Insights Regarding Resistance of 'Nemaguard' Rootstock to the Bacterium Xylella fastidiosa.

'Nemaguard' is a commonly used rootstock for almond and stone fruits due to resistance to nematodes and enhanced scion vigor. Nemaguard also happens to be resistant to strains of Xylella fastidiosa that cause almond leaf scorch disease. Previous research showed that prior to June-budding, this rootstock can prevent infection of almond nursery stock by X. fastidiosa. Further, the rootstock also promotes recovery from infection in susceptible almond scions. Objectives of this study were to 1) compare movement and bacterial populations of X. fastidiosa in almond and Nemaguard, 2) determine whether the metabolic profile of infected versus noninfected plants of each species correspond with differences in pathogen distribution, and 3) evaluate the impact of feeding on Nemaguard on transmission efficiency and pathogen populations in insects. Results showed limited or no movement of X. fastidiosa beyond the point of mechanical inoculation in Nemaguard, whereas X. fastidiosa was detected in susceptible almond and isolated from plant samples distal to the point of inoculation. Large differences in the concentration of phenolic compounds between Nemaguard and almond were also found, although this was not impacted by infection status. After acquiring X. fastidiosa from infected plants, vector access periods of up to 14 days on Nemaguard neither reduced pathogen populations in vectors nor reduced transmission efficiency of X. fastidiosa to susceptible plants when compared with similar vector-access periods on susceptible grapevines. Results suggest Nemaguard, in spite of having high phenolic concentrations in its xylem, does not directly impact X. fastidiosa survival and that future research should focus on identification of potential physical traits that prevent bacterial attachment, multiplication, or movement within the plant.

Postharvest Development of 'Candidatus Liberibacter solanacearum' in Late-Season Infected Potato Tubers under Commercial Storage Conditions.

Zebra chip (ZC) disease of potato is associated with the putative pathogen 'Candidatus Liberibacter solanacearum', which is transmitted by the potato psyllid Bactericera cockerelli (Hem., Triozidae). The present study was initiated to investigate 'Ca. L. solanacearum' development during and following typical commercial storage practices. Using bacteriliferous psyllids, Russet Norkotah potato tubers were infested in field cages 14, 10, and 4 days before harvest. Changes in 'Ca. L. solanacearum' detection rate, 'Ca. L. solanacearum' titer, and concentrations of phenolic compounds were documented throughout storage. 'Ca. L. solanacearum' titer continued to increase during storage. Although significant increases in the frequency of 'Ca. L. solanacearum' detection were observed in all infestation treatments, the impact of 'Ca. L. solanacearum' infection on tuber quality remained comparatively low in plants infected 4 days before harvest, because the majority of the tubers remained asymptomatic. Minimizing storage and retail chain movement durations would help to limit 'Ca. L. solanacearum' impact on tuber quality in tubers infected 14 and 10 days before harvest. This study also demonstrated that 'Ca. L. solanacearum' can relocate from a newly infected leaf to a tuber in as little as 4 days. Psyllid management is recommended until at least 4 days before green harvest, when psyllid pressure is high in fields in which tubers are destined for commercial storage.

Variations in Zebra Chip Disease Expression and Tuber Biochemistry in Response to Vector Density.

Zebra chip (ZC) disease, caused by 'Candidatus Liberibacter solanacearum', which is transmitted by the potato psyllid, has negatively affected potato production in the United States for over a decade. The present study was conducted to evaluate the affect of the number of bacteriliferous psyllids on 'Ca. L. solanacearum' titer, levels of amino acids, carbohydrates, phenolics, and, subsequently, symptom severity in potato tubers. 'Red La Soda' and 'Russet Norkotah' potato were planted in the field and later inoculated with 'Ca. L. solanacearum' using 5, 10, and 30 bacteriliferous potato psyllids. In both cultivars, the increase in the number of psyllids resulted in elevated 'Ca. L. solanacearum' titer and symptom severity. In the cases of amino acids and reducing sugars, responses to vector density appeared to be cultivar specific. Overall, phenolic compounds showed a consistent increase following infection, a response that, in most cases, was positively correlated with the number of infective psyllids. Results quantified the impact of the number of infective psyllids on 'Ca. L. solanacearum' titer and biochemical content of infected tubers previously shown to be correlated with the degree of symptom expression. Managing vector numbers is currently the only effective approach for minimizing losses to ZC in grower's fields. However, our findings on physiological responses to vector density suggest that, in combination with chemical control, development of more tolerant or resistant potato genotypes, with emphasis on interrupting pathways that are associated with increased phenolic activity levels, may lead to more sustainable management of ZC in the future.

