A Recipe for Success: Three Key Strategies Used by Aphids and Pseudomonas syringae to Colonize the Phyllosphere.

Aphids and Pseudomonas syringae are a permanent challenge for agriculture, causing severe losses to the crop industry worldwide. Despite the obvious phylogenetic distance between them, both have become predominant colonizers of the plant kingdom. In this study, we reviewed three key steps of spread and colonization that aphids and P. syringae have mastered to successfully colonize the phyllosphere. These steps involve (i) plant-to-plant movement for locating new nutritional sources, (ii) disruption and modification of the apoplast to facilitate nutrient acquisition, and (iii) suppression of host defenses through effector proteins. In addition, we will provide insights about the direct interaction between aphids and P. syringae and how this yet underrated phenomenon could bring new ecological implications for both organisms beyond their pathogenicity.

Pectin Methylesterases Modulate Plant Homogalacturonan Status in Defenses against the Aphid Myzus persicae.

Because they suck phloem sap and act as vectors for phytopathogenic viruses, aphids pose a threat to crop yields worldwide. Pectic homogalacturonan (HG) has been described as a defensive element for plants during infections with phytopathogens. However, its role during aphid infestation remains unexplored. Using immunofluorescence assays and biochemical approaches, the HG methylesterification status and associated modifying enzymes during the early stage of Arabidopsis (Arabidopsis thaliana) infestation with the green peach aphid (Myzus persicae) were analyzed. Additionally, the influence of pectin methylesterase (PME) activity on aphid settling and feeding behavior was evaluated by free choice assays and the Electrical Penetration Graph technique, respectively. Our results revealed that HG status and HG-modifying enzymes are significantly altered during the early stage of the plant-aphid interaction. Aphid infestation induced a significant increase in total PME activity and methanol emissions, concomitant with a decrease in the degree of HG methylesterification. Conversely, inhibition of PME activity led to a significant decrease in the settling and feeding preference of aphids. Furthermore, we demonstrate that the PME inhibitor AtPMEI13 has a defensive role during aphid infestation, since pmei13 mutants are significantly more susceptible to M. persicae in terms of settling preference, phloem access, and phloem sap drainage.

Oligogalacturonides Enhance Resistance against Aphids through Pattern-Triggered Immunity and Activation of Salicylic Acid Signaling.

The remarkable capacity of the generalist aphid Myzus persicae to resist most classes of pesticides, along with the environmental and human health risks associated with these agrochemicals, has necessitated the development of safer and greener solutions to control this agricultural pest. Oligogalacturonides (OGs) are pectin-derived molecules that can be isolated from fruit industry waste. OGs have been shown to efficiently stimulate plant defenses against pathogens such as Pseudomonas syringae and Botrytis cinerea. However, whether OGs confer resistance against phytophagous insects such as aphids remains unknown. Here, we treated Arabidopsis plants with OGs and recorded their effects on the feeding performance and population of M. persicae aphids. We also identified the defense mechanism triggered by OGs in plants through the analysis of gene expression and histological approaches. We found that OG treatments increased their resistance to M. persicae infestation by reducing the offspring number and feeding performance. Furthermore, this enhanced resistance was related to a substantial accumulation of callose and reactive oxygen species and activation of the salicylic acid signaling pathway.

The TGA Transcription Factors from Clade II Negatively Regulate the Salicylic Acid Accumulation in Arabidopsis.

Salicylic acid (SA) is a hormone that modulates plant defenses by inducing changes in gene expression. The mechanisms that control SA accumulation are essential for understanding the defensive process. TGA transcription factors from clade II in Arabidopsis, which include the proteins TGA2, TGA5, and TGA6, are known to be key positive mediators for the transcription of genes such as PR-1 that are induced by SA application. However, unexpectedly, stress conditions that induce SA accumulation, such as infection with the avirulent pathogen P. syringae DC3000/AvrRPM1 and UV-C irradiation, result in enhanced PR-1 induction in plants lacking the clade II TGAs (tga256 plants). Increased PR-1 induction was accompanied by enhanced isochorismate synthase-dependent SA production as well as the upregulation of several genes involved in the hormone's accumulation. In response to avirulent P. syringae, PR-1 was previously shown to be controlled by both SA-dependent and -independent pathways. Therefore, the enhanced induction of PR-1 (and other defense genes) and accumulation of SA in the tga256 mutant plants is consistent with the clade II TGA factors providing negative feedback regulation of the SA-dependent and/or -independent pathways. Together, our results indicate that the TGA transcription factors from clade II negatively control SA accumulation under stress conditions that induce the hormone production. Our study describes a mechanism involving old actors playing new roles in regulating SA homeostasis under stress.

