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Genome of bacteria Xylella fastidiosa, a threat to fruit and nut crops, is sequenced
By Sharon Guynup

The world's first complete gene sequencing of a plant pathogen has been announced by a consortium of Brazilian researchers in the July 13 issue of Nature. The group mapped one strain of the bacterium Xylella fastidiosa, an insidious organism that devastates fruit, nut, and coffee groves, grape arbors, and alfalfa fields from California to Argentina. This genetic profile may not only help researchers design a cure for a family of serious agricultural diseases, but may also help them gain insight into the mechanisms of bacterial infection and infestation.

Scanning electron micrograph of the bacterium Xylella fastidiosa, causal agent of the citrus variegated chlorosis, present in the xylem vessel of infected sweet orange leaf(ca. X10,000).

The bacteria grows in the gut of certain species of leafhoppers known as sharpshooters—small, winged insects that feed on plant sap. While snacking, these insects carry the bacterial infection from plant to plant, transferring X. fastidiosa directly into the plant's xylem, the vascular tissues. There, the bacteria multiply, clogging the plant's internal plumbing and blocking the flow of water to leaves. Trees and plants weaken, leaves discolor, and fruits appear prematurely, remaining small, hard—and worthless.

The sequenced bacterial strain causes citrus variegated chlorosis (CVC), a disease infecting citrus trees in Brazil. In the state of Sao Paulo, about five million diseased trees are destroyed yearly, causing $50 million in losses. Related bacterial infections threaten other cash crops throughout the Americas.

In California, another X. fastidiosa-induced disease, Pierce's disease, threatens the state's $2.6 billion grape crop. A federal agricultural state of emergency has been declared, and the U.S. Department of Agriculture has contracted the Brazilian consortium, the Organization for Nucleotide Sequencing and Analysis (ONSA), to sequence the strain that causes this scourge. Edwin L. Civerolo, the USDA scientist working with the Brazilians, estimates that the effort should take a year, but they should be able to begin genomic analysis within a few months.

Fruits of sweet orange 'Pera' affected by CVC.

It took the group, comprised of 34 university lab teams, two years to map the 9a5c citrus strain of X. fastidiosa. Like the other 23 known bacterial genomes, X.fastidiosa's genome can be "read" because researchers can identify the DNA sequences of virtually all its genes. They found the bacteria to be a highly specialized organism. "At a basic level, it resembles other known bacteria—no surprises there," said Joao Carlos Setubal, one of the paper's authors. "But we were unable to assign functions to 53 percent of its genes, which shows that there is a great deal of novelty in this genome."

Some of the most interesting discoveries among the 2,904 identified genes were the presence of genes used in the excretion of a gum similar to xanthan gum, which helps the bacteria stick to host cells. And researchers were truly surprised to find genes that produce proteins called adhesins and hemagglutinins, substances that mediate cell invasion—and were previously found only in pathogens that attack animals.

This discovery, Setubal feels, could be of major importance. It may help substantiate the hypothesis that disease-causing organisms share similar methods of infection regardless of their host organism. The ultimate goal in this research, he says, is two-fold. First, agricultural scientists will use the genetic blueprint "to find a cheap and effective means of controlling CVC," and second, "to gain a deep understanding of plant pathogenic processes, which could then be applied to other pathogenic pests."

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Simpson, A.J.G. et al. The genome sequence of the plant pathogen Xylella fastidiosa. Nature 406, 151-159 (July 13, 2000).

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