Development Of Necrotic Lesions Research Articles

Population dynamics of Steneotarsonemus concavuscutum, a neglected mite pest in coconut fruits (Cocos nucifera).

The meristematic region of Cocos nucifera fruits can be colonized by various species of mites, including Steneotarsonemus concavuscutum, Steneotarsonemus furcatus, and Aceria guerreronis. The consequence of this colonization is the development of necrotic lesions on the fruit, and sometimes its abortion. Losses are commonly attributed to A. guerreronis alone, owing to the similarities in the injuries caused and its predominance in coconut plantations. However, S. concavuscutum may be the predominant pest species in some crops. Despite the possible impact of S. concavuscutum, little is known about its bioecological aspects, such as the influence of biotic and abiotic factors on its population dynamics. Our objective was to document macroclimatic abiotic factors (temperature, relative humidity, and precipitation) and biotic factors (interspecific competition and predation) interfere in the population dynamics of S. concavuscutum. We evaluated the diversity and abundance of mites in the perianth of coconut fruit naturally infested by S. concavuscutum for 1 year. The species found in the fruits of bunch 6 of the plant, which is the fruit age at which the mites commonly reach the highest abundance, were counted every 2 weeks. We found mites from nine families and S. concavuscutum was the predominant species, representing about 92% of the individuals collected. Predators represented approximately 2% of the total collection, with Neoseiulus baraki as the predominant species. Steneotarsonemus concavuscutum population density ranged from 60 to 397 mites/fruit. The highest population densities of S. concavuscutum were observed in the hottest and driest periods of the year. The population densities of S. concavuscutum were negatively associated with the presence of N. baraki, suggesting that this predator may have a role in the biological control of this pest.

Relationship between Dubas Bug (Ommatissus lybicus) Infestation and the Development of Fungal-Induced Leaf Spots in Date Palms (Phoenix dactylifera)

The dubas bug (Ommatissus lybicus) (Hemiptera: Tropiduchidae) is a serious pest in date palms in several date-producing countries, including Oman. Infestation results in a severe reduction in yield and a weakening of date palm growth. In addition, egg laying, which causes injuries to date palm leaves, results in the development of necrotic lesions on the leaves. This study aimed at investigating the role of fungi in the development of necrotic leaf spots following dubas bug infestation. Leaf samples developing leaf spot symptoms were collected from dubas-bug-infested leaves, as the leaf spot symptoms were not observed on the non-infested leaves. Isolation from date palm leaves collected from 52 different farms yielded 74 fungal isolates. Molecular identification of the isolates revealed that they belonged to 31 fungal species, 16 genera, and 10 families. Among the isolated fungi, there were five Alternaria species, four species each of Penicillium and Fusarium, three species each of Cladosporium and Phaeoacremonium, and two species each of Quambalaria and Trichoderma. Out of the thirty-one fungal species, nine were pathogenic on date palm leaves and induced varying levels of leaf spot symptoms. The pathogenic species were Alternaria destruens, Fusarium fujikuroi species complex, F. humuli, F. microconidium, Cladosporium pseudochalastosporoides, C. endophyticum, Quambalaria cyanescens, Phaeoacremonium krajdenii, and P. venezuelense, which were reported for the first time as leaf spot causal agents in date palms. The study provided novel information on the effect of dubas bug infestation in date palms on the development of fungal infection and associated leaf spot symptoms.

First Report of Coniella granati Associated with Dieback of Rose (Rosa sp.) in India.

