. This study demonstrates that altered DNA methylation contributes to cross-adaptation.

A Novel Two-Step Method for Screening Shade Tolerant Mutant Plants via Dwarfism

When subjected to shade, plants undergo rapid shoot elongation, which often makes them more prone to disease and mechanical damage. Shade-tolerant plants can be difficult to breed; however, they offer a substantial benefit over other varieties in low-light areas. Although perennial ryegrass (Lolium perenne L.) is a popular species of turf grasses because of their good appearance and fast establishment, the plant normally does not perform well under shade conditions. It has been reported that, in turfgrass, induced dwarfism can enhance shade tolerance. Here we describe a two-step procedure for isolating shade tolerant mutants of perennial ryegrass by first screening for dominant dwarf mutants, and then screening dwarf plants for shade tolerance. The two-step screening process to isolate shade tolerant mutants can be done efficiently with limited space at early seedling stages, which enables quick and efficient isolation of shade tolerant mutants, and thus facilitates development of shade tolerant new cultivars of turfgrasses. Using the method, we isolated 136 dwarf mutants from 300,000 mutagenized seeds, with 65 being shade tolerant (0.022%). When screened directly for shade tolerance, we recovered only four mutants from a population of 150,000 (0.003%) mutagenized seeds. One shade tolerant mutant, shadow-1, was characterized in detail. In addition to dwarfism, shadow-1 and its sexual progeny displayed high degrees of tolerance to both natural and artificial shade. We showed that endogenous gibberellin (GA) content in shadow-1 was higher than wild-type controls, and shadow-1 was also partially GA insensitive. Our novel, simple and effective two-step screening method should be applicable to breeding shade tolerant cultivars of turfgrasses, ground covers, and other economically important crop plants that can be used under canopies of existing vegetation to increase productivity per unit area of land.