Agricultural productivity is severely affected by soil salinity. One possible mechanism by which plants could survive salt stress is to compartmentalize sodium ions away from the cytosol. Overexpression of a vacuolar Na+/H+ antiport from Arabidopsis thaliana in Arabidopsis plants promotes sustained growth and development in soil watered with up to 200 millimolar sodium chloride. This salinity tolerance was correlated with higher-than-normal levels of AtNHX1 transcripts, protein, and vacuolar Na+/H+ (sodium/proton) antiport activity. These results demonstrate the feasibility of engineering salt tolerance in plants.

Contents 32I.32II.34III.38IV.3941References41 Summary Environmental stress conditions such as drought, heat, salinity, cold, or pathogen infection can have a devastating impact on plant growth and yield under field conditions. Nevertheless, the effects of these stresses on plants are typically being studied under controlled growth conditions in the laboratory. The field environment is very different from the controlled conditions used in laboratory studies, and often involves the simultaneous exposure of plants to more than one abiotic and/or biotic stress condition, such as a combination of drought and heat, drought and cold, salinity and heat, or any of the major abiotic stresses combined with pathogen infection. Recent studies have revealed that the response of plants to combinations of two or more stress conditions is unique and cannot be directly extrapolated from the response of plants to each of the different stresses applied individually. Moreover, the simultaneous occurrence of different stresses results in a high degree of complexity in plant responses, as the responses to the combined stresses are largely controlled by different, and sometimes opposing, signaling pathways that may interact and inhibit each other. In this review, we will provide an update on recent studies focusing on the response of plants to a combination of different stresses. In particular, we will address how different stress responses are integrated and how they impact plant growth and physiological traits.

Reactive oxygen species (ROS) play a key role in the acclimation process of plants to abiotic stress. They primarily function as signal transduction molecules that regulate different pathways during plant acclimation to stress, but are also toxic byproducts of stress metabolism. Because each subcellular compartment in plants contains its own set of ROS-producing and ROS-scavenging pathways, the steady-state level of ROS, as well as the redox state of each compartment, is different at any given time giving rise to a distinct signature of ROS levels at the different compartments of the cell. Here we review recent studies on the role of ROS in abiotic stress in plants, and propose that different abiotic stresses, such as drought, heat, salinity and high light, result in different ROS signatures that determine the specificity of the acclimation response and help tailor it to the exact stress the plant encounters. We further address the role of ROS in the acclimation of plants to stress combination as well as the role of ROS in mediating rapid systemic signaling during abiotic stress. We conclude that as long as cells maintain high enough energy reserves to detoxify ROS, ROS is beneficial to plants during abiotic stress enabling them to adjust their metabolism and mount a proper acclimation response.

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Drought, the most prominent threat to agricultural production worldwide, accelerates leaf senescence, leading to a decrease in canopy size, loss in photosynthesis and reduced yields. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during drought, we hypothesized that it may be possible to enhance drought tolerance by delaying droughtinduced leaf senescence. We generated transgenic plants expressing an isopentenyltransferase gene driven by a stress-and maturationinduced promoter. Remarkably, the suppression of drought-induced leaf senescence resulted in outstanding drought tolerance as shown by, among other responses, vigorous growth after a long drought period that killed the control plants. The transgenic plants maintained high water contents and retained photosynthetic activity (albeit at a reduced level) during the drought. Moreover, the transgenic plants displayed minimal yield loss when watered with only 30% of the amount of water used under control conditions. The production of drought-tolerant crops able to grow under restricted water regimes without diminution of yield would minimize drought-related losses and ensure food production in water-limited lands.cytokinins ͉ isopentenyltransferase ͉ water stress ͉ water use efficiency ͉ oxidative stress D rought is the most serious environmental factor limiting the productivity of agricultural crops worldwide, with devastating economical and sociological impact. Climate models have indicated that drought episodes will become more frequent because of the long-term effects of global warming (1, 2), emphasizing the urgent need to develop adaptive agricultural strategies for a changing environment. These range from changes in traditional management and agronomic practices to the use of marker-assisted selection for the improvement of drought-related traits and the development of transgenic crops with enhanced tolerance of drought and improved water use efficiency that would minimize drought-related losses and ensure food production for a growing population.Plants can use a combination of different strategies to avoid or tolerate drought stress (3, 4). In arid regions, for example, winter annuals combine a relatively short life cycle with a high growth rate during the wet season to avoid drought altogether. Other types of avoidance include closing of stomata to minimize water loss, adjusting sink/source allocation by increasing root growth, and decreasing canopy by reducing growth and shedding of older leaves (5). Accelerated leaf senescence and leaf abscission are associated with drought in nature as a means to decrease canopy size. In perennial plants, this strategy contributes to the survival of the plant and the completion of the plant life cycle under drought stress. In contrast, this strategy reduces the yields of annual crops, with concomitant economical loss to farmers. We hypothesized that it is possible to enhance the tolerance of plants of drought stress by delaying the drought-induced...