Role of methyl jasmonate in the expression of mycorrhizal induced resistance against Fusarium oxysporum in tomato plants

Arbuscular mycorrhiza (AM) colonization led to a decrease in the severity of fusarium wilt disease caused by Fusarium oxysporum f. sp. lycopersici in tomato plants. The involvement of two plant defense hormones, namely methyl jasmonate (MeJA) and salicylic acid (SA), in the expression of mycorrhiza induced resistance (MIR) against this vascular pathogen was studied in the AM colonized and non-colonized (controls) plants. Activity of lipoxygenase (LOX), which plays a role in jasmonic acid (JA) biosynthesis, as well as levels of methyl jasmonate (MeJA) increased in AM colonized plants as compared to controls, but did not show any further changes in response to F. oxysporum inoculation. On the other hand, activity of phenylalanine ammonia lyase (PAL), which is an enzyme from salicylic acid (SA) biosynthetic pathway, as well as SA levels, increased in both controls and AM colonized plants in response to application of F. oxysporum spores. Hence the JA and not the SA signalling pathway appeared to play a role in the expression of MIR against this vascular pathogen. The resistance observed in AM colonized plants was completely compromised when plants were treated with the JA biosynthesis inhibitor salicylhydroxamic acid (SHAM). This confirmed that the AM-induced increase in JA levels was involved in the expression of resistance toward F. oxysporum. The SA response gene pathogenesis-related 1 (PR1) showed an increased expression in response to F. oxysporum infection in SHAM treated AM colonized plants as compared to plants that were not treated with this JA inhibitor. This suggested the possibility that JA inhibited SA responses, at least in the roots. AM colonization therefore appeared to prime plants for improved tolerance against the vascular pathogen F. oxysporum, which was mediated through the JA signalling pathway.     

Involvement of jasmonic acid signalling in bacterial wilt disease resistance induced by biocontrol agent Pythium oligandrum in tomato

When the biocontrol agent Pythium oligandrum (PO) colonizes the rhizosphere, it suppresses bacterial wilt disease in tomato (Solanum lycopersicum cv. Micro‐Tom) caused by Ralstonia solanacearum, and a homogenate of its mycelia exhibits elicitor activity, inducing an ethylene (ET)‐dependent defence response in Micro‐Tom. Since salicylic acid (SA) and jasmonic acid (JA) play an important role in plant defence responses to pathogens, the involvement of SA‐ and JA‐dependent signal transduction pathways in resistance to R. solanacearum was investigated in tomato roots treated with a mycelial homogenate of PO. Bacterial wilt disease was also suppressed in tomato cv. Moneymaker treated with the PO homogenate. However, the SA‐inducible PR‐1(P6) gene was not up‐regulated in either Micro‐Tom or Moneymaker. SA did not accumulate in homogenate‐treated roots in comparison with distilled water‐treated controls, even 24 h after inoculation. Induced resistance against R. solanacearum was not compromised in SA‐non‐accumulating NahG transgenic plants treated with the PO homogenate. On the other hand, the expression of the JA‐responsive gene for the basic PR‐6 protein was induced in both tomato cultivars treated with the PO homogenate. Furthermore, quantitative disease assays showed that the induced resistance against R. solanacearum was compromized in PO homogenate‐treated jai1‐1 mutant plants defective in JA signalling. These results indicated that the JA‐dependent signalling pathway is required for PO‐induced resistance against R. solanacearum in tomato.     

Increased resistance to Verticillium dahliae in Arabidopsis plants defective in auxin signalling

Auxin signalling and transport participate in plant–microbe interactions as positive or negative regulators of disease resistance. The present study investigated the responses of Arabidopsis thaliana plants impaired in the auxin receptors TIR1, AFB1 and AFB3 and the auxin transporter AXR4, upon infection by the soilborne root pathogen Verticillium dahliae. Fewer symptoms were recorded in afb1, afb3 and axr4 plants compared to the wild type (wt). qPCR analysis revealed that the decrease in symptom severity in afb1, afb3 and axr4 was correlated with reduction in the growth of the pathogen in the plants. Therefore, afb1, afb3 and axr4 are partially resistant to V. dahliae. Upon V. dahliae infection, the expression of TIR1, AFB1, AFB3 and AXR4 was up‐regulated in roots, while indole‐3‐acetic acid levels were similar to mocks. The partial resistance of afb1, afb3 and axr4 against V. dahliae can be attributed in part to the up‐regulation of defence‐related genes, as it was observed that afb1 and axr4 had higher PR1 levels than wt, while afb3 had higher PDF1.2 levels than wt. The findings of the present study suggest that the auxin signalling defective mutants afb1, afb3 and axr4 show increased resistance against V. dahliae.     

