The role of phytohormones in relation to bakanae disease development and symptoms expression

Fusarium proliferatum, the causal agent of bakanae disease was found to be up-regulated gibberellic acids (GAs) in bakanae diseased rice plants and thereby contributing to the different types of symptoms observed associated with the disease. The up- and down regulation of GAs, IAA and ABA were studied in susceptible (MR 211) and resistant (BR3) varieties artificially inoculated with F. proliferatum at different times after inoculation. The increase in GAs including fungiproduced GA3 and IAA were higher in inoculated MR 211 rice plants (GAs = 26%, IAA = 40.39%) as compared to resistant BR3 (GAs = 19%, IAA = 4.27%), 7 days after inoculation (disease score 1, S-1) in whole plant sample analysis. The increase were higher in both roots and leaves but not in the stem, thus the main symptom observed as stunting of the susceptible plants at disease score 1. However, 14 days after inoculation (disease score 3, S-3), both phytohormones were observed to increase in whole plant sample analysis as well as in all plant parts sampled. This increasing effect was found associated with elongation of internodes and chlorosis of leaves in susceptible plants of MR 211. As the symptoms progressed to disease score 5 (S-5, after 21 days of inoculation) susceptible plants of MR 211 were started to collapse and lodged due to over elongation, and finally dead followed by a decrease in GAs and IAA but an increase in ABA. In resistant variety BR3, marginal up regulation of GAs and IAA were observed only at 21 days after inoculation in stems with no typical symptom of bakanae disease.     

Metabolic profiling in the roots of coffee plants exposed to the coffee root-knot nematode, Meloidogyne exigua

To understand the effect of nematode Meloidogyne exigua infestation on coffee plants, resistant and susceptible coffee seedlings were inoculated with second-stage juveniles of M. exigua, and root metabolites were studied for four time intervals at 0, 24, 48 and 96 h. During this important period for parasite establishment, the concentrations of phenols, carbohydrates, amino acids and alkaloids in the roots were measured, and hydrogen nuclear magnetic resonance spectra of the root extracts were used to identify and quantify other metabolites. One of the most striking changes was the concentration of fumaric acid on resistant plants, which varied from 59 μg g(of root)^?1 to 138 μg g(of root)^?1 during the first 24 h of the nematode inoculation. The same level of variation was observed much later (96 h) in susceptible plants. Similarly, formic and quinic acid concentrations increased more rapidly in the resistant plants compared to the susceptibles. Sucrose concentrations increased to 370 % in the first 48 h in the resistant plants but showed no significant variation in the susceptible plant. Besides, the concentration of alkaloids was much higher at 24 and 48 h in the susceptibles compared to the resistant plants. These results suggest that the higher production of sucrose as well as formic, fumaric and quinic acids, and the lower production of alkaloids by the resistant cultivar in the first 48 h after the nematode inoculation are associated with the resistance of coffee plants to M. exigua.     

Evaluation of resistance of Spanish olive cultivars to Verticillium dahliae in inoculations conducted in greenhouse

The resistance of 28 Spanish olive cultivars to Verticillium dahliae was evaluated in an experiment conducted under greenhouse conditions, by impregnating plant roots with a semisolid fluid mass of a mixture of culture medium and the conidia and mycelium of the fungus. Five-month-old olive plants were inoculated with a cotton defoliating isolate of V. dahliae. ‘Frantoio’ and ‘Picual’ were used as resistant and susceptible reference cultivars, respectively. Cultivars were assessed on the basis of final values of the area under the disease progress curve, mean severity of symptoms, and mortality at 26 weeks following inoculation. Verticillium wilt disease developed more slowly and reached lower values of these parameters than those normally recorded in previous studies conducted in growth chambers, using root-dip inoculation in a conidial suspension of the pathogen. However, most of the evaluated cultivars exhibited susceptible or moderately susceptible reactions to the infections caused by V. dahliae. In particular, a group of eight cultivars, from the same group as ‘Picual’, such as ‘Manzanilla de Abla’, ‘Manzanilla del Centro’ and ‘Negrillo de Iznalloz’, were significantly more susceptible than ‘Frantoio’. Conversely, ‘Escarabajillo’, ‘Menya’ and ‘Sevillana de Abla’ exhibited a high level of resistance to the disease, no dead plants, and vegetative recovery. Field experiments are currently being carried out to confirm the level of resistance assigned to these last genotypes. If confirmed, these genotypes will act as potential resistant genitors for inclusion in current olive breeding programs or for use as resistant rootstocks.     