Class III Peroxidases PRX01, PRX44, and PRX73 Control Root Hair Growth in Arabidopsis thaliana.

Root hair cells are important sensors of soil conditions. They grow towards and absorb water-soluble nutrients. This fast and oscillatory growth is mediated by continuous remodeling of the cell wall. Root hair cell walls contain polysaccharides and hydroxyproline-rich glycoproteins, including extensins (EXTs). Class-III peroxidases (PRXs) are secreted into the apoplastic space and are thought to trigger either cell wall loosening or polymerization of cell wall components, such as Tyr-mediated assembly of EXT networks (EXT-PRXs). The precise role of these EXT-PRXs is unknown. Using genetic, biochemical, and modeling approaches, we identified and characterized three root-hair-specific putative EXT-PRXs, PRX01, PRX44, and PRX73. prx01,44,73 triple mutation and PRX44 and PRX73 overexpression had opposite effects on root hair growth, peroxidase activity, and ROS production, with a clear impact on cell wall thickness. We use an EXT fluorescent reporter with contrasting levels of cell wall insolubilization in prx01,44,73 and PRX44-overexpressing background plants. In this study, we propose that PRX01, PRX44, and PRX73 control EXT-mediated cell wall properties during polar expansion of root hair cells.

Influence of cell wall polymers and their modifying enzymes during plant-aphid interactions.

Aphids are a major issue for commercial crops. These pests drain phloem nutrients and transmit ~50% of the known insect-borne viral diseases. During aphid feeding, trophic structures called stylets advance toward the phloem intercellularly, disrupting cell wall polymers. It is thought that cell wall-modifying enzymes (CWMEs) present in aphid saliva facilitate stylet penetration through this intercellular polymer network. Additionally, different studies have demonstrated that host settling preference, feeding behavior, and colony performance of aphids are influenced by modulating the CWME expression levels in host plants. CWMEs have been described as critical defensive elements for plants, but also as a key virulence factor for plant pathogens. However, whether CWMEs are elements of the plant defense mechanisms or the aphid infestation process remains unclear. Therefore, in order to better consider the function of CWMEs and cell wall-derived damage-associated molecular patterns (DAMPs) during plant-aphid interactions, the present review integrates different hypotheses, perspectives, and experimental evidence in the field of plant-aphid interactions and discusses similarities to other well-characterized models such as the fungi-plant pathosystems from the host and the attacker perspectives.

bZIP17 regulates the expression of genes related to seed storage and germination, reducing seed susceptibility to osmotic stress.

Low temperatures, salinity, and drought cause significant crop losses. These conditions involve osmotic stress, triggering transcriptional remodeling, and consequently, the restitution of cellular homeostasis and growth recovery. Protein transcription factors regulate target genes, thereby mediating plant responses to stress. bZIP17 is a transcription factor involved in cellular responses to salinity and the unfolded protein response. Because salinity can also produce osmotic stress, the role of bZIP17 in response to osmotic stress was assessed. Mannitol treatments induced the transcript accumulation and protein processing of bZIP17. Transcriptomic analyses showed that several genes associated with seed storage and germination showed lower expression in bzip17 mutants than in wild-type plants. Interestingly, bZIP17 transcript was more abundant in seeds, and germination analyses revealed that wild-type plants germinated later than bzip17 mutants in the presence of mannitol, but no effects were observed when the seeds were exposed to ABA. Finally, the transcript levels of bZIP17 target genes that control seed storage and germination were assessed in seeds exposed to mannitol treatments, which showed lower expression levels in bzip17 mutants compared to the wild-type seeds. These results suggest that bZIP17 plays a role in osmotic stress, acting as a negative regulator of germination through the regulation of genes involved in seed storage and germination.

Modulation of Auxin Levels in Pollen Grains Affects Stamen Development and Anther Dehiscence in Arabidopsis.

Auxin regulates diverse aspects of flower development in plants, such as differentiation of the apical meristem, elongation of the stamen, and maturation of anthers and pollen. It is known that auxin accumulates in pollen, but little information regarding the biological relevance of auxin in this tissue at different times of development is available. In this work, we manipulated the amount of free auxin specifically in developing pollen, using transgenic Arabidopsis lines that express the bacterial indole-3-acetic acid-lysine synthetase (iaaL) gene driven by a collection of pollen-specific promoters. The iaaL gene codes for an indole-3-acetic acid-lysine synthetase that catalyzes the conversion of free auxin into inactive indole-3-acetyl-l-lysine. The transgenic lines showed several abnormalities, including the absence of short stamina, a diminished seed set, aberrant pollen tubes, and perturbations in the synchronization of anther dehiscence and stamina development. This article describes the importance of auxin accumulation in pollen and its role in stamina and anther development.

bZIP17 and bZIP60 Regulate the Expression of BiP3 and Other Salt Stress Responsive Genes in an UPR-Independent Manner in Arabidopsis thaliana.

