Uninfected Control Plants Research Articles

Adaptive microbiome responses to anthracnose in sorghum: Enhanced network complexity and disease resistance across plant niches

Anthracnose threatens sorghum productivity globally, with yield losses exceeding 50 % in some cases, making it crucial to understand plant-microbe-pathogen interactions to develop effective, sustainable control strategies. To probe deeper into the complexity of plant-microbe-pathogen relationships, this study used high-throughput sequencing to characterize microbial communities present in association with sorghum leaves and seeds harvested from both anthracnose infected and uninfected control plants. Our analysis shows that the variation in the diversity and composition of the microbiome due to anthracnose is indicative of an adaptive regulation by coexisting microbes with a plant for enhancing resistance against stresses. These changes were not only reflected by community composition, but also had remarkable effects on the topological properties of microbial ecological networks which further influence certain functions as well as key functional guilds related to plant growth. The presence of anthracnose significantly increased network complexity and resistance in the phyllosphere and leaf endosphere meta-microbiome, which may reflect an adaptive microbiome response to pathogen. The seed-associated microbiomes, however, had relatively no change in diversity and network properties indicating a stable background that may repel pathogen invasion. The study also suggests that anthracnose may facilitate its spread through the reorganization of microbial communities and the enrichment of specific beneficial microbes, aiding in plant defense against pathogens. This is achieved through an upregulation of siderophore production, antimicrobial compounds, phosphate solubilization and sulfur metabolism-specifically genes. Our work also uncovers the possibility for microbiome transfer through phyllosphere-and seed associated microbiomes, thereby indicating that pathogen spread between both above and below ground plant niches limit management options. To conclude, the present study reveals fresh insights into how plant-associated microbiomes respond to anthracnose stress and offers insight on their prospective in sustainable agriculture.

Effect of strawberry vein banding virus and strawberry mottle virus co-infection on the growth and development of strawberry

Strawberry mottle virus (SMoV) and strawberry vein banding virus (SVBV) cause diseases on strawberry plants, but the effect of coinfection of SMoV and SVBV on the growth, development, and defense system of strawberry (Fragaria × ananassa Duchesne) remains unknown. We investigated the effect of SMoV and SVBV co-infection on strawberry cultivar ‘Benihope’. The results showed that stem diameter, leaf size, leaf number, relative chlorophyll content, total chlorophyll content, photosynthetic parameters, and stomatal aperture of SMoV and SVBV co-infected strawberry (VIS) plants were in a weaker level than uninfected control plants, indicating that viruses inhibited the growth and photosynthesis of strawberry plants. Furthermore, the initiation of flowering and fruiting stages of VIS plants were delayed by about three weeks compared with the controls, and the fruiting period was shortened, demonstrating that the reproduction of VIS plants was inhibited. Fruit quality was damaged in VIS plants due to a significant increase in fruit firmness and titratable acidity and decrease in total soluble solid content than control fruits. More dead cells and H2O2 accumulated along the veins of VIS leaves, and the content of abscisic acid and catalase activity significantly increased, whereas anthocyanin content was lower than that of control plants. The results demonstrate that SVBV and SMoV coinfection inhibits the growth and development of ‘Benihope’ strawberry plants, and the plants respond to viruses by regulating stomatal aperture, the accumulation of ABA and antioxidants. To our knowledge, this study contributes information to understand how both viruses impair the strawberry growth and development for the first time.

Differential Response of Mycorrhizal Plants to Tomato bushy stunt virus and Tomato mosaic virus Infection.

