Oct 20, 2008

Research Woody Plants

Send

New molecular-biological approaches to apple breeding.

“It works just like traditional apple breeding, but it’s quicker.”

Genetic engineering is of particular interest to apple breeders because it can considerably speed up the lengthy breeding process, which can take several decades. However, consumers, especially those in Europe, do not want genetically modified apples, so research is increasingly focusing on approaches which, although using genetic engineering in the breeding process, ultimately leave the end plant GM-free. And if it is genetically modified, then preferably with apple genes. GMO Safety spoke to Henryk Flachowsky from the Institute for Breeding Research on Horticultural and Fruit Crops about the main areas of research and his work at the institute in Dresden-Pillnitz.

When asked about the benefits of genetic engineering, Henryk Flachowsky has no hesitation in replying: The advantage of genetic engineering is that individual traits of established varieties can be specifically targeted and modified. Conventional breeding methods do not allow this, as the DNA from both parents is recombined at every hybridisation. Since an apple has around 35,000 genes, there is a vast number of potential recombinations, making the outcome of such a crossing difficult to predict. “On the other hand, if we transfer individual genes, we know that any change is linked to this gene transfer. This enables us to preserve the essential nature of the variety.”

A further key advantage of genetic engineering, according to Henryk Flachowsky, is the fact that it can considerably shorten the breeding process. It takes at least 20 to 25 years to develop a new apple variety by conventional means. When dealing with new problems in fruit growing, breeders have to resort to wild species if the genes for the desired characteristics cannot be found in the gene pool of common varieties, and this can take forty or even seventy years.

“Take fire blight, for example. Only a few varieties have a certain degree of resistance to it, but these are not grown commercially,” Henryk Flachowsky explains. Resistance genes are normally found only in wild species, but these tend to have very small fruit. If you cross them with established apple varieties, it takes six to seven back-crosses to obtain a new apple variety with good fruit quality. Furthermore, to produce effective resistance it makes sense to combine different resistance genes to make it more difficult for pathogens to overcome the resistance.

Genetic engineering using genes from apples

In principle it is possible to transfer genes from other organisms into apple plants using genetic engineering techniques. Researchers at Wageningen University have recorded some initial success with apple-scab-resistant Golden Delicious and Elstar apples, by inserting a gene from barley into them. But researchers are increasingly turning to resistance genes from apples themselves as a means of creating resistance to pathogens.

According to Henryk Flachowsky, initial attempts have reached the stage where individual genes from apple variety A have been successfully transferred to apple variety B, so that the new variety contains only natural apple DNA, as with conventional breeding. However, at present it is not entirely possible to determine the site of integration, although preliminary experiments are taking place to replace genes in a specific position with a more effective gene. Many apple varieties that are susceptible to disease do actually carry resistance genes, often in the same position in the genome as the resistant varieties. However, the DNA sequence of these genes often changes slightly as they evolve, rendering them unable to resist certain pathogens.

Scientists at various international research institutes, including Wageningen University in Holland and the ETH Zurich, are working on the production of disease-resistant apple varieties using genes from wild apples. These ‘cisgenic’ approaches are also being pursued in Pillnitz, where the development of new varieties is playing only a minor part at this stage. Henryk Flachowsky is keen to point out that the initial focus is on basic research and on testing procedures, which is also true for the majority of the research projects. Researchers first have to understand the function of individual genes, then identify metabolic pathways and finally find and isolate suitable genes, such as those for disease resistance. The first apple genome, from a Golden Delicious, has now been mapped and publication is expected this year, bringing the researchers a step closer to their goal. This sequence data will provide more accurate information about the location and environment of individual genes.

Bringing apple plants to flower more quickly

It takes six to ten years for an apple tree to flower for the first time. This means that researchers have to wait for at least six years before they are able to assess the fruits of seedlings following hybridisation. The same period of time must also elapse before the next stage of the breeding process can be carried out. In Pillnitz scientists have now succeeded in developing plants which flower in the first year after sowing by transferring a birch gene. These early-flowering apple plants are then used in a conventional breeding programme. “The thinking behind this,” explains Henryk Flachowsky, “is that if these transgenic plants are used for hybridisation, fifty percent of the offspring will be transgenic, in other words they will flower early. If a resistance gene is inserted as well, fifty percent of the offspring will carry this resistance gene and a quarter of the offspring will carry both. A seedling is selected from this quarter for back crossing. This process is repeated in several stages until the fruit quality of the seedlings reaches a certain level. At the end of the breeding process seedlings are selected that are resistant and have good fruit quality, but are no longer transgenic. It works just like traditional breeding, but it’s quicker.”

To further accelerate the breeding process, work is being conducted on molecular markers, i.e. apple seedlings will undergo molecular analysis for certain genes at an early stage, rather than waiting to assess the appearance of the mature plants.

Transgenic rootstock, GM-free fruit

Researchers at the institute in Pillnitz are investigating another method of stimulating apple trees to flower earlier.

Henryk Flachowsky has to go into some detail to explain this approach: In recent years scientists have obtained a greater understanding about which genes initiate flowering in the model plant Arabidopsis. The ‘flowering locus T’ (FT) gene is thought to play a crucial role. “It was previously thought that hormones controlled this process, but we now know that it is a protein. The FT protein is formed in the leaves and is probably carried up towards the shoot tips via the plant’s nutrient pathways, where it causes the vegetative meristem to change into a generative meristem.”

Apples have matching (homologous) genes which are very similar to what is thought to be the ‘flowering gene’ in Arabidopsis – the FT gene. The scientists in Pillnitz want to discover whether stimulating the rootstock to produce an excess of natural apple FT protein will result in the protein being transported to a grafted non-transgenic plant. Could this method be used to produce an apple seedling that flowers after just one year but is not itself transgenic? If it works, the fruit would contain no transgenes, only the natural apple protein, if anything.

Various international research institutes, such as the ETH Zurich, as well as scientists from New Zealand, the USA and Italy are interested in these very promising approaches from Pillnitz.