Plants can be severely affected by salt stress. Since these are sessile organisms, they have developed different cellular responses to cope with this problem. Recently, it has been described that bZIP17 and bZIP60, two ER-located transcription factors, are involved in the cellular response to salt stress. On the other hand, bZIP60 is also involved in the unfolded protein response (UPR), a signaling pathway that up-regulates the expression of ER-chaperones. Coincidentally, salt stress produces the up-regulation of BiP, one of the main chaperones located in this organelle. Then, it has been proposed that UPR is associated to salt stress. Here, by using insertional mutant plants on bZIP17 and bZIP60, we show that bZIP17 regulate the accumulation of the transcript for the chaperone BiP3 under salt stress conditions, but does not lead to the accumulation of UPR-responding genes such as the chaperones Calnexin, Calreticulin, and PDIL under salt treatments. In contrast, DTT, a known inducer of UPR, leads to the up-regulation of all these chaperones. On the other hand, we found that bZIP60 regulates the expression of some bZIP17 target genes under conditions were splicing of bZIP60 does not occur, suggesting that the spliced and unspliced forms of bZIP60 play different roles in the physiological response of the plant. Our results indicate that the ER-located transcription factors bZIP17 and bZIP60 play a role in salt stress but this response goes through a signaling pathway that is different to that triggered by the unfolded protein response.

The UDP-glucose: glycoprotein glucosyltransferase (UGGT), a key enzyme in ER quality control, plays a significant role in plant growth as well as biotic and abiotic stress in Arabidopsis thaliana.

BACKGROUND: UDP-glucose: glycoprotein glucosyltransferase (UGGT) is a key player in the quality control mechanism (ER-QC) that newly synthesized glycoproteins undergo in the ER. It has been shown that the UGGT Arabidopsis orthologue is involved in ER-QC; however, its role in plant physiology remains unclear. RESULTS: Here, we show that two mutant alleles in the At1g71220 locus have none or reduced UGGT activity. In wild type plants, the AtUGGT transcript levels increased upon activation of the unfolded protein response (UPR). Interestingly, mutants in AtUGGT exhibited an endogenous up-regulation of genes that are UPR targets. In addition, mutants in AtUGGT showed a 30% reduction in the incorporation of UDP-Glucose into the ER suggesting that this enzyme drives the uptake of this substrate for the CNX/CRT cycle. Plants deficient in UGGT exhibited a delayed growth rate of the primary root and rosette as well as an alteration in the number of leaves. These mutants are more sensitive to pathogen attack as well as heat, salt, and UPR-inducing stressors. Additionally, the plants showed impairment in the establishment of systemic acquired resistance (SAR). CONCLUSIONS: These results show that a lack of UGGT activity alters plant vegetative development and impairs the response to several abiotic and biotic stresses. Moreover, our results uncover an unexpected role of UGGT in the incorporation of UDP-Glucose into the ER lumen in Arabidopsis thaliana.

Metabolite Profiling Reveals the Effect of Cold Storage on Primary Metabolism in Nectarine Varieties with Contrasting Mealiness.

Chilling injury is a physiological disorder caused by cold storage in peaches and nectarines. The main symptom of chilling injury is mealiness/wooliness, described as a lack of juice in fruit flesh. In this work, we studied two nectarine varieties (Andes Nec-2 and Andes Nec-3) with contrasting susceptibility to mealiness after cold storage. A non-targeted metabolomic analysis was conducted by GC-MS to understand if changes in metabolite abundance are associated with nectarine mealiness induced by cold storage. Multivariate analyses indicated that in unripe nectarines, cold storage promoted a higher accumulation of amino acids in both varieties. Interestingly, for ripe nectarines, cold storage induced an accumulation of fewer amino acids in both varieties and showed an increased abundance of sugars and organic acids. A pathway reconstruction of primary metabolism revealed that in ripe nectarines, cold storage disrupted metabolite abundance in sugar metabolism and the TCA cycle, leading to a differential accumulation of amino acids, organic acids, and sugars in mealy and juicy nectarines.

Identification of grapevine clones via high-throughput amplicon sequencing: a proof-of-concept study.

