By Katharina Schoebi, Checkbiotech

The parasitic plant species Orobanche can cause enormous yield losses. Up to now, there are only few control measures that are successful and affordable. An American-Israeli research team has now been able to genetically engineer tobacco plants to enhance their resistance against Orobanche.

Parasitic plants heavily contribute to the weed problem for agriculture. Plants of the species Orobanche attack the roots of many crops and abstract water, nutrients and photosynthesis products from their host plant, and by so doing can cause enormous yield losses. Since the parasite is closely associated with the host root, its control is very difficult. Thus, crop species that are resistant to the parasite are in great demand.

James Westwood from the Virginia Tech, Department of Plant Pathology, Physiology, and Weed Science in Blacksburg, USA, and his Israeli colleagues recently set out to render tobacco plants resistant to Orobanche. They published their work in the Journal Transgenic Research.

The research team genetically engineered tobacco plants so that they expressed a protein fragment, called sarcotoxin IA, from the flesh fly Sarcophaga peregrine. Sarcotoxin has toxic effects to several plant pathogenic bacteria and fungi.

Already in 1999, Radi Aly from the Agricultural Research Organiszation in Ramat Yishay, Israel, and his colleagues showed that transgenic tobacco plants producing sarcotoxin IA were less parasitized by Orobanche. Yet, resistance was not complete, perhaps due to the low production level of sarcotoxin IA.

Westwood has now combined the so called HMG2-promoter (a plant gene sequence that controls a natural plant defence response) with the sarcotoxin IA gene and found, that the transgenic tobacco plants showed parasitic resistance after O. aegyptiaca had penetrated the plant.

However, sarcotoxin IA confers only an intermediate level of resistance to Orobanche. Though the transgenic plants accumulated a higher biomass than untransformed plants when grown in soil infected with O. aegyptiaca, the added gene did not enable plants to completely avoid damage by the parasite.

Since the number of tubercles of O. aegyptiaca did not differ between transgenic and untransformed plants and parasite biomass was lower in genetically engineered plants, the researchers conclude that sarcotoxin IA first of all affects parasite growth after it has attached to the roots, and second, it does not inhibit the attachment itself.

The researchers write in their publication that the resistance level of the genetically engineered tobacco plants fall short of the levels that would be required for reducing Orobanche infestations in the field.

“We are in the very early stages of research on this line of resistance, and I don’t foresee any of our current generation of plants being planted in the field,” Dr. Westwood told Checkbiotech.

When plants armed with this resistance mechanism would be released, the development of possible resistant Orobanche populations might be a concern. Dr. Westwood answered, “I hypothesize that the sarcotoxin IA mechanism is acting as a general membrane disrupter, and resistance may be slow to develop against such a non-specific mechanism.”

But since resistance is always possible, the best strategy is to combine it with other resistance mechanisms, or effective control measures to further delay the emergence of resistant Orobanche populations.

The researchers are currently investigating the precise mechanism of action of sarcotoxin IA against Orobanche. Dr. Westwood said, “We think we can enhance the activity by modifying the protein, but again, we are in the first steps of this research and it is too early to say what resistance levels can be expected.”

If they are able to increase Orobanche resistance in tobacco plants enough, Dr. Westwood’ team will test it against other parasitic weeds to see if it is generally useful against them as well.

“Genetically engineered plants are not very different from the plants we encounter and consume every day,” explained Dr. Westwood about his research with transgenic plants. He further explained that nearly all crops would have been substantially modified over the years by conventional genetic breeding, in many cases with little knowledge or concern about unintended changes that may have been made along the way.

“When I think about the crop losses suffered due to parasitic plants, and that we still have few good tools to protect these crops, I think we must be open to new approaches.”

Hamamouch et al. A peptide from insets protects transgenic tobacco from a parasitic weed. Transgenic Research (2005) 14, pp. 227-236.

Link to the abstract:
http://springerlink.metapress.com/(i11vqjzjxgifsbr443ibzd55)/app/home/contribution.asp?referrer=parent&backto=issue,1,13;journal,3,52;linkingpublicationresults,1:100225,1

Contact:
James H. Westwood
Virginia Tech
Department of Plant Pathology, Physiology, and Weed Science
Blacksburg VA 24061
USA
E-Mail: westwood@vt.edu