This invention relates to the expression of fructose 1,6 bisphosphate aldolase (FDA) in transgenic plants to increase or improve plant growth and development, yield, vigor, stress tolerance, carbon allocation and storage into various storage pools, and distribution of starch. Transgenic plants expressing FDA have increased carbon assimilation, export and storage in plant source and sink organs, which results in growth, yield and quality improvements in crop plants.
Recent advances in genetic engineering have provided the prerequisite tools to transform plants to contain alien (often referred to as xe2x80x9cheterologousxe2x80x9d) or improved endogenous genes. These genes can lead either to an improvement of an already existing pathway in plant tissues or to an introduction of a novel pathway to modify product levels, increase metabolic efficiency, and or save on energy cost to the cell. It is presently possible to produce plants with unique physiological and biochemical traits and characteristics of high agronomic and crop processing importance. Traits that play an essential role in plant growth and development, crop yield potential and stability, and crop quality and composition include enhanced carbon assimilation, efficient carbon storage, and increased carbon export and partitioning.
Atmospheric carbon fixation (photosynthesis) by plants represents the major source of energy to support processes in all living organisms. The primary sites of photosynthetic activity, generally referred to as xe2x80x9csource organsxe2x80x9d, are mature leaves and, to a lesser extent, green stems. The major carbon products of source leaves are starch, which represents the transitory storage form of carbohydrate in the chloroplast, and sucrose, which represents the predominant form of carbon transport in higher plants. Other plant parts named xe2x80x9csink organsxe2x80x9d (e.g., roots, fruit, flowers, seeds, tubers, and bulbs) are generally not autotrophic and depend on import of sucrose or other major translocatable carbohydrates for their growth and development. The storage sinks deposit the imported metabolites as sucrose and other oligosaccharides, starch and other polysaccharides, proteins, and triglycerides.
In leaves, the primary products of the Calvin Cycle (the biochemical pathway leading to carbon assimilation) are glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), also known as triose phosphates (triose-P). The condensation of G3P and DHAP into fructose 1,6 bisphosphate (FBP) is catalyzed reversibly by the enzyme fructose 1,6 bisphosphate aldolase (FDA), and various isozymes are known. The acidic isoenzyme appears to be chloroplastic and comprises about 85% of the total leaf aldolase activity. The basic isoenzyme is cytosolic. Both isoenzymes appear to be encoded by the nuclear genome and are encoded by different genes (Lebherz et al., 1984).
In the leaf, the chloroplast FDA is an essential enzyme in the Calvin Cycle, where its activity generates metabolites for starch biosynthesis. Removal of more than 40% of the plastidic aldolase enzymatic activity by antisense technology reduced leaf starch accumulation as well as soluble proteins and chlorophyll levels but also reduced plant growth and root formation (Sonnewald et al., 1994). In contrast, the cytosolic FDA is part of the sucrose biosynthetic pathway where it catalyzes the reaction of FBP production. Moreover, cytosolic FDA is also a key enzyme in the glycolytic and gluconeogenesis pathways in both source and sink plant tissues.
In the potato industry, production of higher starch and uniform solids tubers is highly desirable and valuable. The current potato varieties that are used for french fry production, such as Russet Burbank and Shepody, suffer from a non-uniform deposition of solids between the tuber pith (inner core) and the cortex (outer core). French fry strips that are taken from pith tissue are higher in water content when compared to outer cortex french fry strips; cortex tissue typically displays a solids level of twenty-four percent whereas pith tissue typically displays a solids level of seventeen percent. Consequently, in the french fry production process, the pith strips need to be blanched, dried, and par-fried for longer times to eliminate the excess water. Adequate processing of the pith fries results in the over-cooking of fries from the high solids cortex. The blanching, drying, and par frying times of the french fry processor need to be adjusted accordingly to accommodate the low solids pith strips and the high solids cortex strips. A higher solids potato with a more uniform distribution of starch from pith to cortex would allow for a more uniform finished fry product, with higher plant throughput and cost savings due to reduced blanch, dry and par-fry times.
Although various fructose 1,6 bisphosphate aldolases have been previously characterized, it has been discovered that overexpression of the enzyme in a transgenic plant provides advantageous results in the plant such as increasing the assimilation, export and storage of carbon, increasing the production of oils and/or proteins in the plant and improving tuber solids uniformity.
The present invention provides structural DNA constructs that encode a fructose 1,6 bisphosphate aldolase (FDA) enzyme and that are useful in increasing carbon assimilation, export, and storage in plants.