Components of different signalling pathways regulated by a new orthologue of AtPROPEP1 in tomato following infection by pathogens

Plants express different defence mechanisms in response to pathogens. Understanding the recognition of pathogen‐associated molecular patterns (PAMPs) by specific receptors, and the role of endogenous signals such as AtPep1 that regulate expression of genes in Arabidopsis thaliana, has aided the understanding of the defence mechanisms in different species. The aim of this study was to identify possible orthologous sequences of AtPROPEPs in tomato (Solanum lycopersicum) and characterize its role in resistance to necrotrophic pathogens. The presence of an orthologue of the A. thaliana AtPROPEP1 gene in S. lycopersicum, SlPROPEP, by in silico analysis, is reported here. This has 96% identity with the C‐terminal region of a previously described potato peptide, another possible orthologue of AtPep1. A virus‐induced gene silencing (VIGS) system was employed to investigate the role of the SlPROPEP. Silencing of SlPROPEP in tomato made plants more susceptible to Pythium dissotocum; approximately 30% of SlPROPEP‐silenced plants showed stem constriction compared with 4% in control plants. Furthermore, quantification of P. dissotocum by qPCR revealed that the increase in symptom severity in SlPROPEP‐silenced plants was associated with a 15 times increase in growth of the pathogen compared to control plants. Silencing of SlPROPEP also resulted in decreased expression of genes involved in plant defence against pathogens, such as PR‐1, PR‐5, ERF1, LOX‐D and DEF2. These results suggest that SlPROPEP is involved in tomato resistance to P. dissotocum and probably acts as a pathogen‐associated molecular pattern through signalling pathways mediated by jasmonic acid/ethylene (JA/ET).     

Induction of systemic resistance against Cucumber mosaic virus by Penicillium simplicissimum GP17‐2 in Arabidopsis and tobacco

The plant growth‐promoting fungus, Penicillium simplicissimum GP17‐2, was evaluated for its ability to induce resistance against Cucumber mosaic virus (CMV) in Arabidopsis thaliana and tobacco plants. Treatment with barley grain inoculum (BGI) of GP17‐2 significantly enhanced fresh weight, dry weight and leaf number of A. thaliana and tobacco plants 6 weeks after planting. Two weeks after CMV inoculation, all plants treated with BGI of GP17‐2 or its culture filtrate (CF) showed a significant reduction in disease severity compared with non‐treated control plants, which exhibited severe mosaic symptoms by the end of the experiment. The enzyme‐linked immunosorbent assay (ELISA) demonstrated that CMV accumulation was significantly reduced in plants treated with GP17‐2 or its CF relative to control plants. Based on RT‐PCR, plants treated with GP17‐2 (BGI or CF) also exhibited increased expression of regulatory and defence genes involved in the SA and JA/ET signalling pathways. These results suggested that multiple defence pathways in A. thaliana and tobacco were involved in GP17‐2‐mediated resistance to CMV, although neither the transgenic NahG line, nor the npr1, jar1 or ein3 mutants disrupted the response in A. thaliana. This is the first report to demonstrate the induction of systemic resistance against CMV by GP17‐2 or its CF.     

Comparison of root and foliar applications of potassium silicate in potentiating post‐infection defences of melon against powdery mildew

The application of silicon to the roots or leaves reduces the severity of powdery mildew (Podosphaera xanthii) in melon but the latter treatment is less effective. This study compared key biochemical defence responses of melon triggered by P. xanthii after root or foliar treatment with potassium silicate (PS). Treatments consisted of pathogen‐inoculated or mock‐inoculated plants supplied with PS via roots or foliarly, as well as a non‐treated control. The activity of defence enzymes and the concentration of phenolic compounds, lignin and malondialdehyde were determined from leaf samples at different time points after inoculation. Pathogen‐inoculated plants irrigated with PS showed both an accumulation of silicon and primed defence responses in leaves that were not observed in pathogen‐inoculated plants either sprayed with PS or not treated. These responses included the anticipated activity of peroxidase and accumulation of soluble phenols, the activation of chitinase and repression of catalase, and the stronger activation of superoxide dismutase, peroxidase and β‐1,3‐glucanase. Moreover, the lignin concentration increased in response to inoculation, whereas the malondialdehyde concentration decreased. For the foliar treatment, however, only an increase in lignin deposition was observed compared with the control plants. The results show that silicon strongly plays an active role in modulating the defence responses of melon against P. xanthii when supplied to the roots as opposed to the foliage.     

Fungal derived cytokinins are necessary for normal Ustilago maydis infection of maize

Phytohormones derived from fungi play a key role in regulating plant–pathogen interactions; however, deciphering the separate contributions of pathogen and plant during infection has been difficult. Here, the Ustilago maydis–Zea mays pathosystem was used to investigate this chemical exchange. Ustilago maydis, the causative agent of maize smut, produces cytokinins (CK), which are a group of phytohormones responsible for directing plant development. The characteristic symptom of smut disease is the formation of tumours composed of plant and fungal tissue. Isopentenyltransferase (IPT) catalyses the rate‐limiting step in CK biosynthesis, and U. maydis strains in which the sole tRNA‐ipt gene was deleted no longer produced CKs. These deletion strains elicited fewer, smaller tumours than the pathogenic strain SG200. High performance liquid chromatography‐electrospray ionization tandem mass spectrometry (HPLC‐ESI MS/MS) was used to detect and quantify phytohormone levels in infected tissue. This revealed that key hormone changes in SG200 infections were not present in infections by deletion strains, suggesting that CK production by U. maydis is required for the altered phytohormone profile in infected tissue relative to uninfected tissue. Separate analyses indicated that U. maydis tRNA‐ipt mutants might be altered in their ability to metabolize CKs taken up from the environment. Mining the U. maydis genome identified genes encoding putative CK signalling and biosynthesis proteins.     

