The Arabidopsis ELP3/ELO3 and ELP4/ELO1 genes enhance disease resistance in Fragaria vesca L

The devastating oomycete phytopathogen Phytophthora cactorum: Insights into its biology and molecular features

Phytophthora cactorum is one of the most economically important soilborne oomycete pathogens in the world. It infects more than 200 plant species spanning 54 families, most of which are herbaceous and woody species. Although traditionally considered to be a generalist, marked differences of P. cactorum isolates occur in degree of pathogenicity to different hosts. As the impact of crop loss caused by this species has increased recently, there has been a tremendous increase in the development of new tools, resources, and management strategies to study and combat this devastating pathogen. This review aims to integrate recent molecular biology analyses of P. cactorum with the current knowledge of the cellular and genetic basis of its growth, development, and host infection. The goal is to provide a framework for further studies of P. cactorum by highlighting important biological and molecular features, shedding light on the functions of pathogenicity factors, and developing effective control measures.

TAXONOMY

P. cactorum (Leb. & Cohn) Schröeter: kingdom Chromista; phylum Oomycota; class Oomycetes; order Peronosporales; family Peronosporaceae; genus Phytophthora.

HOST RANGE

Infects about 200 plant species in 154 genera representing 54 families. Economically important host plants include strawberry, apple, pear, Panax spp., and walnut.

DISEASE SYMPTOMS

The soilborne pathogen often causes root, stem, collar, crown, and fruit rots, as well as foliar infection, stem canker, and seedling damping off.

Wheat Elongator Subunit 4 Negatively Regulates Freezing Tolerance by Regulating Ethylene Accumulation

Freezing stress is a major factor limiting production and geographical distribution of temperate crops. Elongator is a six subunit complex with histone acetyl-transferase activity and is involved in plant development and defense responses in Arabidopsis thaliana. However, it is unknown whether and how an elongator responds to freezing stress in plants. In this study, we found that wheat elongator subunit 4 (TaELP4) negatively regulates freezing tolerance through ethylene signaling. TaELP4 promoter contained cold response elements and was up-regulated in freezing stress. Subcellular localization showed that TaELP4 and AtELP4 localized in the cytoplasm and nucleus. Silencing of TaELP4 in wheat with BSMV-mediated VIGS approach significantly elevated tiller survival rate compared to control under freezing stress, but ectopic expression of TaELP4 in Arabidopsis increased leaf damage and survival rate compared with Col-0. Further results showed that TaELP4 positively regulated ACS2 and ACS6 transcripts, two main limiting enzymes in ethylene biosynthesis. The determination of ethylene content showed that TaELP4 overexpression resulted in more ethylene accumulated than Col-0 under freezing stress. Epigenetic research showed that histone H3K9/14ac levels significantly increased in coding/promoter regions of AtACS2 and AtACS6 in Arabidopsis. RT-qPCR assays showed that the EIN2/EIN3/EIL1-CBFs-COR pathway was regulated by TaELP4 under freezing stress. Taken together, our results suggest that TaELP4 negatively regulated plant responses to freezing stress via heightening histone acetylation levels of ACS2 and ACS6 and increasing their transcription and ethylene accumulation.

Variations in transfer and ribosomal RNA epitranscriptomic status can adapt eukaryote translation to changing physiological and environmental conditions

The timely reprogramming of gene expression in response to internal and external cues is essential to eukaryote development and acclimation to changing environments. Chemically modifying molecular receptors and transducers of these signals is one way to efficiently induce proper physiological responses. Post-translation modifications, regulating protein biological activities, are central to many well-known signal-responding pathways. Recently, messenger RNA (mRNA) chemical (i.e. epitranscriptomic) modifications were also shown to play a key role in these processes. In contrast, transfer RNA (tRNA) and ribosomal RNA (rRNA) chemical modifications, although critical for optimal function of the translation apparatus, and much more diverse and quantitatively important compared to mRNA modifications, were until recently considered as mainly static chemical decorations. We present here recent observations that are challenging this view and supporting the hypothesis that tRNA and rRNA modifications dynamically respond to various cell and environmental conditions and contribute to adapt translation to these conditions.