Rose (Rosa sp.) is an important floricultural crop largely cultivated in the Karnataka state of southern India. A field survey conducted in Devanahalli, Bangalore rural during October, 2019 revealed rose plants (cv. Arka Pride) showing die-back symptoms with a disease incidence of 7% in an area of 30 hectare rose field. Die-back was characterized by the development of necrotic lesions that started at the tip of shoots and progressed downwards leading to the death of young shoots. Pycnidia were found on young shoots showing necrotic lesions. Die-back affected shoots (n=10) were surface sterilized with 2% NaOCl for 2 min, rinsed thrice in sterile distilled water, and plated on potato dextrose agar (PDA) medium amended with chloramphenicol (40mg/L). The plates were incubated at 28±2 °C and pure cultures were obtained by hyphal tipping. Fungal colonies with white aerial mycelia along with pycnidia were observed on eight samples. Colony growth rate was at 4.2 ± 0.4 mm/day with an average colony diameter of 47.2 ± 3.8 mm after 7 days (12/12 h light and dark period). Pycnidia were solitary and globose. Conidia were hyaline, single celled ellipsoid to fusiform, measured 12.4 - 16.78 × 3.2 - 5.1 μm (n=50). Based on cultural and morphological features, the fungus was identified as Coniella granati (Van Niekerk et al., 2004; Mahadevakumar et al., 2019) (Figure S1). Further, two representative isolates (NLD-2021-1 & NLD-2021-2) were used for molecular identification based on barcoding of ITS-rDNA, LSU and tef1 regions amplified using primer pairs described in Alvarez et al., 2016. PCR amplified products were sequenced and the consensus sequence were analyzed through BLASTn search which showed ITS (NLD-2021-1: 611/611bp - MH860368 & NLD-2021-2: 573/573bp - KU147239), LSU (NLD-2021-1: 783/906bp - MH872113 & NLD-2021-2: 844/1172bp - KX833400) and tef1 (NLD-2021-1 & NLD-2021-2: 548/548bp -KX833682) sequences shared 100% sequence similarity with C. granati respectively. The nucleotide sequences were deposited in GenBank with the accession numbers ITS: MW898436, MW898437; LSU: MZ895475, MZ895476 and tef1 MW916275, MW916276 respectively. The phylogenetic tree constructed based on combined ITS-LSU-tef1 loci confirmed that the sequences shared a common clade with C. granati (Figure S2). Further, pathogenicity tests were conducted on conducted twice on healthy rose plants (Arka Pride; 60 days old after grafting) maintained in a greenhouse at 28±2°C and 70% relative humidity. Whole plant inoculation was followed by spraying the conidial suspension (10ml / plant) of C. granati (NLD-2021-1) (3 × 106 conidia/ml, amended with Tween20 at 2%) on 15 plants. A total of 10 healthy plants inoculated with sterile distilled water amended with Tween20 at 2% served as control. Inoculated plants were covered with polythene bags for 48 h to maintain relative humidity. Die-back symptoms were observed on young shoots after 15-18 days of post inoculation (DPI), and control plants remained disease free after 15 DPI. The pathogen was re-isolated on PDA from 10 diseased plants and identity was confirmed through morphological methods. Coniella granati is an important phytopathogen known to cause die-back and twig blight of pomegranate globally and other economically important crops (Cintora-Martinez et al. 2017). No reports are available on the association of C. granati with dieback of rose (Farr & Rossman, 2021), which should help in developing appropriate disease management strategies.

Comparative Genomics of Potato Common Scab-Causing Streptomyces spp. Displaying Varying Virulence.

Common scab of potato causes important economic losses worldwide following the development of necrotic lesions on tubers. In this study, the genomes of 14 prevalent scab-causing Streptomyces spp. isolated from Prince Edward Island, one of the most important Canadian potato production areas, were sequenced and annotated. Their phylogenomic affiliation was determined, their pan-genome was characterized, and pathogenic determinants involved in their virulence, ranging from weak to aggressive, were compared. 13 out of 14 strains clustered with Streptomyces scabiei, while the last strain clustered with Streptomyces acidiscabies. The toxicogenic and colonization genomic regions were compared, and while some atypical gene organizations were observed, no clear correlation with virulence was observed. The production of the phytotoxin thaxtomin A was also quantified and again, contrary to previous reports in the literature, no clear correlation was found between the amount of thaxtomin A secreted, and the virulence observed. Although no significant differences were observed when comparing the presence/absence of the main virulence factors among the strains of S. scabiei, a distinct profile was observed for S. acidiscabies. Several mutations predicted to affect the functionality of some virulence factors were identified, including one in the bldA gene that correlates with the absence of thaxtomin A production despite the presence of the corresponding biosynthetic gene cluster in S. scabiei LBUM 1485. These novel findings obtained using a large number of scab-causing Streptomyces strains are challenging some assumptions made so far on Streptomyces’ virulence and suggest that other factors, yet to be characterized, are also key contributors.

