Ultrastructural and Cytochemical Aspects of Silicon-Mediated Rice Blast Resistance

Abstract

Although exogenous application of silicon (Si) confers efficient control of rice blast, the probable hypothesis underlying this phenomenon has been confined to that of a mechanical barrier resulting from Si polymerization in planta. However, in this study, we provide the first cytological evidence that Si-mediated resistance to Magnaporthe grisea in rice correlates with specific leaf cell reaction that interfered with the development of the fungus. Accumulation of an amorphous material that stained densely with toluidine blue and reacted positively to osmium tetroxide was a typical feature of cell reaction to infection by M. grisea in samples from Si+ plants. As a result, the extent of fungal colonization was markedly reduced in samples from Si+ plants. In samples from Si- plants, M. grisea grew actively and colonized all leaf tissues. Cytochemi-cal labeling of chitin revealed no difference in the pattern of chitin localization over fungal cell walls of either Si+ or Si- plants at 96 h after inoculation, indicating limited production of chitinases by the rice plant as a mechanism of defense response. On the other hand, the occurrence of empty fungal hyphae, surrounded or trapped in amorphous material, in samples from Si+ plants suggests that phenolic-like compounds or phytoalexins played a primary role in rice defense response against infection by M. grisea. This finding brings new insights into the complex role played by Si in the nature of rice blast resistance.

Eficiência de argila silicatada no controle de Xanthomonas axonopodis pv. passiflorae, in vitro e em mudas de maracujazeiro-amarelo

“…Possivelmente, os mecanismos pelos quais o silício esteja controlando a bacteriose seja uma ação direta contra o patógeno, como constatado no teste in vitro, e também por indução de resistência na planta do maracujazeiro, seja por barreiras estruturais, como o acúmulo desse elemento na parede das células da epiderme e da cutícula, tornando-as menos acessíveis às enzimas de degradação (Barbosa Filho et al, 2000;Korndorfer & Pereira, 2001), seja por acúmulo no local de penetração do patógeno (Rodrigues et al, 2003) ou por ativar barreiras químicas e bioquímicas das plantas, favorecendo maior acúmulo de compostos fenólicos e peroxidases no local da injúria . nesse mesmo sentido, Pozza et al (2004) observaram que o aumento da resistência do cafeeiro (Coffea arabica l.) à cercosporiose foi devido ao maior espessamento da cutícula e ao aumento da absorção de micronutrientes pelas plantas tratadas com silício.…”

Capacidade fotossintética de plantas de algodoeiro infectadas por ramulose e supridas com silício

“…Tem sido relatado que aplicações do elemento Si podem reduzir a intensidade de várias doenças (Ranganathan et al, 2006;Rodrigues et al, 2003), a toxidez de alguns metais pesados, como boro e manganês (Feng et al, 2009;Gunes et al, 2007; Hodson e Sangster, 2002) e o estresse salino (Gong et al, 2003;Liang et al, 2006). Em arroz, o Si diminuiu a transpiração e a taxa da permeabilidade das membranas das plantas sob déficit hídrico (Agarie et al, 1998).…”

Effect of root and leaf applications of soluble silicon on blast development in rice

“…(teleomorph Magnaporthe oryzae (T.T. Hebert) Yaegashi & Udagawa) Barr is the most important fungal disease of rice (Oryza sativa L.) and causes significant yield losses worldwide (Ou, 1985;Skamnioti et al, 2009). The application of silicon (Si) to rice fields to control blast is an alternative, especially when rice is grown in soils with low Si availability Seebold et al, 2000).…”

“…A thick layer of silica beneath the cuticle of rice leaves and sheaths (Yoshida et al, 1962) can impede P. oryzae penetration and then decrease the number of lesions on the leaf blades ). In addition, the soluble Si in plant tissue may somehow be associated with an increase in rice resistance to blast through the production of phenolic-like compounds, diterpenoid phytoalexins and the activation of some PR-genes (Rodrigues et al, 2003;2004;.…”