Epigenomic Regulators Elongator Complex Subunit 2 and Methyltransferase 1 Differentially Condition the Spaceflight Response in Arabidopsis

Background: Plants subjected to the novel environment of spaceflight show transcriptomic changes that resemble aspects of several terrestrial abiotic stress responses. Under investigation here is whether epigenetic modulations, similar to those that occur in terrestrial stress responses, have a functional role in spaceflight physiological adaptation. The Advanced Plant Experiment-04 - Epigenetic Expression experiment examined the role of cytosine methylation in spaceflight adaptation. The experiment was conducted onboard the International Space Station, and evaluated the spaceflight-altered, genome-wide methylation profiles of two methylation-regulating gene mutants [methyltransferase 1 (met1-7) and elongator complex subunit 2 (elp2-5)] along with a wild-type Col-0 control. Results: The elp2-5 plants suffered in their physiological adaptation to spaceflight in that their roots failed to extend away from the seed and the overall development of the plants was greatly impaired in space. The met1-7 plants suffered less, with their morphology affected by spaceflight in a manner similar to that of the Col-0 controls. The differentially expressed genes (DEGs) in spaceflight were dramatically different in the elp2-5 and met1-7 plants compared to Col-0, indicating that the disruptions in these mutants resulted in a reprogramming of their spaceflight responses, especially in elp2-5. Many of the genes comprising the spaceflight transcriptome of each genotype were differentially methylated in spaceflight. In Col-0 the majority of the DEGs were representative of the now familiar spaceflight response, which includes genes associated with cell wall remodeling, pathogen responses and ROS signaling. However, the spaceflight transcriptomes of met1-7 and elp2-5 each presented patterns of DEGs that are almost completely different than Col-0, and to each other. Further, the DEGs of the mutant genotypes suggest a more severe spaceflight stress response in the mutants, particularly in elp2-5. Conclusion: Arabidopsis physiological adaptation to spaceflight results in differential DNA methylation in an organ-specific manner. Disruption of Met1 methyltransferase function does not dramatically affect spaceflight growth or morphology, yet met1-7 reprograms the spaceflight transcriptomic response in a unique manner. Disruption of elp2-5 results in poor development in spaceflight grown plants, together with a diminished, dramatically reprogrammed transcriptomic response.

First Report of Xanthomonas fragariae Causing Angular Leaf Spot on Strawberry (Fragaria x ananassa) in Taiwan