Diversity and Virulence of Streptomyces spp. Causing Potato Common Scab in Prince Edward Island, Canada.

Common scab (CS) is a potato disease that significantly decreases the market value of potato tubers after the development of necrotic lesions on their surface. Streptomyces scabiei is the main causal agent of CS; however, other closely related species, including S. acidiscabies and S. turgidiscabies, have also been shown to cause the disease. In this study, we characterized the genetic and phenotypic diversity of Streptomyces spp. causing CS in Prince Edward Island, the main potato-producing province in Canada. Two hundred and ninety-six pathogenic Streptomyces spp. isolates were retrieved from diseased tubers harvested from six fields located across a longitudinal geographical gradient. Genome fingerprinting analyses using repetitive elements PCR (ERIC- and BOX-PCR) revealed 14 distinct genetic groups. Thirteen groups were taxonomically affiliated with S. scabiei, whereas the fourteenth group was affiliated with S. acidiscabies. Their geographical distribution was characterized and revealed that on average between six and eight different genetic groups were detected per field, with variable abundance. Virulence assays showed strong differences in virulence between the genetic groups, ranging from low to highly virulent. Interestingly, pathogenic Streptomyces spp. populations in each field seem to be dominated by the most virulent genetic groups. The results obtained will contribute to better understanding of the population dynamic of pathogenic Streptomyces spp. causing CS of potato and promoting the development of more efficient detection and intervention tools to manage this important potato disease.

Unraveling the Fungal Community Associated with Leaf Spot on Crataegus sp.

Crataegus sp. is a tree that grows in temperate zones with worldwide distribution and is commonly known in Mexico as tejocote. The use of products derived from Crataegus in traditional medicine, food, and cosmetics has increased over the last few years and the relevance of this plant has also grown. Here, we report a disease that was observed in tejocote plants that grew both in the wild and in greenhouses in Puebla (Mexico). The disease was characterized by necrotic spots on the leaf ranging from brown to reddish tones that were accompanied by structures on the back of the leaf. Furthermore, we investigated the fungal genera associated with infected leaves in wild tejocote plants, from which we recovered Alternaria sp., Aureobasidium sp., Dreschlera sp., Fusarium sp., Paecilomyces sp. and Ulocladium sp. genera. Inoculation on healthy Crataegus sp. plants with isolate UAP140 showed similar symptoms as observed in nature, while inoculation with UAP127 resulted in the development of necrotic lesions in the leaf. The identity of these isolates was further studied through the phylogenetic analysis of the ribosomal DNA internal transcribed spacer (ITS) region, where isolate UAP140 showed the highest identity with Fusarium equiseti and isolate UAP127 was similar to Alternaria arborescens. To our knowledge, this is the first report of a characteristic disease developed in Crataegus sp. plants in Mexico where the fungal community associated to the lesion was analyzed. Further studies would be necessary to determine the ecological and environmental implications of the microbiome on the appearance and development of the disease.

Pathogenicity and Lipid Composition of Mycelium of the Fungus Stagonospora cirsii VIZR 1.41 Produced on Liquid Media with Different Nitrogen Sources

The effect of the nitrogen source (sodium nitrate and soybean meal) on the growth, pathogenicity, and lipid composition of mycelium of Stagonospora cirsii Davis VIZR 1.41, a potential mycoherbicide against Canada thistle (Cirsium arvens (L.) Scop.), was studied. The fungus grew significantly (two times) faster on sucrose-soybean meal medium (SSM) than on modified Czapek medium (CM). The fungal mycelium during the exponential growth phase demonstrated the maximal pathogenicity level on the third day on SSM and on the sixth day on CM. Canada thistle leaf tissues were more susceptible (25% higher development of necrotic lesion) to mycelium obtained on SSM than to mycelium obtained on CM. The nitrogen source strongly affected the lipid composition of S. cirsii mycelium. In S. cirsii mycelium obtained on SSM, the total lipid content was 1.7 times lower; the ratio of nonpolar lipids to polar lipids was three times lower than that for fungal mycelium grown on CM. Considerable differences were found in the composition of nonpolar (sterols and fatty acid) and polar (sphingolipids) lipids with respect to the nitrogen source in the culture medium. The higher contents of sterols, free fatty acids, and some glycoceramides in the mycelium of S. cirsii obtained on SSM may be related to its higher pathogenicity level and can serve as a marker of mycoherbicide quality.

