Illustration by Adelaide Tyrol
Crown gall, the common name for Agrobacterium tumefaciens, is one of the most famous plant diseases in the world. This bacterium can cause galls in more than 600 plant species, some of which are trees. Members of the rose family, including apples, pears, peaches, and cherries, are especially susceptible; willows and poplars are also vulnerable. Vineyards can be troubled by a closely-related gall species.
In scientific circles, crown gall is also famous for its ability to genetically modify its host plant. Like all gall-inducing organisms, A. tumefaciens integrates a segment of its DNA into its host plant’s cells; this causes the cells to proliferate, creating visible, and usually disfiguring, galls.
The “crown” part of the disease’s name doesn’t refer to the tree’s lofty, leafy part but to the area where its roots meet the stem. There, near the soil line, is where galls commonly form, perhaps because the bacteria that cause them reside in the soil, though they also occur on roots, higher on the stem, and in branches. Infection begins at wounds – from pruning, freezing injury, insect damage, or anything that penetrates the surface of a plant. When plants are being grafted en masse in nurseries, precautions against crown gall infection are essential.
Crown galls become evident in late spring or early summer and at first are roughly spherical, spongy, light-colored swellings. As they age, sometimes over many years, they often resemble black walnuts: rough, hard, dark-brown lumps on branches. Sometimes the ugliness of galls is the worst of it; in other cases, the galls affect water and nutrient transport, which can easily result in a sick or dead plant.
The feats of scientists are frequently impressive, and their recently acquired ability to tailor plants to their liking by inserting genes into one organism from another seems astonishing – whatever we might think of the wisdom of this practice. Yet crown gall, a lowly bacterium, has been doing this very thing for millions of years, and without any NSF grants or corporate support. Biologists frequently tweak the age-old procedures of Agrobacterium tumefaciens to create remodeled crop species. The Flavr Savr tomato, NewLeaf potato, and Roundup Ready field crops owe their existence to crown gall – as well as to researchers at Monsanto and Calgene.
Crown gall bacteria move through the soil using flagellae in response to certain chemical compounds that are released when plant tissue is injured. Once inside a plant, the bacterium first anchors itself with some fibrils and then sends out a plasmid – a bit of DNA separate from a chromosome – which contains the machinery to splice itself into the chromosomal DNA of the host plant. This spliced-in bit of DNA causes its host’s cells to divide rapidly, creating a gall. The cleverness doesn’t end there. Inside the gall, the cells have been reprogrammed to manufacture unusual compounds called opines that are the preferred fare of the bacterium.
When Agrobacterium tumefaciens is used to introduce new genes to plants, scientists remove the plasmid genes that induce tumors but retain those that are needed for the transfer. They can then insert new genes, sometimes called transgenes, into the DNA of the plasmid, which become integrated into the plant’s chromosomes. The opportunities this technology presents are endless. Genes can be inserted into a tomato that will slow its ripening, or a firefly’s luciferase gene can make a plant glow.
Crown gall bacteria are most abundant on the outer parts of the galls they form, and as the galls deteriorate, the bacteria fall back to the soil, where they remain viable for at least two or three years.
No chemical control has been found to be effective against crown gall, but there is a biological way to prevent new infections. A particular strain of a close relative of crown gall, called Agrobacterium radiobacter, protects plants from its gall-forming cousin. Although nursery stock that is dipped in a suspension of A. radiobacter at planting time is fully protected, alas, this does not cure existing infections.