Phytoplasma Infection Research Articles

Biochemical changes induced by phytoplasmas in plants

Abstract Phytoplasmas are cell wall-less and pleomorphic bacterial pathogens that cause considerable yield losses in more than 1000 plant species worldwide and transmitted by sap-feeding insects. Phytoplasma-infected plants exhibit symptoms including witches’ broom, phyllody, virescence, yellowing induced by the phytoplasma effector proteins. It has been revealed that phytoplasmas are capable of modulating biochemical responses of plants involved in defense mechanisms. In this respect, phenolic compounds, free amino acids in source leaves, protein contents, some secondary metabolites, and polyamines were found to increase, while chlorophyll content and photosynthetic activity and the number of chemical compounds were reduced as a consequence of phytoplasma infection. Therefore, the objective of this study was to review the chemical composition changes in phytoplasma-infected plants.

Occurrence of cpg islands in differentially methylated Loci in Sesamum indicum L. upon phytoplasma infection

Phytoplasmas are cell wall-less bacteria that inhabit the phloem, transmitted by sap sucking insects and associated with diseases in numerous plants including economically important taxa. Phytoplasmas are associated with the serious disease of sesame known as phyllody in nearly all growing countries. Currently, the disease is a significant threat to the production in the most important sesame-producing regions of the world. Symptoms of sesame phyllody include shoot proliferation, virescence, foliar yellowing, shortened internodes, small leaves, abnormal floral organs, generalized stunting, and phloem necrosis. Studies on changes in the levels of gene expression in response to phytoplasma infection are almost lacking in sesame. DNA methylation is a conserved epigenetic mark that regulates multiple processes, and its disruption can lead to developmental abnormalities. The CpG sites are DNA sequences where a cytosine followed by a guanine is often methylated to form 5-methylcytosines and are known to regulate gene expression through transcriptional silencing. In the present study, seven loci from sesame genome were chosen based on their role in development or defence that were found to be differentially methylated using whole genome bisulfite sequencing followed by in-silico analysis using Meth Primer online tool. All the loci were found to consist of one or more CpG islands. CpG island specific primers were designed to confirm their presence. Expected amplicons were obtained in both healthy and phyllody infected sesame plants.

Occurrence, Distribution and Molecular Characterization of Phytoplasma Infecting Chickpea (Cicer arietinum L.) from Tamil Nadu

Phytoplasma is an obligate prokaryote infecting a wide array of crops such as urdbean, sesame, brinjal and many other vegetable crops in India. Several workers have noticed the recent outbreak of phytoplasma disease in several pulse crops. A study was conducted to identify the phytoplasma infection in chickpea from Tamil Nadu. The average disease incidence due to phytoplasma was recorded with a range of 5-20% under field condition. The infected plants produced symptoms including stunting of plants, bushy appearance, reduced leaf size, chlorosis and reddening of infected leaves. The association of phytoplasma with these symptoms was confirmed by nested PCR assay using the universal primers P1/P7 and R16F2n/R16R2. The infected samples were amplified with an amplicon size of 1.2 kb and sequence analysis showed more than 99% similarity with phytoplasma belongs to Candidatus Phytoplasma aurantifolia. Phylogenetic analysis of nucleotide sequences confirmed the chickpea phyllody phytoplasma forms a single subgroup with other Indian isolates of various crops.

