Suppression of the barley uroporphyrinogen III synthase gene by a Ds activation tagging element generates developmental photosensitivity

Agrobacterium-mediated transformation of B. juncea reveals that BjuLKP2 functions in plant yellowing

KEY MESSAGE

A stable Agrobacterium-mediated transformation system was constructed for B. juncea, and BjuLKP2 was overexpressed, leading to plant yellowing. A stable and efficient transformation system is necessary to verify gene functions in plants. To establish an Agrobacterium-mediated transformation system for B. juncea, various factors, including the explant types, hormone combination and concentration, infection time and concentration, were optimized. Eventually, a reliable system was established, and two BjuLKP2 overexpression (OE) lines, which displayed yellowing of cotyledons, shoot tips, leaves and flower buds, as well as a decrease in total chlorophyll content, were generated. qRT-PCR assays revealed significant upregulation of five chlorophyll synthesis genes and downregulation of one gene in the BjuLKP2 OE line. Furthermore, antioxidant capacity assays revealed reduced activities of APX, CAT and SOD, while POD activity increased in the BjuLKP2 OE26. Additionally, the kinetic determination of chlorophyll fluorescence induction suggested a decrease in the photosynthetic ability of BjuLKP2 OE26. GUS assays revealed the expression of BjuLKP2 in various tissues, including the roots, hypocotyls, cotyledons, leaf vasculature, trichomes, sepals, petals, filaments, styles and stigma bases, but not in seeds. Scanning electron revealed alterations in chloroplast ultrastructure in both the sponge and palisade tissue. Collectively, these findings indicate that BjuLKP2 plays a role in plant yellowing through a reduction in chlorophyll content and changes in chloroplasts structure.

PROGRAMMED CELL DEATH8 interacts with tetrapyrrole biosynthesis enzymes and ClpC1 to maintain homeostasis of tetrapyrrole metabolites in Arabidopsis

Tetrapyrrole biosynthesis (TBS) is a dynamically and strictly regulated process. Disruptions in tetrapyrrole metabolism influence many aspects of plant physiology, including photosynthesis, programmed cell death (PCD), and retrograde signaling, thus affecting plant growth and development at multiple levels. However, the genetic and molecular basis of TBS is not fully understood. We report here PCD8, a newly identified thylakoid-localized protein encoded by an essential gene in Arabidopsis. PCD8 knockdown causes a necrotic phenotype due to excessive chloroplast damage. A burst of singlet oxygen that results from overaccumulated tetrapyrrole intermediates upon illumination is suggested to be responsible for cell death in the knockdown mutants. Genetic and biochemical analyses revealed that PCD8 interacts with ClpC1 and a number of TBS enzymes, such as HEMC, CHLD, and PORC of TBS. Taken together, our findings uncover the function of chloroplast-localized PCD8 and provide a new perspective to elucidate molecular mechanism of how TBS is finely regulated in plants.

Enhanced proteostasis, lipid remodeling, and nitrogen remobilization define barley flag leaf senescence

Leaf senescence is a developmental process allowing nutrient remobilization to sink organs. We characterized flag leaf senescence at 7, 14, and 21 d past anthesis in two near-isogenic barley lines varying in the allelic state of the HvNAM1 transcription factor gene, which influences senescence timing. Metabolomics and microscopy indicated that, as senescence progressed, thylakoid lipids were transiently converted to neutral lipids accumulating in lipid droplets. Senescing leaves also exhibited an accumulation of sugars including glucose, while nitrogen compounds (nucleobases, nucleotides, and amino acids) decreased. RNA-Seq analysis suggested lipid catabolism via β-oxidation and the glyoxylate cycle, producing carbon skeletons and feeding respiration as a replacement of the diminished carbon supply from photosynthesis. Comparison of the two barley lines highlighted a more prominent up-regulation of heat stress transcription factor- and chaperone-encoding genes in the late-senescing line, suggesting a role for these genes in the control of leaf longevity. While numerous genes with putative roles in nitrogen remobilization were up-regulated in both lines, several peptidases, nucleases, and nitrogen transporters were more highly induced in the early-senescing line; this finding identifies processes and specific candidates which may affect nitrogen remobilization from senescing barley leaves, downstream of the HvNAM1 transcription factor.

