of Xylellas Genome--First Step to Its Control
By Judy McBride
June 27, 2001
Researchers now have a complete genome
sequence of another strain of Xylella fastidiosa--the bacterium that has
turned some California vineyards into grape graveyards.
Agricultural Research Service scientists
have created tools to study the genes that enable this plant pathogen to do its
dirty work. Such knowledge will speed up the chances of finding new ways to
Since the insect that most effectively transmits X. fastidiosa
arrived in southern California in 1999, the pathogen has caused $12 million in
losses to grape growers there. In Brazil, which boasts the largest citrus
industry worldwide, X. fastidiosa damages 35 percent of sweet orange
trees in Sao Paulo state. And it infects up to 70 percent of Brazils
coffee bushes, limiting yields.
To learn how each gene functions, researcher knock out that gene, note what
changes occur, then restore the gene to validate its function. ARS plant
pathologists Xiaoting Qin and John S. Hartung at the
Fruit Laboratory in
Beltsville, Md., created two tools from circles of DNA known as plasmids. One
plasmid delivers a knockout punch to existing genes. The other can be used to
haul in functional genes to replace the damaged ones.
Inside most bacteria are several small plasmids in addition to the central
strand of DNA. To produce the knockout tool, Qin and Hartung coaxed one X.
fastidiosa bacterium to accept plasmids from two E. coli bacteria.
This results in foreign DNA being inserted directly into X.
fastidiosas central strand, disabling whatever gene lies at the
The researchers have inserted DNA for a green fluorescent protein at several
different sites in the X. fastidiosa genome. Not only did this knock out
different genes, the fluorescent protein enabled researchers to see
the pathogens movement in the infected plant.
To restore a disabled genes function, the researchers inserted a
critical piece of DNA from X. fastidiosa into an E. coli plasmid.
This hybrid plasmid shuttled a functional gene into the pathogen without
disabling other of its genes.
ARS is the chief scientific research agency of the
U.S. Department of Agriculture.