ОЦЕНКА РАЗНООБРАЗИЯ ВИДОВ РАСТЕНИЙ, ПОРАЖАЕМЫХ ФИТОПЛАЗМОЙ ГРУППЫ Х-БОЛЕЗНИ (16Sr-III) В РОССИИ

Worldwide, there is an increase in the harmfulness of plant diseases transmitted by insects, including phytoplasmas. In Russia, phytoplasma diseases have been studied insufficiently. The aim of the study was to monitor phytoplasma diseases in various economic regions of the Russian Federation and determine the taxonomic affiliation of their pathogens. The task of the study was to identify the species composition of plants affected by phytoplasmas belonging to the 16Sr-III group and their potential vectors. Molecular genetic studies were carried out at the All-Russian Research Institute of Phytopathology (VNIIF) in 2006-2021. Plants with symptoms of phytoplasma infection and leafhoppers collected in the Moscow and Samara regions were the material of the research. The presence of phytoplasma DNA in the samples was determined using sequentially direct and nested PCR with the primer pairs: P1/16S-SR and R16F2n/R16R2, respectively. Phytoplasmas were identified by analyzing restriction fragment length polymorphism (RFLP) after treatment of 1.2 Kb DNA amplicons with restriction endonucleases. In a quarter of the analyzed potato samples and five of the eight surveyed economic regions (Volga, North Caucasian, Ural, Central and West Siberian), phytoplasma of the 16SrIII group was found. Among other solanaseous, phytoplasma of the 16SrIII group was found in pepper and garden petunia, as well as in two species of wild potatoes. In total, the phytoplasma of X-disease was detected in 51 species of plants from 19 families, including Fabaceae (12 species), Asteraceae (10), Rosaceae (6), and Solanaceae (5). In other families, 1–2 plant species infected with this phytoplasma were identified. The phytoplasma of the 16SrIII-B subgroup was most frequently found in plant material. Phytoplasma of the 16SrIII-F subgroup was detected in two plant species: astilbe (Astilbe thunbergii (Siebold & Zucc.) Miq.) and hybrid clover (Trifolium hybridum L.). In potential vectors (leafhoppers Euscelis incisа Kirschbaum, Sonronius binotatus Sahlberg, Macrosteles laevis Rib. and Psammotettix striatus L.), phytoplasma of the 16SrIII group, including the 16SrIII-O and 16SrIII-P subgroups, was found.

Molecular identification of 16SrI-X subgroup phytoplasma associatedwith eggplant in India

Eggplants exhibiting phyllody and virescence symptoms typical of phytoplasma infection were observed in fields in Alandi, Pune, Maharashtra, India. The phytoplasma presence was confirmed in all symptomatic samples using PCR and nested PCR with specific primer pairs P1/P7 and R16F2n/R2. Nucleotide sequence comparison revealed that the amplicon sequence shares more than 99% identity with ‘Candidatus Phytoplasma asteris’ sequences and in the phylogenetic analysis clustered with sequences of similar strains of phytoplasmas. Virtual RFLP analysis of 16S rDNA sequences enabled to enclose the strain into the 16SrI-X subgroup. This is the first report of natural occurrence of a 16SrI-X phytoplasma in eggplant.

Detection and molecular characterization of phytoplasmas associated with stone fruit trees in northwest of Iran

During surveys conducted from 2015 to 2018 in stone fruit orchards of East Azerbaijan province in the northwest of Iran, several stone fruit trees showing phytoplasma typical symptoms such as witches'-broom, little leaf, leaf rolling, leaf scorching, dieback, decline, yellowing, reddening and rosette were sampled. PCR and nested PCR analyses using phytoplasma universal primers confirmed the phytoplasma infection in some of symptomatic almond, peach, plum and sweet cherry trees. Sequence analyses of some 485 bp and 1239 bp 16S–23S rDNA fragments amplified in nested PCR, confirmed that ʻCandidatus Phytoplasma asterisʼ, ʻCa. Phytoplasma aurantifoliaʼ and ʻCa. Phytoplasma phoeniciumʼ were associated with almond, ʻCa. Phytoplasma phoeniciumʼ was associated with peach, ʻCa. Phytoplasma asterisʼ and ʻCa. Phytoplasma trifoliiʼ were associated with plum and ʻCa. Phytoplasma asterisʼ was associated with sweet cherry trees. Virtual RFLP analyses indicated that stone fruit phytoplasmas in East Azerbaijan are belonging to 16SrI–B, 16SrII-A, 16SrIX-C, 16SrXII-A and 16SrVI-A subgroups. This research is the first report of infection by ʻCa. Phytoplasma asterisʼ in almond and infection by ʻCa. Phytoplasma trifolliʼ in plum around the world and the first report of ʻCa. Phytoplasma aurantifoliaʼ and ʻCa. Phytoplasma trifoliiʼ presence in the northwest of Iran.

СТВОРЕННЯ, ПІДТРИМКА ТА ВПОРЯДКУВАННЯ ОЗНАКОВИХ КОЛЕКЦІЙ ВИНОГРАДУ (VITIS VINIFERA L.)

