Prospects for utilising plant-adaptive mechanisms to improve wheat and other crops in drought- and salinity-prone environments

Breeding for adaptation to abiotic stress is extremely challenging due to the complexity of the target environments as well as that of the stress-adaptive mechanisms adopted by plants. While many traits have been reported in the literature, these must be considered with respect to the type of environment for which a cultivar is targeted. In theory, stress-adaptive traits can be divided into groups whose genes and/or physiological effects are likely to be relatively independent such that when parents with contrasting traits are crossed, adaptive genes will be pyramided. Currently the following groups of candidate traits are being considered for drought adaptation in wheat: traits relating to: (i) pre-anthesis Growth. (ii) water extraction, (iii) water use efficiency, (iv) photo-protection. A number of mechanisms relating to root function have potential to ameliorate drought stress. Hydraulic redistribution (HR) of water by roots of dryland shrubs enables even relatively small amounts of rainwater to be moved down into the soil profile actively by the root system before it evaporates from the soil surface. Another example is the symbiotic relationship of plants with mycorrhizal fungi that produce a glycoprotein that has a positive effect on soil structure and moisture characteristics. From an agronomic point of view, crop water use efficiency can be increased by exploiting the stress-adaptive mechanism whereby leaves reduce transpiration rate in response to a chemical root signal in response to drying soil. While there is limited genetic diversity for adaptation to salinity in wheat, tolerance has been found in the ancestral genomes of polyploid wheat and their relatives associated with sodium exclusion into the Xylem. Wide crossing techniques such as production of synthetic hexaploids are being exploited to tap into this source of genetic diversity. Looking further into the future, progress is being made into understanding the regulatory mechanisms that are expressed under abiotic stress to maintain cellular homeostasis, as well as in the ability to genetically transform crop plants with genes from alien species.