Plastid-mediated singlet oxygen in regulated cell death

Reactive oxygen species (ROS) generation within a cell is a natural process of specific subcellular components involved in redox reactions. Within a plant cell, chloroplasts are one of the major sources of ROS generation. Plastid-generated ROS molecules include singlet oxygen (¹ O₂ ), superoxide radical (O₂ ^- ), hydroxyl radical (OH^• ) and hydrogen peroxide (H₂ O₂ ), which are produced mainly during photochemical reactions of photosynthesis and chlorophyll biosynthetic process. Under normal growth and developmental, generated ROS molecules act as a secondary messenger controlling several metabolic reactions; however, perturbed environmental conditions lead to multi-fold amplification of cellular ROS that eventually kill the target cell. To maintain homeostasis between production and scavenging of ROS, the cell has instituted several enzymatic and non-enzymatic antioxidant machineries to maintain ROS at a physiological level. Among chloroplastic ROS molecules, excess generation of singlet oxygen (¹ O₂ ) is highly deleterious to the cell metabolic functions and survival. Interestingly, within cellular antioxidant machinery, enzymes involved in detoxification of ¹ O₂ are lacking. Recent studies suggest that under optimal concentrations, ¹ O₂ acts as a signalling molecule and drives the cell to either the acclimation pathway or regulated cell death (RCD). Stress-induced RCD is a survival mechanism for the whole plant, while the involvement of chloroplasts and chloroplast-localized molecules that execute RCD are not well understood. In this review, we advocate for participation of chloroplasts-generated ¹ O₂ in signalling and RCD in plants.

Up-regulation of phenylpropanoid biosynthesis system in peach species by peach aphids produces anthocyanins that protect the aphids against UVB and UVC radiation

Conspicuous color is a common trait of foliar galls, but their relationship with gall-inducing insects is unknown. Red and green galls were taken from sunny or shady parts of peach species Prunus persica (L.) Batsch. f. rubro-plena Schneid with peach aphid Tuberocephalus momonis (Matsumura) infestation. We found that the loss of photosynthetic pigments was associated with the conspicuous coloration of green gall tissues. The concentrations of anthocyanins significantly increased following ultraviolet (UV) irradiation of green gall tissues, suggesting that accumulation of anthocyanins in red galls is related to ultraviolet B and C (UVB and UVC) radiation. The expression of structural genes related to the biosynthesis of chlorogenic acid and malic acid benzoate was increased in all gall tissues and negatively correlated with the expression profiles of certain genes associated with photosynthetic biosynthesis, indicating that the increased transcript levels of the phenylpropanoid pathway might cause loss of photosynthetic efficiency in the gall tissues. Transcriptome and quantitative reverse transcription PCR analyses revealed that MYB transcription factors that up-regulate the biosynthesis of anthocyanins in red gall tissues might be activated by both UVB and UVC exposure. Comet assays suggest that green and red gall tissues have similar DNA damage following UV irradiation. No obvious effect of the up-regulated compounds on the growth of the peach aphid was observed. Interestingly, peach aphids under leaves painted with anthocyanins had lower mortality following UV irradiation than those in controls. These results suggest that the anthocyanins in red gall tissues have a defensive function for the peach aphid, protecting it against UV radiation.

How protoporphyrinogen IX oxidase inhibitors and transgenesis contribute to elucidate plant tetrapyrrole pathway

Several families of herbicides, especially diphenyl ether (DPE) and pyrimidinedione, target the plant tetrapyrrole biosynthesis pathways and in particular one key enzyme, protoporphyrinogen IX oxidase (PPO). When plants are treated with DPE or pyrimidinedione, an accumulation of protoporphyrin IX, the first photosensitizer of this pathway, is observed in cytosol where it becomes very deleterious under light. Indeed these herbicides trigger plant death in two distinct ways: (i) inhibition of chlorophylls and heme syntheses and (ii) a huge accumulation of protoporphyrin IX in cytosol. Recently, a strategy based on plant transgenesis that induces deregulation of the tetrapyrrole pathway by up- or down-regulation of genes encoding enzymes, such as glutamyl-[Formula: see text]RNA reductase, porphobilinogen deaminase and PPO, has been developed. Against all expectations, only transgenic crops overexpressing PPO showed resistance to DPE and pyrimidinedione. This herbicide resistance of transgenic crops leads to the hypothesis that the overall consumption of herbicides will be reduced as previously reported for glyphosate-resistant transgenic crops. In this review, after a rapid presentation of plant tetrapyrrole biosynthesis, we show how only PPO enzyme can be the target of DPE and how transgenic crops can be further resistant not only to herbicide but also to abiotic stress such as drought or chilling. Keeping in mind that this approach is mostly prohibited in Europe, we attempt to discuss it to interest the scientific community, from plant physiologists to chemists, who work on the interface of photosensitizer optimization and agriculture.