Silicon-Induced Systemic Defense Responses in Perennial Ryegrass Against Infection by Magnaporthe oryzae.

Sustainable integrated disease management for gray leaf spot of perennial ryegrass may involve use of plant defense elicitors with compatible traditional fungicides to reduce disease incidence and severity. Silicon (Si) has previously been identified as a potential inducer or modulator of plant defenses against different fungal pathogens. To this end, perennial ryegrass was inoculated with the causal agent of gray leaf spot, Magnaporthe oryzae, when grown in soil that was nonamended or amended with three different levels of calcium silicate (1, 5, or 10 metric tons [t]/ha). When applied at a rate of 5 t/ha, calcium silicate was found to significantly suppress gray leaf spot in perennial ryegrass, including a significant reduction of disease incidence (39.5%) and disease severity (47.3%). Additional studies observed nonpenetrated papillae or cell-wall appositions harboring callose, phenolic autofluorogens, and lignin-associated polyphenolic compounds in grass grown in the Si-amended soil. Regarding defense-associated enzyme levels, only following infection did grass grown in Si-amended soil exhibit greater activities of peroxidase and polyphenol oxidase than equivalent inoculated control plants. Also following infection with M. oryzae, grass levels of several phenolic acids, including chlorogenic acid and flavonoids, and relative expression levels of genes encoding phenylalanine ammonia lyase (PALa and PALb) and lipoxygenase (LOXa) significantly increased in Si-amended plants compared with that of nonamended control plants. These results suggest that Si-mediated increase of host defense responses to fungal pathogens in perennial ryegrass has a great potential to be part of an effective integrated disease management strategy against gray leaf spot development.

Effects of Glassy-Winged Sharpshooter (Hemiptera: Cicadellidae) Feeding, Size, and Lipid Content on Egg Maturation.

The glassy-winged sharpshooter (Homalodisca vitripennis) is synovigenic and must feed as an adult to produce eggs. Egg maturation rates depend on the host plant species provided to the adult female for feeding and are variable for females provided with the same host plant species. Here, the contribution of female size and lipid content to variation in egg maturation rates among females held on the same host plant species was assessed. To assess effects of female size and lipid content on egg maturation, feeding assays followed by measurements of egg load, female size, and lipid content were conducted. To accomplish this, females were field collected and held on cowpea until producing approximately 0, 12, 25, or 50 ml of excreta. After reaching prescribed excreta thresholds, females were dissected to determine egg load, hind tibia length, and head capsule width. Mature eggs were removed from the abdomen and dry weight of eggs and bodies (head, thorax, and abdomen) were obtained. Lipid content of eggs and bodies were determined using a quantitative colorimetric assay. Rates of body weight gain and body lipid gain were rapid with low levels of feeding (12 ml of excreta) but decelerated with additional feeding (>12 ml of excreta). In contrast, low levels of feeding (12 ml of excreta) resulted in little egg production, with rates of egg production accelerating with additional feeding (>12 ml of excreta). Accordingly, egg production was preceded by an increase in body dry weight and body lipid content. In agreement, probability that a female carried eggs increased with body lipid content in the 0-, 12-, and 25-ml feeding treatments. Across treatments, larger females carried more eggs than smaller females. Collectively, results suggest that variation in glassy-winged sharpshooter egg maturation rates partially may be explained by availability of lipid reserves at the start of a feeding bout and female size.

Characterization of data observing Meloidogyne incognita, Neofusicoccum parvum, and Xylella fastidiosa infection effects on development of grapevine phenolic compound levels and resistance to subsequent Neofusicoccum parvum infections.

