James G. Menzies

Silicon Enhances the Accumulation of Diterpenoid Phytoalexins in Rice: A Potential Mechanism for Blast Resistance

ABSTRACT Although several reports underscore the importance of silicon (Si) in controlling Magnaporthe grisea on rice, no study has associated this beneficial effect with specific mechanisms of host defense responses against this fungal attack. In this study, however, we provide evidence that higher levels of momilactone phytoalexins were found in leaf extracts from plants inoculated with M. grisea and amended with silicon (Si(+)) than in leaf extracts from inoculated plants not amended with silicon (Si(-) ) or noninoculated Si(+) and Si(-) plants. On this basis, the more efficient stimulation of the terpenoid pathway in Si(+) plants and, consequently, the increase in the levels of momilactones appears to be a factor contributing to enhanced rice resistance to blast. This may explain the lower level of blast severity observed on leaves of Si(+) plants at 96 h after inoculation with M. grisea. The results of this study strongly suggest that Si plays an active role in the resistance of rice to blast rather than the formation of a physical barrier to penetration by M. grisea.

The protective role of silicon in the Arabidopsis–powdery mildew pathosystem

The role and essentiality of silicon (Si) in plant biology have been debated for >150 years despite numerous reports describing its beneficial properties. To obtain unique insights regarding the effect of Si on plants, we performed a complete transcriptome analysis of both control and powdery mildew-stressed Arabidopsis plants, with or without Si application, using a 44K microarray. Surprisingly, the expression of all but two genes was unaffected by Si in control plants, a result contradicting reports of a possible direct effect of Si as a fertilizer. In contrast, inoculation of plants, treated or not with Si, altered the expression of a set of nearly 4,000 genes. After functional categorization, many of the up-regulated genes were defense-related, whereas a large proportion of down-regulated genes were involved in primary metabolism. Regulated defense genes included R genes, stress-related transcription factors, genes involved in signal transduction, the biosynthesis of stress hormones (SA, JA, ethylene), and the metabolism of reactive oxygen species. In inoculated plants treated with Si, the magnitude of down-regulation was attenuated by >25%, an indication of stress alleviation. Our results demonstrate that Si treatment had no effect on the metabolism of unstressed plants, suggesting a nonessential role for the element but that it has beneficial properties attributable to modulation of a more efficient response to pathogen stress.

The influence of silicon on cytological interactions between Sphaerotheca fuliginea and Cucumis sativus

Abstract The response of epidermal cells of cucumber leaf tissue infected by Sphaerotheca fuliginea was examined by light microscopy to understand how silicon in infected host cells affects host defence mechanisms. Leaf pieces from plants treated with nutrient solutions containing 0·05, 0·50 or 2·3 m m of silicon (Si) were harvested at intervals of 24, 48, 72, 96 and 120 h after inoculation with the pathogen and examined after staining with toluidine blue or aniline blue. Si treatments significantly reduced the time to initiation of production and/or accumulation of phenolic materials in infected host epidermal cells, and increased the number of infected cells that produced and/or accumulated phenolics. The number of haustoria produced per colony of S. fuliginea was significantly reduced over time, and conidiophore development was delayed on the leaves of cucumber treated with 2·3 m m Si nutrient solution.

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.

Evaluation of plant growth-promoting rhizobacteria for biological control of pythium root rot of cucumbers grown in rockwool and effects on yield

Three strains ofPseudomonas fluorescens (63-49, 63-28, and 15), one strain ofPseudomonas corrugata (13) and one strain ofSerratia plymuthica (R1GC4) were tested on rockwool-grown cucumbers for their ability to reduce Pythium root-rot caused byPythium aphanidermatum. These strains were previously selected for biocontrol ability from collections of >4000 bacteria. Strains 63-49 and 63-28 were tested on cucumber plants grown in rockwool in two replicatedPythium-inoculated trials conducted in British Columbia (B.C). Another inoculated, replicated trial was conducted in Quebec with all five strains. Cucumber yields (fruit number and weight) were measured over a ten-week harvest period. Strain 63-49 caused an early promotion of plant growth and increased cucumber yields at early harvests. No measurable effect ofPythium inoculation on disease development was observed in the Quebec trial, due to unfavourable cool weather. However, 63-49 significantly increased the total number of cucumbers (12%) and cucumber weight (18%), compared to the non-treated control. Strains 13, 15 and R1GC4 slightly increased the cumulative cucumber yields, but strain 63-28 had no effect. In the B.C. trial, inoculation withP. aphanidermatum reduced the number and weight of cucumbers by 27%. Treatments ofPythium-inoculated cucumbers with 63-49 significantly increased fruit number and weight by 18%, compared to thePythium-inoculated control. Strain 63-28 increased the cumulative number of cucumbers over time, compared to thePythium-inoculated control, but the increase was less than with 63-49. The use ofPseudomonas spp. in rockwool-grown cucumbers can increase yields, both in the presence and absence of Pythium root rot, and with variable seasonal conditions and disease pressures.