Tomato bushy stunt virus (TBSV) and Tomato mosaic virus (ToMV) are important economic pathogens in tomato fields. Rhizoglomus irregulare is a species of arbuscular mycorrhizal (AM) fungus that provides nutrients to host plants. To understand the effect of R. irregulare on the infection by TBSV/ToMV in tomato plants, in a completely randomized design, five treatments, including uninfected control plants without AM fungi (C), uninfected control plants with AM fungi (M) TBSV/ToMV-infected plants without AM fungi (V), TBSV/ToMV-infected plants before mycorrhiza (VM) inoculation, and inoculated plants with mycorrhiza before TBSV/ToMV infection (MV), were studied. Factors including viral RNA accumulation and expression of Pathogenesis Related proteins (PR) coding genes including PR1, PR2, and PR3 in the young leaves were measured. For TBSV, a lower level of virus accumulation and a higher expression of PR genes in MV plants were observed compared to V and VM plants. In contrast, for ToMV, a higher level of virus accumulation and a lower expression of PR genes in MV plants were observed as compared to V and VM plants. These results indicated that mycorrhizal symbiosis reduces or increases the viral accumulation possibly via the regulation of PR genes in tomato plants.

First report of field bindweed (Convolvulus arvensis L.) penetrates the roots of pepper (Capsicum annuum L.) plants.

Bindweed (Convolvulus arvensis L) is a root competitor weed that represents a major constraint for several crops production all over the world. Bindweed has adopted a parasitic lifestyle on pepper plants (Capsicum annuum L) in Egypt. Parasitic plants develop a multicellular infectious organ called a haustorium. In March 2017, 42 samples were collected from Minia and Beni Suef governorates, Egypt and evaluated for their virulence against pepper plants. Pathogenicity test was carried out in Sids Research Station. These populations tested were able to attack the pepper roots (Balady variety) appearing polar and dwarf leaves compared to leaves of uninfected control plants. The histological studies of Convolvulus arvensis and pepper roots revealed that the vascular xylem of the bindweed primary and secondary haustoria eventually connected with phloem conducting elements of the host root. By using the light microscope, boundary features of the penetration site and connection of the secondary to the primary root (in healthy pepper plants) were compared histologically with those of the haustoria and the host root cells (in infected pepper plants). These findings are important in assessing the potential of the parasite as an agronomically significant pest in Egypt.

Label-free quantitative proteomic analysis of pre-flowering PMeV-infected Carica papaya L.

Papaya meleira virus (PMeV) infects papaya (Carica papaya L.) and leads to Papaya Sticky Disease (PSD) or “Meleira”, characterized by a spontaneous exudation of latex from fruits and leaves only in the post-flowering developmental stage. The latex oxidizes in contact with air and accumulates as a sticky substance on the plant organs, impairing papaya fruit's marketing and exportation. To understand pre-flowering C. papaya resistance to PMeV, an LC-MS/MS-based label-free proteomics approach was used to assess the differential proteome of PMeV-infected pre-flowering C. papaya vs. uninfected (control) plants. In this study, 1333 proteins were identified, of which 111 proteins showed a significant abundance change (57 increased and 54 decreased) and supports the hypothesis of increased photosynthesis and reduction of 26S-proteassoma activity and cell-wall remodeling. All of these results suggest that increased photosynthetic activity has a positive effect on the induction of plant immunity, whereas the reduction of caspase-like activity and the observed changes in the cell-wall associated proteins impairs the full activation of defense response based on hypersensitive response and viral movement obstruction in pre-flowering C. papaya plants. Biological significanceThe papaya (Carica papaya L.) fruit's production is severely limited by the occurrence of Papaya meleira virus (PMeV) infection, which causes Papaya Sticky Disease (PSD). Despite the efforts to understand key features involved with the plant×virus interaction, PSD management is still largely based on the observation of the first disease symptoms in the field, followed by the elimination of the diseased plants. However, C. papaya develops PSD only after flowering, i.e. about six-months after planting, and the virus inoculum sources are kept in field. The development of PMeV resistant genotypes is impaired by the limited knowledge about C. papaya resistance against viruses. The occurrence of a resistance/tolerance mechanism to PSD symptoms development prior to C. papaya flowering is considered in this study. Thus, field-grown and PMeV-infected C. papaya leaf samples were analyzed using proteomics, which revealed the modulation of photosynthesis-, 26S proteasome- and cell-wall remodeling-associated proteins. The data implicate a role for those systems in C. papaya resistance to viruses and support the idea of a partial resistance induction in the plants at pre-flowering stage. The specific proteins presented in the manuscript represent a starting point to the selection of key genes to be used in C. papaya improvement to PMeV infection resistance. The presented data also contribute to the understanding of virus-induced disease symptoms development in plants, of interest to the plant-virus interaction field.