Catalog of Micro-Tom tomato responses to common fungal, bacterial, and viral pathogens

Lycopersicon esculentum cultivar Micro-Tom is a miniature tomato with many advantages for studies of the molecular biology and physiology of plants. To evaluate the suitability of Micro-Tom as a host plant for the study of pathogenesis, Micro-Tom plants were inoculated with 16 well-known fungal, bacterial, and viral pathogens of tomato. Athelia rolfsii, Botryotinia fuckeliana, Oidium sp., Phytophthora infestans, and Sclerotinia sclerotiorum caused typical symptoms and sporulated abundantly on Micro-Tom. Micro-Tom was resistant to Alternaria alternata, Corynespora cassiicola, and Fusarium oxysporum. When Micro-Tom was inoculated with 17 isolates of Ralstonia solanacearum, many isolates induced wilt symptoms. Agrobacterium tumefaciens also was pathogenic, causing crown galls on stem tissue after needle prick inoculation. In Micro-Tom sprayed with Pseudomonas syringae pv. tomato, P. s. pv. tabaci, or P. s. pv. glycinea, bacterial populations did not increase, and yellow lesions appeared only on leaves sprayed with P. s. pv. tomato. Tomato mosaic virus, Tomato aspermy virus, and Cucumber mosaic virus systemically infected Micro-Tom, which developed symptoms characteristic of other cultivars of tomato after infection with the respective virus. These results indicated that Micro-Tom was generally susceptible to most of the important tomato pathogens and developed typical symptoms, whereas certain pathogens were restricted by either hypersensitive resistance or nonhost resistance on Micro-Tom. Therefore, an assortment of Micro-Tom–pathogen systems should provide excellent models for studying the mechanism of susceptible and resistant interactions between plants and pathogens.     

Reduced paraquat translocation in paraquat resistant Arctotheca calendula (L.) Levyns is a consequence of the primary resistance mechanism, not the cause

Resistance to paraquat has been studied in detail in many weed species for more than a decade, with the precise mechanism of resistance still unclear. Several studies have indicated that reduced movement of the herbicide to the site of action in the chloroplast is at least partly responsible for endowing resistance. Although paraquat translocation studies have been performed in the past it has been rare for these studies to have been conducted on whole plants in the light, despite early observations which clearly showed that paraquat translocation is minimal unless treated plants are exposed to light. This study has addressed this issue in Arctotheca calendula by tracing the movement of ¹⁴C-paraquat in resistant and susceptible plants in both the dark and light. Differences in paraquat translocation between the resistant and susceptible biotypes of A. calendula were only observed when treated plants were exposed to light. It was observed that paraquat translocation was significantly reduced in the resistant compared to the susceptible biotype when plants were exposed to light but not in the dark. It is postulated that paraquat translocation is dependent on light mediated damage. As paraquat-induced damage is less severe in paraquat resistant plants, overall paraquat translocation is reduced in the resistant biotype.     

Structural changes of homogalacturonan,rhamnogalacturonan I and arabinogalactan protein in xylem cell walls of tomato genotypes in reaction to Ralstonia solanacearum

Healthy and Ralstonia solanacearum-inoculated tomato genotypes susceptible or resistant to bacterial wilt including recombinant inbred lines (RILs) deriving from a cross between the resistant genotype Hawaii7996 and the susceptible Wva700 were compared for symptom and bacterial population development, and for the composition and structure of pectic polysaccharides and arabinogalactan proteins (AGPs) of xylem cell walls by immunological staining of tissue prints. Constitutive differences were observed between resistant and susceptible RILs, with a higher degree of methyl-esterification of homogalacturonan (HG) detected by antibody JIM7 in the resistant plants. After inoculation, decreased methyl-esterification of HG indicated by stronger labeling with antibody JIM5 was observed in all susceptible genotypes and in five of eleven resistant genotypes, with a clear increase in the non-blockwise de-esterification pattern of HG (LM7) only in the susceptible lines, indicating the mode of action of the pectinmethylesterase of R. solanacearum. In the susceptible lines infection generally leads to increased branching of rhamnogalacturonan I indicated by the detection of arabinan (LM6) and galactan (LM5) side chains, and of arabinogalactan protein (LM2), while only few of the resistant genotypes reacted with changes in these epitopes. All the resistant, symptomless genotypes contained relatively high pathogen populations in stems. A clear relation between cell wall composition and degree of latent infection of resistant genotypes was not found.     

