Abdelbasset El Hadrami

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.

Breeding Date Palm

Date palm, Phoenix dactylifera L., is a perennial long-lived dioecious monocotyledon of great socio-economic importance especially in North Africa and the Middle East. These countries grow 62 million of the 105 million trees available worldwide on an area of over a million hectares (Fig. 6.1; Table 6.1). These ‘trees’ are cultivated not only for their valuable fruits (dates), but also for producing fuel, fibre and as shelter for ground crops. Production of dates is of approximately 6.5 million metric tons around the world (Table 6.2) and generates an important commercial activity. Countries such as Egypt, Islamic Republic of Iran and Saudi Arabia represent the top three producers worldwide. Furthermore, in the areas where it is cultivated, the date palm contributes to the creation of a micro-climate that enables agricultural development of other species.

Variations in relative humidity modulate Leptosphaeria spp. pathogenicity and interfere with canola mechanisms of defence

Blackleg (phoma stem canker), caused by Leptosphaeria spp., is an important disease of canola (oilseed rape, Brassica napus). Control strategies rely on the use of resistant cultivars, chemical and disease-reducing cropping practices. In Canada, the pathogen population is represented by L. maculans and L. biglobosa, which are considered to be highly and weakly aggressive, respectively. It is largely admitted that L. biglobosa isolates are not able to cause a significant amount of stem canker and develop on the plant only when it becomes senescent, late in the season. The prevalence of L. maculans over L. biglobosa has been considered to be linked to the low aggressiveness of the latter. However, in this study, we show that L. biglobosa isolates could become highly aggressive in terms of lesion appearance on cotyledons, if the right conditions of temperature and relative humidity (RH) are provided. Percent germination of inoculated pycnidiospores was not affected by the RH regimes tested. This is the first study to show the importance of RH as a factor conditioning the pathogenicity of L. biglobosa isolates on canola cotyledons. Concurrent changes in the host defence mechanisms against L. biglobosa isolates in response to variations in the RH were also investigated. Under high RH, the increase in disease caused by the weakly aggressive isolates coincided with a reduced accumulation of lignin at the early stages of infection.

Priming canola resistance to blackleg with weakly aggressive isolates leads to an activation of hydroxycinnamates

Blackleg (Phoma leaf spot), caused by the ascomycete Leptosphaeria spp. (anamorph: Phoma lingam), is a serious yield-limiting factor in canola. Leptosphaeria spp. populations are highly diverse and display differential interactions with canola cultivars, ranging from hypersensitive-like reactions to partial resistance and susceptibility. Leptosphaeria biglobosa encompasses weakly aggressive isolates and can be found late in the season towards maturity stages of the crop while other isolates from Leptosphaeria maculans are highly aggressive and can be detected throughout the season. In an earlier study, we showed the primary involvement of lignin in the containment of the progress of Phoma leaf spot symptoms around the infection sites by comparing the reaction of three different cultivars with a set of isolates with various levels of aggressiveness. The present investigation reports on further characterization of the host responses to L. biglobosa and L. maculans isolates used individually or subsequently in the infection. Hydroxycinnamates were the major phenolics that differentially accumulated locally or systemically from the infection sites in response to either pre- or co-inoculation with the weakly aggressive isolate tested. Given the involvement of these hydroxycinnamates as precursors for the synthesis of lignin and phenylamide phytoalexins, their detected amounts in response to the priming with a weakly aggressive isolate could explain the restricted development of further inoculated highly aggressive isolates. Key words: canola, Leptosphaeria maculans, L. biglobosa, blackleg, phenolics, phenylamides, hydroxycinnamates, ferulates, phytoalexins. Le chancre noir (phoma), causé par l’ascomycète Leptosphaeria spp. (forme imparfaite : Phoma lingam), constitue un facteur limitant au rendement du canola. Les populations de Leptosphaeria spp. sont très diversifiées et montrent des interactions différentielles, variant de l’hypersensibilité à la résistance partielle ou la sensibilité, vis-à-vis des cultivars de canola. Leptosphaeria biglobosa se compose d’isolats peu agressifs qui n’apparaissent que tard dans la saison quand les plantes ont atteint leur maturité. Leptosphaeria maculans comporte des isolats très agressifs et peut être détecté durant toute la saison de production. Dans une étude précédente comparant les réactions différentielles de trois cultivars à une série d’isolats ayant des niveaux différents d’agressivité, nous avions montré le rôle de la lignine comme composante principale dans la restriction de la progression des symptômes de la maladie autour des sites d’infection. La présente étude fait davantage état de la caractérisation des réponses de l’hôte aux isolats de L. biglobosa et de L. maculans, inoculés soit séparément soit l’un après l’autre. Les dérivés hydroxycinnamiques sont les principaux composés phénoliques accumulés de manière différentielle, que ce soit localement autours des sites d’infection ou systémiquement, en réaction à la pré- ou à la co-inoculation avec l’isolat peu agressif étudié. Étant donné le rôle de ces dérivés hydroxycinnamiques comme précurseurs dans la synthèse de la lignine et de celle des phytoalexines de nature phénylamidique, les quantités induites en réaction à l’isolat peu agressif pourraient expliquer le taux relativement limité de symptômes causés par l’inoculation ultérieure d’isolats beaucoup plus agressifs. Mots-clés : canola, Leptosphaeria maculans, L. biglobosa, chancre noir, composés phénoliques, phénylamides, dérivés hydroxycinnamiques, dérivés féruliques, phytoalexines.

