Fouad Daayf

Chitosan in plant protection.

Chitin and chitosan are naturally-occurring compounds that have potential in agriculture with regard to controlling plant diseases. These molecules were shown to display toxicity and inhibit fungal growth and development. They were reported to be active against viruses, bacteria and other pests. Fragments from chitin and chitosan are known to have eliciting activities leading to a variety of defense responses in host plants in response to microbial infections, including the accumulation of phytoalexins, pathogen-related (PR) proteins and proteinase inhibitors, lignin synthesis, and callose formation. Based on these and other proprieties that help strengthen host plant defenses, interest has been growing in using them in agricultural systems to reduce the negative impact of diseases on yield and quality of crops. This review recapitulates the properties and uses of chitin, chitosan, and their derivatives, and will focus on their applications and mechanisms of action during plant-pathogen interactions.

Botrytis cinerea Manipulates the Antagonistic Effects between Immune Pathways to Promote Disease Development in Tomato

Botrytis cinerea is a necrotrophic pathogen that causes grey mould disease in a broad host range, including tomato, grapes, potato, and strawberry. Here, we report that B. cinerea secretes a virulence factor that hijacks the plant’s own crosstalk network to promote disease development. Plants have evolved sophisticated mechanisms to sense and respond to pathogen attacks. Resistance against necrotrophic pathogens generally requires the activation of the jasmonic acid (JA) signaling pathway, whereas the salicylic acid (SA) signaling pathway is mainly activated against biotrophic pathogens. SA can antagonize JA signaling and vice versa. Here, we report that the necrotrophic pathogen Botrytis cinerea exploits this antagonism as a strategy to cause disease development. We show that B. cinerea produces an exopolysaccharide, which acts as an elicitor of the SA pathway. In turn, the SA pathway antagonizes the JA signaling pathway, thereby allowing the fungus to develop its disease in tomato (Solanum lycopersicum). SA-promoted disease development occurs through Nonexpressed Pathogen Related1. We also show that the JA signaling pathway required for tomato resistance against B. cinerea is mediated by the systemin elicitor. These data highlight a new strategy used by B. cinerea to overcome the plant’s defense system and to spread within the host.

The Top 10 oomycete pathogens in molecular plant pathology

Oomycetes form a deep lineage of eukaryotic organisms that includes a large number of plant pathogens which threaten natural and managed ecosystems. We undertook a survey to query the community for their ranking of plant-pathogenic oomycete species based on scientific and economic importance. In total, we received 263 votes from 62 scientists in 15 countries for a total of 33 species. The Top 10 species and their ranking are: (1) Phytophthora infestans; (2, tied) Hyaloperonospora arabidopsidis; (2, tied) Phytophthora ramorum; (4) Phytophthora sojae; (5) Phytophthora capsici; (6) Plasmopara viticola; (7) Phytophthora cinnamomi; (8, tied) Phytophthora parasitica; (8, tied) Pythium ultimum; and (10) Albugo candida. This article provides an introduction to these 10 taxa and a snapshot of current research. We hope that the list will serve as a benchmark for future trends in oomycete research.

Signaling cross-talk in plant disease resistance.

Hormone signaling crosstalk plays a major role in plant defense against a wide range of both biotic and abiotic stresses. While many reviews on plant-microbe interactions have well described the general trends of signaling pathways in shaping host responses to pathogens, few discussions have considered a synthesis of positive versus negative interactions among such pathways, or variations in the signaling molecules themselves. This review deals with the interaction trends between salicylic, jasmonic, and abscisic acids in the signaling pathways, as well as exceptions to such trends. Here we focused on antagonistic versus cooperative interactions between salicylic and jasmonic acids, two major disease resistance signaling molecules, and some interactions with abscisic acid, a known abiotic stress hormone, and another player in plant defense mechanisms. We provide a set of examples materializing either antagonism or cooperation for each interaction between two pathways, thereby showing the trends and pinpointing the exceptions. Such analyses are practical for researchers working on the subject and essential for a better exploitation of the data already available in plant disease resistance signaling, both in Arabidopsis and crop species, toward the development of better disease management strategies for economically important crops.

Differentiation of Verticillium dahliae populations on the basis of vegetative compatibility and pathogenicity on cotton.

Complementary auxotrophic nitrate-nonutilizing (nit) mutants were used to investigate vegetative compatibility within 27 strains ofVerticillium dahliae isolated from several hosts originating from Africa, Asia, Europe and the United States. Using about 500nit mutants generated from these strains, three vegetative compatibility groups, 1, 2 and 4, were identified. Simultaneously, virulence of each strain was assessed on cultivars ofGossypium hirsutum, G. barbadense andG. arboreum, based upon Foliar Alteration Index (FAI) and Browning Index (BI) estimation. The strains in VCG1 were of both the cotton-defoliating pathotype and race 3 (on cotton) but were non pathogenic on tomato; those in VCG2 and VCG4 were of the nondefoliating pathotype and belonged to different races on cotton and on tomato. Hyaline mutants deriving from parental wild-type strain showed differences in pathogenicity but were always assigned to the parental VCG. A relationship was established between VCGs and the taxonomic position of host plants. Data fromnit pairings indicated that the sub-populations ofV. dahliae (VCGs) may not be completely isolated genetically.

