Sclerotinia sclerotiorum causes white mold disease on a wide range of economically important crops such as soybean, canola, tomato, pea and sunflower. As one of the most successful plant pathogens, S. sclerotiorum has the unique ability of adapting to various environmental conditions and effectively suppressing or evading plant defense. Notably, S. sclerotiorum secretes an array of plant cell-wall degrading enzymes (CWDEs) to macerate host cell wall and utilizes the liberated monosaccharides and oligosaccharides as nutrients. One of the major plant cell wall constituents is polygalacturonic acid in pectin, with D-galacturonic acid being the most abundant component. In this research, we identified four S. sclerotiorum genes that encode the enzymes for the D-galacturonic acid catabolism, namely Ssgar1, Ssgar2, Sslgd1 and Sslga1. Gene-knockout mutants were created for all four catabolic genes. When cultured on pectin as the alternative carbon source, Sslgd1- and Sslga1-deletion mutants and Ssgar1/Ssgar2 double deletion mutants exhibited significantly reduced growth. The D-galacturonic acid catabolic genes are transcriptionally induced by either polygalacturonic acid in the culture media or during host infection. Virulence tests of the knockout mutants revealed that Ssgar2, Sslgd1 and Sslga1 all facilitated the effective colonization of S. sclerotiorum to the leaves of soybean and pea, but not of tomato which has the lowest D-galacturonic acid contents in its leaves. In addition to their positive roles in virulence, all four enzymes negatively affect S. sclerotiorum tolerance to salt stress. SsGAR2 has an additional function in tolerance to Congo Red, suggesting a potential role in cell wall stability of S. sclerotiorum. This study is the first report revealing the versatile functions of D-galacturonic acid catabolic genes in S. sclerotiorum virulence, salinity response and cell wall integrity.

核盘菌可在多种具有重要经济意义的作物上引起白霉病，例如大豆、油菜、番茄、豌豆和向日葵。作为最成功的植物病原体之一，核盘菌具有独特的适应各种环境条件并有效抑制或逃避植物防御的能力。值得注意的是，核盘菌分泌一系列植物细胞壁降解酶 (CWDE) 来浸渍宿主细胞壁，并利用释放的单糖和寡糖作为营养物质。主要的植物细胞壁成分之一是果胶中的聚半乳糖醛酸，其中 D-半乳糖醛酸是最丰富的成分。在这项研究中，我们确定了四种核盘菌编码 D-半乳糖醛酸分解代谢酶的基因，即Ssgar1、Ssgar2、Sslgd1和Sslga1。为所有四种分解代谢基因创建了基因敲除突变体。当以果胶作为替代碳源培养时，Sslgd1-和Sslga1-缺失突变体和Ssgar1/Ssgar2双缺失突变体表现出显着降低的生长。D-半乳糖醛酸分解代谢基因由培养基中或宿主感染期间的聚半乳糖醛酸转录诱导。敲除突变体的毒力测试表明Ssgar2、Sslgd1和Sslga1所有这些都促进了核盘菌在大豆和豌豆的叶子上的有效定殖，但对叶子中 D-半乳糖醛酸含量最低的番茄则不然。除了它们在毒力方面的积极作用外，所有四种酶都会对核盘菌对盐胁迫的耐受性产生负面影响。SsGAR2具有耐受性刚果红的附加功能，这表明在细胞壁稳定的潜在作用菌核病。这项研究是第一份揭示 D-半乳糖醛酸分解代谢基因在核盘菌毒力、盐度反应和细胞壁完整性中的多功能功能的报告。