Plants versus Fungi and Oomycetes: Pathogenesis, Defense and Counter-Defense in the Proteomics Era

Plant-fungi and plant-oomycete interactions have been studied at the proteomic level for many decades. However, it is only in the last few years, with the development of new approaches, combined with bioinformatics data mining tools, gel staining, and analytical instruments, such as 2D-PAGE/nanoflow-LC-MS/MS, that proteomic approaches thrived. They allow screening and analysis, at the sub-cellular level, of peptides and proteins resulting from plants, pathogens, and their interactions. They also highlight post-translational modifications to proteins, e.g., glycosylation, phosphorylation or cleavage. However, many challenges are encountered during in planta studies aimed at stressing details of host defenses and fungal and oomycete pathogenicity determinants during interactions. Dissecting the mechanisms of such host-pathogen systems, including pathogen counter-defenses, will ensure a step ahead towards understanding current outcomes of interactions from a co-evolutionary point of view, and eventually move a step forward in building more durable strategies for management of diseases caused by fungi and oomycetes. Unraveling intricacies of more complex proteomic interactions that involve additional microbes, i.e., PGPRs and symbiotic fungi, which strengthen plant defenses will generate valuable information on how pathosystems actually function in nature, and thereby provide clues to solving disease problems that engender major losses in crops every year.

A cupin domain-containing protein with a quercetinase activity (VdQase) regulates Verticillium dahliae's pathogenicity and contributes to counteracting host defenses.

We previously identified rutin as part of potato root responses to its pathogen Verticillium dahliae. Rutin was directly toxic to the pathogen at doses greater than 160 μM, a threshold below which many V. dahliae pathogenicity-related genes were up-regulated. We identified and characterized a cupin domain-containing protein (VdQase) with a dioxygenase activity and a potential role in V. dahliae-potato interactions. The pathogenicity of VdQase knock-out mutants generated through Agrobacterium tumefasciens-mediated transformation was significantly reduced on susceptible potato cultivar Kennebec compared to wild type isolates. Fluorescence microscopy revealed a higher accumulation of flavonols in the stems of infected potatoes and a higher concentration of rutin in the leaves in response to the VdQase mutants as compared to wild type isolates. This, along with the HPLC characterization of high residual and non-utilized quercetin in presence of the knockout mutants, indicates the involvement of VdQase in the catabolism of quercetin and possibly other flavonols in planta. Quantification of Salicylic and Jasmonic Acids (SA, JA) in response to the mutants vs. wild type isolates revealed involvement of VdQase in the interference with signaling, suggesting a role in pathogenicity. It is hypothesized that the by-product of dioxygenation 2-protocatechuoylphloroglucinolcarboxylic acid, after dissociating into phloroglucinol and protocatechuoyl moieties, becomes a starting point for benzoic acid and SA, thereby interfering with the JA pathway and affecting the interaction outcome. These events may be key factors for V. dahliae in countering potato defenses and becoming notorious in the rhizosphere.