Although silicon (Si) has not been recognized as an essential element for plant growth, the beneficial effects of Si have been observed in a wide variety of plant species. The beneficial effects of Si are usually expressed more clearly in Si-accumulating plants under various abiotic and biotic stress conditions. Silicon is effective in controlling various pests and diseases caused by both fungi and bacteria in different plant species. Silicon also exerts alleviative effects on various abiotic stresses including salt stress, metal toxicity, drought stress, radiation damage, nutrient imbalance, high temperature, freezing and so on. These beneficial effects are mainly attributed to the high accumulation of silica on the tissue stirface although other mechanisms have also been proposed. To obtain plants resistant to multiple stresses, genetic modification of the root ability to take up Si has been proposed. In this review, the role of Si in conferring resistance to mutiple stresses is described.

https://www.tandfonline.com/doi/pdf/10.1080/00380768.2004.10408447

Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses

Summary
Some plant terpenes such as sterols and carotenes are part of primary metabolism and found essentially in all plants. However, the majority of the terpenes found in plants are classified as ‘secondary’ compounds, those chemicals whose synthesis has evolved in plants as a result of selection for increased fitness via better adaptation to the local ecological niche of each species. Thousands of such terpenes have been found in the plant kingdom, but each species is capable of synthesizing only a small fraction of this total. In plants, a family of terpene synthases (TPSs) is responsible for the synthesis of the various terpene molecules from two isomeric 5-carbon precursor ‘building blocks’, leading to 5-carbon isoprene, 10-carbon monoterpenes, 15-carbon sesquiterpenes and 20-carbon diterpenes. The bryophyte Physcomitrella patens has a single TPS gene, copalyl synthase/kaurene synthase (CPS/KS), encoding a bifunctional enzyme producing ent-kaurene, which is a precursor of gibberellins. The genome of the lycophyte Selaginella moellendorffii contains 18 TPS genes, and the genomes of some model angiosperms and gymnosperms contain 40–152 TPS genes, not all of them functional and most of the functional ones having lost activity in either the CPS- or KS-type domains. TPS genes are generally divided into seven clades, with some plant lineages having a majority of their TPS genes in one or two clades, indicating lineage-specific expansion of specific types of genes. Evolutionary plasticity is evident in the TPS family, with closely related enzymes differing in their product profiles, subcellular localization, or the in planta substrates they use.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2011.04520.x

The family of terpene synthases in plants: A mid-size family of genes for specialized metabolism that is highly diversified throughout the kingdom

http://www.medicinalgenomics.com/wp-content/uploads/2011/12/Monoterpene-and-sesquiterpene-synthases-and-the-origin-of-terpene-skeletal.pdf

Monoterpene and sesquiterpene synthases and the origin of terpene skeletal diversity in plants.

Terpene synthases and the regulation, diversity and biological roles of terpene metabolism.

The essential micronutrients for field crops are B, Cu, Fe, Mn, Mo, and Zn. Other mineral nutrients at low concentrations considered essential to growth of some plants are Ni and Co. The incidence of micronutrient deficiencies in crops has increased markedly in recent years due to intensive cropping, loss of top soil by erosion, losses of micronutrients through leaching, liming of acid soils, decreased proportions of farmyard manure compared to chemical fertilizers, increased purity of chemical fertilizers, and use of marginal lands for crop production. Micronutrient deficiency problems are also aggravated by the high demand of modern crop cultivars. Increases in crop yields from application of micronutrients have been reported in many parts of the world. Factors such as pH, redox potential, biological activity, SOM, cation-exchange capacity, and clay contents are important in determining the availability of micronutrients in soils. Plant factors such as root and root hair morphology (length, density, surface area), root-induced changes (secretion of H + , OH − , HCO 3 − ), root exudation of organic acids (citric, malic, tartaric, oxalic, phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant demand, plant species/cultivars, and microbial associations (enhanced CO 2 production, rhizobia, mycorrhizae, rhizobacteria) have profound influences on plant ability to absorb and utilize micronutrients from soil. The objectives of this article are to report advances in research on the micronutrient availability and requirements for crops, in correcting deficiencies and toxicities in soils and plants, and in increasing the ability of plants to acquire needed amounts of micronutrient elements.

