Water-soluble exudates from seeds of <i>Kochia scoparia</i> exhibit antifungal activity against <i>Colletotrichum graminicola</i>

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.植物种子渗出液（Plant seed exudates）由复杂的化学混合物组成，其中蕴含具备抗菌活性的生物活性成分。本研究聚焦于地肤（Kochia scoparia）——一种在诸多农业生产系统中被认定为入侵性杂草的植物物种。采用水提法提取其中化合物，所得渗出物质量约为所用初始种子质量的7%。通过纸片扩散法（disk diffusion assays），将水溶性渗出物与16种已知植物病原菌进行拮抗测试，结果发现地肤渗出物可抑制禾谷炭疽菌（Colletotrichum graminicola）——一种引发玉米炭疽病与茎腐病的真菌致病菌。仅能抑制有限种类真菌的测试结果表明，其抑菌机制可能具有特异性，而非广谱抗真菌作用。病原菌细胞在暴露于渗出物后的6小时内，活力下降了四个数量级。该渗出物的最低抑菌浓度为3.125 mg·L⁻¹。禾谷炭疽菌的菌丝形成在暴露于渗出物后受到显著抑制。经气相色谱-质谱联用（GC/MS）分析鉴定的小分子化合物中，果糖、吡喃半乳糖、葡萄糖与山梨醇的相对含量较高，同时还含有一定比例的有机酸与氨基酸。在抑菌测试所用的标准浓度（100 mg·mL⁻¹）下，渗出物的蛋白质含量平均为0.7%。对渗出物进行分子尺寸分级分离，并开展后续纸片扩散测试，结果显示分子量（MW）小于5 kDa的组分具备生物活性。据我们所知，本研究首次证实，从常见杂草物种中提取的水溶性化合物对禾谷炭疽菌具有良好的抑菌活性。研究结果表明，像地肤这类拥有持久种子库的植物物种，其种子有望用于开发抑制真菌病原菌的活性化合物。