Based on the grapevine genotypes of NSC “Tairov Research Institute of Viticulture and Wine-Making” analysis, traits collections have been created and registered, representing genotypes for breeding programs to obtain new varieties resistant to abiotic and biotic environmental factors (frost and phytopathogens) under conditions implementation of climate change scenarios, varieties with improved quality traits and seedless varieties. Genetic ordering of traits collections has begun on the basis of varieties molecular identification using microsatellite DNA markers (from 5 to 9 MC loci) and identification using DNA markers of genes of interest or gene complexes (primarily a trait of seedlessness). Microsatellite analysis made it possible to identify a total of about 80 genotypes, mainly the varieties breeded at the NSC “Tairov Research Institute of Viticulture and Wine-Making”, belonging to the traits collections of resistance and quality. Visual sanitary control of three traits collections showed a practical absence of virus diseases and grapevine crown gall disease symptoms. Sometimes symptoms similar to the phytoplasma infection manifestation were found. The data of virus diseases agents identification by ELISA showed the absence the most harmful viruses — grapevine fanleaf virus (GFLV) and grapevine leafroll virus 1 (GLRaV I) on 19 samples tested. From samples of resistance traits collection one sample was positive on grapevine leafroll virus 3 and two were positive on grapevine fleck virus (GfkV). These samples should be eliminated from further propagation and genetic material moving between germplasm repositories. The causative agent of grapevine crown gall disease (Agrobacterium vitis) was not detected on 14 samples of 3 traits collections tested by PCR. It has been determined that the sanitary status of grapevine traits collections practically corresponds to the European minimum sanitary requirements, genetic ordering of characteristic collections will be continued.

Variability of Phytoplasma Infection Density in Poinsettia and Evaluation of its Association with the Level of Branching in Host Plants.

It has long been established that phytoplasma infection is the cause of the free-branching phenotype in poinsettia. However, relatively little is known about the ecology of the pathogen in planta. The present study evaluated the infection pattern of poinsettia branch-inducing phytoplasma (PoiBI) and its association with the poinsettia phenotype during cutting propagation. The presence of this pathogen in the poinsettia variety Luv U Pink was determined using PCR and sequence analysis. The infection density of PoiBI in distinct tissue types of different plant segments were then determined using quantitative PCR coupled with plasmid-based standard curves. Both vegetative-stage and flowering-stage plants were tested. The results showed that, despite being considerably variable among plants, the infection densities of PoiBI tend to be higher in source leaves located in the lower parts of the plant. The densities were consistently lower in tissues located at the top of the plants, regardless of the tissue type. Analysis of the infection densities among samples collected from six stock plants used in commercial production also revealed significantly different levels of PoiBI load. An association between PoiBI infection density in the stock plants and the level of branching in cutting-propagated plants (derived from the stock plants) was also observed; stock plants with low infection densities tended to produce smaller proportions of plants exhibiting higher degrees of branching both before and after pinching. These data suggest that uneven distribution of PoiBI within and among stock plants may lead to the production of cuttings with variable phytoplasma densities which may, in turn, affect the phenotypic uniformity of the plants produced. Overall, findings from the present work add to the understanding of PoiBI's ecology and could provide implications to commercial poinsettia production.

Phloem and xylem modifications of Vitis vinifera stems in response to flavescence dorée phytoplasma infection

AbstractAnatomical modifications of xylem and phloem tissues of grapevine (Vitis vinifera) stems of shoots infected by the flavescence dorée phytoplasma (FDp) were first observed and described in the 1960s, but never quantified in detail. In this paper, we describe and quantify the impact of FDp on grapevine stem tissues, and relate it to the level of expression of symptoms and to cultivar‐specific FDp susceptibility. For this purpose, we measured and quantified the anatomical parameters of xylem and phloem tissues of a tolerant (Merlot) and a susceptible (Chardonnay) cultivar. For each cultivar, thin sections of eight shoots with symptoms from FDp‐infected grapevines, eight symptomless shoots from the same FDp‐infected grapevines, and eight symptomless shoots from symptomless grapevines (control) were compared. Results showed general inhibition of xylem growth and proliferation of phloem tissues (hyperplasia) with lack or irregular arrangement of the fibre‐sclereids in the axial phloem of the stems from shoots with symptoms, irrespective of the cultivar. Xylem vessels of infected Merlot shoots were partly occluded by tyloses and a higher number of smaller vessels were produced than in control plants. Thus, the anatomical responses confirmed the detrimental effect of FDp on stems of infected grapevine shoots, including impaired stem development and lack of periderm formation. Statistically significant differences were found between the two cultivars with different levels of susceptibility to FDp infection.

