Mohamed Chérif

Silicon induced resistance in cucumber plants against Pythium ultimum

Abstract The effect of the amendment of nutrient solutions with soluble potassium silicate on the response of cucumber (cv. Corona) root and hypocotyl tissues infected by Pythium ultimum was examined by light and electron microscopy, and by energy dispersive X-ray analysis (EDX). Plants were grown in 0 or 1·7 m m Si-amended nutrient solutions, and root and hypocotyl samples were collected at different times after inoculation with P. ultimum . By 48 h after infection, striking differences in the expression of defence reactions were observed between Si-amended and Si-free cucumber plants. Treatment of plants with Si markedly stimulated the accumulation of an electron-dense, phenolic-like material in infected host tissues, and significantly increased the percentage of cells filled with this material. Fungal hyphae colonizing occluded host cells were seriously damaged, and were often reduced to empty hyphal shells. Additionally, Si-treated cucumber plants responded to P. ultimum infection by forming electron-dense layers along primary and secondary cell walls, as well as over pit membranes of xylem vessels. EDX analysis failed to reveal the presence of silica deposits in P. ultimum -infected plants grown in Si-supplemented media. Our results suggest that a relationship exists between Si treatment, resistance to P. ultimum attack, and expression of plant defence mechanisms.

Treatment with the Mycoparasite Pythium oligandrum Triggers Induction of Defense-Related Reactions in Tomato Roots When Challenged with Fusarium oxysporum f. sp. radicis-lycopersici

ABSTRACT The influence exerted by the mycoparasite Pythium oligandrum in triggering plant defense reactions was investigated using an experimental system in which tomato plants were infected with the crown and root rot pathogen Fusarium oxysporum f. sp. radicis-lycopersici. To assess the antagonistic potential of P. oligandrum against F. oxysporum f. sp. radicis-lycopersici, the interaction between the two fungi was studied by scanning and transmission electron microscopy (SEM and TEM, respectively). SEM investigations of the interaction region between the fungi demonstrated that collapse and loss of turgor of F. oxysporum f. sp. radicis-lycopersici hyphae began soon after close contact was established with P. oligandrum. Ultrastructural observations confirmed that intimate contact between hyphae of P. oligandrum and cells of the pathogen resulted in a series of disturbances, including generalized disorganization of the host cytoplasm, retraction of the plasmalemma, and, finally, complete loss of the protoplasm. Cytochemical labeling of chitin with wheat germ agglutinin (WGA)/ovomucoid-gold complex showed that, except in the area of hyphal penetration, the chitin component of the host cell walls was structurally preserved at a time when the host cytoplasm had undergone complete disorganization. Interestingly, the same antagonistic process was observed in planta. The specific labeling patterns obtained with the exoglucanase-gold and WGA-ovomucoid-gold complexes confirmed that P. oligandrum successfully penetrated invading cells of the pathogen without causing substantial cell wall alterations, shown by the intense labeling of chitin. Cytological investigations of samples from P. oligandrum-inoculated tomato roots revealed that the fungus was able to colonize root tissues without inducing extensive cell damage. However, there was a novel finding concerning the structural alteration of the invading hyphae, evidenced by the frequent occurrence of empty fungal shells in root tissues. Pythium ingress in root tissues was associated with host metabolic changes, culminating in the elaboration of structural barriers at sites of potential fungal penetration. Striking differences in the extent of F. oxysporum f. sp. radicis-lycopersici colonization were observed between P. oligandrum-inoculated and control tomato plants. In control roots, the pathogen multiplied abundantly through much of the tissues, whereas in P. oligandrum-colonized roots pathogen growth was restricted to the outermost root tissues. This restricted pattern of pathogen colonization was accompanied by deposition of newly formed barriers beyond the infection sites. These host reactions appeared to be amplified compared to those seen in nonchallenged P. oligandrum-infected plants. Most hyphae of the pathogen that penetrated the epidermis exhibited considerable changes. Wall appositions contained large amounts of callose, in addition to be infiltrated with phenolic compounds. The labeling pattern obtained with gold-complexed laccase showed that phenolics were widely distributed in Fusarium-challenged P. oligandrum-inoculated tomato roots. Such compounds accumulated in the host cell walls and intercellular spaces. The wall-bound chitin component in Fusarium hyphae colonizing P. oligandrum-inoculated roots was preserved at a time when hyphae had undergone substantial degradation. These observations provide the first convincing evidence that P. oligandrum has the potential to induce plant defense reactions in addition to acting as a mycoparasite.

Studies of silicon distribution in wounded and Pythium ultimum infected cucumber plants

Abstract The objective of this study was to investigate the deposition of silicon (Si) in relation to the ability of Si to reduce the severity of Pythium ultimum infection on cucumber. Roots, hypocotyls and leaves of cucumber plants grown in nutrient solutions unamended or amended with 1·7 m m (100 ppm) potassium silicate were inoculated with P. ultimum or pierced with a sharp needle. At 24, 48 and 72 h after treatment, the plants were examined for Si distribution using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) microanalysis. No Si was detected at sites of fungal penetration or in P. ultimum hyphae, regardless of the plant organ studied. Si was also absent from wounded roots and almost absent in wounded leaves and hypocotyls collected from plants maintained under high humidity in a growth chamber. By contrast, a specific and intense deposition of Si was found in cells surrounding the trichome hairs and in wounded leaves and hypocotyls of silicon amended plants. These results reinforce the idea that accumulation and polymerization of silica at fungal penetration sites or in epidermal cell walls has no role as a physical barrier against fungal attack. Thus Si deposition does not appear to be the mechanism by which fungal growth and penetration of plant tissues are hindered.

Chapter 9 Silicon and disease resistance in dicotyledons

Silicon (Si) has been exploited for its prophylactic properties against plant disease for hundreds of years. Its role as a disease-preventing product has been well documented, but the mechanisms by which it exerts its beneficial properties in planta remain poorly understood. For a long time, the observation of a systematic accumulation of silica in cell walls and appositions occurring at pathogen penetration sites led to the conclusion that this parietal strengthening was responsible for the increased resistance of plants to diseases. However, recent evidence suggests that Si. would rather play an active role in reinforcing plant disease resistance by stimulating the expression of its natural defense reactions. Incidentally, in the cucumber ( Cucumis sativus )-powdery mildew ( Sphaerotheca fuliginea ) system, this latter mechanism appears to be predominant, if not exclusive. A better understanding of this rather unique property of Si. could be exploited to optimize its use in agriculture and to help decipher how plants can be naturally stimulated to protect themselves against pathogens.

Chapter 20 Plant-related silicon research in Canada

Research in Canada on the significance of silicon (Si) to higher plants has focused on four main areas: 1) the nature and location of Si deposition in organs and cells of higher plants, 2) the chemistry of Si in biological systems, 3) the role of Si in plant-fungal pathogen interactions and 4) the use of diatomaceous earth and silica aerogels to control insects in post-harvest products. All four provide new and important information which improves our understanding and perhaps, exploitation of Si in agriculture.