Angular leaf spot of strawberry, considered an A2 quarantine pest by the European and Mediterranean Plant Protection Organization (EPPO 2019), is an important bacterial disease in many regions. Since 2017, symptoms similar to angular leaf spot were observed in several strawberry cultivars including 'Taoyuan No. 1' and 'Xiang-Shui'. Early symptoms were angular, water-soaked lesions on the abaxial leaf surface, and later, reddish-brown irregular spots and coalesced lesions developed on the adaxial surface. In the humid conditions, sticky bacterial ooze exuding from lesions was observed. To isolate the causal agent, leaves showing water-soaked lesions were surface sterilized, cut into small pieces and soaked in 5 ml sterile water for at least 15 min. The supernatant from the cut-up pieces was serially diluted followed by spreading on sucrose peptone agar (SPA) (Hayward 1960). After incubating at 20°C for 4-5 days, single colonies grown on SPA were transferred to a new SPA plate and cultured at 20°C until colonies appeared. The yellow, glossy and mucoid colonies, which resembled the colony morphology of Xanthomonas fragariae, were selected as candidates for further confirmation. First, bacterial DNA of four candidate isolates, B001, B003 and B005 from Miaoli County and B004 from Taoyuan City, was PCR amplified with X. fragariae-specific primers: XF9/XF12 (Roberts et al. 1996) and 245A/B and 295A/B (Pooler et al. 1996). All four isolates could be detected by XF9/XF12 primer. Furthermore, isolates B003 and B004 could be detected by both 245A/B and 295A/B primers, while B001 and B005 could be detected by 295A/B only. Next, DNA gyrase subunit B (gyrB) was PCR amplified with the primers XgyrB1F/XgyrB1R (Young et al. 2008). The gyrB sequences of these four isolates were deposited in GenBank with accession numbers MT754942 to MT754945. The gyrB phylogenetic tree was constructed based on Bayesian inference analysis and maximum likelihood analysis. The gyrB sequences of the four isolates from Taiwan clustered in the clade containing the type strain of X. fragariae ICMP5715, indicating that they belong to X. fragariae. B001 and B005 formed a sub-group separated from B003 and B004, suggesting genetic differences between these isolates. To fulfill Koch's postulates, the abaxial surface of strawberry leaves were syringe infiltrated (KJP Silva et al., 2017) or wounded inoculated (Wang et al., 2017) with bacterial suspensions (final OD600 = 1.0-2.0) prepared from colonies of B001 and B003 washed from SPA plates. Inoculated plants were enclosed in a plastic bag (> 90% RH) at 25/20°C (day/night) under a 12-h/12-h photoperiod. After 7-14 days, water-soaked lesions similar to those observed in the field were developed on all inoculated leaves. The bacteria were successfully re-isolated from lesions of inoculated leaves and confirmed by specific primers XF9/XF12, 245A/B and 295A/B. We also found that the disease commonly occurs in the strawberry fields/nurseries with sprinkler irrigation during winter or early spring, and was particularly serious in the windward side or near riverside. To our knowledge, this is the first report of X. fragariae causing angular leaf spot on strawberry in Taiwan. Currently, the disease only occurs severely in certain regions, but establishment of effective management strategies will be needed to prevent spreading of this disease and potential economic loss in the future.

Plant Elongator-Protein Complex of Diverse Activities Regulates Growth, Development, and Immune Responses

Contrary to the conserved Elongator composition in yeast, animals, and plants, molecular functions and catalytic activities of the complex remain controversial. Elongator was identified as a component of elongating RNA polymerase II holoenzyme in yeast, animals, and plants. Furthermore, it was suggested that Elonagtor facilitates elongation of transcription via histone acetyl transferase activity. Accordingly, phenotypes of Arabidopsis elo mutants, which show development, growth, or immune response defects, correlate with transcriptional downregulation and the decreased histone acetylation in the coding regions of crucial genes. Plant Elongator was also implicated in other processes: transcription and processing of miRNA, regulation of DNA replication by histone acetylation, and acetylation of alpha-tubulin. Moreover, tRNA modification, discovered first in yeast and confirmed in plants, was claimed as the main activity of Elongator, leading to specificity in translation that might also result indirectly in a deficiency in transcription. Heterologous overexpression of individual Arabidopsis Elongator subunits and their respective phenotypes suggest that single Elongator subunits might also have another function next to being a part of the complex. In this review, we shall present the experimental evidence of all molecular mechanisms and catalytic activities performed by Elongator in nucleus and cytoplasm of plant cells, which might explain how Elongator regulates growth, development, and immune responses.

A G(enomic)P(ositioning)S(ystem) for Plant RNAPII Transcription

Post-translational modifications (PTMs) of histone residues shape the landscape of gene expression by modulating the dynamic process of RNA polymerase II (RNAPII) transcription. The contribution of particular histone modifications to the definition of distinct RNAPII transcription stages remains poorly characterized in plants. Chromatin immunoprecipitation combined with next-generation sequencing (ChIP-seq) resolves the genomic distribution of histone modifications. Here, we review histone PTM ChIP-seq data in Arabidopsis thaliana and find support for a Genomic Positioning System (GPS) that guides RNAPII transcription. We review the roles of histone PTM 'readers', 'writers', and 'erasers', with a focus on the regulation of gene expression and biological functions in plants. The distinct functions of RNAPII transcription during the plant transcription cycle may rely, in part, on the characteristic histone PTM profiles that distinguish transcription stages.

