Fun­gus Helps Car­niv­o­rous Plant Di­gest Prey

by Kevin Blake

It is well-es­tab­lished that mi­croor­gan­isms help ver­te­brates di­gest food, such as the bac­te­ria in our gut break­ing down com­plex car­bo­hy­drates. But do car­niv­o­rous plants en­gage in sim­i­lar di­ges­tive col­lab­o­ra­tions?

A new study, headed by Isheng Ja­son Tsai of the Acad­e­mia Sinica in Tai­wan, de­scribes how an aci­dophilic fun­gus that re­sides on the leaves of car­niv­o­rous sun­dew plants helps them di­gest in­sect prey.

Fig. 1. (a) Drosera spat­u­lata car­niv­o­rous sun­dew plant. (b) Close-up of stalk glands with se­creted mu­cilage. Source. Fron­tispiece: A sun­dew plant traps a fly. Source

Sun­dews get their name from the sticky drops of mu­cilage that hang from the glan­du­lar ten­ta­cles cov­er­ing their leaves (Fig. 1). Small in­sects are at­tracted by the sweet-smelling drops and get stuck on them, as if it were fly­pa­per.  Sens­ing it has trapped a meal, the sundew's ten­ta­cles then bend in­ward, bring­ing the in­sect into con­tact with more mu­cilage-cov­ered glands. Even­tu­ally the in­sect dies, ei­ther through ex­haus­tion or as­phyx­i­a­tion, and en­zymes within the mu­cilage be­gin di­gest­ing it.

Al­though the mu­cilage is acidic, and as in­hos­pitable to many mi­croor­gan­isms as it is to in­sects, Tsai and col­leagues hy­poth­e­sized that some species could not only sur­vive within the mu­cilage but thrive by help­ing the car­niv­o­rous plant di­gest its in­sect prey.

Us­ing 16S rRNA and ITS am­pli­con se­quenc­ing, the au­thors found that nearly half of the bac­te­ria and fungi liv­ing on the leaves of the sun­dew Drosera spat­u­lata be­longed to the fun­gal species Acrodon­tium cra­ter­i­forme. A. cra­ter­i­forme is aci­dophilic, pre­fer­ring to grow at low pH, such as is found in D. spat­u­lata mu­cilage. In ad­di­tion, us­ing scan­ning elec­tron mi­croscopy, they ob­served the fun­gus grow­ing on the plant's mu­cilage-se­cret­ing glands.

To de­ter­mine if the fun­gus ac­tively con­tributes to prey di­ges­tion, they sup­ple­mented mu­cilage with A. cra­ter­i­forme. This re­sulted in sig­nif­i­cantly faster pro­tein di­ges­tion than ster­ile mu­cilage alone. Fur­ther, they ob­served that sun­dew leaves col­o­nized by A. cra­ter­i­forme cap­ture prey, di­gest it, and then re­open faster than un­col­o­nized leaves.

The au­thors also pro­vided ge­netic ev­i­dence that this fun­gal-plant col­lab­o­ra­tion has under­gone se­lec­tion to in­crease the fit­ness of the pair. The A. cra­ter­i­forme genome has lost genes en­cod­ing en­zymes that de­grade plant cell walls – a sig­na­ture of sym­bi­otic fungi. It also en­codes two pro­teases that are ab­sent in re­lated species and are gen­er­ally rare in fungi but could help in prey di­ges­tion. Ad­di­tion­ally, on the sun­dew side, the pres­ence of the fun­gus primes al­most half of its genes in­volved in di­ges­tion, re­duc­ing the time re­quired to turn those genes on and re­sult­ing in faster di­ges­tion.

These re­sults show that the Drosera-Acrodon­tium re­la­tion­ship is co­op­er­a­tive and mutual­istic. Be­cause of the com­plex in­ter­ac­tions and meta­bolic in­ter­de­pen­den­cies, the pair can be con­sid­ered as a sin­gu­lar en­tity – a "plant holo­biont" – rather than sin­gu­lar or­gan­isms.

The au­thors pro­pose that this part­ner­ship could be an ideal lab­o­ra­tory sys­tem for study­ing plant-mi­cro­bial part­ner­ships. First, both or­gan­isms can be grown sep­a­rately or to­gether. This per­mits di­rect com­par­i­son of their meta­bolic ac­tiv­i­ties and fit­ness when work­ing alone ver­sus to­gether. Sec­ond, al­though the di­ges­tive mu­cilage of other car­niv­o­rous plants – in­clud­ing blad­der­wort, corkscrew, and pitcher plants – also con­tain mi­cro­bial com­mu­ni­ties, they are gen­er­ally highly vari­able, di­verse, and rarely dom­i­nated by a sin­gle mi­croor­gan­ism. In con­trast, the much sim­pler Drosera-Acrodon­tium sys­tem is eas­ier to ma­nip­u­late and likely con­tains stronger sig­nals of co­op­er­a­tion.

This fas­ci­nat­ing sym­bio­sis shows that wher­ever there's digestion—even in its most un­usual forms, such as on the sur­face of plant leaves—microbes will be there help­ing the process along.

 

Kevin Blake is a Sci­en­tific Ed­i­tor at Wash­ing­ton Uni­ver­sity in St. Louis, in the De­part­ment of Pathol­ogy & Im­munol­ogy. He earned his PhD from WashU in 2023 where, in the lab of Dr. Gau­tam Dan­tas, he stud­ied an­tibi­otic re­sis­tance, mi­cro­bio­mes, and mi­cro­bial ecol­ogy.

 

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