The plant growth-promoting rhizobacterium Bacillus cereus AR156 induces resistance in tomato with induction and priming of defence response

In a previous study, we demonstrated the ability of the rhizobacterium Bacillus cereus AR156 (AR156) to protect tomato against bacterial wilt caused by Ralstonia solanacearum and root-knot disease caused by Meloidogyne incognita. Here, we investigate the ability of AR156 to promote plant growth and its role in the systemic protection of tomatoes cultivated in greenhouses against bacterial speck disease caused by Pseudomonas syringae pv. tomato DC3000 (DC3000). In our experiments, the AR156 population reached 10⁵–10⁶ CFU/g rhizosphere soil, and remained at that level in the rhizosphere of tomato plants for more than 2 months. In terms of its ability to promote plant growth, AR156 increased the average biomass of the tomato by 47.7%. AR156 also elicited induced systemic resistance against DC3000, significantly reduced bacterial speck disease severity 1.6-fold, and inhibited proliferation of the pathogen by approximately 15-fold. This strain triggered the accumulation of defence-related genes (PR1 and PIN2) in tomato leaves and primed the leaves for accelerated defence-related gene expression upon challenge with DC3000. That suggested simultaneous activation of the salicylic acid and the jasmonic acid dependent signalling pathways by AR156 against DC3000. In conclusion, B. cereus AR156 was found to form robust colonies in the roots of tomato and had some beneficial effects, including biological control of bacterial speck disease via ISR and promotion of plant growth.     

Evidence on antimicrobial properties and mode of action of a chitosan obtained from crustacean exoskeletons on Pseudomonas syringae pv. tomato DC3000

Pseudomonas syringae pv. tomato DC3000 (Pto DC3000) causes bacterial speck of tomato, a widely spread disease that causes significant economical losses worldwide. It is representative of many bacterial plant diseases for which effective controls are still needed. Despite the antimicrobial properties of chitosan has been previously described in phytopathogenic fungi, its action on bacteria is still poorly explored. In this work, we report that the chitosan isolated from shrimp exoskeletons (70 kDa and 78 % deacetylation degree) exerts cell damage on Pto DC3000. Chitosan inhibited Pto DC3000 bacterial growth depending on its concentration, medium-pH, and presence of metal ion (Mg⁺²). Biochemical and cellular changes resulting in cell aggregation and impaired bacterial growth were also viewed. In vivo studies using fluorescent probes showed cell aggregation, increase in membrane permeability, and cell death, suggesting the chitosan antibacterial activity is due to its interaction as a polycation with Pto DC3000 membranes. Transmission electron microscopic analysis revealed that chitosan also caused morphological changes and damage in bacterial surfaces. Also, the disease incidence in tomato inoculated with Pto DC3000 was significantly reduced in chitosan pretreated seedlings, revealing a promising action of chitosan as nontoxic biopesticide in tomato plants. Indeed, a wider comprehensive knowledge of the mechanism of action of chitosan in phytopathogenic bacterial cells will increase the chances of its successful application to the control of spread disease in plants.     

南方根结线虫对不同砧木嫁接番茄苗活性氧清除系统的影响

采用盆栽人工接种方法,对番茄嫁接苗进行了抗性评价,研究了番茄嫁接苗叶片中抗氧化酶活性和活性氧代谢的动态变化。结果表明,接种南方根结线虫(J2)后,砧木嫁接苗表现为高抗,自根嫁接苗为高感。通过嫁接换根,与自根嫁接苗相比,砧木嫁接苗明显提高了接穗叶片的超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性,降低了超氧阴离子(O.2-)产生速率以及过氧化氢(H2O2)和丙二醛(MDA)含量。表明番茄植株体内的活性氧水平和抗氧化酶活性的高低与其抗根结线虫的能力密切相关,较低的活性氧水平和较高的抗氧化酶活性有利于减轻对膜系统的伤害,提高番茄植株的抗根结线虫能力。     

Greenhouse Studies on the Effect of Marigolds (Tagetes spp.) on Four Meloidogyne Species

The effects of preplanted marigold on tomato root galling and multiplication of Meloidogyne incognita, M. javanica, M. arenaria, and M. hapla were studied. Marigold cultivars of Tagetes patula, T. erecta, T. signata, and a Tagetes hybrid all reduced galling and numbers of second-stage juveniles in subsequent tomato compared to the tomato-tomato control. All four Meloidogyne spp. reproduced on T. signata ''Tangerine Gem''. Several cultivars of T. patula and T. erecta suppressed galling and reproduction of Meloidogyne spp. on tomato to levels lower than or comparable to a fallow control. Phytotoxic effects of marigold on tomato were not observed. Several of the tested marigold cultivars are ready for full-scale field evaluation against Meloidogyne spp.     