Micronutrients in Crop Production

Soluble silicon: its role in crop and disease management of greenhouse crops

The effect of soluble potassium silicate applied to cucumber (Cucumis sativus L.), muskmelon (C. melo L.), and zucchini squash (Cucurbita pepo L.) on the severity of powdery mildew was examined. Application methods included amending nutrient solutions to a concentration of 1.7 mM Si and foliar sprays containing 1.7, 8.5, 17, and 34 mM Si. Untreated plants and plants sprayed with distilled water were used as controls. The leaves of all plants were inoculated with known concentrations of conidia of Sphaerotheca fuliginea (Schlecht.:Fr.) Poll. (cucumber and mum Mathai et al., 1978; Volk et al., 1958) and powdery mildew diseases of barley (Erysiphe gruminis DC. f. sp. hordei E m . Marchal on Hordeum vulgare L.) (Jiang et al., 1989), wheat (E. graminis D.C. f. sp. tritici Em. Marchal on Triticum aestivum L.) (Leusch and Buchenauer, 1989), and cucumber (S. fuliginea) (Menzies et al., 1991a). Previous studies of Si effects on plant disease have involved root absorption of Si from either soil or soilless media. Although the amendment of soilless media with Si can be adapted to greenhouse and similar industries (Menzies et al., 1991a; Samuels et al., 1991b), large amounts of Si must be applied to soils to adequately control diseases. For example, basal dressings of 250 and 500 kg sodium silicate/ha were required to reduce sheath blight of rice (Mathai et al., 1978) and an equivalent of 4500 kg SiO2/ha was necessary to control powdery mildew of wheat (Leusch and Buchenauer, 1989). A reduction of the incidence of a wilt pathogen of cucumber requires the application of 2000 to 4000 kg calcium silicate/ha or 2250 to 4500 kg potassium silicate/ ha (Miyake and Takahashi, 1983b). Foliar application of Si to field plants is potentially a viable alternative to root-zone applifor publication 5 Nov. 1991. Accepted for publication 6 May 1992. esearch Station Contribution no. 440. We acknowlege the assistance muels, D. Vianzon, C. Koch, and T. Helmer, the statistical advice , Research Station, Agriculture Canada, Vancouver, B.C., and the the B.C. Greenhouse Vegetable Research Council and the Science B.C. Mention of a trademark or proprietary product does not imply nt of the products named or criticism of other products that may also . The cost of publishing this paper was defrayed in part by the payage charges. Under postal regulations, this paper therefore must be rked advertisement solely to indicate this fact. Scientist. cation. This method of Si application to control disease has been reported only once, to our knowledge. Aleshin et al. (1986) applied sprays of sodium metasilicate (440 mg·liter-1) and 1-ethoxysilatran (180 mg·liter -l) and reduced the degree of rice blast infestation on both resistant and susceptible rice cultivars. The objective of our study was to determine the extent of control of powdery mildew diseases of cucumber, muskmelon, and zucchini squash achieved through foliar or root applications of soluble potassium silicate, and the duration of control that may be obtained with foliar applications. Materials and Methods Long English cucumber (‘Corona’), muskmelon (‘Earlisweet’) and zucchini squash (‘Select’) were used in the experiments. Effective Si concentrations for foliar application. Cucumber, muskmelon, and zucchini squash plants were grown from seed sown in rockwool cubes (Grodan, Roermond, Holland), arranged at random on three greenhouse benches. Eight randomly chosen plants of each species were watered as needed with a nutrient solution containing ( in mM ): 1.75 Si, 13.0 NO3, 1.5 H 2P O4, 7.5 K, 3.5 Ca, 1.0 Mg, and 1.0 to 1.5 SO4; and (in μ M ) 18.8 Fe, 5.5 Mn, 0.9 Zn, 0.2 Cu, 18.1 B, and 1.0 Mo, with a pH of 6.0. Silicon was added in the form of potassium silicate as product K6 (National Silicates Ltd., Toronto, Ont.). The remaining plants were watered with a solution (pH 6.0) containing the same nutrients but without added potassium silicate (the Si and K contents of the solution were 0.05 and 7.0 m M , respectively). Foliar treatments were applied when cucumber and zucchini squash had two fully expanded leaves and muskmelon had three. Treatments included an untreated control, a distilled water spray (pH 5.5), and Si sprays of 1.7, 8.5 (not applied to the zucchini squash), and 17.0 mM (also containing 0.5, 2.5, and 5.0 mM K, respectively). The Si sprays were prepared by diluting the appropriate amount of potassium silicate in distilled water and adjusting to pH 5.5 with phosphoric acid. A drop of Tween 20 J. Amer. Soc. Hort. Sci. 117(6):902-905. 1992. Table 1. The effect of Si treatments on the number of colonies of powdery mildew on cucumber, muskmelon, and zucchini squash. z Colonies established (no.)

