ABSTRACT The root endophytic fungus Piriformospora indica plays an important role in increasing abiotic stress tolerance of its host plants. To explore the impact of P. indica on freezing tolerance, Arabidopsis seedlings were co-cultivated with P. indica exposed to −6°C for 6 h. Freezing stress decreased the survival rate, electrolyte leakage, leaf temperature, water potential and chlorophyll fluorescence of Arabidopsis plants in comparison to the controls. P. indica colonizion reduced the negative effects of freezing, and the plants contained also higher amounts of soluble proteins, proline and ascorbic acid during the post-thaw recovery period (4°C; 12 h). In contrast, the H2O2 and malondialdehyde levels were reduced in seedlings colonized by the fungus. The brassinolide (BR) and abscisic acid (ABA) levels dramatically increased and the transcript levels of several crucial freezing-stress related genes (CBFs, CORs, BZR1, SAG1 and PYL6) were higher in inoculated plants during the post-thaw recovery period. Finally, inocculated mutants impaired in the freezing tolerance response (such as ice1 for INDUCER OF CBF EXPRESSION1, a crucial basic helix-loop-helix transcription factor for the cold-response pathway in Arabidopsis, cbf1, −2, −3 for C-REPEAT-Binding Factor, cor47 and −15 for COLD-REGULATED and siz1 encoding the SUMO E3 LIGASE) showed better survival rates and higher expression levels of freezing-related target genes after freezing compared to the inocculated controls. Our results demonstrate that P. indica confers freezing tolerance and better post-thaw recovery in Arabidopsis, and stimulates the expression of several genes involved in the CBF-dependent pathway.

中文翻译：

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Piriformospora indica 通过刺激 CBF 基因的表达增强拟南芥的冷冻耐受性和解冻后恢复

摘要 根内生真菌 Piriformospora indica 在提高其寄主植物的非生物胁迫耐受性方面发挥着重要作用。为了探索 P. indica 对冷冻耐受性的影响，将拟南芥幼苗与暴露于 -6°C 的 P. indica 共培养 6 小时。与对照相比，冷冻胁迫降低了拟南芥植物的存活率、电解质泄漏、叶温、水势和叶绿素荧光。P. indica 定植减少了冷冻的负面影响，并且在解冻后恢复期（4°C；12 小时），植物还含有更多的可溶性蛋白质、脯氨酸和抗坏血酸。相比之下，真菌定植的幼苗中的 H2O2 和丙二醛水平降低。在解冻后恢复期，接种植物的芸苔素内酯 (BR) 和脱落酸 (ABA) 水平显着增加，并且几个关键的冷冻胁迫相关基因（CBFs、CORs、BZR1、SAG1 和 PYL6）的转录水平较高。最后，接种的突变体在冷冻耐受性反应中受损（如 INDUCER OF CBF EXPRESSION1 的 ice1，拟南芥冷反应途径的关键基本螺旋-环-螺旋转录因子，cbf1，-2，-3 用于 C-REPEAT与接种的对照相比，-结合因子、COLD-REGULATED 和编码 SUMO E3 LIGASE 的 siz1 的结合因子 cor47 和 -15 显示出更好的存活率和更高的冷冻相关靶基因表达水平。我们的结果表明 P. indica 赋予拟南芥抗冻性和更好的解冻后恢复能力，