Abiotic stress tolerance in grasses. From model plants to crop plants.

Abiotic stresses, notably extremes in temperature, photon irradiance, and supplies of water and inorganic solutes, frequently limit growth and productivity of major crop species such as wheat (Triticum aestivum; http://www.cimmyt.org/Research/Wheat/ map/research_results/wphysio/wphysio.html). Inaddition, more than one abiotic stress can occur at one time. For example, high temperature and high photon irradiance often accompany low water supply, which can in turn be exacerbated by subsoil mineral toxicities that constrain root growth. Furthermore, one abiotic stress can decrease a plant’s ability to resist a second stress. For example, low water supply can make a plant more susceptible to damage from high irradiance due to the plant’s reduced ability to reoxidize NADPH and thus maintain an ability to dissipate energy delivered to the photosynthetic light-harvesting reaction centers. If a single abiotic stress is to be identified as the most common in limiting the growth of crops worldwide, it most probably is low water supply (Boyer, 1982; Araus et al., 2002). However, other abiotic stresses, notably salinity and acidity, are becoming increasingly significant in limiting growth of both forage grasses and the cereals. Globally, low temperature also is a major limitation of plant growth, and this has a major impact on grasses via, for example, vernalization and low temperature damage at anthesis. In this focus issue, there are articles addressing three aspects of these abiotic stresses. Traditional approaches to breeding crop plants with improved abiotic stress tolerances have so far met limited success (Richards, 1996). This is due to a number of contributing factors, including: (1) the focus has been on yield rather than on specific traits; (2) the difficulties in breeding for tolerance traits, which include complexities introduced by genotype by environment, or G 3 E, interactions and the relatively infrequent use of simple physiological traits as measures of tolerance, have been potentially less subject to G 3 E interferences; and (3) desired traits can only be introduced from closely related species. Most cereals are moderately sensitive to a wide range of abiotic stresses, and variability in the gene pool generally appears to be relatively small and may provide few opportunities for major step changes in tolerance. Of potentially larger impact on abiotic stress tolerance is the use of genetic manipulation technologies to generate such step changes. Having said this, more immediately achievable, if modest, increases in tolerance may be introgressed into commercial lines from tolerant landraces using marker-assisted breeding approaches (Dubcovsky, 2004), facilitated by recent breakthroughs with positional cloning (e.g. Yan et al., 2003, 2004) that are likely to enable identification of extant tolerance genes within cereal germplasms (see www.acpfg.com.au). Of course, the sequencing of the rice (Oryza sativa) genome provides an invaluable resource for work on rice and, by exploiting syntenic alignment with many other grasses (Devos and Gale, 2000), facilitates fine mapping in the unsequenced genomes of many other grasses. It is exploitation of this latest resource that, combined with steadily increasing transformation frequencies for many grasses, is making the functional genomics approach to the study and manipulation of abiotic stresses in grasses increasingly tractable. The need to use a model plant such as Arabidopsis (Arabidopsis thaliana) for such work is steadily decreasing, and will continue to do so, as the principles uncovered in this model organism are refined (or even supplanted) by knowledge gained in the plants that are the ones in which this knowledge needs to be applied (this means, of course, primarily the grasses, both cereals and forage species). Furthermore, in addition to the obvious fundamental differences in development and anatomy between monocotyledons and dicotyledons, many of the mechanisms of tolerance to abiotic stresses can have fundamentally different characteristics between these two major plant groups, so transferring knowledge from Arabidopsis to the major crops often is not possible. For example, when grown in saline soils, many dicotyledonous halophytes accumulate much higher concentrations of Na in their shoots than monocotyledonous halophytes, a feature that may be related to the observation that succulence is observed more commonly in dicotyledons than monocotyledons, particularly the grasses. * Corresponding author; [email protected]; fax 61–8– 8303–7102. www.plantphysiol.org/cgi/doi/10.1104/pp.104.900138.