Grapevines encounter many different pathogens throughout their lifespans, including the bacterial pathogen Xylella fastidiosa, which causes Pierce's disease that results in vascular occlusion and eventual plant host death, the fungal pathogen Neofusicoccum parvum, which causes stem cankers that kill individual vines and reduce fruit yields, and the root knot nematode Meloidogyne incognita, which destroys root tissues that impacts host vigour. To date, little research has been conducted to examine how one infection could impact subsequent infections by the same or different pathogens despite this is important to ensure healthy vineyards. Therefore, grapevines initially infected with either X. fastidiosa, N. parvum, or M. incognita were subsequently infected with N. parvum eight weeks later to observe developing lesion lengths, which were assessed to determine grapevine resistance to infections. Collected data shows that when prior infections were present, the N. parvum lesions lengths were smaller. This suggests grapevines had induced resistance to combat infections. Further, defence-associated phenolics were measured by high-performance liquid chromatography to determine roles in observed resistance to the secondary N. parvum infections. Data shows that of the different phenolics examined, only stilbenoids were different due to infections, with lowered levels observed in plants that were infected compared with non-infected controls. These data provide insight into how infections by different pathogens could impact grapevine host resistance to new, subsequent pathogen infections.

Opportunities for optimizing fungal biological control agents for long-term and effective management of insect pests of orchards and vineyards: a review.

Novel tactics for controlling insect pests in perennial fruit and nut crops are needed because target pests often display decreased susceptibility to chemical controls due to overreliance on a handful of active ingredients and regulatory issues. As an alternative to chemical controls, entomopathogenic fungi could be utilized as biological control agents to manage insect pest populations. However, development of field ready products is hampered by a lack of basic knowledge. Development of field ready products requires collecting, screening, and characterizing a greater variety of potential entomopathogenic fungal species and strains. Creation of a standardized research framework to study entomopathogenic fungi will aid in identifying the potential mechanisms of biological control activity that fungi could possess, including antibiotic metabolite production; strains and species best suited to survive in different climates and agroecosystems; and optimized combinations of entomopathogenic fungi and novel formulations. This mini review therefore discusses strategies to collect and characterize new entomopathogenic strains, test different potential mechanisms of biocontrol activity, examine ability of different species and strains to tolerate different climates, and lastly how to utilize this information to develop strains into products for growers.

Pre-inoculation water deficit effects on grapevine physiology, Xylella fastidiosa titers, and Pierce's disease progression.

Drought and Pierce's disease are common throughout many grapevine-growing regions such as Mexico and the United States. Yet, how ongoing water deficits affect infections of Xylella fastidiosa, the causal agent of Pierce's disease, is poorly understood. Symptoms were observed to be significantly more severe in water-stressed plants one month after X. fastidiosa inoculation, and, in one experiment, titers were significantly lower in water-stressed than well-watered grapevines. Host chemistry examinations revealed overall amino acid and phenolic levels did not statistically differ due to water deficits, but sugar levels were significantly greater in water stressed than well-watered plants. Results highlight the need to especially manage Pierce's disease spread in grapevines experiencing drought.

Glassy-Winged Sharpshooters Cease Feeding and Avoid Plants Treated With Sub-Lethal Doses of the Neonicotinoid Insecticide Imidacloprid.