Plant Photodynamic Stress: What's New?

In the 1970's, an unconventional stressful photodynamic treatment applied to plants was investigated in two directions. Exogenous photosensitizer treatment underlies direct photodynamic stress while treatment mediating endogenous photosensitizer over-accumulation pinpoints indirect photodynamic stress. For indirect photodynamic treatment, tetrapyrrole biosynthesis pathway was deregulated by 5-aminolevulenic acid or diphenyl ether. Overall, photodynamic stress involves the generation of high amount of reactive oxygen species leading to plant cell death. All these investigations were mainly performed to gain insight into new herbicide development but they were rapidly given up or limited due to the harmfulness of diphenyl ether and the high cost of 5-aminolevulinic acid treatment. Twenty years ago, plant photodynamic stress came back by way of crop transgenesis where for example protoporphyrin oxidases from human or bacteria were overexpressed. Such plants grew without dramatic effects of photodamage suggesting that plants tolerated induced photodynamic stress. In this review, we shed light on the occurrence of plant photodynamic stress and discuss challenging issues in the context of agriculture focusing on direct photodynamic modality. Indeed, we highlighted applications of exogenous PS especially porphyrins on plants, to further develop an emerged antimicrobial photodynamic treatment that could be a new strategy to kill plant pathogens without disturbing plant growth.

Gene Overexpression Resources in Cereals for Functional Genomics and Discovery of Useful Genes

Identification and elucidation of functions of plant genes is valuable for both basic and applied research. In addition to natural variation in model plants, numerous loss-of-function resources have been produced by mutagenesis with chemicals, irradiation, or insertions of transposable elements or T-DNA. However, we may be unable to observe loss-of-function phenotypes for genes with functionally redundant homologs and for those essential for growth and development. To offset such disadvantages, gain-of-function transgenic resources have been exploited. Activation-tagged lines have been generated using obligatory overexpression of endogenous genes by random insertion of an enhancer. Recent progress in DNA sequencing technology and bioinformatics has enabled the preparation of genomewide collections of full-length cDNAs (fl-cDNAs) in some model species. Using the fl-cDNA clones, a novel gain-of-function strategy, Fl-cDNA OvereXpressor gene (FOX)-hunting system, has been developed. A mutant phenotype in a FOX line can be directly attributed to the overexpressed fl-cDNA. Investigating a large population of FOX lines could reveal important genes conferring favorable phenotypes for crop breeding. Alternatively, a unique loss-of-function approach Chimeric REpressor gene Silencing Technology (CRES-T) has been developed. In CRES-T, overexpression of a chimeric repressor, composed of the coding sequence of a transcription factor (TF) and short peptide designated as the repression domain, could interfere with the action of endogenous TF in plants. Although plant TFs usually consist of gene families, CRES-T is effective, in principle, even for the TFs with functional redundancy. In this review, we focus on the current status of the gene-overexpression strategies and resources for identifying and elucidating novel functions of cereal genes. We discuss the potential of these research tools for identifying useful genes and phenotypes for application in crop breeding.

Genetic Transformation of Hordeum vulgare ssp. spontaneum for the Development of a Transposon-Based Insertional Mutagenesis System

Domestication and intensive selective breeding of plants has triggered erosion of genetic diversity of important stress-related alleles. Researchers highlight the potential of using wild accessions as a gene source for improvement of cereals such as barley, which has major economic and social importance worldwide. Previously, we have successfully introduced the maize Ac/Ds transposon system for gene identification in cultivated barley. The objective of current research was to investigate the response of Hordeum vulgare ssp. spontaneum wild barley accessions in tissue culture to standardize parameters for introduction of Ac/Ds transposons through genetic transformation. We investigated the response of ten wild barley genotypes for callus induction, regenerative green callus induction and regeneration of fertile plants. The activity of exogenous Ac/Ds elements was observed through a transient assay on immature wild barley embryos/callus whereby transformed embryos/calli were identified by the expression of GUS. Transient Ds expression bombardment experiments were performed on 352 pieces of callus (3-5 mm each) or immature embryos in 4 genotypes of wild barley. The transformation frequency of putative transgenic callus lines based on transient GUS expression ranged between 72 and100 % in wild barley genotypes. This is the first report of a transformation system in H. vulgare ssp. spontaneum.