Evolutionary Diversification in Insect Vector–Phytoplasma–Plant Associations

AbstractThe association between insect herbivores and vascular plants represents one of the greatest success stories in terrestrial evolution. Specific mechanisms generating diversity in the association remain poorly understood, but it has become increasingly clear that microbes play important roles in mediating plant–insect interactions. Previous research on phytoplasmas (Acholeplasmatales: Acholeplasmataceae), a diverse group of plant-pathogenic bacteria, and their hemipteran insect vectors suggests that this system provides a new model for understanding how interactions among distantly related but ecologically associated groups of organisms can drive evolutionary diversification. Phytoplasma infections affect the phenotypes of both plants and vectors, altering functional traits (e.g., diet breadth) and mediating host shifts which may, in turn, alter genetic and phylogenetic patterns. This review highlights previous research on the functional ecology and phylogenetic components of phytoplasma-plant-vector (PPV) associations relevant to the evolutionary diversification of this system. Although phytoplasmas and their hosts occur in most terrestrial biomes and have evolved together over the past 300+ million years, major gaps in knowledge of PPV associations remain because most prior research on the system focused on strategies for mitigating effects of phytoplasma diseases in agroecosystems. Study of this system within a broader evolutionary context could help elucidate mechanisms by which interactions between insect herbivores, microbes, and plants drive biological diversification and also help predict the emergence of diseases affecting agriculture. Future research should more thoroughly document PPV associations in natural habitats, examine the relative prevalence of cospeciation versus host shifts in this system, and test possible macroevolutionary consequences of host manipulation by phytoplasmas.

Silencing of ATP synthase β reduces phytoplasma multiplication in a leafhopper vector

The leafhopper Euscelidius variegatus is a natural vector of the chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of the Flavescence dorée phytoplasma (FDp). Previous studies indicated a crucial role for insect ATP synthase α and β subunits during phytoplasma infection of the vector species. Gene silencing of ATP synthase β was obtained by injection of specific dsRNAs in E. variegatus. Here we present the long-lasting nature of such silencing, its effects on the small RNA profile, the significant reduction of the corresponding protein expression, and the impact on phytoplasma acquisition capability. The specific transcript expression was silenced at least up to 37 days post injection with an average reduction of 100 times in insects injected with dsRNAs targeting ATP synthase β (dsATP) compared with those injected with dsRNAs targeting green fluorescent protein (dsGFP), used as negative controls. Specific silencing of this gene was also confirmed at protein level at 15 days after the injection. Total sRNA reads mapping to dsATP and dsGFP sequences in analysed libraries showed in both cases a peak of 21 nt, a length consistent with the generation of dsRNA-derived siRNAs by RNAi pathway. Reads mapped exclusively to the fragment corresponding to the injected dsATPs, probably indicating the absence of a secondary machinery for siRNA synthesis. Insects injected either with dsATP or dsGFP successfully acquired CYp and FDp during feeding on infected plants. However, the average phytoplasma amount in dsATP insects was significantly lower than that measured in dsGFP specimens, indicating a probable reduction of the pathogen multiplication when ATP synthase β was silenced. The role of the insect ATP synthase β during phytoplasma infection process is discussed.

Plant health status effects on arbuscular mycorrhizal fungi associated with Lavandula angustifolia and Lavandula intermedia infected by Phytoplasma in France

We investigated root communities of arbuscular mycorrhizal fungi (AMF) in relation to lavender (Lavandula angustifolia) and lavandin (Lavandula intermedia) health status from organic and conventional fields affected by Phytoplasma infection. The intensity of root mycorrhizal colonization was significantly different between diseased and healthy plants and was higher in the latter regardless of agricultural practice. This difference was more pronounced in lavender. The root AMF diversity was influenced by the plant health status solely in lavender and only under the conventional practice resulting in an increase in the AMF abundance and richness. The plant health status did not influence the distribution of root AMF communities in lavandin unlike its strong impact in lavender in both agricultural practices. Finally, among the most abundant molecular operational taxonomic units (MOTUs), four different MOTUs for each plant species were significantly abundant in the roots of healthy lavender and lavandin in either agricultural practice. Our study demonstrated that the plant health status influences root colonization and can influence the diversity and distribution of root AMF communities. Its effects vary according to plant species, can be modified by agricultural practices and allow plants to establish symbiosis with specific AMF species.