Insecticides are a primary means for suppressing populations of insects that transmit plant pathogens. Application of insecticides for limiting the spread of insect-transmitted plant pathogens is often most effective when applied on an area-wide scale. The glassy-winged sharpshooter is a vector of the bacterial pathogen Xylella fastidiosa, which causes numerous plant diseases including Pierce's disease of grapevine. The glassy-winged sharpshooter has been the subject of an area-wide suppression program in California for nearly two decades. Overreliance on a limited number of active ingredients including the neonicotinoid imidacloprid has resulted in increased levels of resistance to commonly applied products. In California, glassy-winged sharpshooters move between citrus, an important overwintering host, and vineyards. Accordingly, imidacloprid is routinely applied via the irrigation system in vineyards and citrus orchards. For soil applied applications, it may take days to weeks for concentrations in plants to increase to lethal doses. Further, as the dose of imidacloprid required to kill sharpshooters increases due to resistance, so too does the period that sharpshooters are exposed to sub-lethal doses. Response of glassy-winged sharpshooter to cowpea plants treated with sub-lethal doses of imidacloprid was evaluated by conducting no-choice and choice tests. In no-choice feeding assays, glassy-winged sharpshooters caged on plants treated with sub-lethal doses of imidacloprid ceased feeding and produced little excreta. Further, sub-lethal exposure to a range of doses over a 4-d period did not affect viability over a 9-wk post-exposure holding period on untreated plants. In choice-tests, glassy-winged sharpshooters avoided treated plants and were observed predominately on untreated plants. Results suggest that application of imidacloprid to vineyards and citrus orchards may push glassy-winged sharpshooters out of treated habitats rather than kill them.

Grapevine phenolic compounds in xylem sap and tissues are significantly altered during infection by Xylella fastidiosa.

Pierce's disease of grapevine (PD), caused by the bacterial pathogen Xylella fastidiosa, remains a serious problem for grape production in California and elsewhere. This research examined induction of phenolic compounds in grapevines ('Thompson Seedless') infected with X. fastidiosa over a 6-month period. Two months postinoculation with X. fastidiosa, catechin, digalloylquinic acid, and astringin were found at greater levels in xylem sap; multiple catechins, procyanidins, and stilbenoids were found at greater levels in xylem tissues; and precursors to lignin and condensed tannins were found at greater levels in xylem cell walls. However, such large-scale inductions of phenolic compounds were not observed 4 months after inoculation. Six months after inoculation, infected plants had significantly reduced phenolic levels in xylem sap and tissues when compared with control plants, including lowered levels of lignin and condensed tannins. At 6 months, PD symptoms were severe in infected plants and most photosynthetic tissue was abscised. These results suggest that, even though grapevine hosts may initially respond to X. fastidiosa infections with increased production of phenolic compounds, ultimately, PD causes grapevines to enter a state of decline whereby diseased hosts no longer have the resources to support secondary metabolite production, including defense-associated phenolic compounds.

Potential effects of Grapevine leafroll-associated virus 3 (genus Ampelovirus; family Closteroviridae) or Grapevine red blotch virus (genus Grablovirus; family Geminiviridae) infection on foliar phenolic and amino acid levels.

OBJECTIVE: Grapevine (Vitis spp.) viral infections, including those by Grapevine leafroll-associated virus 3 (GLRaV-3) and Grapevine red blotch virus (GRBV), greatly reduce fruit yields and quality. Evidence exists that host chemistry shifts result in reductions in fruit quality. However, changes over the season in foliar chemistry has not been well examined. Therefore, phenolic and amino acid levels were examined in leaves collected in grapevines with different rootstocks that were healthy or were infected with GLRaV-3 or GRBV. This was part of an effort to assess changes that different pathogens cause in grapevine tissues. RESULTS: Month and year appeared to account for the greatest variability in grapevine foliar phenolic or amino acid levels, followed by differences in rootstock, and then differences in infection status. GLRaV-3 infection significantly lowered levels of total and individual hydroxycinnamic acid derivatives, and GRBV lowered total phenolic levels, total and individual hydroxycinnamic acids. Amino acid levels were increased over controls in vines infected by GLRaV-3, but not with GRBV. Overall, changes within grapevine leaves due to viral infection were likely too small to overcome variability due to sampling time or rootstock cultivar, and therefore such factors should be considered in determining infection effects on plant foliar chemistry.

Exploring potential induction of grapevine (Vitis spp.) root phenolic compounds by ring nematodes, Mesocriconema xenoplax.

