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Depending on the setting, the ability of a crucial bacterium in biotechnology—Agrobacterium tumefaciens—to transfer its DNA to a host plant can make it either a pathogen that damages crops or a powerful method for genetically enhancing them.

New research by an Iowa State University team found the effectiveness of Agrobacterium's virulence also varies, depending on how its chromosome is arranged.

The study this month in Science Advances showed the bacterium is more effective at infecting plants when in its natural two-chromosome state, but it grows faster and handles stress better when its densely coiled genetic material is fused into a single chromosome.

"Our work is the first to directly test how chromosome structure affects bacterial growth, survival, and ability to cause disease, and it opens the door for similar studies in many other microbes," said Kan Wang, Charles F. Curtiss Distinguished Professor of agronomy and Global Professor in Biotechnology.

Knowing that chromosomal architecture affects Agrobacterium's balance between fitness and infectiousness could help genetic engineers optimize its use as a crop improvement tool or devise new ways to protect crops vulnerable to crown galls, the tumor-like growths the bacterium can cause on roots and stems, said Wang, corresponding author of the study.

A rare configuration

Agrobacterium is widespread in soil and attacks crops such as fruit and nut trees, grapevines and sugar beets. Since the 1980s, scientists have harnessed its infection-causing DNA transfer mechanism to insert customized genetic sequences into plants. Agrobacterium-driven crop transformations have produced herbicide-tolerant soybeans, insect-resistant corn and cotton, and vitamin-enriched Golden Rice.

While its usefulness in plant biotechnology was part of the reason researchers focused the study on Agrobacterium, Wang said they also were intrigued by its unusual chromosomal configuration that features both circular and linear shapes.

"This rare genomic architecture makes Agrobacterium an excellent model to investigate how chromosome shape and organization influence fundamental traits," she said.

Some naturally occurring variants of Agrobacterium also have one larger chromosome fused into a linear shape. Using CRISPR gene-editing tools, the researchers constructed two other Agrobacterium strains with distinct chromosomal architectures. The common dual-chromosome type was edited to have two circular chromosomes instead of one circle and one linear. The variant with one fused linear chromosome was edited to instead have one circular chromosome.

Lab testing of all four types, which were genetically identical other than their chromosome configuration, showed the fused types had fitness and replication advantages but weren't as effective at infecting host plants.

Gene expression patterns matched the observations. Transcriptome analysis tools, which can show an organism's full set of RNA at a given moment, found greater activation of genes linked to stress tolerance and other survival traits in the fused, single-chromosome types. Genes related to virulence were more active in the dual-chromosome types.

Far-reaching impact

For scientists who use Agrobacterium to enhance crops and other plants, it's valuable to be aware of how the bacterium's strengths vary based on the organization of its chromosomes, Wang said.

"It helps us fine-tune strains depending on our goals. We can keep the natural chromosome split for strong gene transfer into plants or use fused versions when growth stability is more important in the lab," she said.

The findings also could lead to new strategies for controlling crown gall disease in crops, such as pushing pathogenic strains toward less effective chromosomal setups, Wang said.

Future work examining the functional impact of chromosome design in other bacteria could even lead to improved prevention or treatment of infections in humans, she said.

"It also gives us a window into evolution, showing how bacteria adjust their DNA organization to adapt and thrive," she said. "The impact of studying genomic architecture in the coming years could be far-reaching."