1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding polypeptides that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding polypeptides that confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on development, growth, and yield of most crop plants are profound. Continuous exposure to drought conditions causes major alterations in the plant metabolism which ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold, and salt tolerance in model drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerance plants using biotechnological methods.
Drought and cold stresses, as well as salt stresses, have a common theme important for plant growth, and that is water availability. Plants are exposed during their entire life cycle to conditions of reduced environmental water content, and most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to higher salt concentrations in the soil. Because high salt content in some soils results in less water being available for cell intake, high salt concentration has an effect on plants similar to the effect of drought on plants. Additionally, under freezing temperatures, plant cells lose water as a result of ice formation that starts in the apoplast and withdraws water from the symplast. A plant's molecular response mechanisms to each of these stress conditions are common, and protein kinases play an essential role in these molecular mechanisms.
Protein kinases represent a superfamily, and the members of this superfamily catalyze the reversible transfer of a phosphate group of ATP to serine, threonine, and tyrosine amino acid side chains on target polypeptides. Protein kinases are primary elements in signaling processes in plants and have been reported to play crucial roles in perception and transduction of signals that allow a cell (and the plant) to respond to environmental stimuli. In particular, receptor protein kinases (RPKs) represent one group of protein kinases that activate a complex array of intracellular signaling pathways in response to the extracellular environment (Van der Gear et al., 1994, Annu. Rev. Cell Biol. 10:251-337). RPKs are single-pass transmembrane polypeptides that contain an amino-terminal signal sequence, extracellular domains unique to each receptor, and a cytoplasmic kinase domain. Ligand binding induces homo- or hetero-dimerization of RPKs, and the resultant close proximity of the cytoplasmic domains results in kinase activation by transphosphorylation. Although plants have many polypeptides similar to RPKs, no ligand has been identified for these receptor-like kinases (RLKs). The majority of plant RLKs that have been identified belong to the family of Serine/Threonine (Ser/Thr) kinases, and most have extracellular Leucine-rich repeats (Becraft, P W., 1998, Trends Plant Sci. 3:384-388).
Another type of protein kinase is the Ca+-dependent protein kinase (CDPK). This type of kinase has a calmodulin-like domain at the COOH terminus which allows response to Ca+ signals directly without calmodulin being present. Currently, CDPKs are the most prevalent Ser/Thr polypeptide kinases found in higher plants. Although their physiological roles remain unclear, they are induced by cold, drought, and abscisic acid (ABA) (Knight et al., 1991, Nature 352:524; Schroeder, J. J. and Thuleau, P., 1991, Plant Cell 3:555; Bush, D. S., 1995, Annu. Rev. Plant Phys. Plant Mol. Biol. 46:95; Urao, T. et al., 1994, Mol. Gen. Genet. 244:331).
Another type of signaling mechanism involves members of the conserved SNF1 Serine/Threonine polypeptide kinase family. These kinases play essential roles in eukaryotic glucose and stress signaling. Plant SNF1-like kinases participate in the control of key metabolic enzymes, including HMGR, nitrate reductase, sucrose synthase, and sucrose phosphate synthase (SPS). Genetic and biochemical data indicate that sugar-dependent regulation of SNF1 kinases involves several other sensory and signaling components in yeast, plants, and animals.
Additionally, members of the Mitogen-Activated Protein Kinase (MAPK) family have been implicated in the actions of numerous environmental stresses in animals, yeasts and plants. It has been demonstrated that both MAPK-like kinase activity and mRNA levels of the components of MAPK cascades increase in response to environmental stress and plant hormone signal transduction. MAP kinases are components of sequential kinase cascades, which are activated by phosphorylation of threonine and tyrosine residues by intermediate upstream MAP kinase kinases (MAPKKs). The MAPKKs are themselves activated by phosphorylation of serine and threonine residues by upstream kinases (MAPKKKs). A number of MAP Kinase genes have been reported in higher plants.
Another major type of environmental stress is lodging, which refers to the bending of shoots or stems in response to wind, rain, pests or disease. Two types of lodging occur in cereals: root-lodging and stem breakage. The most common type of lodging is root lodging, which occurs early in the season. Stem-breakage, by comparison, occurs later in the season as the stalk becomes more brittle due to crop maturation. Stem breakage has greater adverse consequences on crop yield, since the plants cannot recover as well as from the earlier root-lodging.
Lodging in cereal crops is influenced by morphological (structural) plant traits as well as environmental conditions. Lodging in cereals is often a result of the combined effects of inadequate standing power of the crop and adverse weather conditions, such as rain, wind, and/or hail. Lodging is also variety (cultivar) dependent. For example, a tall, weak-stemmed wheat cultivar has a greater tendency to lodge than a semi-dwarf cultivar with stiffer straw. In addition, the tendency of a crop to lodge depends on the resistance especially of the lower internodes. This is because the lower internodes have to resist the greatest movement of force. The weight of the higher internodes of the stems plus leaves and heads in relation to the stem (culm) will affect the resistance of a crop to lodging. The heavier the higher parts of the stem are and the greater the distance from their center of gravity to the base of the stem, the greater is the movement of the forces acting upon the lower internodes and the roots. Supporting this argument, it was found that the breaking strength of the lowest internode and shoot per root ratio were the most suitable indices of lodging. Furthermore, plant morphological (structural) characteristics such as plant height, wall thickness, and cell wall lignification can affect the ability of the plant to resist a lateral force.
Severe lodging is very costly due to its effects on grain formation and associated harvesting problems and losses. It takes about twice the time to harvest a lodged crop than a standing one. Secondary growth in combination with a flattened crop makes harvesting difficult and can subsequently lead to poor grain quality. Yield loss comes from poor grain filling, head loss, and bird damage. Yield losses are most severe when a crop lodges during the ten days following head emergence. Yield losses at this stage will range between 15% and 40%. Lodging that occurs after the plant matures will not affect the yield but it may reduce the amount of harvestable grain. For instance, when lodging occurs after the plant matures, neck breakage and the loss of the whole head can result; these often lead to severe harvest losses. In theses cases, farmers who straight combine their grain will likely incur higher losses than those who swath them. Accordingly, it is desirable to identify genes expressed in lodging resistant plants that have the capacity to confer lodging resistance to the host plant and to other plant species.
Although some genes that are involved in stress responses in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance remains largely incomplete and fragmented. For example, certain studies have indicated that drought and salt stress in some plants may be due to additive gene effects, in contrast to other research that indicates specific genes are transcriptionally activated in vegetative tissue of plants under osmotic stress conditions. Although it is generally assumed that stress-induced proteins have a role in tolerance, direct evidence is still lacking, and the functions of many stress-responsive genes are unknown.
There is a need, therefore, to identify genes expressed in stress tolerant plants that have the capacity to confer stress tolerance to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as an increased range in which the crop plants can be cultivated, by for example, decreasing the water requirements of a plant species. Other desirable advantages include increased resistance to lodging, the bending of shoots or stems in response to wind, rain, pests, or disease.