Investigations on yellowing of chestnut crowns in Trentino (Alps, Northern Italy)

Yellowing of part or the whole canopy of chestnut trees was observed during the summer of 2014 in the areas of Tenno, Pranzo and Drena in Trentino, where chestnut cultivation is traditional on the lateral moraine deposit in a context of a prevalent limestone lithological substrate. Symptoms were observed on chestnut trees of different ages, either grafted or not, scattered or in stripes along the maximum slope. Investigations were carried out in the form of field surveys, chemical and molecular analyses of soil and leaves and greenhouse trials, to assess the damage evolution and its causes. No known pathogens emerged during the field surveys, and laboratory tests excluded the presence of phytoplasma infections. A comparison of yellow and asymptomatic leaves evidenced significant deficiencies of manganese (Mn) and iron (Fe) in the yellow leaves, though the level of the two nutrients was the same in the soil below green and yellowed trees. Experimental run-off trials in the greenhouse reproduced the foliage damage with a continuous basic water regime; open-field fertilizations with Mn and Fe led to a slightly recovery on the part of some treated leaves, thus confirming the hypothesis of a probable, sudden deficiency of the mentioned microelements. A possible explanation of this phenomenon could be, on one side, the high precipitation level of 2014, and, on the other side, the previous outbreaks of the Asian chestnut gall wasp, that had reduced foliage volume and thus the organic substance in the soil. The natural recovery from symptoms was clearly visible in trees after four years in a context of normal rainfall regime and increasing organic acid in the soil, after effective biological control of wasp. Even if related to a particular and rare geological condition, this is the first description of Mn deficiency in Castanea sativa.

Stolbur and Clover Proliferation Phytoplasma Infections in Tomato from Bingöl province, Turkey

Phytoplasma-type symptoms were noted in tomato (Lycopersicum esculentum L.) in Bingöl province of Turkey. The remarkable symptoms include witch’s broom, rosetting, purple curled and crispy leaves, small leaves in the upper branches and excessively elongated calyx. Genomic DNA isolation was performed to identify possible pathogens from the leaves of 11 plants with and without symptom. In the Nested-PCR test performed using universal primer sets, DNA bands of approximately 1200 bp size were obtained in 4 of 11 samples. Randomly chosen two DNA bands primed R16F2n/R16R2 were cloned into an appropriate plasmid vector to further characterizations. The recombinant plasmid DNAs purified were sequenced in both directions. Molecular assays of the 16S rRNA sequence confirmed the existence of the “Canditatus Phytoplasma solani” (16SrXII-A group) (similarity coefficient 1.00) (Accession no: MT279680) and the “Canditatus Phytoplasma trifolii” (16SrVI-A group) (similarity coefficient 1.00) (Accession no: MT279852) in the infected tomato samples. The isolates associated with tomato-phytoplasma were named as ‘Bingöl D11’ and ‘Bingöl D90’ isolates, respectively. The phylogenetic dendrogram created also confirmed where both pathogens belong. This current paper is documented in the first record of “Ca. P. solani” (16SrXII-A) and “Ca. P. trifolii” (16SrVI-A) in naturally diseased tomato in Bingol of Turkey.