Selection pressure effects on the proportion and movement of resistance alleles introgressed from wheat in Aegilops cylindrica

In winter wheat in the USA, Aegilops cylindrica is one of the most troublesome weeds, while the pathogen Oculimacula spp. causes foot rot disease. Imazamox‐resistant (IR) and foot rot‐resistant (FR) wheat cultivars represent effective tools to control the weed and prevent disease infection. However, resistance allele (RA) movement between wheat and A. cylindrica facilitates the introgression process under herbicide and disease selection pressure. Field experiments using IR and FR A. cylindrica plants intermixed with susceptible plants were conducted to measure the proportion of the RAs in the progeny and RA movement with and without herbicide and disease selection. Yield components of A. cylindrica plants were determined across treatments. The herbicide RA proportion in the progeny was greater when plants were treated with the herbicide imazamox in both years. Disease RA proportion was greater with disease occurrence only in one year. Herbicide RA movement from resistant to susceptible plants was greater with herbicide than without it only in one year. Plants carrying the RAs had greater total spikelet weight and 1000‐spikelet weight compared with susceptible plants with or without selection. However, susceptible plants produced more spikelets than the resistant ones in the absence of selection. If plants within an A. cylindrica population acquire the herbicide RA, its proportion will increase each generation under selection. These findings contribute to the understanding of crop allele introgression into related species and the evolution of increased weediness, with weed management implications.     

Identification of allele specific AFLP markers linked with bacterial wilt [Ralstonia solanacearum (Smith) Yabuuchi et al.] resistance in hot peppers (Capsicum annuum L.)

Even though the bacterial wilt is identified as the most destructive disease in hot peppers world-wide, robust molecular markers that facilitate marker assisted selection are absent till date. Kerala Agricultural University (India) has released two hot peppers named Ujwala and Anugraha which show high level field resistance to this pathogen. The variety Anugraha was developed through backcross breeding between a high yielding but highly susceptible variety Pusa Jwala with the highly resistant Ujwala, using Pusa Jwala as a recurrent parent. Thus, Pusa Jwala and Anugraha are near isogenic lines (NILs) differing for the resistance to bacterial wilt only and the resistance is governed by a homozygous recessive (rr) gene action. The F₁s of Anugraha × Pusa Jwala were selfed to generate the segregating F₂ population. The F₂ population has been field screened, 10 highly susceptible and 10 most resistant plants were identified and DNA from these plants were bulked separately. Bulked segregant analysis with AFLP primer combination EcoACT + MseCAC was done using the DNA from donor parent Ujwala, susceptible parent Pusa Jwala, resistant parent Anugraha, bulked susceptible F₂ and bulked resistant F₂ plants. On resolution using capillary electrophoresis system in genetic analyzer, the AFLP products have yielded three polymorphic bands (103, 117, and 161 bp) which were linked with the resistant recessive allele and three polymorphic bands (183, 296, 319 bp) linked with the dominant susceptible allele of the bacterial wilt resistance gene. The results were confirmed through co-segregation analysis in most resistant and susceptible plants of F₂ segregating population.     

