Previous work with model transgenic plants has demonstrated that cellular accumulation of mannitol can alleviate abiotic stress. Here, we show that ectopic expression of the mtlD gene for the biosynthesis of mannitol in wheat improves tolerance to water stress and salinity. Wheat (Triticum aestivum L. cv Bobwhite) was transformed with the mtlD gene of Escherichia coli. Tolerance to water stress and salinity was evaluated using calli and T 2 plants transformed with (ϩmtlD) or without (ϪmtlD) mtlD. Calli were exposed to Ϫ1.0 MPa of polyethylene glycol 8,000 or 100 mm NaCl. T 2 plants were stressed by withholding water or by adding 150 mm NaCl to the nutrient medium. Fresh weight of ϪmtlD calli was reduced by 40% in the presence of polyethylene glycol and 37% under NaCl stress. Growth of ϩmtlD calli was not affected by stress. In ϪmtlD plants, fresh weight, dry weight, plant height, and flag leaf length were reduced by 70%, 56%, 40%, and 45% compared with 40%, 8%, 18%, and 29%, respectively, in ϩmtlD plants. Salt stress reduced shoot fresh weight, dry weight, plant height, and flag leaf length by 77%, 73%, 25%, and 36% in ϪmtlD plants, respectively, compared with 50%, 30%, 12%, and 20% in ϩmtlD plants. However, the amount of mannitol accumulated in the callus and mature fifth leaf (1.7-3.7 mol g Ϫ1 fresh weight in the callus and 0.6-2.0 mol g Ϫ1 fresh weight in the leaf) was too small to protect against stress through osmotic adjustment. We conclude that the improved growth performance of mannitol-accumulating calli and mature leaves was due to other stress-protective functions of mannitol, although this study cannot rule out possible osmotic effects in growing regions of the plant.Water stress and salinity are major abiotic factors that limit crop productivity in drought-prone areas. One way of increasing productivity in stressful environments is to breed crops that are more tolerant to stress. However, success in breeding for tolerance has been limited because (a) tolerance to stress is controlled by many genes, and their simultaneous selection is difficult (Richards, 1996;Yeo, 1998;Flowers et al., 2000); (b) tremendous effort is required to eliminate undesirable genes that are also incorporated during breeding (Richards, 1996); and (c) there is a lack of efficient selection procedures particularly under field conditions (Ribaut et al., 1997). Genetic engineering offers an alternative approach for developing tolerant crops. Unlike classical breeding, genetic engineering is a faster and more precise means of achieving improved tolerance (Cushman and Bohnert, 2000) because it avoids the transfer of unwanted chromosomal regions. Moreover, through genetic engineering, multiple genes can be assembled and simultaneously introduced to the crop of interest. There are many functional targets for engineering tolerance to water stress and salinity, one of them being accumulation of osmoprotectants (Rathinasabapathi, 2000).The osmolyte mannitol is normally synthesized in numerous plant species, but not in wheat (Triticum aes...

Three tomatoes, Lycopersicon esculentum Mill. cv UC82B, a droughttolerant wild related species, Lycopersicon pennellii (Cor.) D'Arcy, and their F, hybrid, were grown in containers maintained at three levels of soil moisture. Season-long water use was obtained by summing over the season daily weight losses of each container corrected for soil evaporation. Plant biomass was determined by harvesting and weighing entire dried plants. Season-long Indirect means to evaluate WUJE,, accurately in field-grown plants are needed to improve our understanding of water relations and adaptation in native plants and to serve as selection criteria in crop improvement programs. Light-weight portable equipment for measuring rates of photosynthesis and transpiration ofindividual leaves or canopies are becoming commonplace. Such a measurement (4, 8, 13, 31) yields the WUEF of the plant part under measurement, and it represents the performance of that plant part at a specific time only. The relationship between such a point measurement and long-term plant performance is uncertain. Micrometeorological approaches to determine the water use efficiency (1) proportionally less than the rate of transpiration (2,3,6,7,20), except perhaps under very rare field conditions of extremely stagnant air (large boundary layer resistance) (1,3,6,7,31). Reduced stomatal aperture also simultaneously decreases the concentration of CO2 in the intercellular air spaces, Co. The isotopic discrimination at any moment in time against '3C02 as a substrate is inherent with the primary carboxylating enzyme of C3 photosynthesis, Rubisco. It is great when Ci is high, and the discrimination is reduced with decreasing Ci (2,16,21,22,29).Thus, 613C of a plant or plant part will reflect the integral of Ci over the time of tissue formation as well as the water use efficiency averaged over the growth period, WUEg.The enzymic discrimination against 13C is the major cause for the difference in plant 613C and a 13C of CO2 in the air, although minor isotopic fractionation occurs due to different diffusivities and solubilities of 13C02 and '2CO2 (14,16,21). Note also that the relationship between 6'3C and WUE, would be similar also if these two parameters were affected by photosynthetic perturbations of the mesophyll (photochemistry, biochemistry), rather than by stomatal behavior, as the theoretical relationships worked out by Farquhar and Richards (17) and Francey and Farquhar (18) show.The variability in a '3C around the mean of a given C3 species is just a few parts per thousand. Yet, Farquhar and Richards (17) were able to show a strong correlation between 613C and WUE', in a study on four dwarf wheat varieties, as were Hubick et al. 213www.plantphysiol.org on May 11, 2018 -Published by Downloaded from

Reactive oxygen species (ROS) are formed in biological systems as part of normal metabolism. Adverse environmental factors like drought stress result in increased levels of ROS that are detrimental to the plant (1, 2). To avoid damage caused by these excess ROS, plants have developed elaborate mechanisms to manage them at sustainable levels. Enzymes play an important role in lowering the ROS levels and helping avoid oxidative stress. Superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase play a vital role in combating oxidative stress. Measuring these enzyme activities spectrophotometrically provides researchers an easy and precise way to study and understand an important part of the defense against oxidative stress. In this chapter we provide details of the assays we used to determine the enzyme activities spectrophotometrically. Antioxidant enzyme responses to moderate water-deficit stress were studied. All enzyme assays were conducted using wheat leaf tissue.

Summary The phytotoxin coronatine (COR), which is produced by Pseudomonas syringae pv. tomato DC3000 (DC3000), has multiple roles in virulence that lead to chlorosis and a reduction in chlorophyll content. However, the physiological significance of COR‐induced chlorosis in disease development is still largely unknown. Global expression analysis demonstrated that DC3000 and COR, but not the COR‐defective mutant DB29, caused reduced expression of photosynthesis‐related genes and result in a 1.5‐ to 2‐fold reduction in maximum quantum efficiency of photosystem II (FV/FM). Tomato (Solanum lycopersicum) seedlings inoculated with DC3000 and incubated in a long daily photoperiod showed more necrosis than inoculated seedlings incubated in either dark or a short daily photoperiod. The accumulation of reactive oxygen species (ROS) was detected in cotyledons inoculated with either purified COR or DC3000 but not in tissues inoculated with DB29. Interestingly, COR‐induced ROS accumulated only in light and was inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea and diphenylene iodonium, which function to inhibit electron transport from PSII. Furthermore, COR and DC3000 suppressed expression of the gene encoding the thylakoid Cu/Zn superoxide dismutase but not the cytosolic form of the same enzyme. In conclusion, these results demonstrate a role for COR‐induced effects on photosynthetic machinery and ROS in modulating necrotic cell death during bacterial speck disease of tomato.