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Bacteria as a weapon against wood fungi

Wood fungi infect the grapevine and produce phytotoxins, substances that are toxic to the plant. There are no curative agents available, which makes these wood diseases difficult to control. However, bacteria in the vineyard bravely resist wood fungi and appear to have the solution for this problem.

Bacteria occur naturally on the grapevines in the vineyard. Often both the vine and the bacteria benefit from this, they live in “symbiosis”. This means that the bacterium, for example, feeds on the organic compounds that the vine secretes and in return helps the plant collecting nutrients, reducing environmental stress, or producing antibodies that improve disease resistance. Bacteria from the Bacillus family are often found on grapevines in (healthy) vineyards and are known to provide the grapevine with increased systemic resistance to the Botrytis cinerea fungus. However, as of yet, there are no bacteria known to work against the wood fungi that cause Eutypa dieback, Phomopsis dieback, Botryosphaeria dieback and the esca complex.

Pinto, 2018 Bacillus amyloliquefaciens subsp planatarum strain Fito_F321 2um (adapted)
Bacterium from the Bacillus family.
Adapted from Pinto, 2018 via CC BY 4.0

Botryosphaeria dieback Neofusioccum parvum is one of the fungi that cause the wood disease Botryosphaeria dieback. In the infected vine, this fungus produces the phytotoxins (-) – terremutin and (R) -mellein. However, it is unclear exactly which symptoms of Botryosphaeria dieback are caused by these phytotoxins. It is known that the more aggressive the fungal infection is, the more of these phytotoxins are present in the wood of the vine. There are currently no treatments that can be used in the vineyard to prevent an infection of Neofusioccum parvum or one of the other fungi that cause Botryosphaeria dieback. There are also no drugs against the phytotoxins of these fungi. A new means to combat these fungi and their toxic phytotoxins is therefore highly desirable.

READ ALL ABOUT WOOD FUNGI: Fungi in the grapevine, that’s rotten!

Bacterium vs. fungus in the laboratory

The bacterium Bacillus subtilis PTA-271 is found on the roots of healthy Chardonnay vines in the Champagne region in France. Because this bacterium lives in symbiosis with the vine, it is suspected to have some beneficial effects to the vine. To see if Bacillus subtilis PTA-271 has an effect on Neofusioccum parvum, they are grown together in a petri dish. Remarkably, the bacterium suppresses the growth of Neofusioccum parvum, although this effect is more apparent in culture at 28 ℃ than at 22 ℃. Nevertheless, after a few days, the fungus still wins the battle from the bacteria in the petri dish.

Fungus and bacterium together in culture
Fungi and bacteria together in culture in Petri dishes. The bacterium Bacillus subtilis PTA-271 (Bs) inhibits the growth of the fungus Neofusioccum parvum (Bt67). The culture at 28 degrees (A) shows a reduced growth of the fungus at all times. At 22 degrees (B) a clear inhibition of fungal growth can be seen after four days.
Trotel-Aziz, 2019 via CC BY 4.0

In addition to the inhibition of growth, Bacillus subtilis PTA-271 also ensures the breakdown of the phytotoxins produced by Neofusioccum parvum. This is demonstrated by the addition of the phytotoxins (-) – terremutin and (R) – melamine to a petri dish containing only the bacteria. After three days the total amount of phytotoxins is decreased by 50-60%. The bacterium breaks down the phytotoxins and can therefore also protect the grapevine against the fungus in this way.

Bacterium vs. fungus in the grapevine

The effects of the bacterium Bacillus subtilis PTA-271 on the growth of the Neofusioccum parvum fungus and the production of phytotoxins has been tested in vines of the Chardonnay grape. The grapevines are manually contaminated with the fungus. The symptoms of Botryosphaeria dieback are already visible ten days after the fungal infection. However, the grapevines whose roots have been previously treated with Bacillus subtilis PTA-271 show fewer symptoms of this fungal disease. The bacteria-treated vines have less mortality of the shoot, the canker at the site of infection (on the outside of the shoot) is smaller, and the damage to the wood inside the shoot was reduced up to 75% by Bacillus subtilis PTA-271. This bacterium therefore appears to have an efficient way of protecting the Chardonnay vine against the Neofusioccum parvum fungus.

Botryosphaeria dieback symptoms caused by Neofusioccum parvum are counteracted by the bacterium Bacillus subtilis PTA-271.
Botryosphaeria dieback symptoms caused by Neofusioccum parvum are counteracted by the bacterium Bacillus subtilis PTA-271. There is no death of the shoots (A) and the damage due to the fungal infection is decreased on both the outside (B) and inside of the shoots (C). Adapted from Trotel-Aziz, 2019 via CC BY 4.0

Battle for the immune system

When the Chardonnay grapevine comes into contact with the Neofusioccum parvum fungus its immune system is triggered, and it starts with the production of specific proteins. However, the production of these proteins only happens when it is in contact with Bacillus subtilis PTA-271. Therefore, the bacterium strengthens the immune system of the vine. The fungus on the other hand tries to eliminate the immune system of the grapevine by producing phytotoxins. These phytotoxins suppress the production of the same proteins that are produced under the influence of the bacteria. By doing so, the phytotoxins weaken the immune system of the vine.

The Bacillus subtilis PTA-271 bacterium therefore fights the fungus on three fronts; it stimulates the immune system of the vine, reduces the growth of the fungus, and breaks down the phytotoxins produced by the fungus.

Graphic summary

Graphic summary Bacterium vs fungus

What can the winegrower do with this?

Future studies will focus on how the harmful phytotoxins are broken down by Bacillus subtilis PTA-271 and how the bacterium causes an inhibition of fungal growth. When these mechanisms are known, it is easier to produce a substance (or bacterium strain) that works curatively. This would greatly help to combat wood diseases such as Botryosphaeria dieback in the vineyard.

Further, this research demonstrates (again) that a healthy vineyard is essential for a natural resistance against fungal diseases in the grapevine (but winegrowers already know that).

Contact Koen Klemann, WineScience

Reference:
Trotel-Aziz P, Abou-Mansour E, Courteaux B, Rabenoelina F, Clément C, Fontaine F, Aziz A. Bacillus subtilis PTA-271 Counteracts Botryosphaeria Dieback in Grapevine, Triggering Immune Responses and Detoxification of Fungal Phytotoxins. Front Plant Sci. 2019 Jan 24;10:25. https://doi.org/10.3389/fpls.2019.00025

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