The present invention relates to isolated polypeptides, polynucleotides encoding same, transgenic plants expressing same and methods of using same. Specifically the present invention can be used to increase fertilizer use efficiency and stress resistance as well as biomass, vigor and yield of transgenic plants.
Fertilizers are the fuel behind the “green revolution”, directly responsible for the exceptional increase in crop yields during the last 40 years. The dramatic rise in crop yields could never have occurred without a parallel increase in fertilizer use. However, in recent years there has been a growing concern with the environmental impact of fertilizer use, particularly nitrogen fertilizers, on water and atmospheric pollution. Limits on fertilizer use have been legislated in several countries, and further restrictions are expected in the future. Greater use of fertilizers will be necessary in the future to support food and fiber production for rapid population growth on limited land resources.
Fertilizer is often mentioned as the number one overhead expense in agriculture. Of the three macronutrients provided as main fertilizers [Nitrogen (N), Phosphate (P) and Potassium (K)], nitrogen is the only one that usually needs to be replenished every year, particularly for cereals, which comprise more than half of the cultivated areas worldwide.
A common approach to promoting plant growth has been, and continues to be, the use of nutrients (fertilizers), natural as well as synthetic. Synthetic nutrients usually provide a macronutrient in a plant-usable form, such as urea for example, and/or inorganic nitrates, phosphates, or the like compounds. While such nutrients may be applied, more or less, at the convenience of the farmer, and may be applied as often as deemed desirable, the overuse of synthetic nutrients and the inefficient use of synthetic nutrients are major factors responsible for environmental problems such as eutrophication of groundwater, nitrate pollution, phosphate pollution, and the like.
Nitrogen is an essential macronutrient for the plant, responsible for biosynthesis of amino and nucleic acids, prosthetic groups, plant hormones, plant chemical defenses, etc. Nitrogen is often the rate-limiting element in plant growth and all field crops have a fundamental dependence on inorganic nitrogenous fertilizer. Since fertilizer is rapidly depleted from most soil types, it must be supplied to growing crops two or three times during the growing season. Nitrogenous fertilizer, which is usually supplied as ammonium nitrate, potassium nitrate, or urea, typically accounts for 40% of the costs associated with crops such as corn and wheat. It has been estimated that by 2050, more than 150 million tons of nitrogenous fertilizer will used worldwide annually. Increased use efficiency of nitrogen by plants should enable crops to be cultivated with lower fertilizer input, or alternatively on soils of poorer quality and would therefore have significant economic impact in both developed and developing agricultural systems. An overview of the undesirable effects of nitrogen fertilizer is presented by Byrnes, Fertilizer Research, 26, pp. 209-215 (1990). Although plants are able to take up organic nitrogen from the environment, the major part of the nitrogen utilized comes usually from the uptake of inorganic nitrogen in the form of ammonium (NH4+) and nitrate (NO3−) and its later conversion to organic nitrogen in a process known as assimilation.
The Nitrogen assimilation process begins with NO3− being converted to NH4+ sequentially by the enzymes Nitrate Reductase (NR) and Nitrite Reductase (NiR). The nitrogen is then incorporated into Glutamate (Glu) by Glutamine Synthase (GS) to obtain Glutamine (Gln). The major pathway of nitrogen assimilation is the GS/GOGAT cycle (Glutamine Synthase/Glutamate-Oxoglutarate Amine Transferase). The remaining amino acids are synthesized from Gln, Glu and Asn by transamination.
Nitrogen (as amino acids or in the form of nitrates) is translocated to the shoot, where it is stored in the leaves and stalk during the rapid step of plant development and up until flowering. In corn for example, plants accumulate the bulk of their organic nitrogen during the period of grain germination, and until flowering. Once fertilization of the plant has occurred, grains begin to form and become the main sink of plant Nitrogen. The stored Nitrogen can then be redistributed from the leaves and stalk that served as storage compartments until grain formation.
