Silicon (Si) plays a pivotal role in the nutritional status of a wide variety of monocot and dicot plant species and helps them, whether directly or indirectly, counteract abiotic and/or biotic stresses. In general, plants with a high root or shoot Si concentration are less prone to pest attack and exhibit enhanced tolerance to abiotic stresses such as drought, low temperature, or metal toxicity. However, the most remarkable effect of Si is the reduction in the intensities of a number of seedborne, soilborne, and foliar diseases in many economically important crops that are caused by biotrophic, hemibiotrophic, and necrotrophic plant pathogens. The reduction in disease symptom expression is due to the effect of Si on some components of host resistance, including incubation period, lesion size, and lesion number. The mechanical barrier formed by the polymerization of Si beneath the cuticle and in the cell walls was the first proposed hypothesis to explain how this element reduced the severity of plant diseases. However, new insights have revealed that many plant species supplied with Si have the phenylpropanoid and terpenoid pathways potentiated and have a faster and stronger transcription of defense genes and higher activities of defense enzymes. Photosynthesis and the antioxidant system are also improved for Si-supplied plants. Although the current understanding of how this overlooked element improves plant reaction against pathogen infections, pest attacks, and abiotic stresses has advanced, the exact mechanism(s) by which it modulates plant physiology through the potentiation of host defense mechanisms still needs further investigation at the genomic, metabolomic, and proteomic levels.

Silicon's Role in Abiotic and Biotic Plant Stresses.

Abstract Silicon (Si) is the second most abundant element in the earth’s crust and plays a number of important roles in the mineral nutrition of plants. In the past 20 years, the scientific documentation on the benefits of Si to crops has helped establish Si fertilization as an agronomic practice in many agricultural lands worldwide. However, very little information has been consolidated on the use of Si specifically for US agriculture. Consequently, the objectives of this review are to provide (1) information on the dynamics of Si in soil, use, and sources; (2) a history and up-to-date documentation on Si-related research in many areas of US production agriculture; and (3) perspectives on Si as a plant beneficial nutrient and the potential of Si fertilization as an agronomic practice in US crop production systems. The Si-driven mechanisms enhancing the productivity of a wide array of crops under stressed conditions are discussed in this review. Based on the recent 10-year average production level and published shoot Si content, the principal crops grown in the United States can collectively take up 9.55 million tons of Si annually, whereas the annual Si removal rate for the entire US cropland area is estimated at 21.1 million tons. On the basis of this projected annual Si removal rate, adoption of continuous intensive farming systems in the country, low solubility of soil Si, and complex chemical dynamics of Si in soil, increasing plant-available Si levels through fertilization is therefore foreseen a logical agronomic practice for US agriculture.

A Review of Silicon in Soils and Plants and Its Role in US Agriculture: History and Future Perspectives

In Brazil, coffee leaf rust, a fungal disease caused by Hemileia vastatrix Berk. et Br., was first detected in Coffea arabica in January 1970, in the southern region of Bahia state. Today, the disease is present in virtually all arabica and conilon (Coffea canephora) coffee-growing areas of Brazil, and continues to threaten coffee production with losses that range from 30 to 50 %. The disease is usually less severe at elevations above 1,200 m, where the environment is less conducive for the rust. Disease risk is increased in arabica coffee compared to conilon, and lower production is expected in the year following an epidemic due to early defoliation and drying of branches. Several varieties were developed in the country using sources of resistance from germplasm collections in Portugal. However, very few are completely resistant, instead exhibiting various levels of partial resistance. The disease is currently managed through the use of protectant and systemic fungicides including copper, triazoles and strobilurins that should be applied following rules of decision that vary according to the risk scenario. A review of the biology and epidemiology of coffee rust in Brazil is presented and the best management practices for controlling the disease based on advances in breeding for resistance and crop protection are discussed.

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Current status and management of coffee leaf rust in Brazil

A cercosporiose do cafeeiro (Coffea arabica), causada por Cercospora coffeicola, e uma das mais importantes doencas na fase de viveiro. O objetivo desse trabalho foi avaliar o efeito da aplicacao de silicio (1 g de CaSiO3 incorporada em 1 kg do substrato utilizado para encher os tubetes) no controle dessa doenca em tres variedades de cafeeiro (catuai, mundo novo e icatu) e, determinar quais os possiveis fatores de resistencia associados. As plantas com dois pares de folhas foram inoculadas com suspensao de 1,4 x 104 conidios de C. coffeicola por ml. Aos sete meses apos a inoculacao avaliaram-se a porcentagem de folhas lesionadas e o numero total de lesoes por planta. Apos essa avaliacao, retiraram-se amostras das folhas para o estudo em Microscopia Eletronica de Varredura (MEV) e Microanalise de Raios-X (MAX). As plantas da variedade catuai tratadas com silicato, tiveram reducao de 63,2% nas folhas lesionadas e de 43% no total de lesoes por planta, em relacao a testemunha. A MAX e o mapeamento para Si indicaram distribuicao uniforme do elemento em toda a superficie abaxial das folhas de cafeeiro nas tres variedades tratadas. Nas folhas das plantas nao tratadas, o Si foi raramente encontrado. Nas imagens de MEV tambem foi observada camada de cera bem desenvolvida na superficie inferior das folhas originarias de todas as plantas tratadas, sendo esta mais espessa em catuai e rara ou ausente nas nao tratadas.

Observações microscópicas do efeito do silício no controle da cercosporiose em três variedades de cafeeiro

All essential nutrients can affect the incidence and severity of plant diseases. Although silicon (Si) is not considered as an essential nutrient for plants, it stands out for its potential to decrease disease intensity in many crops. The mechanism of Si action in plant resistance is still unclear. Si deposition in plant cell walls raised the hypothesis of a possible physical barrier to pathogen penetration. However, the increased activity of phenolic compounds, polyphenol oxidases and peroxidases in plants treated with Si demonstrates the involvement of this element in the induction of plant defense responses. The studies examined in this review address the role of Si in disease control and the possible mechanisms involved in the mode of Si action in disease resistance in plants.

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Silicon in plant disease control

Coffee leaf rust, caused by Hemileia vastatrix, is the most important disease on coffee. This study aimed to evaluate the effect of silicon (Si) on some components of coffee resistance to rust and to determine if it could increase the activity of chitinases (CHI), β-1,3-glucanases (GLU), and peroxidases (POX). Coffee plants (cultivar "Catuai vermelho 44") were grown on aerated Clarck’s nutrient solution containing 0 (-Si) or 2 mmol L-1 Si (+Si) for 45 days and inoculated with H. vastatrix. There was no significant difference between the -Si and +Si treatments for Si content in leaf tissue, but Si content in root tissue was significantly higher for the +Si treatment. The -Si and +Si treatments were not significantly different for incubation period, latent period, number of pustules per leaf, pustule size, pustule expansion, and area under rust progress curve. The activity of CHI, GLU, and POX increased after inoculation with H. vastatrix, even with increase or decrease in activities during the sampling times. There was not a pronounced effect of Si in increasing the activity of these enzymes, except for rare increases in specific sampling times. Results from this study indicated that coffee plants were inefficient in uptaking Si from the nutrient solution and translocating it to the shoot. Silicon was exclusively restricted to the root system, which did not guarantee an increase in coffee resistance to rust.

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Sandra C. Pereira

Papers

Inefficiency of silicon in leaf rust control on coffee grown in nutrient solution