Three to Tango

by Elio

How many sym­bioses that in­volve three part­ners can you think of? Surely, not many. But here is one, in­volv­ing a path­o­genic fun­gus, a bac­terium, and a fun­gal virus. The virus be­ing the new­comer, let me start with the well-known part­ner­ship, that be­tween the fun­gus and the bac­terium. The fun­gus, Rhi­zo­pus mi­crosporus, is a pathogen of a va­ri­ety of plants, such as maize, sun­flow­ers, and rice, where it causes var­i­ous rots and blights. It can even in­fect hu­mans, caus­ing necrotic le­sions es­pe­cially in in­fants. To bal­ance this out a bit, a closely re­lated species, R. oligosporus, is used in the prepa­ra­tion of fer­mented soy prod­ucts such as tem­peh.

Fig­ure 1. Rhi­zo­pus mi­crospo­rum. Source

To do its dam­age, R. mi­crosporus part­ners with a bac­terium, one that used to be called Burk­holde­ria but is now known as Myce­to­hab­i­tans ("fun­gus dweller"). This sym­biont is typ­i­cally found in path­o­genic strains of the fun­gus and is ver­ti­cally trans­mit­ted. The bac­terium pro­duces a num­ber of tox­ins that af­fect the hosts, among them the an­tim­i­totic agent rhi­zoxin.

En­ter the viruses

Fig­ure 2. Quan­tifi­ca­tion of the vi­­ruses RmNV-20S and RmNV-23S along deve­lopment and re­pro­duc­tion of R. mi­crosporus. a. Ab­solute quan­tifi­ca­tion of RmNV-­20S and Rm­N­V-­23S by RT-qPCR in 814 wt and 814 b‑strains grown on potas­sium ac­etate agar (low nut­rient) and half-strength PDA (high nut­rient). Each dot cor­res­ponds to a sin­gle vi­ral measure­ment. Bars indi­cate the arith­metical mean, dot­ted li­nes show the ten­dency, let­ters and let­ters with prime sym­bol above dots rep­resent stat­istical dif­ferences be­tween develop­ment stages in RmNV-20S (20S) or RmNV-23S (23S), res­pecti­vely, by one-way ANOVA fol­lowed by Tukey's post-hoc test, P < 0.05, n = 5 to 31. b. Ver­tical trans­mission of narna­viruses through asex­ual and sex­ual spores. No stat­istical dif­ferences bet­ween Narna­virus species were found in spo­r­angio­spores or zygo­spores, but in vi­ral loads in spor­angio­spores prod­uced in dif­ferent nut­rient con­ditions by one-way ANOVA, *P < 0.05, n = 3. c. Micro­­graphs of reprod­uctive fun­gal struc­tur­es. From left to right: spor­angio­phore, iso­lated spor­angio­spore, zygo­spores formed by mat­ing, and iso­lated zygo­spore. Scale bars = 50 µm. Source

Prob­ing into the bac­te­r­ial-fun­gus in­ter­ac­tions led the au­thors of a re­cent pa­per to a de­tailed RNA se­quence analy­sis. A com­men­tary by the se­nior au­thor is found here. To their sur­prise, these in­ves­ti­ga­tors, mainly from labs in Mex­ico, dis­cov­ered the pres­ence of yet other part­ners, two viruses. Phy­lo­ge­netic analy­sis told them that they be­long to the Nar­naviruses. Mem­bers of a fas­ci­nat­ing group that are known to in­fect yeasts, other fungi, and even Caenorhab­di­tis el­e­gans. They are re­mark­able in sev­eral ways, no­tably not be­ing en­dowed with a cap­sid, a topic dis­cussed in de­tail here. They de­rive their name from NAked RNA viruses. Their viri­ons, if you want to call them that (why not vi­roids?), are ex­ceed­ingly tiny. They con­sist of a small dou­ble stranded lin­ear genome, 2.3 to 2.9 kb in size that en­codes a sin­gle pro­tein, 80–140 kDa, which looks and acts like an RNA-de­pen­dent RNA poly­merase. This vi­ral RNA can be found in the cy­to­plasm in very high copy num­bers (un­der stress con­di­tions, up to 100,00 units per cell). Rel­a­tives of this group, the Mi­tovirus, in­fect fun­gal mito­chon­dria.

These three play­ers, fun­gus, bac­terium, and virus, hold an an­i­mated con­ver­sa­tion. Tran­scrip­tion from the vi­ral se­quences is in­flu­enced by many fac­tors, such as the fun­gal re­pro­duc­tion and its de­vel­op­ment, the medium, and the stage of fun­gal de­vel­op­ment.  To con­ve­niently study these in­ter­ac­tions, the au­thors used an­tibac­te­r­ial and an­tivi­ral drugs to cre­ate strains of fun­gus that are free of both bac­te­ria and viruses. The fungi could then be re­in­fected at will. One in­ter­est­ing re­sult sug­gests that bac­te­r­ial col­o­niza­tion is re­duced in the ab­sence of the viruses. If so, the viruses would in­flu­ence the de­gree of path­o­genic­ity of the fun­gus, some­thing that is in the process of be­ing as­cer­tained di­rectly (per­sonal com­mu­ni­ca­tion by the last au­thor of the pa­per). But there is more: The pres­ence of the viruses de­creases the fun­gus' asex­ual re­pro­duc­tion (spo­ran­giospore for­ma­tion). On the other hand, sex­ual sporu­la­tion (zy­gospore for­ma­tion) is nearly abol­ished in the ab­sence of the bac­te­ria and the viruses. I can't say that I fully un­der­stand what's go­ing on here but in­ter­est­ing it is. 

The data sug­gest that the me­tab­o­lism of the fun­gus is "rewired" by the growth and pres­ence of the sym­bionts, pos­si­bly to en­hance its meta­bolic plas­tic­ity. The au­thors ask sev­eral good ques­tions such as which sym­bio­sis arose first? Did one sym­biont, bac­te­r­ial or vi­ral, fa­cil­i­tate the es­tab­lish­ment of the other? Did ei­ther alone or to­gether in­flu­ence the evo­lu­tion of the fun­gus? Surely, we will hear more about this mé­nage à trois.