Florida has a multi-billion dollar orange industry and it is in danger from Citrus Greening. Citrus Greening Disease, or in Chinese: huánglóngbìng in English this means Yellow Dragon Disease is caused by Candidatus Liberibacter spp. and transferred by insects.
If we want to kill this bacteria we should look to nature. How are bacteria killed in nature? One of the most effective ways, bacteria dies is a bacteria virus. Bacteria like animals have viruses. Viruses for bacteria are called bacteriophage, meaning to devour bacteria.
Virus for the Bacteria
Meet Larry a Florida Orange farmer.
Larry: “So, does citrus greening have a virus?”
Larry: “How do I know this?”
Scientists have sequenced the DNA for Candidatus Liberibacter spp, Citrus Greening, the bacteria we want to get rid of. In the bacteria’s DNA, there is also DNA for a virus.
Larry: “How can you have a virus DNA in a bacteria?”
There are two different types of viruses, virulent, and temperate. Virulent viruses inject their genetic information into their host and cause the host to immediately create copies of the virus until the cell explodes with copies of the virus.
Temperate viruses inject genetic information into the host and waits. The host cell replicates with the virus genes inside it. The virus genes are then expressed at a later point and causes a viral infection. When the dormant genes of the temperate virus become active it is called induction. There are different things that trigger induction one of them is damage to the host DNA. The temperate virus genes are regulated by the host. When this regulation is disrupted like in the case of damaged DNA then the virus genes are induced. So all that would be needed for a bacteriophage to devour the bacteria is induction, or to damage the DNA of the bacteria.
Larry: “How can we cause DNA damage in citrus?”
One way would be to use UV light. UV-B light is known to cause damage to DNA.
Application in the Field
Larry: “Ok, so we just put some UV lights on my tractor and drive around the orchard then the tree and it will be cured?”
Not quite. Citrus greening moves in the phloem of the tree. Phloem during the summer time moves down from source (Leaves) to sink (Roots). The roots are typically effected before the leaves. The UV light should not be shined on the leaves but should be focused on the roots.
Larry: “I got my UV light, I’m going to put it into the soil and shine the light on the roots. And then the tree will be cured?”
Sorry, this won’t work. Not in Florida right now. The varieties of citrus greening in Florida do not have a complete temperate virus in the genome.
Larry: “Would it work somewhere else? If so why doesn’t it work here?”
Yes, the UV light trick would work in a place where there is complete virus DNA genome in Candidatus Liberibacter spp, citrus greening. This would likely work in South China where the disease is believed to originate from.
It doesn’t work everywhere because of ecology. In ecology when a species is abundant and out of balance with the ecosystem, it is called invasive. Citrus greening is a disease caused by an invasive species, Candidatus Liberibacter spp. There are many theories to how invasive species get started. One of them is predator escape. And example of this is rabbits in Australia. There were no natural predators to rabbits in Australia, so when they were introduced they became invasive and the rabbit population quickly increased. For citrus greening this is also seen in the variety that was sequenced in Japan. That sequence had less virus DNA than other versions of the bacteria found in China or Florida. The versions of citrus greening that are causing diseases in the world have escaped their predator(s), the viruses.
Larry: “How do we know citrus greening has predators?”
We know that there is or was a virus for citrus greening because there are virus fragments in the DNA of in the genome.
Larry: “So how is this going to help me in Florida?”
We will need to reintroduce the virus from the bacteria’s natural environment. This could be done by purposely moving infected trees with citrus greening that has the full virus genome from China to other parts of the world. Then use the UV light to induce the virus and devour the bacteria.
This method is not just limited to citrus greening. Any bacteria infection with a temperate virus could also use this method of using UV light or a chemical to induce a virus to kill bacteria.
Unique Features of a Japanese ‘Candidatus Liberibacter asiaticus’ Strain Revealed by Whole Genome Sequencing
Katoh H, Miyata Si, Inoue H, Iwanami T (2014) Unique Features of a Japanese ‘Candidatus Liberibacter asiaticus’ Strain Revealed by Whole Genome Sequencing. PLOS ONE 9(9): e106109. doi: 10.1371/journal.pone.0106109