Comparative screening of bacteria for biological control of potato late blight (strain US-8), using in- vitro, detached-leaves, and whole-plant testing systems

Forty-three bacteria were isolated from the phylloplane and (or) rhizosphere of potato and canola plants and tested for their ability to control the pathogen Phytophthora infestans (strain US-8) causing late blight on potato. This study revealed the benefit of using more than one system when searching for biocontrol activity. In this regard, the complementarity of the three systems chosen (in-vitro culture media, detached leaves, and whole plants) in selecting and identifying potential modes of action, provided a useful insight into the different types of biocontrol activity present. Bacteria with biocontrol activity were from the genera Bacillus, Pseudomonas, Rahnella, and Serratia. Mechanisms of inhibition characterized included those occurring directly, through antibiosis, and (or) indirectly, through the induction of plant defense systems.

Elicitation of soluble phenolics in date palm (Phoenix dactylifera) callus by Fusarium oxysporum f. sp. albedinis culture medium

Abstract A system using callus cultures from two cultivars of date palm ( Phoenix dactylifera ), resistant (BSTN) and susceptible (JHL) to ‘Bayoud disease’, caused by Fusarium oxysporum f.sp. albedinis (Foa), was established as a suitable system for this host-pathogen interaction study. De novo accumulation of phenolic compounds occurred in date palm callus in response to elicitation with filtrates from Foa cultures. Based on their chemical characteristics, most of these compounds were shown to be hydroxycinnamic derivatives. Besides, an increase was recorded in the concentration and the degree of polymerization of flavans. All of these reactions were greater in the resistant than the susceptible cultivars of date palm, which indicates a possible role of phenolic compounds in the resistance to the Bayoud disease.

Proteomic analysis of the phytopathogenic soilborne fungus Verticillium dahliae reveals differential protein expression in isolates that differ in aggressiveness

Verticillium dahliae is a soilborne fungus that causes a vascular wilt disease of plants and losses in a broad range of economically important crops worldwide. In this study, we compared the proteomes of highly (Vd1396‐9) and weakly (Vs06‐14) aggressive isolates of V. dahliae to identify protein factors that may contribute to pathogenicity. Twenty‐five protein spots were consistently observed as differential in the proteome profiles of the two isolates. The protein sequences in the spots were identified by LC‐ESI‐MS/MS and MASCOT database searches. Some of the identified sequences shared homology with fungal proteins that have roles in stress response, colonization, melanin biosynthesis, microsclerotia formation, antibiotic resistance, and fungal penetration. These are important functions for infection of the host and survival of the pathogen in soil. One protein found only in the highly aggressive isolate was identified as isochorismatase hydrolase, a potential plant‐defense suppressor. This enzyme may inhibit the production of salicylic acid, which is important for plant defense response signaling. Other sequences corresponding to potential pathogenicity factors were identified in the highly aggressive isolate. This work indicates that, in combination with functional genomics, proteomics‐based analyses can provide additional insights into pathogenesis and potential management strategies for this disease.

Early vascular defense reactions of cotton roots infected with a defoliating mutant strain of Verticillium dahliae

Susceptible and resistant cotton lines were cytologically and histochemically investigated for their defense reactions to a highly aggressive and defoliating strain of Verticillium dahliae, a fungus responsible for vascular wilt. Cytochemistry showed that early responses consisted of reinforcement in structural barriers with polysaccharides, including callose and cellulose. Ultrastructural modifications of parenchyma cells of the vascular tissues were associated with strong production of terpenoids and phenolics. These defense reactions were detected early in roots of the resistant line, one to four days after inoculation, while they were seen later in roots of the susceptible line.

Biocontrol Treatments Confer Protection Against Verticillium dahliae Infection of Potato by Inducing Antimicrobial Metabolites

Verticillium wilt, caused by Verticillium dahliae Kleb., is a serious potato (Solanum tuberosum L.) disease worldwide, and biocontrol represents a promising eco-friendly strategy to reduce its impact. We used extracts from Canada milk vetch (CMV) and a set of four V. dahliae-antagonistic bacterial strains to coat potato seeds at planting and examined the degree of protection provided against V. dahliae as well as accumulation of soluble phenolics as markers for induced resistance. All tested treatments were effective in reducing disease severity, and CMV showed the highest level of protection. In this treatment, flavonol-glycoside rutin was a highly abundant compound induced in potato tissues, with levels two to three times higher than those detected in noninoculated controls and V. dahliae-inoculated plants. We investigated dose-dependent effects of rutin on V. dahliae growth and sporulation in vitro and in planta. The effect of rutin on mycelial growth was inconsistent between disk assay and amended medium experiments. On the other hand, significant reduction of V. dahliae sporulation in vitro was consistently observed starting at 300 and 100 μM for isolates Vd-9 and Vd-21, respectively. We successfully detected 2-protocatechuoylphloroglucinolcarboxylic acid (2-PCPGCA) using ultra-performance liquid chromatography tandem mass spectrometry, indicating that V. dahliae dioxygenally oxidizes quercetin. Quercetin, as an aglycone, is freed from the sugar moiety by glucosidases and rhamnosidases produced by the fungus and is a substrate for quercetinases. The occurrence of quercetinases in V. dahliae provides a background to formulate a hypothesis about how by-product 2-PCPGCA may be interfering with potato defenses.