Comparison of different inoculating methods to evaluate the pathogenicity and virulence of Aspergillus niger on two maize hybrids

A two-year field study was conducted to determine the effects of artificial inoculation techniques on the pathogenicity and virulence of Aspergillus niger kernel infection on two maize hybrids. Test plants included in the study were hybrids resistant and susceptible to Aspergillus flavus to determine if the host resistance mechanisms that limited A. flavus infection would also suppress A. niger infection. Ears were inoculated with the silk-channel, side-needle, and spray techniques 7?days after midsilk (50% of the plants in a plot had silk emerging). Ears were also inoculated with a modified-pinbar technique 21?days after midsilk. Kernel infection in 2008 in inoculated plants ranged from 2% to 11% and from 2% to 45% in the resistant and susceptible hybrids, respectively. In 2009, kernel infection in inoculated plants ranged from 13% to 32% and from 10% to 67% in the resistant and susceptible, respectively. The silk-channel, side-needle, and modified-pinbar techniques produced significantly higher levels of kernel infection in the susceptible hybrid in both years than the spray technique. When hybrids were compared, the silk-channel, side-needle, and modified-pinbar techniques induced significantly higher levels of infections in the susceptible hybrid than in the resistant hybrid in 2008 and 2009. The level of A. niger pathogenicity and virulence increased when conidia were placed inside the husks of developing ears by wounding (modified-pinbar and side-needle techniques) or non-wounding (silk-channel technique) inoculation methods. Although A. niger kernel infection was significantly lower in the A. flavus resistant hybrid compared to the A. flavus susceptible hybrid, A. niger infection levels were much higher than A. flavus infection levels typically observed in both of these hybrids in past studies.     

Sweet potato stem blight caused by Alternaria sp.: A new disease in Ethiopia

A new sweet potato disease was discovered in Ethiopia. The disease mainly affects the stems and petioles of sweet potato and the name sweet potato stem blight is proposed. The pathogen is a species ofAlternaria, for which no definite name has been found so far. Sweet potatoes were most susceptible, tomatoes were slightly susceptible, and muskmelons and chilli peppers were resistant. Thorn apples and onions showed slight to moderate symptoms, butAlternaria was not reisolated from these two species. Although all sweet potato varieties tested were susceptible, there were significant differences in sensitivity.     

The development of juveniles of Heterodera schachtii in roots of resistant and susceptible genotypes of Sinapis alba,Brassica napus,Raphanus sativus and hybrids

The development ofHeterodera schachtii Schm. (beet cyst nematode, BCN) juveniles in roots of resistant and susceptible genotypes belonging to cruciferous crop species and hybrids was studied from 4 to 28 days after inoculation. No difference in root penetration by larvae was observed between resistant and susceptible plants.The development of nematodes in roots from resistant plants ofRaphanus sativus L., resistant xBrassicoraphanus Sageret and a resistant hybrid xBrassicoraphanus x Brassica napus L. was similar. BCN resistance in these three sources of plant material appeared to be related to an increased male:female nematode ratio as compared to the ratio found in susceptibleR. sativus plants.Also in resistant plants ofSinapis alba L. and a resistant intergeneric hybridS. alba x B. napus the increase in male:female nematode ratio, as compared to the ratio found for susceptibleS. alba cultivars and a susceptible intergeneric hybridS. alba x B. napus, seemed to be related with the observed resistance. In roots of the resistantS. alba and of a resistant hybridS. alba x B. napus, however, BCN resistance might also be due to a slower development of larvae and increased necrosis of root cells at the site of larval penetration.     

Polyamines can inhibit paraquat toxicity and translocation in the broadleaf weed Arctotheca calendula

Reduced paraquat transport from the site of application to the site of action in the chloroplast seems a likely mechanism for paraquat resistance in several weed species including Arctotheca calendula. Recently, it has been shown that paraquat translocation in A. calendula is correlated with paraquat-induced injury and is reduced in paraquat-resistant A. calendula. Studies with leaf slices have shown that some polyamines when applied concomitantly with paraquat can reduce the toxic effects of paraquat. This study examined the effects of three polyamines, putrescine, cadaverine, and spermidine, on paraquat translocation to examine the possibility that paraquat translocation in susceptible plants would be reduced in the presence of polyamines due to competition of the polyamines with cellular paraquat uptake. Two polyamines, spermidine and cadaverine were effective in reducing paraquat translocation in susceptible A. calendula inducing these plants to perform more like resistant A. calendula in terms of translocation. Quantification of the polyamine contents of resistant and susceptible A. calendula showed that resistant plants have higher constitutive spermidine levels than susceptible plants, which infers a possible role of either polyamines or a polyamine transporter in paraquat resistance.     

