External conditions that adversely affect growth, development or productivity trigger a wide range of plant responses, such as altered gene expression, cellular metabolism and changes in growth rates and crop yields. There are two types of stress: biotic stress is imposed by other organisms, such as a pathogen, whereas abiotic stress arises from an excess or deficit in the physical or chemical environment, such as drought, salinity, high or low temperature or high light. Biotic and abiotic stresses can reduce average plant productivity by 65% to 87%, depending on the crop.
An example of biotic stress is pathogen infection. Plants have evolved defensive mechanisms, such as the induction of the expression of specific resistance genes upon infection. It is known that resistance is heritable and plant breeders have been breeding varieties of crop plants with disease resistance ever since. However, pests and pathogens have also developed ways to compromise plant resistance. Pathogens are adaptive by their ability to evolve strains that defeat the resistance genes deployed in crop plants by plant breeders. This has led to the need of continually updating and replacing varieties with different genes or combinations of genes for resistance in response to the ever-changing pathogen populations. Therefore, new ways of improving pathogen resistance are needed (Crute et al 1998, Cook et al 1996).
In addition to pathogen infection, plants are exposed to varying environmental conditions. One important factor in the development of plants and thus in agriculture is the availability of water. Water is essential for crop production because plants require water for growth and tissue expansion. Thus, the supply of fresh water is essential for all forms of agriculture, although the amount of water required varies greatly between different agricultural types and climatic regions.
There has been limited success with conventional breeding to improve the way in which plants use the water resources available. Genetic engineering is therefore considered an alternative. Several genes that regulate drought response have been identified in the model plant Arabidopsis. These are categorised as responsive to dehydration and early response to dehydration genes (Valliyodan et al 2006). One of the factors identified in regulating cold and drought stress responsive gene expression in Arabidopsis is a family of transcription factors termed DREB, which interact with a dehydration responsive element. Overexpression of DREB results in significant drought tolerance under water limited conditions. However, resistance to drought often compromises development of these transgenic plants under normal conditions. It has been shown that overexpression of DREB1/CBF and DREB2A driven by the CaMV 35S promoter causes growth retardation under normal conditions (Valliyodan et al 2006, Sakuma et al 2006, Qiang et al 2000). Thus, there has so far been no success in genetically modifying plants so that they show improved and more efficient use of water under normal non-drought conditions as well as under water deficit conditions.
Although increasing drought tolerance is desirable in the face of global warming, from an agricultural point of view, drought resistance is usually linked to low productivity, and is thus of limited use in agricultural production. Also, severe water deficits are generally rare in viable agriculture. Therefore, reducing the amount of water used per unit yield is now seen as the most promising way forward.
This is increasingly important due to the rising amounts of water which are used in agriculture and the changing climate. Globally, some 2.7×103 km3 of water were used in agriculture in 2000. It is estimated that the production of 1 kg of wheat requires 1 m3 of water, and 1 kg of rice requires at least 1.2 m3 of water. In the 15 countries of the EU in 2003, an area equivalent to 15.5% of the arable and permanent crop area was irrigated, and irrigation comprised over half of the total water consumption (EEA 2003). Even within the humid, temperate climate of England, 147 kha of outdoor crops were irrigated in 2001 (about 3% of the cropped area), using 131×106 m3 of water (Morison et al 2008; Rijsberman. 2004; Richards 2004; Food and Agriculture Organisation (FAO) 2003; Parry et al 2005.)
Thus, how to reduce agricultural water use and make water resources more sustainable is an increasingly urgent question. There is a need to develop crops that require less water to produce sufficient yield under normal conditions in addition to showing improved drought resistance. The amount of yield produced per unit water used is referred to as ‘water productivity’, a well known term in agriculture (Morison et al., 2008).
All eukaryotic organisms respond to an increase in the ambient temperature with the expression of a group of proteins known as heat shock proteins (HSPs). Key factors in the regulation of the expression of Hsp genes are the heat shock transcription factors (Hsfs) that act by binding to a highly conserved palindromic heat shock response sequence in the promoters of the target genes. In addition to mediating the response to heat stress, Hsfs are thought to be involved in cellular responses to oxidative stress, heavy metals and other stress responses (Panchuk et al 2002, Panikulangara et al 2004).
It is known that the basic structure of Hsfs and of their promoter recognition site is conserved throughout the eukaryotic kingdom (Kotak et al 2004, Miller and Mittler 2006). Hsfs have a modular structure with a highly conserved N-terminal DNA binding and a C-terminal activation domain. Other conserved domains include an oligomerisation domain, a nuclear localisation sequence and a nuclear export sequence. Thus, Hsfs are easily recognised by their conserved motifs essential for their function as transcription factors (Kotak et al 2004, Miller and Mittler 2006, Nover et al 2001).
Yeast and Drosophila contain only one Hsf gene, while vertebrates are thought to have three Hsf genes. In plants, Hsf genes have been identified in many species, for example maize, the model plant Arabidopsis thaliana (21 Hsfs), soybean (34 Hsfs), rice (23 Hsfs), barley, potato, tomato (18 Hsfs) and others. Hsfs within the plant kingdom are highly conserved and divided into three classes (A, B and C). For example, it has been found that a class of Hsfs in Arabidopsis is closely related to Hsf from rice and to Hsfs identified from ESTs in barley, potato, tomato and soy bean (Nover et al 2001 and Kotak et al 2004).
The invention is aimed at solving or at least mitigating the problems discussed above by introducing and expressing a gene sequence encoding a plant heat shock transcription factor.
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