By Stacy Kish

Scientists are unlocking the process of how bacteria transfer genetic
material into the plant genome. This information may lead to crop plants
with improved resistance to pests and disease.

With funding from USDA's Cooperative State Research, Education, and
Extension Service (CSREES), a project team in New York systematically
investigates and uncovers fundamental biological principles behind the
action of Agrobacterium, which is known for its ability to transfer DNA
between itself and plants, making it an important tool for genetic
engineering.

Agrobacterium serves as both a genetic engineering tool for crop production
and an experimental system to study basic cellular reactions in genetic
transformation. It represents the only known natural example of how genetic
information can be transferred between different kingdoms of life, in this
case from bacterium to plants. In nature, Agrobacterium promotes
uncontrolled growth of cells in the infected plants. In the hands of
scientists, it serves as a living nanomachine for genetic transformation of
plants in biotechnology industry and research laboratories.

To genetically transform plants, Agrobacterium transports its genetic
material, in a form of a nucleic acid-protein complex termed the T-complex,
into the host cell nucleus. Once inside the nucleus, the T-complex must
reach the host chromosomes to complete the integration into the genetic
material of the host cell. Agrobacterium employs a diverse number of
biological processes to genetically transform the host cell. This makes it a
unique model system to examine genetic material transport between bacteria
and eukaryotic cells, cells with internal structures enclosed in membranes.
It also works as a model for the transport of genetic material into the cell
nucleus.

Vitaly Citovsky and colleagues at the State University of New York at Stony
Brook, demonstrated that the plant protein VIP1 helps transfer the invading
bacterial T-complex into the host cell nucleus. Then, VIP1 targets the
T-complex to the integration site in the plant genome. The targeting process
occurs from the direct interaction of VIP1 with the host nucleosomes, the
protein-DNA structures that make up chromosomes.

Having arrived to the host chromosome, the T-complex must lose its coat of
protein components to allow T-DNA integration. The Citovsky team showed that
this uncoating likely occurs when the T-complex proteins begin to decompose;
the research team identified bacterial and plant factors that make this
decomposition possible. These experiments also revealed that many of the
cell functions involved in the genetic transformation process are mimicked
and/or augmented by bacterial virulence proteins exported into the host
cell.

"Besides this important contribution to our understanding of the
Agrobacterium-mediated genetic transformation of plant cells, our work has
practical implications for agriculture," Citovsky said.

Using Agrobacterium, the studies discovered proteins essential for the
transformation process that may facilitate genetic manipulation of crops
that are currently difficult to transform. On the other hand, this knowledge
is also critical for production of agronomically important plants resistant
to Agrobacterium, Citovsky said.

CSREES funded this research project through the National Research
Initiative's Developmental Processes of Crop Plants program. Through federal
funding and leadership for research, education and extension programs,
CSREES focuses on investing in science and solving critical issues impacting
people's daily lives and the nation's future. For more information, visit
www.csrees.usda.gov.