The association of Boeremia lilacis with necrotic lesions on shoots and leaf petioles and its pathogenicity towards Fraxinus excelsior

This paper reports the results of the study on the new type of necrotic lesion observed on epicormic shoots and leaf petioles of Fraxinus excelsior showing ash decline symptoms in stands of the Miechów Forest District, southern Poland. The performed symptom analysis included: shape and size of necroses, discoloration of necrotic tissue and the occurrence of fungal fructification. The same species of fungus, which was initially identified as Boeremia exigua, was isolated from 97% of epicormic shoots and from 82% of leaf petioles. However, detailed morphological and physiological (NaOH test) analyses and combined ITS, ACT, TUB and EF1-α phylogeny proved these isolates to belong to Boeremia lilacis, the species known so far as Syringa vulgaris pathogen occurring occasionally also on Forsythia hybrids or Philadelphus. Fraxinus excelsior is reported for the first time as a host for this fungus. To clarify the possible role of B. lilacis in development of necrotic lesions on ash, 10 randomly selected isolates were tested for their pathogenicity toward young F. excelsior plants using the method of artificial wound inoculation under field conditions. The isolates used in the test caused tissue necroses on all 60 inoculated shoots and all 60 inoculated leaf petioles. Necrotic lesions developed as a result of artificial inoculation corresponded largely to the symptoms following natural infections.

Biological Control of Rice Bakanae by an Endophytic Bacillus oryzicola YC7007.

In our previous study, we reported that a novel endophytic bacterium Bacillus oryzicola YC7007 has suppressed bacterial diseases of rice via induced systemic resistance and antibiotic production. This endophytic strain, B. oryzicola YC7007 was used as a biological control agent against bakanae disease of rice caused by Fusarium fujikuroi, and its mechanism of interaction with the pathogen and the rice was further elucidated. Root drenching with B. oryzicola YC7007 suspension reduced the disease severity of bakanae significantly when compared with the untreated controls. The treatments of B. oryzicola YC7007 suspension (2.0 × 107 cfu/ml) to the rice rhizosphere reduced bakanae severity by 46–78% in pots and nursery box tests containing autoclaved and non-autoclaved soils. Moreover, in the detached rice leaves bioassay, the development of necrotic lesion and mycelial expansion of F. fujikuroi were inhibited significantly by spraying the culture filtrate of B. oryzicola YC7007. Drenching of ethyl acetate extracts of the culture filtrate to the rhizosphere of rice seedlings also reduced the bakanae disease severity in the plant culture dish tests. With the root drenching of B. oryzicola YC7007 suspension, the accumulation of hydrogen peroxide was observed at an early stage of rice seedlings, and a hormonal defense was elicited with and without pathogen inoculation. Our results showed that the strain B. oryzicola YC7007 had a good biocontrol activity against the bakanae disease of rice by direct inhibition, and was also capable of inducing systemic resistance against the pathogen via primed induction of the jasmonic acid pathway.

Digital Ulcers in Systemic Sclerosis – Frequency, Subtype Distribution and Clinical Outcome

