Department of Plant Pathology, Washington State University, Pullman, WA 99164, USA.
Department of Plant Pathology, Washington State University, Pullman, WA 99164, USA.
Fungal Genet Biol. 2020 Dec;145:103482. doi: 10.1016/j.fgb.2020.103482. Epub 2020 Oct 31.
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.
核盘菌可引起大豆、油菜、番茄、豌豆和向日葵等多种重要经济作物的白霉病。作为最成功的植物病原体之一，核盘菌具有适应各种环境条件的独特能力，并能有效抑制或逃避植物防御。值得注意的是，核盘菌分泌一系列植物细胞壁降解酶（CWDEs）来软化宿主细胞壁，并利用释放的单糖和寡糖作为营养物质。植物细胞壁的主要成分之一是果胶中的多聚半乳糖醛酸，其中 D-半乳糖醛酸是最丰富的成分。在这项研究中，我们鉴定了四个编码 D-半乳糖醛酸分解代谢的核盘菌基因，即 Ssgar1、Ssgar2、Sslgd1 和 Sslga1。创建了所有四个分解代谢基因的基因敲除突变体。当在果胶作为替代碳源进行培养时，Sslgd1 和 Sslga1 缺失突变体以及 Ssgar1/Ssgar2 双缺失突变体的生长明显受到抑制。D-半乳糖醛酸分解代谢基因受到培养基中多聚半乳糖醛酸或宿主感染过程中的转录诱导。敲除突变体的毒力测试表明，Ssgar2、Sslgd1 和 Sslga1 均有助于核盘菌有效定殖大豆和豌豆的叶片，但不能定殖番茄的叶片，因为番茄叶片中的 D-半乳糖醛酸含量最低。除了在毒力方面的积极作用外，这四种酶都对核盘菌耐受盐胁迫产生负面影响。SsGAR2 在耐受刚果红方面具有额外的功能，这表明其在核盘菌细胞壁稳定性方面可能具有潜在作用。这项研究首次报道了 D-半乳糖醛酸分解代谢基因在核盘菌毒力、盐度响应和细胞壁完整性方面的多种功能。