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Foliar applications of potassium silicate reduce severity of powdery mildew on cucumber, muskmelon, and zucchini squash

The effect of root or leaf applications of soluble Si on severity of grape (Vitis vinifera L.) powdery mildew (Uncinula necator (Schwein) Burrill) was determined. On potted plants, root-feeding at 1.7 m M Si had no effect on disease severity, but foliar sprays at 17 m M Si substantially reduced the number of mildew colonies that developed on inoculated leaves. Scanning electron micrographs showed that, on Si-sprayed leaves, hyphae did not develop in areas where thick Si deposits were present on the leaf surface; and where surface deposits were not present, Si was translocated laterally through the leaf and surrounded the appressoria. Leaves on plants that were fed Si via roots showed a similar deposition of Si surrounding the appressoria. On water-sprayed leaves and leaves from untreated plants, internal deposition of Si was more variable and generally less than on Si-sprayed or root-fed plants. Conidia germination and germtube development on agar media were weakly promoted by the presence of Si. Reduced severity of grape mildew by Si sprays may be partly due to a physical barrier to hyphal penetration and to a resistance response involving the lateral movement of Si and its deposition within the leaf at fungal penetration sites. In monocot crops, the association between Si and reduced severity of fungal diseases has been known for some time. Ger- mar (1934) reported that wheat (Triticum aestivum L.) plants supplied with Si are more resistant to powdery mildew (Erysiphe graminis f. sp. hordei) than control plants. Since then, Si has been implicated in several other monocot disease resistance re- sponses, including sorghum (Sorghum vulgare Pert.) resistance to anthracnose (Colletotricum graminicolum) (Narwal, 1973), barley (Hordium vulgare L.) and wheat resistance to powdery mildew (E. graminis f. sp. hordei) (Jiang et al., 1989; Kunoh and Ishizaki, 1976; Leusch and Buchenauer, 1989; Sargent and Gay, 1977), and rice (Oryza sativa L.) resistance to blast (Pir- icularia oryzae Cav.), brown spot (Bipolaris oryzae Shoemaker) and sheath blight (Corticium sasakii Shiriai) (Aleshin et al., 1986; Datnoff and Snyder, 1991; Mathai et al., 1978; Volk et al., 1958). The exact role silica plays in enhancing disease re- sistance in monocots is as yet undetermined, but localized de- posits of Si have been found in host tissue surrounding fungal haustoria (Kunoh and Ishizaki, 1976; Sargent and Gay, 1977). In dicots, less attention has been paid to the association be- tween Si and resistance to fungal infection. Although Wagner (1940) reported that cucumber powdery mildew severity was reduced by supplying plants with Si, current interest in this phenomenon was not spawned until the mid-1980s, when it was reported that the natural incidence of powdery mildew was re- duced by feeding Si to solution-cultured cucumbers (Adatia and Besford, 1986; Miyake and Takahashi, 1983). The reduction in

/pdf/soluble-silicon-sprays-inhibit-powdery-mildew-development-on-w3nxong7zd.pdf

Soluble silicon sprays inhibit powdery mildew development on grape leaves

Commercial perennial agriculture is prone to declining productivity due to negative plant-soil feedback. An alternative to costly and environmentally harmful conventional treatment such as soil fumigation could be to manipulate soil microbial diversity through careful selection and management of cover crop mixtures. Although cover crops are already used in these systems for other reasons, their capacity to influence soil biota is unexploited. Here, we examine the role of plant diversity and identity on plant-soil feedbacks in the context of perennial agriculture. We identify key microorganisms involved in these feedbacks and explore plant-based strategies for mitigating decline of perennial crop plants. We conclude that (1) increasing plant diversity increases soil microbial diversity, minimizing the proliferation of soil-borne pathogens; (2) populations of beneficial microbes can be increased by increasing plant functional group richness, e.g., legumes, C4 grasses, C3 grasses, and non-leguminous forbs; (3) brassicas suppress fungal pathogens and promote disease-suppressive bacteria; (4) native plants may further promote beneficial soil microbiota; and (5) frequent tillage, herbicide use, and copper fungicides can harm populations of beneficial microbes and, in some cases, contribute to greater crop decline. Non-crop vegetation management is a viable and cost-effective means of minimizing crop decline in perennial monocultures but is in need of more direct experimental investigation in perennial agroecosystems.

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Cover crops to increase soil microbial diversity and mitigate decline in perennial agriculture. A review

Vitis vinifera terpenoid cyclases: functional identification of two sesquiterpene synthase cDNAs encoding (+)-valencene synthase and (-)-germacrene D synthase and expression of mono- and sesquiterpene synthases in grapevine flowers and berries.

Pat Bowen

Papers

Soluble silicon: its role in crop and disease management of greenhouse crops

Foliar applications of potassium silicate reduce severity of powdery mildew on cucumber, muskmelon, and zucchini squash

Soluble silicon sprays inhibit powdery mildew development on grape leaves

Cover crops to increase soil microbial diversity and mitigate decline in perennial agriculture. A review

Vitis vinifera terpenoid cyclases: functional identification of two sesquiterpene synthase cDNAs encoding (+)-valencene synthase and (-)-germacrene D synthase and expression of mono- and sesquiterpene synthases in grapevine flowers and berries.