Resistance against Botrytis cinerea in smooth leaf pruning wounds of tomato does not depend on major disease signalling pathways

In high‐tech, heated tomato glasshouses, stem infections caused by Botrytis cinerea usually end up girdling the stem, resulting in plant death and consequently high economic losses. Such infections originate primarily from wounds created during leaf pruning, a common cultural practice in which it is intended to remove leaves completely, resulting in smooth stem wounds. However, hasty leaf pruning often results in numerous petiole stubs accidentally left behind. In this study analysis of disease incidences clearly proved that pruning leaves flush to the stem resulted in absolute resistance of the stem wounds, whereas petiole stubs displayed a high level of susceptibility to B. cinerea. Postponing inoculation of wounds after pruning indicated that development of nearly complete resistance occurs within 48 h after deleafing. Monitoring of the wound wetness period showed that drying of the wound surface is not the cause of the decreased susceptibility, contrary to what was commonly believed. Tomato mutants deficient in disease signalling showed altered phenotypes for susceptibility to B. cinerea, indicating that defences against this pathogen in petiole stubs depend on ethylene signalling. Additionally, the decreased susceptibility of mutants deficient in the biosynthesis of jasmonates and abscisic acid suggest an antagonistic effect of these signal molecules. On the other hand, resistance of smooth stem wounds could not be altered by disruption of salicylic acid, ethylene, jasmonate or abscisic acid signalling. This indicates that this remarkable absolute resistance to B. cinerea does not depend on the major disease signalling pathways.     

Biological control of Pythium,Rhizoctonia and Sclerotinia in lettuce: association of the plant protective activity of the bacterium Paenibacillus alvei K165 with the induction of systemic resistance

The soilborne fungi Sclerotinia sclerotiorum, Rhizoctonia solani and the oomycete Pythium ultimum are among the most destructive pathogens for lettuce production. The application of the biocontrol agent Paenibacillus alvei K165 to the transplant soil plug of lettuce resulted in reduced S. sclerotiorum, R. solani and P. ultimum foliar symptoms and incidence compared to untreated controls, despite the suppressive effect of the pathogens on the rhizosphere population of K165. In vitro, K165 inhibited the growth of S. sclerotiorum and R. solani but not P. ultimum. Furthermore, the expression of the pathogenesis‐related (PR) gene PR1, a marker gene of salicylic acid (SA)‐dependent plant defence, and of the Lipoxygenase (LOX) and Ethylene response factor 1 (ERF1) genes, markers of ethylene/jasmonate (ET/JA)‐dependent plant defence was recorded. K165‐treated plants challenged with P. ultimum showed up‐regulation of PR1, whereas challenge with R. solani resulted in up‐regulation of LOX and ERF1, and challenge with S. sclerotiorum resulted in up‐regulation of PR1, LOX and ERF1. This suggests that K165 triggers the SA‐ and the ET/JA‐mediated induced systemic resistance against P. ultimum and R. solani, respectively, while the simultaneous activation of the SA and ET/JA signalling pathways is proposed for S. sclerotiorum.     

Fungal endophyte protects Atractylodes lancea from root rot caused by Fusarium oxysporum

Endophytic fungi, which stimulate a variety of defence reactions in host plants without causing visible disease symptoms, have been isolated from almost every plant. However, beneficial interactions between fungal endophytes and pathogens from the same habitat remain largely unknown. An inoculation of Atractylodes lancea plantlets with Gilmaniella sp. AL12 (AL12) prior to infection with Fusarium oxysporum prevented the necrotization of root tissues and plant growth retardation commonly associated with fusarium root rot. Quantification of F. oxysporum infections using real‐time PCR revealed a correlation between root rot symptoms and the relative amount of fungal DNA. Pretreatment with AL12 reduced the accumulation of reactive oxygen species stimulated by F. oxysporum. An in vitro analysis of their interactions under axenic culture conditions showed AL12 could inhibit F. oxysporum growth. Additionally, F. oxysporum infections were shown to decrease salicylic acid (SA) production compared with control plantlets. SA biosynthesis inhibitors, 2‐aminoindan‐2‐phosphonic acid and paclobutrazol, abolished the inhibition of F. oxysporum growth in A. lancea even after inoculation with AL12. The results indicated that the fungal endophyte protected A. lancea not only by direct antibiosis, but also by reversing the F. oxysporum‐mediated suppression of SA production.