The present invention relates to new plasmids and methods for the preparation of transgenic plants, as well as the plants, that are modified through the transfer and the expression of genes which influence the sugar metabolism or the sugar partitioning within a plant, and which are localized on these plasmids.
The growth, the development and the yield of a crop or an ornamental plant depends on the energy that the plant gains through the fixing of CO.sub.2 in carbohydrates during photosynthesis. The primary loci for photosynthesis are the leaves and to a lesser extent the stem tissue. The other organs of the plant, such as roots, seeds or tubers, do not make a material contribution to the formation of photoassimilates, but on the contrary are dependent for their growth on the supply of the photoassimilates from the photosynthetically active organs. This means that there is a flow in photosynthetically gained energy from photosynthetically active tissues to photosynthetically inactive parts of a plant.
The photosynthetically active tissues are generally known as sources. They are defined as net exporters of fixed carbon dioxide. The photosynthetically inactive parts of a plant are designated as sinks. They are defined as net importers of photosynthetically fixed carbon dioxide.
It is believed that the sinks have a strong influence in several ways, particularly in the efficient use of photosynthetic products as well as in their distribution within a plant. One example is the habit of the plant. Newly developing organs, such as very young leaves or other areas such as roots and seeds, are fully dependent on the photosynthesis performance of the sources. That means that the development of such organs is dependent on the distribution of the photoassimilates from sources within the plants. The influence on the formation of young leaves and also the formation of roots can have drastic effects on the habit of a plant, such as, for example, the size of a plant, the internode separation, the size and shape of a leaf, the appearance of a leaf and the number and shape of the roots. Further, the distribution of photoassimilates is quite critical to determining the yield of a plant.
Although the harvestable yield of a wheat plant has increased in the last decades, the total photosynthesis performance of wheat has not changed significantly. This can be explained by changes in the sink to source relationship, wherein the sinks which are important for the yield, such as seeds, take up essentially more photoassimilates than other parts of the plant which are unimportant as far as yield is concerned, such as the stem. In this case, through a shortening of the stem, a much more valid sink to source relationship in wheat could be achieved. This underlines the importance of the distribution of photoassimilates which are formed in the primary sources in higher plants in relation to both the habit and also the yield of plants.
It is not known which biochemical mechanism regulates the relationship of sink and source and the corresponding distribution of photoassimilates.
New biotechnological processes for the genetic change of dicotyledonous and monocotyledonous plants are known (Gasser and Fraley, 1989, Science 244, 1293-1299).
In most plants, photoassimilates are distributed within a plant in the form of sugars and preferentially in the form of sucrose. The distribution of sucrose between the source and sink tissues occurs by transport of sucrose via the phloem. One of the important determinants for the strength of a sink is the unloading of the phloem in the sink. In order to achieve a strong unloading of sucrose from the phloem into the sink, the sucrose should be transformed as soon as possible after leaving the phloem into a different chemical component that no longer has a chemical relationship to sucrose.
Changes of the plant habit can mean important improvements in known plants. For example, this can lead to a shortening of the stem to produce varieties which have greater wind resistance. A preferable distribution of the photoassimilates in harvestable organs such as seeds, e.g. of barley, wheat, soya beans or maize; leaves, for example tobacco; stems, for example sugar cane; tubers, for example potatoes; beets, for example animal feed beets and sugar beet; and fruit, for example tomatoes, lead to a higher yield of a plant. The redistribution of photoassimilates can also be performed in ornamental and garden plants, so as to produce plants with a completely new habit.