Disruption of Ethylene Responses by Turnip mosaic virus Mediates Suppression of Plant Defense against the Green Peach Aphid Vector

Plants employ diverse responses mediated by phytohormones to defend themselves against pathogens and herbivores. Adapted pathogens and herbivores often manipulate these responses to their benefit. Previously, we demonstrated that Turnip mosaic virus (TuMV) infection suppresses callose deposition, an important plant defense induced in response to feeding by its aphid vector, the green peach aphid (Myzus persicae), and increases aphid fecundity compared with uninfected control plants. Further, we determined that production of a single TuMV protein, Nuclear Inclusion a-Protease (NIa-Pro) domain, was responsible for changes in host plant physiology and increased green peach aphid reproduction. To characterize the underlying molecular mechanisms of this phenomenon, we examined the role of three phytohormone signaling pathways, jasmonic acid, salicylic acid, and ethylene (ET), in TuMV-infected Arabidopsis (Arabidopsis thaliana), with or without aphid herbivory. Experiments with Arabidopsis mutants ethylene insensitive2 and ethylene response1, and chemical inhibitors of ET synthesis and perception (aminoethoxyvinyl-glycine and 1-methylcyclopropene, respectively), show that the ET signaling pathway is required for TuMV-mediated suppression of Arabidopsis resistance to the green peach aphid. Additionally, transgenic expression of NIa-Pro in Arabidopsis alters ET responses and suppresses aphid-induced callose formation in an ET-dependent manner. Thus, disruption of ET responses in plants is an additional function of NIa-Pro, a highly conserved potyvirus protein. Virus-induced changes in ET responses may mediate vector-plant interactions more broadly and thus represent a conserved mechanism for increasing transmission by insect vectors across generations.

Ethyl methane sulfonate induces disease resistance in Begonia × hiemalis Fotsch.

To evaluate the disease resistance of Begonia × hiemalis Fotsch. ‘Schwabenland Red’ mutants induced by ethyl methane sulphonate (EMS), we investigated the incidence and severity of stem rot caused by Rhizoctonia solani Kuhn, and their morphological changes. In this study, we obtained three mutants with highly resistant disease after two rounds of screening when the inoculated amount was 3 sheets/plant. The severity scale was 0 and there were no obvious symptoms of stem rot in highly resistant Begonia × hiemalis. However, most of EMS mutagenized plants were susceptible to stem rot caused by R. solani under this inoculated amount. In addition, these resistant mutants had different phenotypes as compared with the uninfected control plants. The color of leaves of the R. solani-resistent mutants was different with the control plants and their leaves were bigger than that of the control. The shape and number of their flowers were obviously different, as compared to the control plants.

Phenotypic screening of pecan seedling rootstocks in search of nematode resistance

Open-pollinated seedstocks for pecan vary in phenology and composition predictably based on their provenance of origin in ways that impact performance. Open-pollinated rootstocks of pecan (Carya illinoinensis), water hickory (Carya aquatica), and their hybrids (Carya × lecontei) were screened for nematode resistance in outdoor above-ground box-plots. Seedstocks were selected to represent the broad geographic range of species diversity. Seedlings were inoculated with eggs of Meloidogyne partityla, the primary nematode pest of Carya, and were harvested after 1 year. All seedlings, except one, manifested nematode damage at moderate to high levels. Evidence of galling was greatest in seedlings from the southern provenance (Mexico), which rated comparably with seedlings from ‘Elliott’. No sources of resistance to Meloidogyne partityla were observed. The box structure allowed harvest of complete root systems and evaluation of plant composition in greater detail than previously observed. Seedlings from the southern provenance were generally distinguishable from other provenances in timing of seasonal growth, stem diameter and seedling height, which is consistent with previous observations. Root and stem dry weights were greatest in seedlings from the southern provenance, as expected based on size measurements. Percent root water varied significantly as a function of seedstock origin, and was negatively correlated with leaf weight. Leaf weights were positively correlated with dates of growth initiation. Uninfected control plants were not observed in this screening effort, and their absence limits the interpretation of patterns. Implications of these observations, as evidence of regional adaptation, merit further exploration by research.

