Tannin tolerance and resistance in dark septate endophytes

Abstract

Woody roots are colonized by a wide range of fungi, including the dark septate endophytes (DSEs) which are common on the roots of plants growing under environmental stress. Since plants under these conditions tend to produce relatively large amounts of protective phenolics such as tannins, we expect that DSEs should be relatively tolerant to these compounds. We assessed the minimum inhibitory concentrations (MICs) of tannic acid for 23 species (35 isolates) of root associated fungi, including five DSEs. We also investigated fungal growth on tannins extracted from spruce roots. Tannin MICs were higher for the DSEs (15–35 mg/mL) than for the other root associated fungi tested (1–10 mg/mL) and DSE growth was superior to other fungi on spruce root extract. All DSEs produced large amounts of polyphenol oxidase and some species produced small amounts of tannase. However, their tolerance to short term tannic acid exposure combined with a susceptibility to freeze-thaw damage, implies an underlying, non-membrane associated, physical mechanism that allows DSEs to at first tolerate high tannin levels, and then actively resist them with tannin modifying enzymes.