Infection of Rice Plants with the Sheath Blight Fungus Causes an Activation of Pentose Phosphate and Glycolytic Pathways

The response of key regulatory enzymes of the pentose phosphate and glycolytic pathways in disease development was assessed in genetically-related rice plants resistant and susceptible to the sheath blight fungus, Rhizoctonia solani. The plants were grown and maintained under greenhouse conditions and inoculated at 50% flowering. Uninoculated healthy plants served as controls. The activities of pentose phosphate pathway enzymes (glucose-6 phosphate dehydrogenase and 6-phosphogluconate dehydrogenase) increased more than two-fold in both the resistant and susceptible plants. Activities of ATP- and pyrophosphate-dependent phosphofructokinase and phosphoenolpyruvate phosphatase increased in infected plants while activity of phosphoenolpyruvate carboxylase in infected plants was lower than in the healthy plants. Furthermore, for enzymes with increased activity, the levels were higher in the resistant line than in the susceptible line. The enhancement of the enzyme activities correlated well with the post infection period. These data suggest that altered carbohydrate metabolism in sheath blight infections may play an important role in modulating the rice plant's response to infection. The isolation of an infection-induced gene encoding a basic enzyme of pentose phosphate and glycolytic pathways could be used to develop plants with more resistance towards sheath blight disease.     

Incidence of Soil-borne Wheat Mosaic Virus in Mixtures of Susceptible and Resistant Wheat Cultivars

The multiplication of Soil-borne wheat mosaic virus (SBWMV) was studied in mixtures of two winter wheat (Triticum aestivum) cultivars, one susceptible (Soissons) and the other resistant (Trémie). Two seed mixtures of susceptible and resistant varieties in ratios of 1 : 1 and 1 : 3 and their component pure stands, i.e. each variety grown separately, were grown in a field infected with SBWMV. The presence of the virus was detected using DAS-ELISA from January to May. The resistant cultivar Trémie showed no foliar symptoms nor could the virus be detected in the leaves or roots. In May, about 88% of plants of susceptible cultivar Soissons grown in pure stands were infected. At this time, the disease reduction relative to pure stands was 32.2% in the 1 : 1 mixture and 39.8% in the 1 : 3 mixture. Optical density (OD) values from ELISA of the infected plants in the two mixtures were consistently lower than that of the infected plants in cultivar Soissons in pure stands. The ELISA index (EI) calculated using three scales of OD values was 65.5% in the susceptible cultivar in pure stands. The value for this index was 19.1% in the 1 : 1 mixture and 7.9% in the 1 : 3 mixture. The plants of the resistant cultivar Trémie infected in the same field and transferred in January to a growth cabinet at 15 °C multiplied the virus and produced viruliferous zoospores. These results show that the resistant cultivar Trémie plays a role in disease reduction in the cultivar mixtures in field conditions. Possible reasons for this are discussed.     

Coat protein-mediated transgenic resistance in tomato against a IB subgroup Cucumber mosaic virus strain

Transgenic tomato plants containing the coat protein (CP) gene of Cucumber mosaic virus (CMV) of subgroup IB were developed through Agrobacterium-mediated transformations. The progenies of transgenic plants showed the presence of transgene, its expression and translation of 26 KDa CP. The T₁ and T₂ generation plants were evaluated for resistance against challenge inoculations by a homologous strain of CMV. Visual observations of challenged transgenic plants categorized them into resistant, tolerant and susceptible as compared with untransformed control plants. Out of 33 plants of the T₁ generation, 36.3% showed resistance and remained symptomless throughout their life, 48.4% showed tolerance which developed delayed symptoms of mild mosaic, and 15.1% showed susceptibility to CMV which developed severe systemic mosaic and leaf distortion symptoms after 30?days of virus challenge. Out of 120 plants of the T₂ generation, 60% showed resistance, 26.6% were tolerant and only 13.3% were found susceptible to challenge inoculations of CMV. Resistant transgenic plants also showed less CP accumulation in systemic upper leaves as compared with challenged untransformed plants. In this study, CP of a CMV subgroup IB strain has demonstrated a significant level of resistance in transgenic tomato plants against the CMV strain. The strategy may be applied for better quality and productivity of tomato crops.     

