About the Author
John P. Helgeson received his Ph.D. from the University of Wisconsin in Botany in 1964 and was a National Science Foundation Postdoctoral Fellow in the Department of Chemistry at the University of Illinois from 1964 to 1966. He joined the faculty of the Departments of Plant Pathology and Botany at the University of Wisconsin in 1966 as a USDA Plant Physiologist. He is presently Professor and Research leader of the USDA/ARS Resistance Research Unit in the Department of Plant Pathology. Helgeson has had several special assignments and visiting scientist assignments both in the United States and abroad, served as USDA Competitive Research Grants Manager for Biological Stress, and is presently the Faculty Director for the University of Wisconsin Biotech Center Plant Biotech Facility. His fields of research interest include genetic variation in cultured cell progeny, cellular genetic modification for improved disease resistance, and molecular analyses of DNA introgression from wild species into cultivars.
Late Blight
Resistance in Potato
- The Molecular Outlook
John P. Helgeson
USDA/ARS, Department of Plant Pathology
University of Wisconsin, Madison, WI
We talk about the molecular "Outlook" for late blight resistance because, as yet, the molecular "Solution" has not been achieved. However, there are many reasons to believe that such may be achieved in the relatively near future.
First, although the early model for molecular transformation was the tobacco plant, potato appears to be just as malleable. Transformation of leaves and stem segments with Agrobacterium spp. succeeds very well. Plants are readily regenerated from potato protoplasts, tissue cultures and plant parts. The transformed clones can be maintained readily in vitro without genetic changes and any number of desired exact clones can be generated from the basic stocks. The characters introduced by transformation appear to be retained and expressed in the plants. Thus, the techniques to "Add" resistance to a given potato variety are ready to be used when the right genes become available.
Second, some useful genes are already available. Positive results have been obtained with plants genetically engineered to produce Bt toxin as a defense against Colorado potato beetle. Plants expressing coat protein genes for various virus such as potato leaf roll virus and potato Y virus are resistant to the viruses. These successes with bacterial and viral genes that either poison the pathogen or disrupt an essential reproductive or metabolic process have spurred efforts to find plant genes that may confer similar resistances. Current efforts include searches for chemicals that might inhibit the fungus, enzymes that might specifically disrupt the fungus, and enhanced triggers that might turn on general defense mechanisms in plants early enough to be effective against Phytophthora infestans.
Some wild species related to potato appear to quite resistant to late blight. Thus, if these genes can be isolated, cloned and then inserted into desirable cultivars the solution comparable to Bt or virus coat protein expressing plants may be obtained. However, much needs to be done in order to map and isolate resistance genes. The situation is very complex as one has to deal with the huge genomes of potato rather than the smaller ones of bacteria or viruses.
Because of crossing incompatibilities, capturing resistances genes by crossing into potato breeding lines doesn't always work. This has led to strategies such as somatic hybridization or embryo rescue for introgressing the resistance genes. These techniques can provide materials for potato breeders and gene mappers as well.
Progress is being made toward the goal of producing late blight resistant cultivars and it is likely that we will soon have plants the will not require extensive pesticide applications in order to produce good yields of satisfactory tubers. Other diseases such as early blight will then be critically important, but these are also yielding to the research efforts. Thus, today there is a real possibility that a molecular solution to susceptibilities of potato cultivars to foliar fungal pathogens can be achieved.
In conclusion, it is worth noting that the genomes of potato and tomato are very similar and that tomato molecular probes have been instrumental in advancing genome mapping in potato. Thus it can be hoped that as the solution to late blight of potato is obtained, a comparable solution to late blight of tomato can also be reached.
He puesto como titulo la "perspectiva" molecular para la resistencia a tizon tardio porque como todavia la "Solucion" molecular no ha sido logrado. Sin embargo, hay muchas razones para creer que esto podria aslcansarse en un futuro relativamente cercano.
Primeramente, aunque un modelo temprano de transformacion molecular fue la planta de tabaco, la papa parece ser una planta maleable. Transformacion de segmentos de hojas y tallos con species de Agrobacterium sucede muy bien. Facilmente son regenerados plantas de papa partiendo de un protoplasma de la papa, a traves de cultivo de tejidos, y atraves de partes de la papa. Clones transformados pueden ser mantenidos facilmente en vitro sin que ocurran cambios geneticos y cualquier numero de clones deseados pueden ser generados a partir de un progenitor base. Las caracteristicas introducidas por transformacion pueden ser retenidos y expresados en las plantas. Esta tecnica, the "Agregar" resistencia a una variedad disponible es usado con facilidad cuando un gene conveniente esta disponible.
Segundo, cuando algunos genes utiles actualmente estan disponibles. Resultados positivos se ha estado obteniendo en plantas con ingenieria genetica para producir Bt toxina como una defenza contra el escarabajo de Colorado de la papa. Plantas con genes expresados en una envoltura proteica de varios virus como PLRV y PVY son resistentes a los virus. Estos sucesos con genes de virus y bacterias que son toxicos al patogeno o desorganizan el proceso reproductivo o metabolico es un estimulo a los esfuerzos encaminados a encontrar genes que pueden conferir resistencia similar. Esfuerzos actuales incluyen investigaciones quimicas que pueden inhibir al hongo, enzimas que pueden especificamente desorganizar al hongo, y acresentar los mecanismos que pueden cambiar el mecanismo de defenza de las plantas, esto es aun bastante precos para ser efectivo contra P. infestans.
Algunas especies de papas silvestres muestran una efectiva resistencia al tizon tardio. Si estos genes pueden ser aislados, clonados y despues puestos en cultivares deseables la solucion comparable con Bt o de la envoltura proteica que expresan las plantas puede ser obtenido. Sin embargo, se necesita realizar mucho trabajo para poder mapear y aislar genes resistentes. La situacion es muy compleja como el de lidiar con un genomio enorme de la papa comparado con las bacterias y virus que son pequenos.
Debido a la incompatibilidad de cruzamientos, capturando genes resistentes por crusamientos dentro de las lineas de mejoramiento de papa no siempre trabaja. Esto ha conducido a elaborar estrategias como el de hibridizaciones somaticas o liberar embriones para introgreesing genes resistentes. Esta tecnica puede proveer material a los mejoradores de papas tanto como a los que mapean genes.
Con el objetivo de producir cultivares de papa resistentes al tizon tardio se ha logrado significativos progresos, esto es prometedor porque muy pronto las plantas no van ha requerir aplicaciones extensivas de pecticidas para producir buenas cosechas y de tuberculos satisfactorios. Tizon temprano otra de las enfermedades puede ser critica, pero esta enfermedad tambien esta siendo sometido a esfuersos de investigacion. Esto, por ahora es una real posibilidad de que la solucion molecular de los cultivares de papa suceptibles a hongos patogenicos de la hoja pueden ser logrados.
En conclusion, es importante que los genomios de la papa y el tomate sean muy similares y que esta perspectiva molecular del tomate es una prueba que puede ser un instrumento de avance en el mapeo del genomio de la papa. Esto puede ser una esperanza de solucion al tizon tardio de la papa, una solucion comparable al tizon tardio del tomate que tambien puede ser logrado.
