Canola is an oilseed species formed by the cross between two Brassica species, B. oleracea L. and B. rapa L. Although different cultivars of rapeseed have been grown globally as oilseeds for condiments and industrial lubricants, rapeseed was considered a minor oilseed crop mainly because of its poor oil and meal quality. The major breakthrough came from intensive breeding programs in the 1970s to reduce erucic acid to less than 2% of total seed oil content as well as to reduce the glucosinolates to less than 30 µmol g–1 of defatted meal. Today, along with soybean and oil palm, canola is one of the largest vegetable oil crops, with 46 million Mg of seed production from approximately 25 million ha worldwide. The majority of this production is from the European Union, China, Canada, India, and Australia.
Due to its economic importance, canola has been bred to increase seed production. In the past 40 years, the average yield of canola in Canada has increased from roughly 1000 to 1500 kg ha–1, with an average increase of 1.25% per year. Besides traditional breeding methods, genetic engineering approaches have also been widely used to increase and maintain canola crop production. Much of the current research efforts are focused on improvement of seed oil and meal quality, as well as on herbicide, pest, and disease resistance. In Canada, it is estimated that 95% of the commercially grown canola possesses herbicide resistance, and of these, about 82% achieved resistance through transgenic introduction of herbicide resistant genes.
Canola is sensitive to many environmental stresses, and in particular, water stress (drought) is one of the main factors that limits crop yield. In Europe, the winter annual forms of canola produce an average yield of about 3000 kg ha–1, which is approximately double the average yield of the Canadian and Australian spring canola. The key reason for the difference is believed to be that the European canola are typically grown under cooler temperatures with sufficient precipitation during the growth season.
Drought has a great impact on canola yield in Canada, especially in the primary canola growth provinces in the western prairies. In the summers of 2001 and 2002, for example, dry spells caused significant canola yield reductions in many regions of Saskatchewan and Alberta. In Australia, canola production is mainly in the regions with an average annual rainfall greater than 450 mm, and it is almost absent from the semiarid agricultural regions averaging less than 325-mm annual precipitation. Development of drought-tolerant canola is considered an important and urgent mandate for the canola industry. A breakthrough in this area should increase yield and permit expansion of canola growth regions and in doing so, address the growing demand.
Plants have evolved various mechanisms to adapt to and cope with water stress conditions. Over the last decade, tremendous efforts have focused on understanding these mechanisms. The progress in this area has been extensively reviewed. Even though it is clearly important to understand how plants respond to water stress, often the genes that are responsive to the stress are not necessarily the most effective targets for enhanced water stress tolerance and yield protection. Furthermore, to be truly effective, an engineered drought-tolerant crop plant must produce normal yields under nondrought conditions and produce greater yields than conventional varieties under stress conditions. Thus far, according to these authors, research efforts to create drought-tolerant crop plants have been intensive, but the results remain incremental. There is still a significant distance between the unearthing of the genetic and cellular components involved in drought tolerance in the laboratory and commercial application in the field.
Ninety percent of soil water used by plants is lost through the process of transpiration via the opening of stomata, which is controlled by a pair of guard cells in the leaves and stems . One of the best understood mechanisms of the plant's adaptation to water stress is the modulation of stomatal aperture by abscisic acid (ABA). Under water stress conditions, the endogenous concentration of ABA rises, which, through a complex signaling cascade, causes guard cell shrinkage and stomatal closure and results in a reduction of transpirational water loss. When soil water content returns to optimal conditions, the level of ABA decreases, reversing the process. Since similar ABA biosynthesis and signal pathways exist in all crops, this hormone is an excellent target for improving drought tolerance.
In this review, the authors summarize efforts to develop drought-tolerant canola through both traditional breeding and genetic engineering methods. They focus on what has been learned about ABA and ABA sensing and how they can apply this knowledge to engineer drought-tolerant canola.
Development of Drought-Tolerant Canola (Brassica napus L.) through Genetic Modulation of ABA-mediated Stomatal Responses by Jiangxin Wana, Rebecca Griffithsa, Jifeng Yinga, Peter McCourtb and Yafan Huanga
Published online 7 August 2009 in Crop Sci 49:1539-1554 (2009) © 2009 Crop Science Society of America, 677 S. Segoe Rd., Madison, WI 53711 USA
Canola photo courtesy of Syngenta