Bacterial wilt-resistant tomato rootstock suppresses migration of <Emphasis Type="Italic">ralstonia solanacearum</Emphasis> into soil

Ralstonia solanacearum, the causal agent of bacterial wilt of tomato, grows in infected plants and migrates from the roots into the soil. We investigated the effectiveness of bacterial wilt-resistant tomato rootstock in reducing the migration of R. solanacearum from susceptible scions into the soil. Rootstock stems were either 3–5 cm tall (low-grafted, LG) or ≥?10 cm tall (high-grafted, HG). After inoculation of scions of the susceptible cultivar (SC) with R. solanacearum below the first flower, there was no difference in disease progression among LG, HG, and ungrafted SC plants, and plants had wilted by 2 weeks. However, the rate of detection of R. solanacearum in the soil of wilted plants was reduced by grafting. The size of the R. solanacearum population in the soil of fully wilted plants increased in the order of HG?<?LG?<?SC. These results show that grafting onto resistant rootstock strongly suppressed the migration of R. solanacearum into the soil by the time of full wilting, and the effect was stronger with a longer rootstock. Migration of R. solanacearum into soil increased with increasing disease severity in SC, LG and HG. These facts suggest that early uprooting of slightly infected plants could control the spread of the bacteria into the soil.     

Resistance to vascular wilt parasites

When tomato plants resistant toVerticillium albo-atrum become infected with this fungus the parasite grows very little in the stem. The stem, however, responds by producing many tyloses. In susceptible tomato plants in which the parasite is more abundant few tyloses occur, but after some weeks when the parasite begins to disappear from the lower parts of the shoots, vessels with tyloses increase in number. Severity of disease can be altered by treating plants with growth regulating substances.These changes are usually accompanied by alterations in numbers of vessels with hyphae and tyloses in the stems in such a way, that an increase in number of vessels with hyphae is accompanied by a decrease in number of vessels with tyloses, and vice versa.Extracts and diffusates from healthy susceptible plants affect germination of conidia and germ tube growth in the same way as those from resistant plants; some evidence was obtained which suggests that after infection, fungitoxic substances appear in resistant but not in susceptible plants.     

Present status of verticillium wilt of olive in Andalucía (southern Spain)1

Verticillium wilt caused by Verticillium dahliae is a disease highly prevalent in newly established olive orchards in Andalucía, southern Spain. Two syndromes of the disease occur in Andalucia, namely apoplexy and slow decline. Apoplexy is characterized by quick dieback of twigs and branches while slow decline consists of rapid drying out of inflorescences together with leaf chlorosis and necrosis. Systematic disease observations carried out in two experimental orchards planted with susceptible cv. Picual indicated that natural recovery of diseased trees occurred over time. Infection and vascular colonization of olive plants by V. dahliae were studied in susceptible (Picual) and resistant (Oblonga) cultivars inoculated with a mildly virulent or a highly virulent cotton-defoliating isolate of V. dahliae. Disease symptoms developed 24–32 days after inoculation in cv. Picual, but at that time plants of cv. Oblonga remained free from symptoms. However anatomical observations and isolations indicated that systemic infections by the two isolates had occurred to a large extent in both cultivars.     

Victoria Blight,defense turned upside down

Genes that confer disease resistance to biotrophic pathogens typically encode nucleotide-binding, leucine-rich-repeat proteins (NB-LRRs). These proteins confer resistance by detecting the presence of virulence effectors secreted by biotrophic pathogens. Recognition triggers NB-LRR activation and subsequently, the defense response which often includes localized host cell death. The fungus, Cochliobolus victoriae, is a necrotrophic pathogen that causes a disease called Victoria Blight. Virulence of this fungus is dependent on its production of a peptide called “victorin” that has been traditionally described as a toxin. Only plants that respond to victorin are susceptible to Cochliobolus victoriae whereas those that do not are resistant to the fungus. Genetic and molecular analyses have revealed that victorin functions like a biotrophic effector recognized by a NB-LRR resistance protein in Arabidopsis. Further, numerous plant species express victorin sensitivity suggesting there are numerous NB-LRRs that recognize victorin. Thus, through expression of victorin, C. victoriae is able to exploit plant defense to cause disease and is capable of evoking this response in an array of different plants.