Pathological Interaction of a Combination of Heterodera schachtii and Meloidogyne hapla on Tomato

Increased culturing of a tomato population of Heterodera schachtii (UT1C) on tomato for 480 days (eight inoculation periods of 60 days each) significantly increased virulence to ''Stone Improved'' tomato. A synergistic relationship existed between Meloidogyne hapla and H. schaehtii on tomato. A combination of H. schachtii (UTIC) and M. hapla significantly reduced tomato root weights by 65, 64, and 61% below root weights of untreated controls, and single inoculations of M. hapla and H. schachtii, respectively. This corresponded to root reductions of 42, 44, and 46% from a combination of H. schachtii (UT1B) and M. hapla. Antagonism existed between H. schachtii and M. hapla with regard to infection courts and feeding sites. The root-knot galling index dropped from 6.0 with a single inoculation of M. hapla to 4.3 and 3.3 with combined inoculations of M. hapla plus UT1B and M. hapla plus UTIC cyst nematode populations. The pathological virulence of H. schachtii to sugarbeet was not lost by extended culturing on tomato; there were no differences in penetration, maturation, and reproduction between sugarbeet populations continually cultured on sugarbeet and the population continually cultured on tomato.     

The phytotoxin coronatine induces light-dependent reactive oxygen species in tomato seedlings

The phytotoxin coronatine (COR), which is produced by Pseudomonas syringae pv. tomato DC3000 (DC3000), has multiple roles in virulence that lead to chlorosis and a reduction in chlorophyll content. However, the physiological significance of COR-induced chlorosis in disease development is still largely unknown. Global expression analysis demonstrated that DC3000 and COR, but not the COR-defective mutant DB29, caused reduced expression of photosynthesis-related genes and result in a 1.5- to 2-fold reduction in maximum quantum efficiency of photosystem II (F(V)/F(M)). Tomato (Solanum lycopersicum) seedlings inoculated with DC3000 and incubated in a long daily photoperiod showed more necrosis than inoculated seedlings incubated in either dark or a short daily photoperiod. The accumulation of reactive oxygen species (ROS) was detected in cotyledons inoculated with either purified COR or DC3000 but not in tissues inoculated with DB29. Interestingly, COR-induced ROS accumulated only in light and was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea and diphenylene iodonium, which function to inhibit electron transport from PSII. Furthermore, COR and DC3000 suppressed expression of the gene encoding the thylakoid Cu/Zn superoxide dismutase but not the cytosolic form of the same enzyme. In conclusion, these results demonstrate a role for COR-induced effects on photosynthetic machinery and ROS in modulating necrotic cell death during bacterial speck disease of tomato.     

Varietal Response of Tomato to the Interaction of Salinity and Meloidogyne incognita Infection

Response of tomato (Lycopersicon esculentum) cultivars to a range of conductivity levels was tested in the presence and absence of Meloidogyne incognita. The conductivity levels were produced by appropriate adjustment of a 1:1 solution of sodium chloride and calcium chloride. The growth of M. incognita resistant (''Beefmaster'' and ''Atkinson'') and susceptible (''Hunts 2580'' and ''Ronita'') tomato plants was inversely related to soil salinity between EC_e 0 and 5 mmhos/cm. Nematode inoculation of salt-stressed plants significantly reduced plant height, fresh and dry weight, number of flowers, and fruit weight in most cultivars. In Hunts 2580, flower number and fruit weight increased; apparently flower production shifted from determinate to indeterminate, with negative implications for mechanical harvesting. Nematode reproduction on susceptible varieties also decreased with increase in salinity.     

Auxin increases the hydrogen peroxide (H2O2) concentration in tomato (Solanum lycopersicum) root tips while inhibiting root growth

Background and Aims

The hormone auxin and reactive oxygen species (ROS) regulate root elongation, but the interactions between the two pathways are not well understood. The aim of this study was to investigate how auxin interacts with ROS in regulating root elongation in tomato, Solanum lycopersicum.

Methods

Wild-type and auxin-resistant mutant, diageotropica (dgt), of tomato (S. lycopersicum ‘Ailsa Craig’) were characterized in terms of root apical meristem and elongation zone histology, expression of the cell-cycle marker gene Sl-CycB1;1, accumulation of ROS, response to auxin and hydrogen peroxide (H₂O₂), and expression of ROS-related mRNAs.