Analyses of the xylem sap proteomes identified candidate Fusarium virguliforme proteinacious toxins.

BackgroundSudden death syndrome (SDS) caused by the ascomycete fungus, Fusarium virguliforme, exhibits root necrosis and leaf scorch or foliar SDS. The pathogen has never been identified from the above ground diseased foliar tissues. Foliar SDS is believed to be caused by host selective toxins, including FvTox1, secreted by the fungus. This study investigated if the xylem sap of F. virguliforme-infected soybean plants contains secreted F. virguliforme-proteins, some of which could cause foliar SDS development.ResultsXylem sap samples were collected from five biological replications of F. virguliforme-infected and uninfected soybean plants under controlled conditions. We identified five F. virguliforme proteins from the xylem sap of the F. virguliforme-infected soybean plants by conducting LC-ESI-MS/MS analysis. These five proteins were also present in the excreted proteome of the pathogen in culture filtrates. One of these proteins showed high sequence identity to cerato-platanin, a phytotoxin produced by Ceratocystis fimbriata f. sp. platani to cause canker stain disease in the plane tree. Of over 500 soybean proteins identified in this study, 112 were present in at least 80% of the sap samples collected from F. virguliforme-infected and -uninfected control plants. We have identified four soybean defense proteins from the xylem sap of F. virguliforme-infected soybean plants. The data have been deposited to the ProteomeXchange with identifier PXD000873.ConclusionThis study confirms that a few F. virguliforme proteins travel through the xylem, some of which could be involved in foliar SDS development. We have identified five candidate proteinaceous toxins, one of which showed high similarity to a previously characterized phytotoxin. We have also shown the presence of four soybean defense proteins in the xylem sap of F. virguliforme-infected soybean plants. This study laid the foundation for studying the molecular basis of foliar SDS development in soybean and possible defense mechanisms that may be involved in conferring immunity against F. virguliforme and other soybean pathogens.

Phytochemicals and antioxidative enzymes defence mechanism on occurrence of yellow vein mosaic disease of pumpkin (Cucurbita moschata).

Pumpkin (Cucurbita moschata) samples showing yellow vein mosaic disease in Varanasi region were identified with begomovirus infection using PCR amplification. A sequencing analysis of the full genome revealed that it is a strain of Tomato leaf curl Palampur virus (GenBank ID. FJ931537). Phytochemical composition and antioxidative enzyme levels were compared in infected and healthy plants. The study revealed that the amount of total protein declined in the infected leaves but elevated up to 135 % in the fruits of infected plants, whereas vitamin C and antioxidants declined in infected leaves as well as fruits. There was substantial increase in total phenol content in leaves (72 %) and fruits (300 %) of infected plants. In infected samples, substantial increase in activities of superoxide dismutases (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and catalase (CAT) was observed as compared to the uninfected control plants. The native PAGE showed alterations in the intensities of isozyme bands in the infected plants. The APX, GPX, CAT, SOD and glutamate dehydrogenase (GDH) bands were intense in the infected plants, whereas the GR isozyme showed reduced intensity in diseased plants.

Mitochondrial plasmids of sugar beet amplified via rolling circle method detected during curtovirus screening

Crops of sugar beet have been considerably impaired by infection with Beet curly top virus (BCTV) during the past decades. Quick and reliable diagnostic techniques are therefore desirable to detect this circular single-stranded DNA-containing geminivirus. Techniques combining either tissue printing or blot hybridization, or rolling circle amplification (RCA) and restriction fragment length polymorphism (RFLP) were compared. Although they easily detected BCTV with certainty, both exhibited apparent false positive results which have been scrutinized in closer detail. Uninfected control plants revealed unspecific signals due to probe attachment on tissue blots, and dominant fragment patterns upon RCA/RFLP which did not hybridize with BCTV-specific probes. Cloning and sequencing of these DNA fragments showed that they were amplified from mitochondrial plasmids. Examination of their genome structure revealed no relationship with geminiviruses or their satellites.

