The sur­pris­ing give-and-take be­tween healthy crops and in­sect-killing fungi

Canadian Geographic - - CONTENTS - By Nick Walker

IIn the first few cen­time­tres of top­soil, a bat­tle for nu­tri­ents is rag­ing that’s bred a sur­pris­ing al­liance be­tween plants and Me­tarhiz­ium — a com­mon but deadly “en­to­mopathogenic” (in­sec­ti­ci­dal) fun­gus that grows at their roots. Be­cause all 12 va­ri­eties of Me­tarhiz­ium kill, mum­mify and feed on soil-dwelling, po­ten­tially de­struc­tive in­sects, agri­cul­tural com­pa­nies mass-pro­duce it as a bioin­sec­ti­cide for use on crops. In 2012, how­ever, re­searchers at Brock Univer­sity in St. Catharines, Ont., dis­cov­ered it also nat­u­rally fer­til­izes the plants above by feed­ing them ni­tro­gen from in­sects’ bod­ies. “That eco­log­i­cal as­pect of Me­tarhiz­ium was ig­nored for a long time,” says Larissa Barelli, part of the Brock team study­ing the fun­gus. “We needed to know more about how it in­ter­acts with plant sys­tems.” And their work is still show­ing how much there is to learn: in 2017, they proved that the re­la­tion­ship is sym­bi­otic — the plants give back. Here’s how.

Cana­dian sci­en­tists are un­lock­ing how an in­sect-killing fun­gus could be used to grow health­ier crops


Me­tarhiz­ium has a world­wide dis­tri­bu­tion and broad tastes. The va­ri­ety tested by the Brock team, M. robert­sii, alone in­fects more than 200 in­sect species, in­clud­ing moth lar­vae and bee­tle grubs (but is harm­less to hu­mans, bees and other an­i­mals). “If it’s soil-dwelling, it’s pretty much a tar­get,” says Barelli.


Plants can’t ab­sorb ni­tro­gen in its abun­dant at­mo­spheric form, so it must some­how be “fixed” (bound to other chem­i­cals). Me­tarhiz­ium sucks us­able ni­tro­gen ( N) from poi­soned in­sects and pumps it into the roots. In re­turn, the plants send down pho­to­syn­thates — sug­ary, high-en­ergy car­bon com­pounds ( C).


Mi­cro­scopic Me­tarhiz­ium spores in the top­soil ger­mi­nate when they make con­tact with an in­sect. They in­ject them­selves through the bug’s outer mem­branes and start grow­ing rapidly, steal­ing nu­tri­ents and re­leas­ing tox­ins. The host dies in about five days and is then co­cooned in spores and mycelia (masses of thread­like, nu­tri­ent-mov­ing hy­phae) that ex­tend out­ward to form net­works con­nect­ing the fun­gus and plant roots.


Me­tarhiz­ium strains have been tweaked to cre­ate more vir­u­lent, faster­work­ing biopes­ti­cides of­ten since the early 1900s. Brock’s re­search is in­stead look­ing at how the fun­gus’s nat­u­ral role in the car­bon and ni­tro­gen cy­cles could also be har­nessed — and at how wide­spread com­mer­cial fer­til­iz­ers might be dis­rupt­ing this ben­e­fi­cial plant-fun­gus re­la­tion­ship.

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