OBJECTIVE: Ring nematodes can decrease vineyard productivity when plated in conditions favorable for their survival. Resistant rootstocks are available to combat harm due to ring nematodes, and compounds called phenolics were hypothesized as imparting this resistance. Therefore, this study measured phenolic compound levels in four different rootstocks and attempted to find associations with ring nematode populations. Furthermore, select phenolics called stilbenoids were tested in a bioassay to observe if these compounds affect ring nematode survival. This was part of a larger effort to assess the role of phenolics in protecting grapevines from nematodes and other pathogens or pests. DATA DESCRIPTION: This study was conducted over 2 years, 2018 and 2019, and phenolic levels were much greater in 2019 than 2018 likely due to uncontrolled differences in climatic controls. Ring nematode infected grapevines also did not have different phenolic compound levels than healthy controls. Bioassays of different stilbenoid polymers revealed no significant effects on ring nematode survival. These results suggest that analyzed root phenolic compounds were not involved in resistance or susceptibility to ring nematodes. These data should steer future researchers into analyzing other potential sources of nematode resistance.

Microplate bioassay to examine the effects of grapevine-isolated stilbenoids on survival of root knot nematodes.

OBJECTIVES: Root knot nematodes can be major pests in vineyards and cause significant yield losses over time. Control involves the use of different resistant grapevine rootstocks, but it remains unclear the mechanisms that such rootstocks possess to limit root knot nematode infections. Defense-associated compounds called stilbenoids, a type of phenolic compound, are present in relatively substantial amounts in grapevine root tissues. Therefore, experiments were performed to assess how different stilbenoid compounds impact nematode survival in microplate assays. Data generated were part of a larger effort to understand potential mechanisms that resistant grapevine rootstocks have to limit root knot infections. DATA DESCRIPTION: The percentage of surviving root knot nematodes was assessed 1, 3, and 5 days after J2 juveniles were placed into microplate wells amended with 0, 1.25, 2.5, 5, or 10 ppm of piceid, ε-viniferin, a resveratrol trimer putatively identified as miyabenol C, or a putative mixture of resveratrol tetramers putatively identified as vitisin B and hopeaphenol. Both ε-viniferin and the resveratrol tetramers significantly reduced root knot survival at the higher concentrations. These data provide insight about one potential mechanism that grapevine rootstocks might possess to combat nematodes.

Characterization of grapevine fungal canker pathogens Fatty Acid Methyl Ester (FAME) profiles.

Fatty acid methyl ester (FAME) analyses can be useful for distinguishing microbial species. This study conducted FAME analyses on 14 fungal species known to cause grapevine trunk diseases. FAME profiles were dominated by oleic acid, albeit profiles were characteristic enough to separate species. Discriminant analyses suggested that palmitoleic acid/sapienic acid, pentadecylic acid, and an unsaturated 17-carbon fatty acid (17:1ω8 c)could explain 79.8% of the variance in the profiles among species in the first three discriminant functions. FAME profile libraries were created for use in a commercialized software, which was able to accurately identify isolates to the species level, with a low rate (9.4%) of samples to be reassessed. Dendrograms created using neighbor-joining cluster analyses with data from FAME profiles were compared with those using internal transcribed spacer (ITS) region sequences. This revealed that FAME profiles, albeit useful for tentative species identification, should not be used for determining phylogenetic relationships because the dendrograms were significantly unconcordant. Regardless, these results demonstrated the potential of FAME analyses in quickly and initially identifying closely related fungal species or confirming conclusions from other species identification techniques that would require independent validation.

Mixed infections of fungal trunk pathogens and induced systemic phenolic compound production in grapevines.

As grapevines mature in California vineyards they accumulate chronic wood infections by the Ascomycete fungi that cause trunk diseases, including Botryosphaeria dieback (caused by Diplodia seriata and Neofusicoccum parvum) and Esca (caused by Phaeomoniella chlamydospora). It is thought that such mixed infections become localized to separate internal lesions/cankers of the permanent, woody structure of an individual vine, but nonetheless the fungi all colonize the same vascular system. In response to infection by one pathogen, the host may initiate systemic biochemical changes, which in turn may affect the extent of subsequent infections by other pathogens. To test this hypothesis, we measured changes in phenolic compounds in the wood and lesion lengths of the pathogens, during sequential co-inoculations with different or identical pair-wise sequences of infection by D. seriata, N. parvum, or P. chlamydospora. Prior fungal infections only affected the development of subsequent D. seriata infections. Effects of fungal infections on phenolic compounds were variable, yet initial infection by D. seriata was associated with significantly higher concentrations of most phenolic compounds distally, compared to all other initial inoculation treatments. It was hypothesized that pre-existing phenolic levels can slow initial lesion development of fungal trunk pathogens, especially for D. seriata, but over time the pathogens appeared to overcome or neutralize phenolic compounds and grow unimpeded. These results demonstrate that effects of one fungal trunk pathogen infection is generally unable to distally affect another long-term, albeit shifts in host phenolics and other plant defenses do occur.

