Date Published: June 19, 2019
Publisher: Public Library of Science
Author(s): Adam J. Houlihan, Peter Conlin, Joanne C. Chee-Sanford, Vijai Gupta.
Plant seed exudates are composed of complex mixtures of chemicals with potential for bioactive compounds with antimicrobial properties. This study focused on kochia (Kochia scoparia), one of many weedy plant species considered invasive in many agricultural systems. Extraction of compounds in water yielded an exudate mass equivalent to 7% of the original seed mass used. Water-soluble exudates were tested against 16 known plant pathogens in disk diffusion assays and kochia exudates were found to inhibit Colletotrichum graminicola, the fungal causative agent of anthracnose and stalk rot in maize. The narrow range of fungi found as targets suggested the mechanism of inhibition may be specific rather than broadly antifungal. A decline in viability of cells over four orders of magnitude occurred within six hours of exposure to exudate. The minimum inhibitory concentration was 3.125 mg L-1. Hyphae formation in C. graminicola appeared inhibited following exposure to the exudate. Small molecular weight compounds as determined by GC/MS analysis showed high relative amounts of the sugars fructose, galactopyranose, glucose, and sorbitol, along with moderate proportions of organic acids and amino acids. Protein content averaged 0.7% in the standard concentration (100 mg mL-1) used for inhibition assays. Size fractionation of the exudate and subsequent disk diffusion assays revealed bioactive fractions with compounds in the MW range <5 kDa. To the best of our knowledge, this study is the first to show promising bioactivity against C. graminicola that was associated with water-extractable compounds from a common weed species. The results suggest that seeds of persistent plant species with long-lived seed banks like kochia may have potential for use in the discovery of compounds active in inhibiting fungal pathogens.
The seed stage of the plant growth cycle is critical to plant establishment , yet relatively few studies have investigated seed-derived chemicals beyond their important role in plant development. Seed exudates are known to have natural bioactive compounds that consist of complex mixtures of molecules , and are recognized for their protective role against soil pathogens during germination [3–5], affecting microbial growth [6–11], allelopathic interactions , and pharmaceutical and industrial product development [13–15]. In contrast to root exudate studies where the importance of chemicals in the interactions between plants, soil and microbes are well known [16–18], the significance of seed exudates in the spermosphere is still not well-studied and few investigations have been made on the extent of these bioactive compounds in plant species considered invasive or weedy.
The inhibition of C. graminicola and Taphrina deformans by as yet unspecified water-extractable compounds from seeds of kochia is an intriguing finding that demonstrates a potential source of natural compounds with bioactivities of interest. The rather narrow spectrum of fungi affected by the kochia exudate suggests an antifungal mechanism that does not broadly affect other fungi. C. graminicola and T. deformans are two phylogenetically- and physiologically-unrelated fungal phytopathogens . C. graminicola is a member of the Phyllachoraceae family and causes leaf anthracnose in maize and stalk rot [44–46], with other Colletotrichum species also noted as pathogens of fruit . Colletrotrichum spp. has been listed as one of the top ten fungal pathogens to control [48, 49]. T. deformans is a member of the Taphrinaceae family that causes leaf curl in certain fruit trees , and while not further pursued in this study, the inhibition by kochia extract for this species also suggests a promising area for further study. It is not known if these fungi are pathogenic to K. scoparia. While numerous discoveries of natural compounds with antimicrobial activities have led to their pursuit as treatments for plant pathogens, e.g. [51–54], few studies have investigated weedy plant species and their seeds as potential sources for bioactive compounds.