There are three main parameters of efficiency used to define plant Nitrogen metabolism:
Nitrogen-uptake efficiency: is the amount of N in above-ground biomass (gr Nt) divided by the amount of N applied (gr/hectare);
Nitrogen utilization efficiency: is the Grain Yield (gr/plant) divided by the amount of N in above-ground biomass (gr Nt); and
Nitrogen-use efficiency: is the Grain Yield (gr/plant) divided by the amount of N applied (gr/Ha).
The Nitrogen-uptake efficiency [the amount of N in above ground biomass (gr Nt)/N applied (gr/hectare)] is the total amount of nitrogen incorporated by the plant and is a function of the “uptake” (the plant's transport capacity), the metabolic efficiency of the assimilation process and the rate of plant size development, since the mass of stalk and leaves created during growth are the actual Nitrogen-storage organs. The fraction of the assimilated Nitrogen found in a shoot that is ultimately transferred to the grain (yield) is controlled enzymatically, and thus a potential site for transgenic manipulation. This parameter is, in effect, equal to the Nitrogen Utilization efficiency (NUE). Better grain-to-shoot N-partitioning most likely will improve yield and protein content of the grain.
Similarly, the same calculations of use and utilization efficiencies can be made for other macronutrients such as Phosphorous (P) and Potassium (K), which have a direct correlation with yield and general plant tolerance.
The NUE for the main crops ranges from 30-70% only, having a direct negative impact on input expenses for the farmer, due to the excess fertilizer applied, which quickly becomes an ecological burden. Thus, nitrate-containing wastes represent an environmental problem of global significance. Nitrate seepage in water causes eutrophication of lakes, rivers and seas (waters endangered because of algae growth that leads to hypoxia and destruction of marine fauna). Nitrate contamination in drinking water can cause methemoglobinemia, which is especially detrimental to infants and nursing mothers. In fact, the Farming Industry is considered as the largest nitrate polluter of surface and coastal waters and drinking water supplies.
Genetic improvement of Fertilizer Use Efficiency (FUE) in plants can be generated either via traditional breeding or via genetic engineering. However, to date, neither transgenic products nor classically bred enhanced FUE material have been released for commercial use. Among the reasons for this, the most important is that breeders select their elite lines under the most favorable fertilizer conditions, thus overlooking improvements in FUE (yield being the main driver of sales and not reduction in the input costs). Attempts at transgenic solutions for improved FUE are being carried out by companies such as Monsanto (see, for example, US Patent Applications 20020046419 to Choo, et al.; U.S. Pat. Appl. 2005010879 to Edgerton et al.; and U.S. Pat. Appl. 2006 0179511 to Chomet et al), Arcadia Biosciences and Biogemma.
Recently, a review summarizing attempts to improve FUE by transgenic means that have been undertaken by academic laboratories was published (Good A G et al. Trends Plant Sci. 2004 December; 9(12):597-605). Encouraging results were reported by Yanagisawa and coworkers (Proc Natl Acad Sci USA. 2004 May 18; 101(20):7833-8) who found that a genetically engineered increase in carbon skeleton production (2-Oxoglutarate, OG from the GS/GOGAT cycle) sustained growth of transgenic Arabidopsis under low nitrogen conditions. As many enzymes are involved in carbon skeleton production, the transgene was a key transcriptional factor (Dofl) that activated multiple genes involved in the pathway. Nitrogen content was higher in the Arabidopsis transgenic plants by approximately 30% under low nitrogen conditions. U.S. Pat. No. 6,084,153 to Good et al. discloses the use of a stress responsive promoter to control the expression of Alanine Amine Transferase (AlaAT). Good et al. further disclosed that transgenic canola plants improved drought and nitrogen deficiency tolerance when compared to control plants. However, neither the Dofl constructs of Yanagisawa et al, nor the drought-induced AlaAT constructs of Good et al. have been evaluated in commercial lines, under true field conditions. Hence the economic relevance of the results is yet to be proven.
There is thus a widely recognized need for, and it would be highly advantageous to identify polynucleotides and polypeptides which improve fertilizer use/uptake efficiency in transgenic plants expressing same, which are devoid of the above limitations.