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Phymatotrichopsis omnivora (Duggar) Hennebert causes a destructive root rot in cotton, alfalfa (Medicago sativa), and many other dicot species. No consistently effective control measures or resistant host germplasm for Phymatotrichum root rot (PRR) are known. The relative genetic intractability of cotton and alfalfa precludes their use as model pathosystem hosts for P. omnivora. Therefore, we used the model legume M. truncatula and its available genetic and genomic resources to investigate PRR. Confocal imaging of P. omnivora interactions with M. truncatula roots revealed that the mycelia do not form any specialized structures for penetration and mainly colonize cortical cells and, eventually, form a mycelial mantle covering the root's surfaces. Expression profiling of M. truncatula roots infected by P. omnivora identified several upregulated genes, including the pathogenesis-related class I and class IV chitinases and genes involved in reactive oxygen species generation and phytohormone (jasmonic acid and ethylene) signaling. Genes involved in flavonoid biosynthesis were induced (2.5- to 10-fold over mock-inoculated controls) at 3 days postinoculation (dpi) in response to fungal penetration. However, the expression levels of flavonoid biosynthesis genes returned to the basal levels with the progress of the disease at 5 dpi. These transcriptome results, confirmed by real-time quantitative polymerase chain reaction analyses, showed that P. omnivora apparently evades induced host defenses and may downregulate phytochemical defenses at later stages of infection to favor pathogenesis.

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In greenhouse production, most floricultural crops are cultivated in soilless substrates, which often supply limited amounts of plant-available silicon (Si). The goal of this study was to determine the effects of Si supplementation on greenhouse-produced ornamental sunflower (Helianthus annuus L. ‘Ring of Fire’). Potassium silicate (KSiO3) substrate incorporation or weekly substrate drenches, sodium silicate (NaSiO3) foliar applications, and rice husk ash substrate incorporation were used as Si supplements. Silicon content of Si-treated plants increased compared with untreated controls. Depending on the source and concentration of silicon supplied, several horticultural traits were improved as a result of Si supplementation. Thick, straight stems, increased flower and stem diameters, and increased height were observed in some of the treatments, upgrading sunflower quality compared with untreated controls. However, growth abnormalities were observed when concentrations of 100 and 200 mg·L−1 Si were supplied as KSiO3 substrate drenches. In these treatments, plants appeared stunted with deformed flowers and were delayed in flowering. Consequently, Si supplementation effects on greenhouse-produced sunflowers can vary from beneficial to detrimental depending on the applied source and concentration.

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