Digital ulcers (DUs) are frequent and recurrent complication in systemic sclerosis (SSc) and are the main cause of pain, impaired function of the hand and disability in SSc. The current study is a retrospective analysis of 60 SSc patients (47 patients with limited cutaneous SSc, 8 patients with diffuse cutaneous SSc and 5 patients with overlap syndrome, mean age 54.5±14.2 years, 52 women and 8 men). The frequency and evolution of DUs as well as the applied therapeutic strategies were analyzed. During the follow-up for a period between 6 months and 6 years, DUs at the fingers were registered in 35% of patients (21/60), more often in patients with diffuse cutaneous SSc (75%, 6/8) as compared with patients with limited cutaneous SSc (29%, 14/47, p<0.05) and overlap syndrome (20%, 1/5). The most frequently observed DUs were ischemic lesions at the fingerpads (85.7%, 18/21) and ulcerations over bony prominences of the fingers (23%, 5/21), which may be found simultaneously. More rare types of DUs were necrotic lesions (14%, 3/21). Thirty-eight percents (8/21) of the patients with DUs showed signs of inflammation. In one patient (4.76%, 1/21) an osteomyelitis developed and an amputation of a finger's distal phalanx was performed. DUs at the toes were significantly less frequent as compared with DUs at the fingers (10%, 6/60, p<0.05). The period of healing of the DUs is prolonged and in the studied group was 3.39±2.39 months. The treatment regimen in SSc patients with DUs included vasodilators, local antiseptic treatment, antiplatelet drug; anticoagulant in cases with development of necrotic lesions, antibiotics in cases of infection or necrotic lesions, and other symptomatic therapies. In conclusion, DUs are a common complication in SSc and require complex therapeutic measures for achievement of a positive outcome.

Genetic diversity of geographically distinct Streptococcus dysgalactiae isolates from fish

Streptococcus dysgalactiae is an emerging pathogen of fish. Clinically, infection is characterized by the development of necrotic lesions at the caudal peduncle of infected fishes. The pathogen has been recently isolated from different fish species in many countries. Twenty S. dysgalactiae isolates collected from Japan, Taiwan, Malaysia and Indonesia were molecularly characterized by biased sinusoidal field gel electrophoresis (BSFGE) using SmaI enzyme, and tuf gene sequencing analysis. DNA sequencing of ten S. dysgalactiae revealed no genetic variation in the tuf amplicons, except for three strains. The restriction patterns of chromosomal DNA measured by BSFGE were differentiated into six distinct types and one subtype among collected strains. To our knowledge, this report gives the first snapshot of S. dysgalactiae isolates collected from different countries that are localized geographically and differed on a multinational level. This genetic unrelatedness among different isolates might suggest a high recombination rate and low genetic stability.

AB0507 Frequency and evolution of digital ulcers in systemic sclerosis

BackgroundDigital ulcers are frequent and recurrent complication in systemic sclerosis (SSc) and are the main cause of pain, impaired function of the hand and disability in SSc.ObjectivesTo evaluate the frequency...

Sulfate supply influences compartment specific glutathione metabolism and confers enhanced resistance to Tobacco mosaic virus during a hypersensitive response

Sufficient sulfate supply has been linked to the development of sulfur induced resistance or sulfur enhanced defense (SIR/SED) in plants. In this study we investigated the effects of sulfate (S) supply on the response of genetically resistant tobacco (Nicotiana tabacum cv. Samsun NN) to Tobacco mosaic virus (TMV). Plants grown with sufficient sulfate (+S plants) developed significantly less necrotic lesions during a hypersensitive response (HR) when compared to plants grown without sulfate (−S plants). In +S plants reduced TMV accumulation was evident on the level of viral RNA. Enhanced virus resistance correlated with elevated levels of cysteine and glutathione and early induction of a Tau class glutathione S-transferase and a salicylic acid-binding catalase gene. These data indicate that the elevated antioxidant capacity of +S plants was able to reduce the effects of HR, leading to enhanced virus resistance. Expression of pathogenesis-related genes was also markedly up-regulated in +S plants after TMV-inoculation. On the subcellular level, comparison of TMV-inoculated +S and −S plants revealed that +S plants contained 55–132 % higher glutathione levels in mitochondria, chloroplasts, nuclei, peroxisomes and the cytosol than −S plants. Interestingly, mitochondria were the only organelles where TMV-inoculation resulted in a decrease of glutathione levels when compared to mock-inoculated plants. This was particularly obvious in −S plants, where the development of necrotic lesions was more pronounced. In summary, the overall higher antioxidative capacity and elevated activation of defense genes in +S plants indicate that sufficient sulfate supply enhances a preexisting plant defense reaction resulting in reduced symptom development and virus accumulation.