Scanning Electron Microscopy Reveals Different Response Pattern of Four Vitis Genotypes to Xylella fastidiosa Infection.

The xylem-limited bacterium Xylella fastidiosa causes Pierce's disease (PD), whose disease symptoms are primarily the result of xylem vessel blockage in susceptible grapevines. Stem internode and petiole tissues from infected and uninfected control plants of four grape genotypes (Vitis vinifera, V. rufotomentosa, V. smalliana, and V. arizonica/candicans) differing in PD susceptibility were examined using scanning electron microscopy (SEM). Tyloses, fibrillar networks, and gum plugs were observed in lumens of tracheary elements in petioles and internodes of both water-inoculated control plants and X. fastidiosa-inoculated plants of all genotypes. Bacteria were not observed in control plants. In both petiole and internode tissues, the greatest number of occluded xylem vessels were observed in V. vinifera and the smallest number in V. arizonica/candicans. The number of xylem vessels infested with X. fastidiosa was greatest in V. vinifera and did not differ among the other three genotypes. Systemic infection was found in all genotypes. The frequency with which X. fastidiosa infested vessels were observed using SEM corresponded well with bacterial levels estimated by enzyme-linked immunosorbent assay. Among infected plants, tylose formation in internodes was lowest in V. arizonica/candicans and did not differ among the other three genotypes. Infection with X. fastidiosa strongly induced tylose formation in V. vinifera and V. smalliana but not in V. arizonica/candicans. Analysis across tissues and genotypes indicated an induction of fibrillar networks and gum occlusions in response to X. fastidiosa infection, whereas treatment comparisons within genotypes were not significant except for V. vinifera petioles. Limiting the spread of X. fastidiosa infection by xylem conduit occlusions does not appear to be the mechanism conferring PD resistance or tolerance to V. arizonica/candicans, V. smalliana, or V. rufotomentosa. In contrast, the strong induction of tyloses may be detrimental rather than beneficial for V. vinifera survival after X. fastidiosa infection.

Isolation of Diaporthe phaseolorum var. caulivora from Abutilon theophrasti in Croatia

During 2000–03 a weed survey was carried out in fields of soybean, sunflower and sugar beet in Croatia. Abutilon theophrasti (velvetleaf), found in soybean fields, showed symptoms of black lesions, several centimetres in length, on the stems and side branches. A fungus was isolated from stem lesions on potato dextrose agar (PDA) and incubated at 25°C, with a 12 h dark/light regime. On PDA the fungus quickly formed a dense white mycelium, which later became fluffy. The mycelium was yellow-white at first, turning yellow-brown later. After 10 days black stromata were produced. These were irregularly shaped and measured 2–5 mm in diameter. Numerous black perithecia were formed within the stromata. Ten perithecia, 50 asci and 100 ascospores were measured. The sizes of perithecia were 200–360 × 295–450 µm and these had a protruding neck which was highly variable in length (300–1200 µm). The sizes of the asci were 25·1–42·0 × 4·2–8·3 µm. The ascospores were two-celled, hyaline, elongate to ellipsoidal, biguttulate, constricted at the septum, and measured 8·0–13·1 × 2·5–4·4 µm. Based on the morphological characteristics, the pathogen was identified as Diaporthe phaseolorum var. caulivora. Velvetleaf plants were infected in the field by applying a mycelium plug to the plant stem. The inoculation point was covered with a piece of wet cotton wool and aluminium foil to retain moisture. Lesions 5–19 mm long were observed 10 days postinfection. Diaporthe phaseolorum var. caulivora was reisolated out of the lesions of inoculated plants, fulfilling Koch's postulates. A pathogenicity test was also done on soybean seedlings. Mycelium plugs were put on the hypocotyl, which was previously wounded with a sterile scalpel. After 10 days the pathogen was reisolated from the stem lesion of inoculated plants. Uninfected control plants did not exhibit any symptoms. To confirm pathogen identification, molecular analysis was performed. DNA was extracted from a monoconidial culture and amplified with ITS4 and ITS5 universal primers (White et al., 1990). The ITS was sequenced and submitted to GenBank (Accession No. AY857867). Comparison of the ITS sequences available on the database revealed that it was almost identical, with only two deletions in positions 24 and 542, to D. phaseolorum var. caulivora (AJ312360). It is known that weeds can act as a reservoir for infection by D. phaseolorum (Black et al., 1996). Abutilon theophrasti has been recorded before as a host for D. phaseolorum var. sojae (Hepperly et al., 1980) and Phomopsis longicolla (Li et al., 2001). Each year in Croatia the presence of fungi in the Diaporthe/Phomopsis complex can be observed on soybean, but these fungi do not appear to cause great economic damage under local conditions. This is, however, the first report of A. theophrasti as a host for D. phaseolorum var. caulivora in Croatia.