Key Results

The dgt mutant exhibited histological defects in the root apical meristem and elongation zone and displayed a constitutively increased level of hydrogen peroxide (H₂O₂) in the root tip, part of which was detected in the apoplast. Treatments of wild-type with auxin increased the H₂O₂ concentration in the root tip in a dose-dependent manner. Auxin and H₂O₂ elicited similar inhibition of cell elongation while bringing forth differential responses in terms of meristem length and number of cells in the elongation zone. Auxin treatments affected the expression of mRNAs of ROS-scavenging enzymes and less significantly mRNAs related to antioxidant level. The dgt mutation resulted in resistance to both auxin and H₂O₂ and affected profoundly the expression of mRNAs related to antioxidant level.

Conclusions

The results indicate that auxin regulates the level of H₂O₂ in the root tip, so increasing the auxin level triggers accumulation of H₂O₂ leading to inhibition of root cell elongation and root growth. The dgt mutation affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H₂O₂ homeostasis in the root tip.     

Disease resistance to Pectobacterium carotovorum is negatively modulated by the Arabidopsis Lectin Receptor Kinase LecRK-V.5

Plant stomata function in disease resistance by restricting bacteria entry inside leaves. During plant-bacteria interactions, stomatal closure is initiated by the recognition of Microbe-Associated Molecular Patterns (MAMPs). Recently, we have shown that the Lectin Receptor Kinase V.5 (LecRK-V.5) negatively regulates bacterium- and MAMP-induced stomatal closure upstream of Reactive Oxygen Species (ROS) production mediated by abscisic acid signaling. Closed stomata in lecrk-V.5 mutants are correlated with constitutive high level of ROS in guard cells. Consequently, lecrk-V.5 mutants are more resistant to hemi-biotrophic pathogen Pseudomonas syringae pv tomato DC3000 (Pst DC3000). In this report, we further investigate the role of LecRK-V.5 in resistance against necrotrophic bacteria Pectobacterium carotovorum ssp. carotovorum (Pcc). Upon surface-inoculation lecrk-V.5 mutants exhibited enhanced resistance against Pcc whereas a wild-type level of resistance was observed using infiltration-inoculation, an inoculation method that bypasses the epidermal barrier. Enhanced resistance of dip-inoculated lecrk-V.5 mutants against necrotrophic bacteria, that induce different defense responses than hemi-biotrophic bacteria, further suggests a possible role for LecRK-V.5 in stomatal immunity.     

Host Differences among Florida Populations of Belonolaimus longicaudatus Rau

Three populations of Belonolaimus longicaudatus from Gainesville, Fuller''s Crossing, and Sanford, all in Florida, were tested for parasitism on Citrus jambhiri (rough lemon), Arachis hypogaea (''Early Runner'' peanut), Fragaria sp. (''Florida 90'' hybrid strawberry), and Lycopersicon esculentum (''Rutgers'' tomato). The three populations were found to be three different physiological races because: (i) the Fuller''s Crossing population reproduced well on and caused injury to rough lemon and tomato but not strawberry; (ii) the Gainesville population reproduced well on and caused injury to peanut, strawberry and tomato but not on rough lemon; (iii) the Sanford population reproduced well on and caused injury to peanut, reproduced well on tomato, but neither reproduced upon nor caused injury to strawberry or rough lemon. Morphologically, females of the Sanford population have a lower "c" value and a greater number of tail annules than those of the Gainesville and Fuller''s Crossing populations. Females and males of the Fuller''s Crossing population had higher "a" values than those of the other two populations.     

ROS-PIASγ cross talk channelizes ATM signaling from resistance to apoptosis during chemosensitization of resistant tumors

With the existing knowledge of ATM''s role in therapeutic resistance, the present study aimed at identifying the molecular mechanisms that influence ATM to oscillate between chemoresistance and chemosensitivity. We observed that the redox status of tumors functions as a major determinant of ATM-dependent ‘resistance-to-apoptosis'' molecular switch. At a low reactive oxygen species (ROS) condition during genotoxic insult, the ATM/sumoylated-IKKγ interaction induced NFκB activation that resisted JNK-mediated apoptosis, whereas increasing cellular ROS restored ATM/JNK apoptotic signaling. A search for the upstream missing link revealed that high ROS induces oxidation and ubiquitin-mediated degradation of PIASγ, thereby disrupting PIASγ-IKKγ cross talk, a pre-requisite for IKKγ sumoylation and subsequent NFκB activation. Interruption in the PIASγ-mediated resistance pathway channels ATM signaling toward ATM/JNK pro-death circuitry. These in vitro results also translated to sensitive and resistant tumor allograft mouse models in which low ROS-induced resistance was over-ruled in PIASγ knockout tumors, while its overexpression inhibited high ROS-dependent apoptotic cues. Cumulatively, our findings identified an unappreciated yet critical combinatorial function of cellular ROS and PIASγ in regulating ATM-mediated chemosensitization of resistant tumors. Thus, therapeutic strategies employing ROS upregulation to inhibit PIASγ during genotoxic therapy may, in future, help to eliminate the problems of NFκB-mediated tumor drug resistance.     