In accomplishing the foregoing, there is provided, in accordance with one aspect of the present invention, a method of producing genetically transformed plants that have elevated carbon assimilation, storage, export, and improved solids uniformity comprising the steps of:
(a) Inserting into the genome of a plant a recombinant, double-stranded DNA molecule comprising
(i) a promoter that functions in the cells of a target plant tissue,
(ii) a structural DNA sequence that causes the production of an RNA sequence that encodes a fructose 1,6 bisphosphate aldolase enzyme,
(iii) a 3xe2x80x2 non-translated DNA sequence that functions in plant cells to cause transcriptional termination and the addition of polyadenylated nucleotides to the 3xe2x80x2 end of the RNA sequence;
(b) obtaining transformed plant cells; and
(c) regenerating from transformed plant cells genetically transformed plants that have elevated FDA activity.
In another aspect of the present invention there is provided a recombinant, double-stranded DNA molecule comprising in sequence
(i) a promoter that functions in the cells of a target plant tissue,
(ii) a structural DNA sequence that causes the production of an RNA sequence that encodes a fructose 1,6 bisphosphate aldolase enzyme,
(iii) a 3xe2x80x2 non-translated DNA sequence that functions in plant cells to cause transcriptional termination and the addition of polyadenylated nucleotides to the 3xe2x80x2 end of the RNA sequence.
In a further aspect of the present invention, the structural DNA sequence that causes the production of an RNA sequence that encodes a fructose 1,6 bisphosphate aldolase enzyme is coupled with a chloroplast transit peptide to facilitate transport of the enzyme to the plastid.
In accordance with the present invention, an improved means for increasing carbon assimilation, storage and export in the source tissues of various plants is provided. Further means of improved carbon accumulation in sinks (such as roots, tubers, seeds, stems, and bulbs) are provided, thus increasing the size of various sinks (larger roots, tubers, etc.) and subsequently increasing yield and crop productivity. The increased carbon availability to these sinks would also improve composition and use efficiency in the sink (oil, protein, starch and/or sucrose production, and/or solids uniformity).
Various advantages may be achieved by the aims of the present invention, including:
First, increasing the expression of the FDA enzyme in the chloroplast would increase the flow of carbon through the Calvin Cycle and increase atmospheric carbon assimilation during early photoperiod. This would result in an increase in photosynthetic efficiency and an increase in chloroplast starch production (a leaf carbon storage form degraded during periods when photosynthesis is low or absent). Both of these responses would lead to an increase in sucrose production by the leaf and a net increase in carbon export during a given photoperiod. This increase in source capacity is a desirable trait in crop plants and would lead to increased plant growth, storage ability, yield, vigor, and stress tolerance.
Second, increasing FDA expression in the cytosol of photosynthetic cells would lead to an increase in sucrose production and export out of source leaves. This increase in source capacity is a desirable trait in crop plants and would lead to increased plant growth, storage ability, yield, vigor, and stress tolerance.
Third, expression of FDA in sink tissues can show several desirable traits, such as increased amino acid and/or fatty acid pools via increases in carbon flux through glycolysis (and thus pyruvate levels) in seeds or other sinks and increased starch levels as result of increased production of glucose 6-phosphate in seeds, roots, stems, and tubers where starch is a major storage nonstructural carbohydrate (reverse glycolysis). This increase in sink strength is a desirable trait in crop plants and would lead to increased plant growth, storage ability, yield, vigor, and stress tolerance.
Fourth, the invention is particularly desirable for use in the commercial production of foods derived from potatoes. Potatoes used for the production of french fries and other products suffer from a non-uniform distribution of solids between the tuber pith (inner core) and the cortex (outer core). Thus, french fry strips from the pith regions of such tubers have a low solids content and a high water content in comparison to cortex strips from the same tubers. Therefore, the french fry processor attempts to adjust the processing parameters so that the final inner strips are sufficiently cooked while the outer cortex strips are not overcooked. The results of such adjustments, however, are highly variable and may lead to poor quality product. Transgenic potatoes expressing fda will provide to the french-fry and potato chip processor a raw product that consistently displays a higher tuber solids uniformity with acceptable agronomic traits. In the french fry plant production process, inner pith fry strips from higher solids uniformity tubers will require less time to blanch, less time to dry to a specific solids content, and less time to par-fry before freezing and shipping to retail and institutional end-users.
Therefore, with respect to potatoes, the present invention provides 1) a higher quality, more uniform finish fry product in which french fries from all tuber regions, when processed, are nearly the same, 2) a higher through-put in the french fry processing plant due to lower processing times, and 3) processor cost savings due to lower energy input required for lower blanch, dry, and par-fry times. A raw tuber product that displays a higher solids uniformity will also produce a potato chip that has a reduced saddle curl, and a reduced tendency for center bubble, which are undesirable qualities in the potato chip industry. Reduced fat content would also result; this would contribute to improved consumer appeal and lower oil use (and costs) for the processor. The increase in solids uniformity will also translate to an increase in overall tuber solids. For both the french fry and chipping industries, this overall tuber solids increase will also result in higher through-put in the processing plant due to lower processing times, and cost savings due to lower energy input for blanching, drying, par-frying, and finish frying.