Barley Yellow Dwarf Disease in Natural Populations of Dominant Tallgrass Prairie Species in Kansas.

The grasses Sorghastrum nutans (Indian grass), Schizachyrium scoparium (little bluestem), Panicum virgatum (switchgrass), and Andropogon gerardii (big bluestem) are four of the most common plant species present in a tallgrass prairie (1). Infection with barley yellow dwarf virus (BYDV, family Luteoviridae) is of interest in these species because of the potential effects of the virus on tallgrass prairie plant communities and the potential for tallgrass prairie to function as a reservoir of the virus for infection in wheat or barley fields. In a previous inoculation experiment, an unidentified strain of BYDV transmitted by the aphid species Rhopalosiphum padi was reported to infect S. scoparium but none of the other three grass species (2). We sampled for the presence of five virus strains in at least 50 blooming plants of each grass species in a natural tallgrass prairie stand in August 2000. Samples were collected in watersheds that were designated 1B, 1D, K1A, 20B, and 20C at Konza Prairie Biological Station in the Flint Hills near Manhattan, KS. To detect the virus, we used enzyme-linked immunosorbent assay (ELISA) with antibodies purchased from Agdia (Elkhart, IN). For the PAV, MAV, RMV, and SGV strains, we used double-antibody sandwich ELISA with alkaline phosphatase label. For Cereal yellow dwarf virus (RPV), we used compound direct ELISA with alkaline phosphatase label. The scoring of ELISA results was based on comparison with infected and uninfected control plants of the same species. Symptoms of infection in the field were difficult to interpret visually, since plants in this natural environment often showed multiple symptoms of stress. None of the five strains were detected in 51 individuals of S. nutans. For 50 individuals of S. scoparium, the incidence of infection by the different strains was 4% for MAV, 0% for PAV, 2% for RMV, 0% for RPV, and 58% for SGV. For 51 individuals of P. virgatum, the incidence of infection was 31% for MAV, 0% for PAV, 0% for RMV, 0% for RPV, and 4% for SGV. For 64 individuals of A. gerardii, the incidence of infection was 59% for MAV, 0% for PAV, 0% for RMV, 0% for RPV, and 3% for SGV. The impact of BYDV on these tallgrass prairie species remains to be determined. The PAV strain is the most commonly reported strain in wheat in Kansas but was not recovered from these grass species.

Effect of geminivirus infection and Bemisia infestation on accumulation of pathogenesis-related proteins in tomato.

The whitefly, Bemisia tabaci biotype B, has been shown to cause pathogenesis-related (PR) proteins to accumulate in plants as a result of direct feeding, but their specific role in plant defensive systems is unclear. Our objective was to compare accumulation of tomato PR proteins (beta-1,3-glucanase, chitinase, peroxidase, P2 and P4) in response to whitefly, with or without tomato mottle virus (ToMoV) infection. Tomato PR protein response was measured over time in plants divided into three treatments: uninfected controls (with or without whiteflies) and plants infested with viruliferous (ToMoV) whiteflies. Five- to six-leaf plants were infested with approximately 5 adult whitefly per leaf. Plants were sampled prior to whitefly infestation and at 14, 28, 42, and 56 days. By 56 days, plants infested with viruliferous whiteflies had significantly more eggs (2.5-fold) and nymphs (4.5-fold) than plants with nonviruliferous whiteflies. A significant increase in the enzymatic activity of all measured PR proteins, as compared to control plants, was only seen in viruliferous whitefly-infested plants. No significant difference was observed in enzyme activities between the uninfected control plants either with or without whiteflies. The greatest differences for all PR proteins assayed were observed 42 days after treatment initiation. Protein blot analyses showed that the differences in PR protein activities among the treatments were due to changes in specific enzyme levels within the plant and were associated with concomitant increases in levels of P2 and P4 PR proteins. Under our experimental conditions, it is clear that PR protein response is much more intense when it is attacked by whiteflies carrying ToMoV than by whitefly alone.