Mycosphaerella graminicola: from genomics to disease control

This Mycosphaerella graminicola pathogen profile covers recent advances in the knowledge of this ascomycete fungus and of the disease it causes, septoria tritici blotch of wheat. Research on this pathogen has accelerated since publication of a previous pathogen profile in this journal in 2002. Septoria tritici blotch continues to have high economic importance and widespread global impact on wheat production. Mycosphaerella graminicola (Fuckel) J. Schröt. In Cohn (anamorph: Septoria tritici Roberge in Desmaz.). Kingdom Fungi, Phylum Ascomycota, Class Loculoascomycetes (filamentous ascomycetes), Order Dothideales, Genus Mycosphaerella, Species graminicola. Bread and durum wheat (Triticum aestivum L. and T. turgidum ssp. durum L.). Disease symptoms: Initially leaves develop a chlorotic flecking, which is followed by the development of necrotic lesions which contain brown-black pycnidia. Necrosis causes a reduction in photosynthetic capacity and therefore affects grain yield. Disease control: The disease is primarily controlled by a combination of resistant cultivars and fungicides. Rapid advances in disease control, especially in resistance breeding, are opening up new opportunities for the management of the disease. http://genome.jgi-psf.org/Mycgr3/Mycgr3.home.html.

Chitosan-induced ethylene-independent resistance does not reduce crop yield in bean

Some unexplored mechanisms of chitosan (CHT)-induced resistance have been investigated by using the pathosystem Tobacco necrosis virus (TNV) – Phaseolus vulgaris L. In particular, it was shown that CHT does not exert a direct antiviral activity and that stomatal uptake of the compound is determinant for the induction of plant defence mechanisms against TNV. By a chemical approach, using ethylene inhibitors or donors, it was also demonstrated that these defence mechanisms do not involve ethylene pathway. Conversely, the infection with TNV stimulated the production of this hormone in leaf tissues, associated with the development of necrotic lesions. Finally, in open field trials, CHT treatments did not incur fitness costs in bean plants, probably due to the activation of priming mechanisms as opposed to the direct resistance induction.

Application of Congo Red agar for detection ofStreptococcus dysgalactiaeisolated from diseased fish

Summary The strong clinical similarity between Lancefield group C Streptococcus dysgalactiae subsp. dysgalactiae (GCSD) and Lactococcus garvieae infections, characterized by development of necrotic lesions in the caudal peduncle of infected fish, has hindered differentiation of these two strains, making rapid and accurate diagnosis of diseased fish in fish farms difficult. GCSD from diseased fish were presumptively identified and isolated using Todd-Hewitt agar containing 30 μg ml−1 of Congo Red dye (TH-CR). TH-CR agar was also used to detect and presumptively identify the GCSD obtained from artificially or naturally infected fish. Orange GCSD colonies distinct from the L. garvieae colonies were observed on the TH-CR agar; thus, TH-CR agar can be used to detect and identify GCSD isolated from infected fish.

Alternaria Toxins—Potential Selective Agents in Biotechnology Approaches for Plant Improvement

ABSTRACTIncreasing sustainable agricultural production has always been related with cultivation of crops that are better able to tolerate biotic stresses (pests, diseases and weeds) and abiotic stresses (drought, salinity, and temperature stress). During last decades breeding programs apply different biotechnology approaches to improve the performance and productivity of crops, incl. increased resistance to plant diseases caused by toxin produced fungal pathogens. Fungi from genus Alternaria produce a group of mycotoxins as alternariols, altertoxins, tenuasonic acid etc. Host specific toxins of Alternaria spp. plant pathogens play an important role in pathogenesis and they could be applied as selective agents in in vitro selection at the cellular level for disease resistance. Toxin compounds' effect on development of necrotic lesions on tobacco and tomato leaves was observed. The correlation between plant susceptibility to the pathogen and sensitivity to the toxin substances is discussed in the light of possibilities for application in the crop breeding programmes.