Amino acid, sugar, phenolic, and terpenoid profiles are capable of distinguishing Citrus tristeza virus infection status in citrus cultivars: Grapefruit, lemon, mandarin, and sweet orange.

Citrus tristeza virus (CTV) is the most severe viral disease for citrus production. Many strains of CTV have been characterized and their symptomology widely varies, ranging from asymptomatic or mild infections to severe symptomology that results in substantial yield loss or host death. The capacity of the different CTV strains to affect the biochemistry of different citrus species has remained largely unstudied, despite that associated metabolomic shifts would be relevant toward symptom development. Thus, amino acid, sugar, phenolic, and terpenoid levels were assessed in leaves of healthy and CTV-infected grapefruit, lemon, mandarin, and two different sweet orange cultivars. Both mild [VT-negative (VT-)] and severe [VT-positive (VT+)] CTV genotype strains were utilized. When looking at overall totals of these metabolite classes, only amino acid levels were significantly increased by infection of citrus with severe CTV strains, relative to mild CTV strains or healthy plants. No significant trends of CTV infection on summed amounts of all sugar, phenolic, or terpenoid compounds were observed. However, individual compound levels were affected by CTV infections. Subsequent canonical discriminant analysis (CDA) that utilized profiles of individual amino acids, terpenoids, or phenolics successfully distinguished leaf samples to specific citrus varieties and identified infection status with good accuracy. Collectively, this study reveals biochemical patterns associated with severity of CTV infections that can potentially be utilized to help identify in-field CTV infections of economic relevance.

Ecosystem, location, and climate effects on foliar secondary metabolites of lodgepole pine populations from central British Columbia.

Lodgepole pines, Pinus contorta Douglas ex Louden var. latifolia Engelm. ex S. Watson, are encountering increased abiotic stress and pest activity due to recent increases in temperature and changes in precipitation throughout their range. This tree species counters these threats by producing secondary metabolites, including phenolics and terpenoids. We examined foliar levels of lignin, soluble phenolics, monoterpenoids, sesquiterpenoids, and diterpenoids in 12 stands in British Columbia, Canada. We used these data to assess associations among foliar secondary metabolite levels and ecosystem, geographic, and climatic variables. Regressions were also performed to observe which combinations of variables best explained secondary metabolite variance. Stands of P. c. latifolia in the Coastal Western Hemlock and Interior Cedar/Hemlock biogeoclimatic zones had consistently greater foliar levels of almost all measured secondary metabolites than did other stands. Lignin was present in greater amounts in Boreal White/Black Spruce ecosystem (i.e., northern) stands than in southern stands, suggesting a role for this metabolite in pine survival in the boreal forest. Attempts to develop regression models with geographic and climatic variables to explain foliar secondary metabolite levels resulted in multiple models with similar predictive capability. Since foliar secondary metabolite levels appeared to vary most between stand ecosystem types and not as much due to geographic and climatic variables, metabolic profiles appeared best matched to the stress levels within local environments. It is unknown if differences in secondary metabolite levels are the result of genetic adaptation or phenotypic plasticity, but results from this and other studies suggest that both are important. These results are interpreted in light of ongoing efforts to assist in the migration of certain populations of P. c. latifolia northward in an effort to counter predicted effects of climate change.

Grapevine (Vitis spp.) rootstock stilbenoid associations with host resistance to and induction by root knot nematodes, Meloidogyne incognita.

