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Fungus Versus Plant

The crop-destroying gray mold fungus uses RNA weapons to disable plant defenses and invade.

By | October 3, 2013

Botrytis cinerea on Riesling grapesWIKIMEDIA, TOM MAACKStrike, counter-strike, and so on, ad infinitum. The relationship between pathogens and their hosts is a never-ending, continuously evolving battle. And scientists have now discovered that one plant pathogen even hijacks its host’s defense system for its own use. According to a report published online today (October 3) in Science, the gray mold fungus, Botrytis cinerea, deploys small, non-coding RNAs (sRNAs) that subvert the plant’s silencing machinery, forcing it to suppress the expression of host immune system genes.

“We know we’re always going to see defense and counter-defense,” said David Baulcombe, a professor of plant sciences at Cambridge University, who was not involved in the work. “But what is novel about this one is that . . . the counter-defense system is firstly an RNA molecule, which is new, and secondly . . . it moves from a fungal to a plant cell and that, too, is quite novel.”

Hailing Jin, a professor of plant pathology and microbiology at the University of California, Riverside, who led the new study, was intrigued to discover that a fungus uses RNAs to attack plants. She had been searching for sRNAs induced upon infection, but had assumed that they would be produced by the plants themselves. After all, RNA silencing, in which sRNAs bind to protein-coding messenger RNAs and suppress their expression, is a mechanism used by plants to inhibit the activity of invading foreign genes. “I was focusing on the plants’ immune responses against pathogens,” she explained, “so I was looking at plant endogenous small RNAs that are induced by pathogen infection.”

But she found that a small proportion of the infection-induced sRNAs were “a 100 percent match to the Botrytis sequence,” she said. “It was a surprise to us.”

Of 832 Botrytis sRNAs that Jin found were induced upon infection of tomato or Arabidopsis plants, 73 were predicted to silence genes of the host plants. Jin focused on three particular sRNAs whose targets were indicated to be mRNAs involved in both plants’ immune responses.

First, the researchers showed that the target immune mRNAs were indeed suppressed upon infection of the plants by the fungus, and that blocking the target mRNAs experimentally made the plants more susceptible to infection. Further, when the sRNA binding sites in the target mRNAs were mutated, their suppression by the Botrytis sRNAs was abolished.

The team then showed that the fungal sRNAs associated with and utilized the plant’s own mRNA silencing machinery, and that plants in which silencing machinery was mutated were consequently more resistant to Botrytis infection.

The ultimate confirmation that the fungal sRNAs target and suppress the host plants’ immune system genes, however, would be “to test whether a plant engineered to express alleles of the [target] genes that are refractory to the action of the [fungal] small RNAs are more resistant, ” said Jonathan Jones, a plant scientist at The Sainsbury Laboratory in Norwich, UK, who did not participate in the work. Not only would this be “the acid test of their hypothesis,” he said, it could point to an effective disease-control strategy for agricultural use.

And coming up with resistant plants could be extremely lucrative. Botrytis cinerea, or gray mold, attacks more than 200 species of fruit and vegetables, and causes billions of dollars of damage worldwide each year, said Jin.

Although Botrytis is  costly and destructive, the fungus does have one saving grace. It rots grapes to a degree that’s perfect for producing exceptional desert wines, such as Sauternes—hence the pest’s other name: nobel rot.  Jones said the new findings suggest  “that, heaven forefend, Botrytis is genetically manipulating the grapes before we drink them.”

A. Weiberg et al., "Fungal small RNAs suppress plant immunity by hijacking host RNA interference pathways," Science, 342: 118-123, 2013.

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Comments

Avatar of: Geziel_Aguilar

Geziel_Aguilar

Posts: 1

October 7, 2013

This is really cool stuff. It is also known from mildews that the opposite (HIGS: Host-Induced Gene Silencing) also happens during infection. 

October 9, 2013

The negative economic importance of Botrytis cinerea far outweighs the seemingly positive economic importance.The fact that it attacks more than 200 species of fruits and vegetables, and causes billions of dollars of damage worldwide each year is a cause for concern.

Avatar of: Matt Chew

Matt Chew

Posts: 4

October 18, 2013

This article provides a useful example of the mixed and partial metaphors that not only color science popularization but often constitute scientific conceptualizing. Within the first paragraph we encounter strike, counterstrike, host, hijack, deploy, subvert, silence, machine and suppress.

It can't accurately be said that B. cinerea are attacking fruit without suggesting that the fungi have hostile intentions and recognize fruits as discrete objects (rather than simply responding to the presence of certain chemicals by producing others). Nor do fruits in any useful sense offer hospitality to fungi that are thus mining them for nutrients. There is no reason to suppose that either is, recognizes the other as, an opponent or as a morally considerable individual. Both conceptions are too complex and teleological — too anthropomorphic — to describe the action here.

Subversion, suppression and hijacking each require more than a demonstrable fungus' sense of process and product. It seems unlikely that fruit or fungus have any conception of altering an ongoing activity in order to change its outcome.

Is any such loose talk 'close enough for practical purposes'? That depends on what—and whose—those purposes are. Fruits and fungi don't have detectable purposes of their own. If interests are under attack and being defended, they are those of the people making the claims. What is the dramatic overlay accomplishing? It doesn't seem to be inciting sympathy for fruit, but it does reinforce antipathy for fungi.

By the way, that should be "noble" rot.

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