Phytoplasmal diseases in India and its management

Phytoplasmas are phloem-limited pleomorphic bacteria which lacks the cell wall, mainly transmitted through leafhoppers but also by plant propagation materials and seeds. In India evidences showed that phytoplasma have been found associated with 129 plant species including Cereals, vegetables, fruits, trees, ornamental, sugarcane, grasses and weeds which is increasing at alarming rate. Sesame phyllody, brinjal little leaf, sugarcane grassy shoot, sandal spike, coconut root wilt, areca nut yellow leaf and many diseases in ornamental plants which causes the most severe economic losses in India. In earlier days, very few phytoplasma diseases were identified in India merely based on the bright-field, fluorescence, electron microscope observations, tetracycline treatment and to a lesser extent by serological assays. Recently detection of phytoplasma based on molecular methods including nested PCR assays are efficiently carried out and on that basis several plant species are reported to have phytoplasma infections. Identified phytoplasmas are related to ‘Candidatus Phytoplasma asteris’, ‘Ca. P. pruni’, ‘Ca. P. ziziphi’, ‘Ca. P. trifolii’, ‘Ca. P. solani’, ‘Ca. P. cynodontis’, ‘Ca. P. oryzae’, ‘Ca. P. phoenicium’, ‘Ca. P. australasia’ and ‘Ca. P. pini’. At present the suggested effective management practices in India includes growing resistant varieties, control of insect vectors, weed species as alternative hosts and use of healthy planting materials. It is moreover essential to understand the molecular basis of phytoplasma-vector interaction and other factors involved in disease development in order to reduce the outbreak of phytoplasma. In this chapter, we have discussed overall progress on phytoplasma disease on plant species in India in terms of detection, transmission and management of phytoplasma.

Bacterial microbiota associated with insect vectors of grapevine Bois noir disease in relation to phytoplasma infection

Bois noir is a grapevine disease causing severe yield loss in vineyards worldwide. It is associated with 'Candidatus Phytoplasma solani', a phloem-limited bacterium transmitted by polyphagous insects. Due to its complex epidemiology, it is difficult to organize effective containment measures. This study aimed to describe the bacterial microbiota associated with 'Candidatus Phytoplasma solani' infected and non-infected insect hosts and vectors to investigate if phytoplasma presence can shape the microbiota. Alpha-diversity analysis showed a low microbiota diversity in these insects, in which few genera were highly abundant. Beta-diversity analysis revealed that the xylem- and phloem-feeding behavior influences the microbiota structure. Moreover, it highlighted that phytoplasma infection is associated with a restructuring of microbiota exclusively in Deltocephalinae insect vectors. Obtained data showed that 'Candidatus Phytoplasma solani' may have adverse effects on the endosymbionts Sulcia and Wolbachia, suggesting a possible fitness modification in the insects. The phytoplasma-antagonistic Dyella was not found in any of the examined insect species. The results indicate an interesting perspective regarding the microbial signatures associated with xylem- and phloem-feeding insects, and determinants that could be relevant to establish whether an insect species can be a vector or not, opening up new avenues for developing microbial resource management-based approaches.

The phytopathogen \u2018Candidatus Phytoplasma mali\u2019 alters apple tree phloem composition and affects oviposition behavior of its vector Cacopsylla picta

Apple proliferation disease is caused by the phloem-dwelling bacterium ‘Candidatus Phytoplasma mali’, inducing morphological changes in its host plant apple, such as witches’ broom formation. Furthermore, it triggers physiological alterations like emission of volatile organic compounds or phytohormone levels in the plant. In our study, we assessed phytoplasma-induced changes in the phloem by sampling phloem sap from infected and non-infected apple plants. In infected plants, the soluble sugar content increased and the composition of phloem metabolites differed significantly between non-infected and infected plants. Sugar and sugar alcohol levels increased in diseased plants, while organic and amino acid content remained constant. As ‘Ca. P. mali’ is vectored by the phloem-feeding insect Cacopsylla picta (Foerster, 1848), we assessed whether the insect–plant interaction was affected by ‘Ca. P. mali’ infection of the common host plant Malus domestica Borkh. Binary-choice oviposition bioassays between infected and non-infected apple leaves revealed C. picta’s preference for non-infected leaves. It is assumed and discussed that the changes in vector behavior are attributable to plant-mediated effects of the phytoplasma infection.

Molecular characterization of group 16SrI and 16SrIII phytoplasmas associated with loofah witches’ broom disease in Sinaloa, Mexico