Do Striga hermonthica‐induced changes in soil matric potential cause the reduction in stomatal conductance and growth of infected maize plants?

Maize (Zea mays L.) plants parasitized by the root hemi‐parasitic angiosperm, Striga hermonthica (Del.) Benth., consistently display a range of symptoms similar to those found in droughted plants. The mechanisms by which these changes occur are largely unknown. However, S. hermonthica has unusually high rates of transpiration, and stomata which are relatively insensitive to water deficit. Consequently, it has often been suggested that the parasite might cause a severe depletion of the available water in the host's rooting zone. To determine whether the lower stomatal conductance and retarded growth of infected plants could be a result of parasite‐induced water deficit, we have monitored the matric potential of the growth medium, water use, growth and stomatal conductance of infected vs. uninfected maize plants.Host plant height and stomatal conductance of parasitized plants were significantly lower than those of control plants from 31 or 37 d after planting (d.a.p.) respectively. However, there was no indication of an increase in the rate of water depletion in the rooting zone of infected plants until approx. 63 d into the parasitic association. In fact, from 39 until 59 d.a.p. infected plants used less water than uninfected control plants, probably the result of the plants having fewer expanded leaves during part of this period, combined with the lower stomatal conductance exhibited by the infected plants from day 37 onwards. Leaf RWC of infected plants was unchanged in comparison with that of uninfected plants, therefore the change in stomatal conductance was not a response to dehydration of the leaf tissue. Our results indicate that parasitism by S. hermonthica does not cause an increase in water uptake/use in the host until well after most of the symptoms of infection have become fully established. It is highly unlikely, therefore, that the observed effects on the host are primarily due to soil water deficit.

Transport of Salicylic Acid in Tobacco Necrosis Virus-Infected Cucumber Plants.

The transport of salicylic acid (SA) was studied in cucumber (Cucumis sativus L.) using 14C-labeled benzoic acid that was injected in the cotyledons at the time of inoculation. Primary inoculation with tobacco necrosis virus (TNV) on the cotyledons led to an induction of systemic resistance of the first primary leaf above the cotyledon against Colletotrichum lagenarium as early as 3 d after inoculation. [14C]SA was detected in the phloem or in the first leaf 2 d after TNV inoculation, whereas [14C]benzoic acid was not detected in the phloem during the first 3 d after TNV inoculation of the cotyledons, indicating phloem transport of [14C]SA from cotyledon. In leaf 1, the specific activity of [14C]SA decreased between 1.7 and 8.6 times compared with the cotyledons, indicating that, in addition to transport, leaf 1 also produced more SA. The amount of SA transported after TNV infection of the cotyledon was 9 to 160 times higher than in uninfected control plants. Thus, SA can be transported to leaf 1 before the development of systemic acquired resistance, and SA accumulation in leaf 1 results both from transport from the cotyledon and from synthesis in leaf 1.