Wine cultivars are available to growers in multiple clonal selections with agronomic and enological differences. Phenotypic differences between clones originated from somatic mutations that accrued over thousands of asexual propagation cycles. Genetic diversity between grape cultivars remains unexplored, and tools to discriminate unequivocally clones have been lacking. This study aimed to uncover genetic variations among a group of clonal selections of 4 important Vitis vinifera cultivars: Cabernet sauvignon, Sauvignon blanc, Chardonnay, and Merlot, and use this information to develop genetic markers to discriminate the clones of these cultivars. We sequenced with short-read sequencing technology the genomes of 18 clones, including biological replicates for a total of 46 genomes. Sequences were aligned to their respective cultivar's reference genome for variant calling. We used reference genomes of Cabernet sauvignon, Chardonnay, and Merlot and developed a de novo genome assembly of Sauvignon blanc using long-read sequencing. On average, 4 million variants were detected for each clone, with 74.2% being single nucleotide variants and 25.8% being small insertions or deletions (InDel). The frequency of these variants was consistent across all clones. From these variants, we validated 46 clonal markers using high-throughput amplicon sequencing for 77.7% of the evaluated clones, most of them small InDel. These results represent an advance in grapevine genotyping strategies and will benefit the viticulture industry for the characterization and identification of the plant material.

Molecular and Genomic Characterization of the Pseudomonas syringae Phylogroup 4: An Emerging Pathogen of Arabidopsis thaliana and Nicotiana benthamiana.

Environmental fluctuations such as increased temperature, water availability, and air CO2 concentration triggered by climate change influence plant disease dynamics by affecting hosts, pathogens, and their interactions. Here, we describe a newly discovered Pseudomonas syringae strain found in a natural population of Arabidopsis thaliana collected from the southwest of France. This strain, called Psy RAYR-BL, is highly virulent on natural Arabidopsis accessions, Arabidopsis model accession Columbia 0, and tobacco plants. Despite the severe disease phenotype caused by the Psy RAYR-BL strain, we identified a reduced repertoire of putative Type III virulence effectors by genomic sequencing compared to P. syringae pv tomato (Pst) DC3000. Furthermore, hopBJ1Psy is found exclusively on the Psy RAYR-BL genome but not in the Pst DC3000 genome. The plant expression of HopBJ1Psy induces ROS accumulation and cell death. In addition, HopBJ1Psy participates as a virulence factor in this plant-pathogen interaction, likely explaining the severity of the disease symptoms. This research describes the characterization of a newly discovered plant pathogen strain and possible virulence mechanisms underlying the infection process shaped by natural and changing environmental conditions.

Draft Genome Sequence of Pseudomonas syringae RAYR-BL, a Strain Isolated from Natural Accessions of Arabidopsis thaliana Plants.

Here, we report the genome sequence of the P. syringae strain RAYR-BL, isolated from natural accessions of Arabidopsis plants. The draft genome sequence consists of 5.85 Mbp assembled in 110 contigs. The study of P. syringae RAYR-BL is a valuable tool to investigate molecular features of plant-pathogen interaction under environmental conditions.

Transcriptome and Gene Regulatory Network Analyses Reveal New Transcription Factors in Mature Fruit Associated with Harvest Date in Prunus persica.

Harvest date is a critical parameter for producers and consumers regarding agro-industrial performance. It involves a pleiotropic effect controlling the development of other fruit quality traits through finely controlling regulatory mechanisms. Fruit ripening is a process in which various signals and biological events co-occur and are regulated by hormone signaling that produces the accumulation/degradation of multiple compounds. However, the regulatory mechanisms that control the hormone signaling involved in fruit development and ripening are still unclear. To investigate the issue, we used individuals with early, middle and late harvest dates from a peach segregating population to identify regulatory candidate genes controlling fruit quality traits at the harvest stage and validate them in contrasting peach varieties for this trait. We identified 467 and 654 differentially expressed genes for early and late harvest through a transcriptomic approach. In addition, using the Arabidopsis DAP-seq database and network analysis, six transcription factors were selected. Our results suggest significant hormonal balance and cell wall composition/structure differences between early and late harvest samples. Thus, we propose that higher expression levels of the transcription factors HB7, ERF017 and WRKY70 in early harvest individuals would induce the expression of genes associated with the jasmonic acid pathway, photosynthesis and gibberellins inhibition. While on the other hand, the high expression levels of LHY, CDF3 and NAC083 in late harvest individuals would promote the induction of genes associated with abscisic acid biosynthesis, auxins and cell wall remodeling.

Unravelling the Molecular Regulation Mechanisms of Slow Ripening Trait in Prunus persica.

