Serendipita indica (former Piriformospora indica) is a non-obligate endophytic fungus and generally a plant growth and defence promoter with high potential to be used in agriculture. However, S. indica may switch from biotrophy to saprotrophy losing its plant growth promoting traits. Our aim was to understand if the free-living stage growth conditions (namely C availability) regulate S. indica’s phenotype, and its potential as plant-growth-promoting-microbe (PGPM). We grew S. indica in its free-living stage under increasing C availabilities (2–20 g L^–1 of glucose or sucrose). We first characterised the effect of C availability during free-living stage growth on fungal phenotype: colonies growth and physiology (plasma membrane proton pumps, stable isotopic signatures, and potential extracellular decomposing enzymes). The effect of the C availability during the free-living stage of the PGPM was evaluated on wheat. We observed that C availability during the free-living stage regulated S. indica’s growth, ultrastructure and physiology, resulting in two distinct colony phenotypes: compact and explorer. The compact phenotype developed at low C, used peptone as the major C and N source, and displayed higher decomposing potential for C providing substrates; while the explorer phenotype developed at high C, used glucose and sucrose as major C sources and casein and yeast extract as major N sources, and displayed higher decomposing potential for N and P providing substrates. The C availability, or the C/N ratio, during the free-living stage left a legacy to the symbiosis stage, regulating S. indica’s potential to promote plant growth: wheat growth promotion by the explorer phenotype was ± 40% higher than that by the compact phenotype. Our study highlights the importance of considering microbial ecology in designing PGPM/biofertilizers. Further studies are needed to test the phenotypes under more extreme conditions, and to understand if the in vitro acquired characteristics persist under field conditions.

印end （前任的 印度梨形孢）是一种非专性的内生真菌，通常是具有高潜力的植物生长和防御促进剂，可用于农业。然而，S.籼稻可能会从生物养分转向腐生而失去其促进植物生长的性状。我们的目的是了解自由生活阶段的生长条件（即C可用性）是否在调节S.籼稻的表型及其作为促进植物生长的微生物（PGPM）的潜力。我们成长了S.籼稻在C利用率增加（2-20 g L ^–1葡萄糖或蔗糖）的自由生活阶段。我们首先表征了自由活动阶段生长过程中C有效性对真菌表型的影响：菌落生长和生理学（质膜质子泵，稳定的同位素特征以及潜在的细胞外分解酶）。在小麦上评估了PGPM自由生活阶段碳可利用性的影响。我们观察到自由生活阶段的C可用性受到调节S.籼稻的生长，超微结构和生理学，导致两种明显的菌落表型：紧密型和探索型。紧凑的表型在低碳下发展，使用蛋白as作为主要的碳和氮源，对提供碳的底物显示出更高的分解潜能。而探索者的表型是在高温下发展的，使用葡萄糖和蔗糖作为主要的C来源，而酪蛋白和酵母提取物作为主要的N来源，显示出提供N和P的底物更高的分解潜力。自由生活阶段的C可用性或C / N比为共生阶段留下了遗产，S.籼稻促进植物生长的潜力：探索者表型对小麦生长的促进作用比紧密表型高±40％。我们的研究突出了在设计PGPM /生物肥料时考虑微生物生态学的重要性。需要进一步研究以测试更极端条件下的表型，并了解体外 获得的特性在野外条件下仍然存在。