OBJECTIVE: The root knot nematodes (RKN) Meloidogyne incognita can severely reduce grapevine yields over time. Grapevine rootstocks have been developed from wild Vitis species that provide resistance to nematode infections. However, the potential biochemical or mechanical mechanisms of resistance have not been thoroughly explored. Therefore, this study measured levels of stilbenoids in roots of non-infected and RKN-infected grapevines with Cabernet Sauvignon scion grafted to susceptible (O39-16) or resistant (Freedom) rootstocks. This was part of a larger effort to assess phenolic compound levels within grapevine rootstocks to determine roles of stilbenoid compounds in improving nematode resistance and overall plant health. RESULTS: None of the assessed compounds were consistently greater in RKN infected plants versus healthy controls. Stilbenoids putatively identified as pallidol, ɑ-viniferin, miyabenol C, and hopeaphenol were overall much greater in Freedom than O39-16 rootstocks. By contrast, the stilbenoids ampelopsin A, ω-viniferin, and vitisin B were greater in O39-16 than Freedom. O39-16 and Freedom had similar levels of other stilbenoids especially monomers and dimers. Potentially the greater levels of specific stilbenoids present in Freedom than O39-16 provided RKN resistance. If validated, breeding programs could utilize the increased presence of these compounds as a marker for increased resistance to nematodes.

Phenolic Compound Induction in Plant-Microbe and Plant-Insect Interactions: A Meta-Analysis.

Plants rely on a variety of ways to protect themselves from being fed upon, including de novo production of specific compounds such as those termed as phenolics. Phenolics are often described as important in plant health and numerous studies have concluded they increase as a result of insect feeding, pathogen infection, or beneficial microorganism colonization. However, there are some studies reaching differing conclusions. Therefore, meta-analyses were conducted to observe whether common trends in phenolic induction in plants can be made when they become hosts to insects or microorganisms. Four hypotheses were tested. The first was that total phenolics increase as a generic response, and meta-analyses confirmed that this occurs when plants are infested with insects or colonized by bacterial or fungal microorganisms, but not for oomycetes. The second hypothesis was that phenolic induction is different when a beneficial microorganism colonizes a plant vs. when a plant is infected by a pathogen. Beneficial bacteria, pathogenic bacteria, and beneficial fungi produced increased phenolic levels in plant hosts, but fungal pathogens did not. The third hypothesis was that insect feeding method on plant hosts determines if phenolics are induced. Chewing induced phenolics but piercing-sucking and wood-boring did not. Lastly, we used meta-analyses to determine if annual or perennials rely on phenolic induction in different amounts, and even though annuals had significantly increased phenolic levels but perennials did not, it was observed that phenolic induction was not statistically different when plant type was considered. These results demonstrate that phenolic induction is a common response in plant hosts exposed to feeding or colonization, with specific exceptions such a pathogenic fungi and piercing-sucking insects.

Grapevine phenolic compounds influence cell surface adhesion of Xylella fastidiosa and bind to lipopolysaccharide.

Bacterial phytopathogen Xylella fastidiosa specifically colonizes the plant vascular tissue through a complex process of cell adhesion, biofilm formation, and dispersive movement. Adaptation to the chemical environment of the xylem is essential for bacterial growth and progression of infection. Grapevine xylem sap contains a range of plant secondary metabolites such as phenolics, which fluctuate in response to pathogen infection and plant physiological state. Phenolic compounds are often involved in host-pathogen interactions and influence infection dynamics through signaling activity, antimicrobial properties, and alteration of bacterial phenotypes. The effect of biologically relevant concentrations of phenolic compounds coumaric acid, gallic acid, epicatechin, and resveratrol on growth of X. fastidiosa was assessed in vitro. None of these compounds inhibited bacterial growth, but epicatechin and gallic acid reduced cell-surface adhesion. Cell-cell aggregation decreased with resveratrol treatment, but the other phenolic compounds tested had minimal effect on aggregation. Expression of attachment (xadA) and aggregation (fimA) related genes were altered by presence of the phenolic compounds, consistent with observed phenotypes. All four of the phenolic compounds bound to purified X. fastidiosa lipopolysaccharide (LPS), a major cell-surface component. Information regarding the impact of chemical environment on pathogen colonization in plants is important for understanding the infection process and factors associated with host susceptibility.