Sampangine (a Copyrine Alkaloid) Exerts Biological Activities through Cellular Redox Cycling of Its Quinone and Semiquinone Intermediates

The cananga tree alkaloid sampangine (1) has been extensively investigated for its antimicrobial and antitumor potential. Mechanistic studies have linked its biological activities to the reduction of cellular oxygen, the induction of reactive oxygen species (ROS), and alterations in heme biosynthesis. Based on the yeast gene deletion library screening results that indicated mitochondrial gene deletions enhanced the sensitivity to 1, the effects of 1 on cellular respiration were examined. Sampangine increased oxygen consumption rates in both yeast and human tumor cells. Mechanistic investigation indicated that 1 may have a modest uncoupling effect, but predominately acts by increasing oxygen consumption independent of mitochondrial complex IV. Sampangine thus appears to undergo redox cycling that may involve respiratory chain-dependent reduction to a semi-iminoquinone followed by oxidation and consequent superoxide production. Relatively high concentrations of 1 showed significant neurotoxicity in studies conducted with rat cerebellar granule neurons, indicating that sampangine use may be associated with potential neurotoxicity.

A root-specific wall-associated kinase gene, HvWAK1, regulates root growth and is highly divergent in barley and other cereals

Wall-associated receptor-like kinases (WAKs) are important candidates for directly linking the extracellular matrix with intracellular compartments and are involved in developmental processes and stress response. WAK gene family has been identified in plants such as Arabidopsis and rice. Here, we present a detailed analysis of the WAK1 gene from barley cv. Golden Promise, mapped to chromosome 5H. Three BAC clones corresponding to the WAK fragment were sequenced and the full-length WAK1 gene was characterized. The gene has three exons and two short introns with a coding region of 2,178 bp encoding a protein of 725 amino acids. A regulatory region was analyzed in -1,000 bp sequence upstream to start codon. Using conserved domains database and SMART, various conserved domains such as GUB WAK Bind, epidermal growth factor CA, and protein kinase C as well as other regions like signal peptides, active sites, and transmembrane domains were identified. The gene organization of HvWAK1 was compared with wheat (TaWAK1) and Arabidopsis (AtWAK1), suggesting that the WAK1 gene organization has remained highly conserved. Nonetheless, WAK1 was found to be highly divergent when compared with sequences available from barley cv. Haruna Nijo (50 %), rice (46 %), wheat (21 %), Arabidopsis (25 %), and maize (19 %). This divergence may have facilitated a better adaptation to surrounding environments due to its role in communication between the extracellular matrix, cell, and outer environment. Semiquantitative RT-PCR-based expression analysis indicates HvWAK1 expression is specific to roots. Significant differences in root growth between GP wild type and GP-Ds mutant seedlings were observed under control and salt stress conditions.

Reactive oxygen species generation and signaling in plants

The introduction of molecular oxygen into the atmosphere was accompanied by the generation of reactive oxygen species (ROS) as side products of many biochemical reactions. ROS are permanently generated in plastids, peroxisomes, mitochiondria, the cytosol and the apoplast. Imbalance between ROS generation and safe detoxification generates oxidative stress and the accumulating ROS are harmful for the plants. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses. Here, we summarize the generation of ROS in the different cellular compartments and the signaling processes which are induced by ROS.

Activation tagging

Insertional mutagenesis is one of the most effective approaches to determine the function of plant genes. However, due to genetic redundancy, loss-of-function mutations often fail to reveal the function of a member of gene families. Activation tagging is a powerful gain-of-function approach to reveal the functions of genes, especially those with high sequence similarity recalcitrant to loss-of-function genetic analyses. Activation tagging randomly inserts a T-DNA fragment containing engineered four copies of enhancer element into a plant genome to activate transcription of flanking genes. We recently generated a new binary vector, pBASTA-AT2, which has been efficiently used to discover genes involved in BR biosynthesis, metabolism, and signal transduction. Compared to pSKI015, a commonly used activation tagging vector, pBASTA-AT2, contains a smaller size of T-DNA and a bigger number of unique restriction sites within the T-DNA region, making cloning of the flanking sequence a lot easier. Our analysis indicated that pBASTA-AT2 gives dramatically improved transformation efficiency relative to pSKI015. In this article, detailed information about this activation tagging vector and the protocol for its application are provided. Three recommended gene cloning approaches based on the use of pBASTA-AT2, including inverse PCR, thermal asymmetric interlaced PCR, and adaptor ligation-mediated PCR, are described to identify T-DNA insertion sites after selection of activation-tagged mutant plants.