New antifungal proteins from sugar beet (Beta vulgaris L.) showing homology to non-specific lipid transfer proteins

Two novel, nearly identical antifungal proteins, IWF1 and IWF2, were isolated from the intercellular washing fluid (IWF) of sugar beet leaves. The proteins were purified to homogeneity and their amino acid sequences were determined. They are basic, monomeric proteins of 91 amino acid residues, 89 of which are identical. Both proteins show strong in vitro antifungal activity against Cercospora beticola, the casual agent of leaf spot disease in sugar beet. Based on primary sequence homology, including the presence of 8 conserved cysteine residues, IWF1 and IWF2 are related to the family of plant non-specific lipid transfer proteins (nsLTPs). Antibodies were raised against IWF2 after conjugation to diphtheria toxoid. The amino acid sequence data was used to generate a polymerase chain reaction (PCR) clone, employed for the isolation of a cDNA clone encoding a closely related isoform IWFA, which differs from IWF1 by two amino acid substitutions only. The induction and subcellular localization of these proteins were studied by western and northern blotting analyses after treatment with 2,6-dichloroisonicotinic acid (INA), a compound capable of inducing resistance against C. beticola, and after fungal infection. The following observations were made: (1) the proteins were present in leaves of non-INA-treated and uninfected control plants, (2) they were only slightly induced by INA treatment and during infection with C. beticola, and (3) they were present both intra- and extracellularly. However, their strong antifungal potentials together with immunohistological investigations, the proteins accumulating in contact with the fungus and in autolysing cells, suggested a role of these proteins in plant defence. Finally, immunohistology revealed a remarkable expression pattern of the IWF1 and IWF2 proteins, or serologically related proteins, in sugar beet styles, in that single or a few scattered papillae and a few cells in the lower transmitting tissue strongly and specifically reacted with the antibody.

Reproductive fitness and pathogenicity of selected Radopholus populations on two banana cultivars

Ten populations of Radopholus similis from various locations and one population of Radopholus sp. from Indonesia were tested for their reproductive fitness and specific pathogenicity on Musa AAA, Cavendish cv. Poyo under controlled experimental conditions in a constant environment chamber. In addition, five of these populations were tested on Musa AAA, Ibota cv. Yangambi. Reproductive fitness of the populations tested on the two cultivars, measured as the ratio of the final number of nematodes per root system (Pf) to the number of nematodes inoculated (Pi), differed significantly. Greatest fitness was observed among R. similis populations collected from banana in different African countries (Cameroon, Uganda and the Ivory Coast) and one population from arecanut in Sri Lanka. In contrast, a population from tea in Sri Lanka and the population of Radopholus sp. from turmeric in Indonesia were the least fit. Specific pathogenicity was estimated at 8 weeks and 12 weeks after inoculation using three plant growth parameters: fresh root weight, fresh shoot weight and plant height, compared to uninfected control plants. Reduction in plant root weight was the best indicator of pathogenicity. While the R. similis populations from Uganda and the Ivory Coast were highly pathogenic, other populations with great reproductive fitness (i.e. isolates from Cameroon and Sri Lanka) did not significantly reduce root weight. In cv. Poyo, no linear correlation was found between final numbers of nematodes per gram of root (Pf) and the decrease of root weight.

The parasitic angiospermStriga hermonthicacan reduce photosynthesis of its sorghum and maize hosts in the field

Two cultivars of sorghum (CK60 and Ochuti) and one cultivar of maize (H511) were grown in field plots in western Kenya in the presence or absence of the parasitic angiosperm Striga hermonthica, with or without a single addition of nitrogen fertilizer (150 kg N ha -1 ) using a factorial design. A progressive decline in rates of photosynthesis of Striga-infected plants were observed for the sorghum cultivar CK60 from 30 d after planting (DAP) and for maize from 40 DAP, until measurements ended 63 DAP. At this time photosynthetic rates were 46% and 31% lower in the Striga-infected sorghum and maize cultivars, respectively, compared to uninfected control plants. No decline in photosynthesis was observed in the second sorghum cultivar studied, Ochuti, a local land race reported to show some tolerance to the parasite. The trends in photosynthesis reflected stunting of the cereals, as determined by the height of the youngest emerged ligule, however, only the grain yield of the sorghum cultivar CK60 was significantly reduced by the presence of the parasite. The nitrogen application influenced neither the growth nor the photosynthetic parameters measured, and possible explanations for the absence of responses are discussed. It is concluded that S. hermonthica can reduce photosynthetic rates of field-grown sorghum and maize, and suggest that an ability to maintain high rates of photosynthesis whilst infected may be an important correlate of tolerance to the parasite.