Salicylic acid-primed defence response in octoploid strawberry 'Benihoppe' leaves induces resistance against Podosphaera aphanis through enhanced accumulation of proanthocyanidins and upregulation of pathogenesis-related genes

BACKGROUND

Podosphaera aphanis, a predominately biotrophic fungal pathogen, causes significant yield losses of strawberry. China is the largest strawberry producer in the world, and selecting for powdery mildew-resistant cultivars is desirable. However, the resistance mechanism against P. aphanis in the octoploid strawberry remains unclear.

RESULTS

To understand possible mechanisms of disease resistance, we inoculated strawberry leaves with P. aphanis, and examined the expression profiles of candidate genes and the biochemical phenotypes in strawberry leaves of two groups. The unigenes obtained from ddH₂O- and SA-pretreated leaves resulted in a total of 48,020 and 45,896 genes, respectively. KEGG enrichment showed that phenylpropanoid biosynthesis and plant hormone signal transduction pathways were enriched to a noticeable extent. DEG analysis showed that key TFs genes associated with the SA signaling pathway could play important role in the strawberry-P. aphanis interaction. In particular, FaWRKY70, FaJAZ1 and FaMYC2-like, involved in regulating the antagonistic effect of SA and JA signaling pathway, leading to increased expression of SA-responsive genes (in particular PR1, PR2, PR3, and PR5) compared to a decline in expression of JA-responsive genes (FaJAR1, FaAOS, and FaLOX2). Furthermore, SA pretreatment induced accumulation of PAs by activating the MBW complex and inhibit powdery mildew growth.

CONCLUSIONS

This study describes the role of the proanthocyanidins (PAs), pathogenesis-related (PR) genes, SA, and transcription factors in regulatory model against P. aphanis, which coincided with an early activation of defense, leading to the accumulation of PAs and the PR proteins.

The Arabidopsis Elongator Subunit ELP3 and ELP4 Confer Resistance to Bacterial Speck in Tomato

Although production of tomato (Solanum lycopersicum) is threatened by a number of major diseases worldwide, it has been difficult to identify effective and durable management measures against these diseases. In this study, we attempted to improve tomato disease resistance by transgenic overexpression of genes encoding the Arabidopsis thaliana Elongator (AtELP) complex subunits AtELP3 and AtELP4. We show that overexpression of AtELP3 and AtELP4 significantly enhanced resistance to tomato bacterial speck caused by the Pseudomonas syringae pv. tomato strain J4 (Pst J4) without clear detrimental effects on plant growth and development. Interestingly, the transgenic plants exhibited resistance to Pst J4 only when inoculated through foliar sprays but not through infiltration into the leaf apoplast. Although this result suggested possible involvement of stomatal immunity, we found that Pst J4 inoculation did not induce stomatal closure and there were no differences in stomatal apertures and conductance between the transgenic and control plants. Further RNA sequencing and real-time quantitative PCR analyses revealed a group of defense-related genes to be induced to higher levels after infection in the AtELP4 transgenic tomato plants than in the control, suggesting that the enhanced disease resistance of the transgenic plants may be attributed to elevated induction of defense responses. Additionally, we show that the tomato genome contains single-copy genes encoding all six Elongator subunits (SlELPs), which share high identities with the AtELP proteins, and that SlELP3 and SlELP4 complemented the Arabidopsis Atelp3 and Atelp4 mutants, respectively, indicating that the function of tomato Elongator is probably conserved. Taken together, our results not only shed new light on the tomato Elongator complex, but also revealed potential candidate genes for engineering disease resistance in tomato.

Contributions of biotechnology to meeting future food and environmental security needs

Biotechnology, including genetic modifications, can play a vital role in helping to meet future food and environmental security needs for our growing population. The nature and use of biotechnology crops are described and related to aspects of food security. Biotechnological applications for food and animal feed are described, together with trends on global adoption of these crops. The benefits of biotechnology crops through increased yield, reduced pesticide use and decreased environmental damage are discussed. Examples of biotechnology crops which do not involve genetic modification are also described. Applications of biotechnology to drought and salt tolerance, and biofortification in which micronutrient content is enhanced are discussed. Emergent technologies such as RNA spraying technology, use of genome editing in agriculture and future targets for improved food and environmental security are considered.