A Comprehensive Transcriptomic Analysis of the Effect of Silicon on Wheat Plants Under Control and Pathogen Stress Conditions

The supply of soluble silicon (Si) to plants has been associated with many benefits that remain poorly explained and often contested. In this work, the effect of Si was studied on wheat plants under both control and pathogen stress (Blumeria graminis f. sp. tritici) conditions by conducting a large transcriptomic analysis (55,000 unigenes) aimed at comparing the differential response of plants under four treatments. The response to the supply of Si on control (uninfected) plants was limited to 47 genes of diverse functions providing little evidence of regulation of a specific metabolic process. Plants reacted to inoculation with B. graminis f. sp. tritici by an upregulation of many genes linked to stress and metabolic processes and a downregulation of genes linked to photosynthesis. Supplying Si to inoculated plants largely prevented disease development, a phenotypic response that translated into a nearly perfect reversal of genes regulated by the effect of B. graminis f. sp. tritici alone. These results suggest that Si plays a limited role on a plants transcriptome in the absence of stress, even in the case of a high-Si-accumulating monocot such as wheat. On the other hand, the benefits of Si in the form of biotic stress alleviation were remarkably aligned with a counter-response to transcriptomic changes induced by the pathogen B. graminis f. sp. tritici.

Recent advances in cultural management of diseases of greenhouse crops

Disease management in greenhouse crops offers a particularly difficult challenge because the environmental conditions for optimum crop production are often also conducive to disease development. With the current restrictions in pesticide usage in greenhouses, cultural and biological methods are often the only alternatives for disease management. In the last few years, several new technologies which offer more natural ways of controlling pathogens have been studied and proposed for commercial use. Canadian scientists have been involved directly in the development and/or the advancement of a considerable number of pest management technologies with potential application in greenhouses, some of which are described here. These include the use of natural products such as soluble silicon and plant extracts for disease control on long English cucumber; the use and development of biological control agents, such as Sporothrix flocculosa, Tilletiopsis spp., and Ampelomyces quisqualis, to control powdery mildews of greenhouse crops; and the sterilization of hydroponic solutions using different physical methods to prevent the introduction and spread of root and crown pathogens in closed recirculating hydroponic systems. The effectiveness of these different management practices, their possible mode of action, and their commercial potential are presented.

Silicon‐mediated resistance of Arabidopsis against powdery mildew involves mechanisms other than the salicylic acid (SA)‐dependent defence pathway

On absorption by plants, silicon (Si) offers protection against many fungal pathogens, including powdery mildews. The mechanisms by which Si exerts its prophylactic role remain enigmatic, although a prevailing hypothesis suggests that Si positively influences priming. Attempts to decipher Si properties have been limited to plants able to absorb Si, which excludes the model plant Arabidopsis because it lacks Si influx transporters. In this work, we were able to engineer Arabidopsis plants with an Si transporter from wheat (TaLsi1) and to exploit mutants (pad4 and sid2) deficient in salicylic acid (SA)-dependent defence responses to study their phenotypic response and changes in defence expression against Golovinomyces cichoracearum (Gc) following Si treatment. Our results showed that TaLsi1 plants contained significantly more Si and were significantly more resistant to Gc infection than control plants when treated with Si, the first such demonstration in a plant transformed with a heterologous Si transporter. The resistant plants accumulated higher levels of SA and expressed higher levels of transcripts encoding defence genes, thus suggesting a role for Si in the process. However, TaLsi1 pad4 and TaLsi1 sid2 plants were also more resistant to Gc than were pad4 and sid2 plants following Si treatment. Analysis of the resistant phenotypes revealed a significantly reduced production of SA and expression of defence genes comparable with susceptible controls. These results indicate that Si contributes to Arabidopsis defence priming following pathogen infection, but highlight that Si will confer protection even when priming is altered. We conclude that Si-mediated protection involves mechanisms other than SA-dependent defence responses.

Use of Inter-Simple Sequence Repeats and Amplified Fragment Length Polymorphisms to Analyze Genetic Relationships Among Small Grain-Infecting Species of Ustilago

ABSTRACT In the smut fungi, few features are available for use as taxonomic criteria (spore size, shape, morphology, germination type, and host range). DNA-based molecular techniques are useful in expanding the traits considered in determining relationships among these fungi. We examined the phylogenetic relationships among seven species of Ustilago (U. avenae, U. bullata, U. hordei, U. kolleri, U. nigra, U. nuda, and U. tritici) using inter-simple sequence repeats (ISSRs) and amplified fragment length polymorphisms (AFLPs) to compare their DNA profiles. Fifty-four isolates of different Ustilago spp. were analyzed using ISSR primers, and 16 isolates of Ustilago were studied using AFLP primers. The variability among isolates within species was low for all species except U. bullata. The isolates of U. bullata, U. nuda, and U. tritici were well separated and our data supports their speciation. U. avenae and U. kolleri isolates did not separate from each other and there was little variability between these species. U. hordei and U. nigra isolates also showed little variability between species, but the isolates from each species grouped together. Our data suggest that U. avenae and U. kolleri are monophyletic and should be considered one species, as should U. hordei and U. nigra.

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.