RLIN1, encoding a putative coproporphyrinogen III oxidase, is involved in lesion initiation in rice

Lesion mimic is necrotic lesions on plant leaf or stem in the absence of pathogenic infection, and its exact biological mechanism is varied. By a large-scale screening of our T-DNA mutant population, we identified a mutant rice lesion initiation 1 (rlin1), which was controlled by a single nuclear recessive gene. Map-based cloning revealed that RLIN1 encoded a putative coproporphyrinogen III oxidase in tetrapyrrole biosynthesis pathway. Sequencing results showed that a G to T substitution occurred in the second exon of RLIN1 and led to a missense mutation from Asp to Tyr. Ectopic expression of RLIN1 could rescue rlin1 lesion mimic phenotype. Histochemical analysis demonstrated that lesion formation in rlin1 was light-dependent accompanied by reactive oxygen species accumulated. These results suggest that tetrapyrrole participates in lesion formation in rice.

Activation tagging and insertional mutagenesis in barley

The process of activation tagging in plants involves the random distribution of plant regulatory sequences throughout the genome. The insertion of a regulatory sequence in the vicinity of an endogenous gene can alter the transcriptional pattern of this gene resulting in a mutant phenotype that arises from excess functional gene product. Activation tagging has been undertaken extensively in a number of dicot plants and also in rice. This has been achieved primarily by high-throughput plant transformation using T-DNA sequences that encode regulatory elements. Apart from rice, most cereals do not have a suitably efficient transformation system for high-throughput transformation. In this article, we detail an activation tagging system in barley that exploits the mobility of the maize Ac/Ds transposable element system to distribute a highly expressed promoter throughout the barley genome. The advantage of this approach in this species is that a relatively small number of primary transgenics are required to generate an activation tagging population. Insertion of this transposable element into genes can also generate insertional inactivation mutants enabling both gene overexpression and gene knockout mutants to be identified in the same population.

Genetic analyses of cell death in maize (Zea mays, Poaceae) leaves reveal a distinct pathway operating in the camouflage1 mutant

Controlled cell death is vital for many physiological processes in plants, such as xylem development, the hypersensitive response (HR), and senescence; however, the pathways governing cell death are incompletely understood. Studies of mutants that display a cell-death phenotype have greatly contributed to our knowledge of how this process is regulated. The maize camouflage1 (cf1) mutant displays the novel phenotype of cell-specific death of bundle sheath (BS) cells in discrete yellow leaf tissues. To investigate the BS cell death in cf1 mutants, we characterized potential underlying factors. Hydrogen peroxide (H(2)O(2)) is known to be involved in many cell-death events in plants, including the HR. However, in vivo staining found no accumulation of H(2)O(2) in cf1 mutant leaves. Additionally, genetic analyses determined that functional chloroplasts are required for cf1 BS cell death. These results demonstrate that cf1 BS cell death occurs via a distinct pathway from that seen in a functionally related maize mutant or in the HR, suggesting that cell death in maize leaves can be caused by multiple mechanisms.

Transposon-based activation tagging in cereals

Advances in DNA sequencing technologies have produced an ever increasing number of sequenced genomes. However, many of the genes identified in these sequencing efforts have unknown functions or functions inferred based upon sequence homology, highlighting the necessity for functional gene analysis. Mutagenesis combined with phenotypic analyses remains a key mechanism for identifying and establishing gene function. Activation tagging is a mutagenic process that uses altered gene expression, usually gene overexpression, to generate mutant phenotypes. We have developed an activation tagging system in barley (Hordeum vulgare L.) based upon a maize (Zea mays L.) transposable element that carries two highly expressed cereal promoters. Insertion of this mobile genetic element in the genome can lead to insertional gene inactivation, gene overexpression and gene silencing through the production of antisense transcripts. This transposable element system has also been introduced into both wheat (Triticum aestivum L.) and maize and transposon mobility observed.