Reactive Oxygen Species Play a Role in the Infection of the Necrotrophic Fungi,Rhizoctonia solani in Wheat

Rhizoctonia solani is a nectrotrophic fungal pathogen that causes billions of dollars of damage to agriculture worldwide and infects a broad host range including wheat, rice, potato and legumes. In this study we identify wheat genes that are differentially expressed in response to the R. solani isolate, AG8, using microarray technology. A significant number of wheat genes identified in this screen were involved in reactive oxygen species (ROS) production and redox regulation. Levels of ROS species were increased in wheat root tissue following R. solani infection as determined by Nitro Blue Tetrazolium (NBT), 3,3''-diaminobenzidine (DAB) and titanium sulphate measurements. Pathogen/ROS related genes from R. solani were also tested for expression patterns upon wheat infection. TmpL, a R. solani gene homologous to a gene associated with ROS regulation in Alternaria brassicicola, and OAH, a R. solani gene homologous to oxaloacetate acetylhydrolase which has been shown to produce oxalic acid in Sclerotinia sclerotiorum, were highly induced in R. solani when infecting wheat. We speculate that the interplay between the wheat and R. solani ROS generating proteins may be important for determining the outcome of the wheat/R. solani interaction.     

Influence of Meloidogyne incognita on Fusarium Wilt of Tomato at or below the Minimum Temperature for Wilt Development

''Bonny Best'' tomato plants were grown at 16, 21, or 24 C for 28 d in soil infested with either of two isolates of Fusarium oxysporum f. sp. lycopersici race 1 and Meloidogyne incognita. Significant levels of fusarium wilt occurred at all temperatures including 16 C, which has not been reported previously. One Fusarium isolate resulted in the highest levels of disease incidence at 21 and 24 C in the presence of root-knot nematodes, and at 24 C when the nematodes were not present. At 16 C there was no significant difference in the number of plants infected by the second Fusarium isolate alone or in combination with root knot nematodes, although the presence of nematodes resulted in a significant increase in the percentage of disease occurrence and vessel infection at 21 C.     

Inhibition of Syncytia Formation and Root-knot Nematode Development on Cultures of Excised Tomato Roots

Two different defined growth media were used to culture aseptically the root-knot nematode, Meloidogyne incognita, on excised roots of tomato, Lycopersicon esculentum cv ''Marglobe.'' One of these media, STW, was a formulation by Skoog, Tsui, and White and the other, MS, a formulation by Murashige and Skoog. From 1 through 4 weeks, inoculated tissues were fractured to observe root infection, giant-cell formation, and nematode development with the scanning electron microscope (SEM). Four weeks after inoculation, the fresh weights of roots and developmental stages of nematodes were recorded. SEM observations indicated that roots cultured on the STW medium had normal growth and infection sites with galls that supported the development of mature females by 4 weeks. Roots cultured on the MS medium were less vigorous and had infection sites with galls containing only one to four syncytialike cells that did not support the development of mature females. Eighty percent of the larvae infecting roots cultured on the MS medium failed to develop into mature females. To determine which factor(s) affected root growth and nematode development, inoculated and uninoculated roots were grown on media consisting of different combinations of the organic and inorganic fractions of the STW and MS formulations. These experiments indicated that the organic fraction of STW was essential for normal root growth; however, the inorganic fraction of MS inhibited normal gall formation and nematode development. Further testing of the inorganic fractions revealed that the high concentration of ammonium nitrate in the MS medium was a factor that inhibited giant-cell formation and nematode development.     

Hatch and Reproduction of Globodera tabacum tabacum in Response to Tobacco,Tomato, or Black Nightshade

The effects of broadleaf tobacco, tomato, and black nightshade on juvenile hatch and reproduction of Globodera tabacum tabacum were determined in laboratory and greenhouse experiments. Root exudates from nightshade stimulated greater egg hatch than those from either ''Rutgers'' tomato or ''86-4'' tobacco. Hatch was greater at higher proportions of root exudates for all three plant species. Root exudates from plants greater than 3 weeks old stimulated more hatch than younger plants. No regression relationships existed between plant age and nematode batch. In other experiments, hatch from eggs in cysts was higher for tomato and nightshade after 10 weeks in greenhouse pots compared to tobacco and bare soil. Numbers of second-stage juveniles in eggs in cysts produced from a previous generation on the same host were highest on nightshade and less on tomato and tobacco. Cysts of variable age recovered from field soil had increased hatch in both root exudates or water compared to recently produced cysts from plants in growth chambers. Globodera t. tabacum may be subject to both host and environmentally mediated diapause.