Identification and characterization of potato protease inhibitors able to inhibit pathogenicity and growth of Botrytis cinerea

Different tissues of potato, tobacco, and bean plants were screened for anti-fungal protease inhibitor (PI) activity, also following fungal pathogen inoculation or mechanical wounding. A potato ( Solanum tuberosum var. Desireé) sprout protein extract showed a strong inhibitory activity against chymotrypsin and Botrytis cinerea fungal proteases, but also on spore germination, hyphal elongation, and development of necrotic lesions. An active mixture of different proteins was affinity column purified and sequenced. Two new anti-fungal genes, PKI1 and PPI3B2, coding, respectively, for a Kunitz-type inhibitor and a Proteinase Inhibitor 1 capable of reducing fungal lesion development, were cloned and partially characterized. Direct effect on leaf necrosis formation was found to be dependent on the anti-chymotrypsin activity of both selected inhibitors. The PKI1 transcript was found to accumulate in untreated sprout tissues, although homologues of this gene seemed to accumulate following Bemisia tabaci attack. In the case of PPI3B2, we provide preliminary evidence that a member of the Proteinase Inhibitor 1 family is active against not only herbivorous insects but also phytopathogenic fungi and foliar lesions caused by them.

A single amino acid insertion in the WRKY domain of the Arabidopsis TIR–NBS–LRR–WRKY‐type disease resistance protein SLH1 (sensitive to low humidity 1) causes activation of defense responses and hypersensitive cell death

In this study we characterized the sensitive to low humidity 1 (slh1) mutant of Arabidopsis ecotype No-0 which exhibits normal growth on agar plate medium but which on transfer to soil shows growth arrest and development of necrotic lesions. cDNA microarray hybridization and RNA gel blot analysis revealed that genes associated with activation of disease resistance were upregulated in the slh1 mutants in response to conditions of low humidity. Furthermore, the slh1 mutants accumulate callose, autofluorescent compounds and salicylic acid (SA). We demonstrate that SA is required for the slh1 phenotype but not PAD4 or NPR1. SLH1 was isolated by map-based cloning and it encodes a resistance (R)-like protein consisting of a domain with Toll and interleukin-1 receptor homology (TIR), a nucleotide-binding domain (NB), leucine-rich repeats (LRR) and a carboxy-terminal WRKY domain. SLH1 is identical to the R gene RRS1-R of the Arabidopsis ecotype Nd-1, a gene which confers resistance to the bacterial pathogen Ralstonia solanacearum GMI1000 and also functions as an R gene to this pathogen in No-0. We identified a 3 bp insertion mutation in slh1 that results in the addition of a single amino acid in the WRKY domain; thereby impairing its DNA-binding activity. Our data suggest that SLH1 disease resistance signaling may be negatively regulated by its WRKY domain in the R protein and that the constitutive defense activation conferred by the slh1 mutation is inhibited by conditions of high humidity.

Tulip virus X (TVX) associated with lemon balm (Melissa officinalis) variegation: first report of TVX in the USA

Lemon balm (Melissa officinalis) has been grown for centuries as an ornamental and for its medicinal properties. One of the most popular clones shows bright variegation symptoms. Mechanical inoculations onto Gomphrena globosa, using variegated leaf tissue as the inoculum source, resulted in development of necrotic lesions about 5 days postinoculation, which suggested a viral aetiology of the variegation. Negatively stained leaf dips, examined with an electron microscope, revealed particles ≈500 × 13–14 nm in size. Double-stranded RNA was extracted from a variegated clone of M. officinalis and cloned as described elsewhere (Tzanetakis et al., 2005). Sequence data (GenBank Accession Nos AY842508–AY842510) revealed that the plant was infected with Tulip virus X (TVX). Oligonucleotide primers TVX 1F (5′-GACAYTCTAACCCCTTCGC-3′), TVX 1R (5′-GCCCTCTGTGGAAGTATCT-3′) and TVX 2F (5′-GAACAAGCACACCTCCACCA-3′), TVX 2R (5′-AGTGTGGTTTTCCCGGC-3′) were developed for detection of the virus, and were used in RT-PCR to test for TVX in four variegated clones of M. officinalis from Oregon and Washington, as well as G. globosa indicators with symptoms. The above-mentioned plants all tested positive for TVX by RT-PCR, while noninoculated G. globosa or M. officinalis plants without symptoms were negative. The amplicons were sequenced, which confirmed their identity as TVX. This is the first report of TVX in association with variegation in lemon balm and the first report of the presence of the virus in the USA.