Transgenic Arabidopsis plants expressing the rice dehydroascorbate reductase gene are resistant to salt stress.

Vitamin C (l-ascorbate) is important for antioxidative and metabolic functions in both plants and humans. Ascorbate itself is oxidized to dehydroascorbate during the process of antioxidation, and dehydroascorbate reductase (DHAR, EC 1.8.5.1) re-reduces the oxidized ascorbate. Therefore, this enzyme is assumed to be critical for ascorbate recycling. Here we show that the expression of rice DHAR in transgenic Arabidopsis thaliana enhanced resistance to salt stress. Salt tolerance was remarkably improved despite slight increases in DHAR activity and total ascorbate. This study provides direct evidence for the importance of DHAR in salt tolerance.

Enhanced formation of flowers in salt-stressed Arabidopsis after genetic engineering of the synthesis of glycine betaine.

Previously, we showed that transformation with the codA gene for choline oxidase allows plants to synthesize glycine betaine (GB) and enhances their ability to tolerate various kinds of stress during germination and vegetative growth. In this study, we examined the tolerance of transformed plants to salt stress at the reproductive stage, which is the stage at which plants are most sensitive to environmental stress. Salt-shock treatment of wild-type plants for 3 days resulted in the abortion of flower buds and decreased the number of seeds per silique. These deleterious effects were clearly visible 6 days after the termination of salt-shock treatment. Microscopic examination of floral structures revealed that salt stress inhibited the development of anthers, pistils, and petals. In particular, the production of pollen grains and ovules was dramatically inhibited. These effects of salt stress were significantly reduced by transformation with the codA gene, and our observations suggested that the enhanced tolerance of the transgenic plants was a result of the accumulation of GB in the reproductive organs. Indeed, levels of GB in flowers, siliques, and inflorescence apices were about five times higher than in leaves.

Structural consequences of genetically engineered saturation of the fatty acids of phosphatidylglycerol in tobacco thylakoid membranes. An FTIR study.

The role of phosphatidylglycerol (PG) in protein-lipid interactions and membrane dynamics has been studied in the thylakoids of wild type and manipulated tobacco plants transformed with complementary DNAs for glycerol-3-phosphate acyltransferases (GPATs) from squash and Arabidopsis. The expression of the foreign enzymes resulted in the level of saturation of the PG molecules being higher in the squash and lower in the Arabidopsis transformants, as compared with the level in wild-type tobacco. For the analysis of fatty acyl chain dynamics in the thylakoid membranes, the nu(sym)CH(2) vibration bands of the infrared specta were decomposed into two components, corresponding to ordered and disordered fatty acyl chain segments. With this approach, it was shown that in squash GPAT-transformed tobacco thylakoids a rigid lipid domain exists below 25 degrees C. Above 25 degrees C, the dynamics of all thylakoid membranes were very similar, regardless of the manipulations. PG seems to tune the dynamics at the protein-lipid interface rather than to affect the structure of the proteins directly. Above 50 degrees C, the frequencies of the disordered nu(sym)CH(2) component bands were decreased. This lipid-related phenomenon correlated with protein denaturing. It is demonstrated that the protein aggregation appearing upon heat denaturing changes the conformational distribution of the disordered lipid population. The data also reveal that the protein stability does not depend on the fatty acid composition of the PG molecules; other lipids should provide the environment governing the protein stability in the thylakoid membrane. This is the first such detailed analysis of the infrared spectra of biological membranes that permits a differentiation between structurally different lipid populations within a membrane.

SHEPHERD is the Arabidopsis GRP94 responsible for the formation of functional CLAVATA proteins.

The Arabidopsis shepherd (shd) mutant shows expanded shoot apical meristems (SAM) and floral meristems (FM), disorganized root apical meristems, and defects in pollen tube elongation. We have discovered that SHD encodes an ortholog of GRP94, an ER-resident HSP90-like protein. Since the shd phenotypes in SAM and FM are similar to those of the clavata (clv) mutants, we have explored the possibility that CLV complex members could be SHD targets. The SAM and FM morphology of shd clv double mutants are indistinguishable from those of clv single mutants, and the wuschel (wus) mutation is completely epistatic to the shd mutation, indicating that SHD and CLV act in the same genetic pathway to suppress WUS function. Moreover, the effects of CLV3 overexpression that result in the elimination of SAM activity were abolished in the shd mutant, indicating that CLV function is dependent on SHD function. Therefore, we conclude that the SHD protein is required for the correct folding and/or complex formation of CLV proteins.