Loofah [Luffa acutangula (L.) Roxb.] is grown in some regions of Asia, Africa and the Americas. This vegetable is well known for its high nutritional and medicinal value. Loofah plants showing typical symptoms of phytoplasma infection, such as witches’ broom, fruit deformation, flower proliferation and phyllody were observed in a commercial field in Sinaloa, Mexico, in January 2015. Nineteen symptomatic loofah plants and seven asymptomatic weeds (Helianthus annuus, Sarcostemma sp., Chenopodium sp., Amaranthus palmeri, Rumex sp., Ambrosia sp., and Raphanus sp.) growing nearby were collected and analyzed by phytoplasma-specific nested PCR. Phytoplasmas were detected in 79% of symptomatic loofah plants and in two of the seven weeds. Conventional and virtual restriction fragment length polymorphism (RFLP) analyses of the 16S rRNA-encoding gene sequences indicated the presence of strains of the 16SrIII and 16SrI phytoplasma groups in loofah, which occurred as both single and mixed infections. The phylogenetic analysis confirmed four new phytoplasma strains associated with loofah diseases, two ‘Candidatus Phytoplasma asteris’-related strains (16SrI-AG and 16SrI-AH), and two ‘Candidatus Phytoplasma pruni’-related strains (16SrIII-AA, and 16SrIII-AB). Furthermore, phytoplasmas belonging to groups 16SrI and 16SrIII also were identified in the weeds H. annuus and A. palmeri, respectively; 16SrIII-AC represents a new phytoplasma subgroup described in A. palmeri in Sinaloa. To our knowledge, this is the first report of loofah disease associated with phytoplasmas belonging to the 16SrI and 16SrIII groups in Mexico.

Pre-symptomatic modified phytohormone profile is associated with lower phytoplasma titres in an Arabidopsis seor1ko line

The proteins AtSEOR1 and AtSEOR2 occur as conjugates in the form of filaments in sieve elements of Arabidopsis thaliana. A reduced phytoplasma titre found in infected defective-mutant Atseor1ko plants in previous work raised the speculation that non-conjugated SEOR2 is involved in the phytohormone-mediated suppression of Chrysanthemum Yellows (CY)-phytoplasma infection transmitted by Euscelidius variegatus (Ev). This early and long-lasting SEOR2 impact was revealed in Atseor1ko plants by the lack of detectable phytoplasmas at an early stage of infection (symptomless plants) and a lower phytoplasma titre at a later stage (fully symptomatic plants). The high insect survival rate on Atseor1ko line and the proof of phytoplasma infection at the end of the acquisition access period confirmed the high transmission efficiency of CY-phytoplasma by the vectors. Transmission electron microscopy analysis ruled out a direct role of SE filament proteins in physical phytoplasma containment. Time-correlated HPLC–MS/MS-based phytohormone analyses revealed increased jasmonate levels in midribs of Atseor1ko plants at an early stage of infection and appreciably enhanced levels of indole acetic acid and abscisic acid at the early and late stages. Effects of Ev-probing on phytohormone levels was not found. The results suggest that SEOR2 interferes with phytohormonal pathways in Arabidopsis midrib tissues in order to establish early defensive responses to phytoplasma infection.

Genetic diversity of phytoplasma strains infecting chrysanthemum varieties in India and their possible natural reservoirs

Symptoms typical of phytoplasma infection such as phyllody, virescence, witches’ broom and yellowing were observed in 12 varieties of Chrysanthemum morifolium in floral nurseries and experimental fields at New Delhi, Karnataka, Maharashtra and Andhra Pradesh, India, during surveys made from 2015 to 2017. Disease incidence ranged from 15 to 30%. Phytoplasma presence was confirmed in all symptomatic chrysanthemum varieties by molecular identification assays. Sequence comparison, phylogenetic and in silico RFLP analyses of 16S rDNA sequences allowed the identification of the chrysanthemum infecting phytoplasma strains into different ribosomal groups and subgroups, namely 16SrI, 16SrII-D, 16SrVI-D and 16SrXIV. Detection of phytoplasma strains of 16SrII-D subgroup were also confirmed in symptomatic Chenopodium album and Parthenium hysterophorus plants grown in and around the surveyed chrysanthemum fields at New Delhi, whereas 16SrVI-D phytoplasma strains were detected in symptomatic Cannabis sativa weed and leafhopper Hishimonus phycitis individuals collected from the symptomatic chrysanthemum fields at New Delhi. This is the first report on the presence of 16SrVI and 16SrXIV groups of phytoplasmas in chrysanthemum plants. Studies on genetic diversity of phytoplasmas infecting the major chrysanthemum varieties in India and their epidemiological aspects had previously not been reported. The detection and identification of phytoplasmas in different chrysanthemum varieties could contribute to increase the awareness among farmers in the management of these diseases.