Feed me! A carnivorous grass?

Grasses get their carbon from the air, transforming it into carbohydrates via photosynthesis. But grasses need other elements too, including nitrogen. Unfortunately, the nitrogen in the air is in a form that is too tightly bonded to be harvested directly, and so grasses get this element from nitrogen compounds that are the by-products of decomposition in the soil, or via nitrogen fixing bacteria (such as those in legumes). But what happens if you live in nitrogen poor soils?  

Carnivorous plants are not the norm in plants, and have mostly been restricted to species that live in nitrogen poor environments and cannot get their N requirements the old fashioned (and less energy intensive) way, by absorption of N compounds through the soil or N-fixing bacteria. Instead, such plants kill and eat animals to get this essential element from their victims.

The fascination with such carnivorous plants as Nepenthes and Dionaea made me wonder whether some grasses have made the leap to carnivory, at least to supplement their normal nitrogen intake. 

Incredibly enough, there is some evidence that some grasses have made the leap to utilizing N from animal sources, albeit indirectly, through partnerships with endophytic fungi. The way they do this is simple and quite interesting, and involves fungi that kill insects in the soil. Such fungi bore thorough the insect cuticle, proliferate within, and ultimately kill it.

Roach killed by Metarhizium fungi. By Chengshu Wang and Yuxian Xia - PLoS Genetics, January 2011

One type of fungi in the genus Metarhizium (M. robertsii) is also an endophyte of the grass Panicum virgatum (switch grass), one of the native species in the old prairies of North America. It lives within the plant roots and in fact is usually found clustered in space close to such grasses. 

Researchers found that P. virgatum root hairs proliferated in the presence of this fungus (Sasan and Bidochka, 2012), and that amazingly enough, the grass "feeds" its fungal boarders carbon photosynthate (Behie et al, 2017).

In return, the grass gets a significant amount of nitrogen from the insects killed by the fungus! 

Researchers tagged test insects with the stable radioactive isotope N-15, and these insects were then exposed to M. robertsii associated with the roots of P. virgatum plants. The researchers discovered that after about a month, up to 48% of the nitrogen in the grass was directly derived from the killed insects, an astonishing percentage that points to the importance of this symbiotic relationship (Behie et al, 2012).

So, there may not be real carnivorous grasses (yet!), but thanks to symbiotic relationships, some grasses get the same positive benefits of killing animals anyways ;-)

Literature Cited

Behie, S., Moreira, C., Sementchoukova, I. et al. Carbon translocation from a plant to an insect-pathogenic endophytic fungus. Nat Commun 8, 14245 (2017). https://doi.org/10.1038/ncomms14245

Behie SW, Zelisko PM, Bidochka MJ. Endophytic insect-parasitic fungi translocate nitrogen directly from insects to plants. Science. 2012 Jun 22;336(6088):1576-7. doi: 10.1126/science.1222289. PMID: 22723421.

Sasan RK, Bidochka MJ. The insect-pathogenic fungus Metarhizium robertsii (Clavicipitaceae) is also an endophyte that stimulates plant root development. Am J Bot. 2012 Jan;99(1):101-7. doi: 10.3732/ajb.1100136. Epub 2011 Dec 14.