Fruit development is a complex process that involves the interplay of cell division, expansion, and differentiation. As a model to study fruit development, nectarines incapable of ripening were described as slow ripening. Slow ripening fruits remained firm and exhibited no rise in CO2 or ethylene production rates for one month or more at 20 °C. Different studies suggest that this trait is controlled by a single gene (NAC072). Transcriptome analysis between normal and slow ripening fruits showed a total of 157, 269, 976, and 5.224 differentially expressed genes in each fruit developmental stage analyzed (T1, T2, T3, and T7, respectively), and no expression of NAC072 was found in the slow ripening individuals. Using this transcriptomic information, we identified a correlation of NAC072 with auxin-related genes and two genes associated with terpene biosynthesis. On the other hand, significant differences were observed in hormonal biosynthetic pathways during fruit development between the normal and slow ripening individuals (gibberellin, ethylene, jasmonic acid and abscisic acid). These results suggest that the absence of NAC072 by the direct or indirect expression control of auxins or terpene-related genes prevents normal peach fruit development.

WRKY7, -11 and -17 transcription factors are modulators of the bZIP28 branch of the unfolded protein response during PAMP-triggered immunity in Arabidopsis thaliana.

Plants must defend themselves against pathogens. The defense response requires greater protein synthesis, which generates endoplasmic reticulum (ER) stress, yet failure to attenuate this stress has detrimental effects. WRKY7/11/17 transcription factors (TFs) are negative regulators of immunity since mutants are more resistant to Pseudomonas syringae pv tomato (Pst) infection. Here, we reveal a connection between ER-stress and the molecular mechanisms underlying the wrky mutant phenotype. The bZIP28 TF upregulates ER-chaperone expression (BiP1/2, ERdj3B, and SDF2) upon exposure of Arabidopsis to a bacterial defense elicitor, flagellin 22 (Flg22). Also, the activation of ER-chaperones is more sustained in double and triple wrky mutants treated with Flg22, suggesting that WRKY7/11/17 TFs downregulate these genes. Moreover, wrky mutants accumulate more bZIP28 transcripts in response to Flg22, indicating that WRKY7/11/17 transcriptionally repress this TF. Using Arabidopsis protoplasts, we also demonstrate that WRKYs bind to the bZIP28 promoter via W-box elements. Additionally, triple wrky mutants are more resistant, whilst bzip28 mutants are more susceptible, to Pst infection. Finally, we postulate a model of PAMP-Triggered Immunity regulation, where Flg22 activates bZIP28-signaling inducing the expression of ER-stress genes, as well as WRKY7/11/17 expression, which in turn inhibits PTI by downregulating bZIP28, controlling physiological responses in the Arabidopsis-Pst interaction.

The Root Hair Specific SYP123 Regulates the Localization of Cell Wall Components and Contributes to Rizhobacterial Priming of Induced Systemic Resistance.

Root hairs are important for nutrient and water uptake and are also critically involved the interaction with soil inhabiting microbiota. Root hairs are tubular-shaped outgrowths that emerge from trichoblasts. This polarized elongation is maintained and regulated by a robust mechanism involving the endomembrane secretory and endocytic system. Members of the syntaxin family of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) in plants (SYP), have been implicated in regulation of the fusion of vesicles with the target membranes in both exocytic and endocytic pathways. One member of this family, SYP123, is expressed specifically in the root hairs and accumulated in the growing tip region. This study shows evidence of the SYP123 role in polarized trafficking using knockout insertional mutant plants. We were able to observe defects in the deposition of cell wall proline rich protein PRP3 and cell wall polysaccharides. In a complementary strategy, similar results were obtained using a plant expressing a dominant negative soluble version of SYP123 (SP2 fragment) lacking the transmembrane domain. The evidence presented indicates that SYP123 is also regulating PRP3 protein distribution by recycling by endocytosis. We also present evidence that indicates that SYP123 is necessary for the response of roots to plant growth promoting rhizobacterium (PGPR) in order to trigger trigger induced systemic response (ISR). Plants with a defective SYP123 function were unable to mount a systemic acquired resistance in response to bacterial pathogen infection and ISR upon interaction with rhizobacteria. These results indicated that SYP123 was involved in the polarized localization of protein and polysaccharides in growing root hairs and that this activity also contributed to the establishment of effective plant defense responses. Root hairs represent very plastic structures were many biotic and abiotic factors can affect the number, anatomy and physiology of root hairs. Here, we presented evidence that indicates that interactions with soil PGPR could be closely regulated by signaling involving secretory and/or endocytic trafficking at the root hair tip as a quick way to response to changing environmental conditions.