Some biochemical defense responses enhanced by soluble silicon in bitter gourd-powdery mildew pathosystem

Silicon (Si) has been exploited for its beneficial effects in terms of disease control in many plants. Added Si was found to accumulate beneath the cuticle and act as a barrier against pathogen penetration, and also induce biochemical defense responses in plants. This study investigated the effect of soluble silicon (Si) supply on biochemical defense responses against powdery mildew pathogen (Erysiphe sp.) in bitter gourd (Momordica charantia L.), an intermediate Si accumulator. Supplying the sand-based growth medium with 200 ppm potassium silicate (Si+) significantly reduced the severity of powdery mildew infections, elevated the activities of enzymes peroxidase, polyphenol oxidase and pathogenesis-related proteins; chitinase and beta-1,3-glucanase in bitter gourd leaves after challenged by Erysiphe sp. compared with those grown in control mix (Si-). After 7 weeks growth in Si-amended medium, leaves accumulated nearly seven times as much silicon (3.36 % dry weight) as those grown in control mix (0.44 % dry weight). The total Si content in leaves gradually decreased after the Si amendment was ceased. Si + plants exhibited a stronger antifungal activity against Cladosporium cladosporioides. Three zones with antifungal activity were revealed after separation of methanolic leaf extracts on thin-layer chromatography plates, out of which one compound was found to have induced by Si and/or Erysiphe infection, and another induced only by Si amendment. These results suggest that Si would play an active role in strengthening resistance in bitter gourd plants against powdery mildew by stimulating expression of several biochemical defense reactions.