Cranberry sauce is never the headliner of the Thanksgiving table, but a holiday meal without the tart and sweet accoutrement just feels wrong. It cuts the richness of the meal, which allows us to pile that plate high and feel less gluttonous. After this year’s Thanksgiving festivities, I did some research on cranberries and learned that they are placed within the plant order Ericales. These plants pair with a unique mycorrhizal symbiont aptly named ericoid mycorrhizae (ERM). I thought it would be cool to have the most widespread cranberry mutualist, Rhizoscyphus ericae, headline this week’s edition of Fungi Friday. Without the fungus, cranberry sauce would not be the popular holiday condiment it is today.
Cranberries and their ERM are native to acidic sphagnum bogs throughout cooler, northern regions. They are widely consumed after the growing season of most plants because these bog loving plants are evergreen, photosynthesizing year round and producing tart red fruits well into late fall and early winter. These wet and acidic environments are nutrient poor places that over millennia have nurtured the development of fungal-plant mutualistic interactions. Interactions worth studying.
Ten years ago, Kevin Kosola and his team wanted to look at the nutrient functioning of cranberry plants with intact ERM, vs. plants without the fungal symbiont. Experimental plants were grown hydroponically within a range of NO3 concentrations found in bogs inhabited by cranberries throughout the world.
Overall, Rhizoscyphus ericae enhanced the nitrate influx by eightfold in ERM colonized plants. This nutritional function of fungal symbionts in the environments inhabited by cranberry are imperative for the species growth and reproduction. The extractible NO3 in these acidic bogs is extremely low, so the plants in these systems need all the help they can get. Rhizoscyphus ericae greatly increases the capability of cranberry to utilize nitrate, in these nitrogen limited bogs.
Interestingly, cranberry along with other plants in the Ericaceae have lost the ability to uptake nitrate themselves. Nitrate in bogs has been a rare form of nitrogen within these systems up until human cultivation, because we introduce nitrate to enhance production. Thus, selection for plants to use this pool has not occurred, because nitrate over evolutionary time has been rare, and they maintain a fungal mutualism that can easily tap into this pool. This study shows that cranberry plants can utilize ammonium but depend on their fungal mutualist to allocate pools of nitrate into their roots. It’s wild to think that some mutualist overtime can reduce the nutrient absorption activities of their host. Throughout nature, we see how mutualisms heavily influence a species evolutionary trajectory. With the strong ERM dependency in cranberry, we can thank Rhizoscyphus ericae for helping the small bog fruit grow. Without the fungus, our holiday meals wouldn’t be the same.