Prediction of the secretomes of endophytic and nonendophytic fungi reveals similarities in host plant infection and colonization strategies

Endophytic fungi are microorganisms that inhabit internal plant tissues without causing apparent damage. During the infection process, both endophytic and phytopathogenic fungi secrete proteins to resist or supplant the plant’s defense mechanisms. This study analyzed the predicted secretomes of six species of endophytic fungi and compared them with predicted secretomes of eight fungal species with different lifestyles: saprophytic, necrotrophic, hemibiotrophic, and biotrophic. The sizes of the predicted secretomes varied from 260 to 1640 proteins, and the predicted secretomes have a wide diversity of CAZymes, proteases, and conserved domains. Regarding the CAZymes in the secretomes of the analyzed fungi, the most abundant CAZyme families were glycosyl hydrolase and serine proteases. Several predicted proteins have characteristics similar to those found in small, secreted proteins with effector characteristics (SSPEC). The most abundant conserved domains, besides those found in the SSPEC, have oxidation activities, indicating that these proteins can protect the fungus against oxidative stress, against domains with protease activity, which may be involved in the mechanisms of nutrition, or against lytic enzymes secreted by the host plant. This study demonstrates that secretomes of endophytic and nonendophytic fungi share an arsenal of proteins important in the process of infection and colonization of host plants.

内生真菌（Endophytic fungi）是一类栖息于植物内部组织且不会引发明显病害的微生物。在侵染过程中，内生真菌与植物病原真菌均会分泌蛋白质，以抵御或突破植物的防御机制。本研究对6种内生真菌的预测分泌组（predicted secretomes）进行了分析，并将其与8种具有不同生活方式的真菌（腐生、死体营养型、半活体营养型、活体营养型）的预测分泌组进行了对比。预测分泌组的蛋白数量介于260至1640之间，且包含丰富多样的碳水化合物活性酶（CAZymes）、蛋白酶以及保守结构域。在所分析真菌的分泌组中，丰度最高的CAZyme家族为糖基水解酶与丝氨酸蛋白酶。部分预测蛋白质具有与具备效应蛋白特征的小分泌蛋白质（SSPEC）相似的特性。除SSPEC相关保守结构域外，丰度最高的保守结构域具有氧化活性，提示这些蛋白质可帮助真菌抵御氧化应激；部分带有蛋白酶活性的结构域可能参与营养获取过程，或可抵御宿主植物分泌的裂解酶。本研究表明，内生真菌与非内生真菌的分泌组共享一套在宿主植物侵染与定殖过程中发挥关键作用的蛋白质库。