No abstract

L-myo-inositol 1-phosphate synthase (MIPS; EC 5.5.1.4) catalyzes the rate-limiting step in the synthesis of myo-inositol, a critical compound in the cell. Plants contain multiple MIPS genes, which encode highly similar enzymes. We characterized the expression patterns of the three MIPS genes in Arabidopsis thaliana and found that MIPS1 is expressed in most cell types and developmental stages, while MIPS2 and MIPS3 are mainly restricted to vascular or related tissues. MIPS1, but not MIPS2 or MIPS3, is required for seed development, for physiological responses to salt and abscisic acid, and to suppress cell death. Specifically, a loss in MIPS1 resulted in smaller plants with curly leaves and spontaneous production of lesions. The mips1 mutants have lower myo-inositol, ascorbic acid, and phosphatidylinositol levels, while basal levels of inositol (1,4,5)P 3 are not altered in mips1 mutants. Furthermore, mips1 mutants exhibited elevated levels of ceramides, sphingolipid precursors associated with cell death, and were complemented by a MIPS1-green fluorescent protein (GFP) fusion construct. MIPS1-, MIPS2-, and MIPS3-GFP each localized to the cytoplasm. Thus, MIPS1 has a significant impact on myo-inositol levels that is critical for maintaining levels of ascorbic acid, phosphatidylinositol, and ceramides that regulate growth, development, and cell death.

Nonsense-mediated mRNA decay, the accelerated turnover of mRNAs transcribed from genes containing early nonsense mutations, is dependent on the product of the UPFI gene in yeast. Mutations that inactivate UPFI lead to the selective stabilization of mRNAs containing early nonsense mutations but have no effect on the half-lives of almost all other mRNAs. Since the transcripts ofnonsense alleles are not typical cellular constituents, we sought to identify those RNAs that comprise normal substrates of the nonsense-mediated mRNA decay pathway. Many yeast pre-mRNAs contain early in-frame nonsense codons and we considered it possible that a role of this pathway is to accelerate the degradation ofpre-mRNAs present in the cytoplasm. Consistent with this hypothesis, we find that, in a strain lacking UPFI function, the CYH2, RPSIB, and MER2 pre-mRNAs are stabilized 2-to 5-fold and are associated with ribosomes. We conclude that a major source of early nonsense codon-containing cytoplasmic transcripts in yeast is pre-mRNAs and that the UPFI protein may be part ofa cellular system that ensures that potentially deleterious nonsense fragments of polypeptides do not accumulate.In eukaryotes and prokaryotes nonsense mutations in a gene can enhance the decay rate of the mRNA transcribed from that gene (1-12), a phenomenon we describe as nonsensemediated mRNA decay (1). Trans-acting factors that are essential for nonsense-mediated mRNA decay have been identified in experiments that characterized a class of nonsense suppressors in the yeast Saccharomyces cerevisiae. Mutants in the UPFI gene, originally isolated on the basis of their ability to enhance the suppression of a frameshift mutation that led to premature translational termination (13), selectively stabilize mRNAs containing early nonsense mutations without affecting the decay rates of most other mRNAs (ref. 2; S.W.P., A. H. Brown, and A.J., unpublished data).The existence of trans-acting factors that promote rapid decay of nonsense-containing mRNAs raises the question of whether such mRNAs are the sole substrates of these factors-i.e., whether the cell has an apparatus to specifically degrade nonsense-containing mRNAs. It seemed unlikely that the normal function ofthe UPFF gene was anticipatoryi.e., solely involved in the degradation of mRNAs derived from nonsense alleles-so we sought to determine whether these factors have additional substrates. Since introns generally lack contiguous open reading frames, and yeast introns are almost always at the 5' ends of their genes (14), we considered it possible that the UPFI gene product (Upflp) might also be involved in controlling the abundance of yeast pre-mRNAs. If this supposition were correct, the presence of unspliced introns within a pre-mRNA would lead to premature translational termination and accelerated RNA decay in The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate ...

Differential sensitivity to the oxidant paraquat was observed in pea (Pisum sativum L.) based on cultivar and leaf age. To assess contributions of inductive responses of the antioxidant enzymes in short-term resistance to oxidative damage, activities of glutathione reductase (CR), superoxide dismutase (SOD), and ascorbate peroxidase (APX) and transcript levels for plastidic CR, Cu,Zn SOD, and cytosolic APX were determined. Responses to paraquat exposure from three different leaf age classes of pea were studied. Resistance was correlated with leaf age, photosynthetic rates, enzyme activities, and pretreatment levels of plastid CR and plastid Cu,Zn SOD transcripts. In response to paraquat, small increases in activities of CR and APX were observed in the more resistant leaves. These changes were not reflected at the mRNA leve1 for the plastidic CR or Cu,Zn SOD. Paraquat-mediated increases in cytosolic APX mRNA occurred in all leaf types, irrespective of resistance. Developmentally controlled mechanisms determining basal antioxidant enzyme activities, and not inductive responses, appear to be critical factors mediating short-term oxidative stress resistance.

The imposition of oxidative stress leads to increased production of reactive oxygen species (ROS) in plant cells. Orchestrated defense processes ensue that have much in common between stresses, yet are also particular to the site of action of the stress and its concentration. Possible functional roles of these responses include, but are not restricted to, the protection of the photosynthetic machinery, the preservation of membrane integrity and the protection of DNA and proteins. Superimposed upon our understanding of cellular mechanisms for protection against abiotic stress is a newly discovered role of ROS in signalling and defense response to pathogens (J. L. Dangl, R. A. Dietrich and M. S. Richberg. 1996. Plant Cell 8: 1793–1807). Evidence to date suggests a coordinated response to ROS among different members of the superoxide dismutase (SOD) gene families. A further layer of complexity is afforded by reports of coordination of expression between ascorbate peroxidase and SOD genes. Our understanding of the signalling mechanisms that underlie these coordinated events is in its infancy. An exciting future lies ahead in which the orchestration of successful antioxidant stress responses will be gradually revealed. Current data suggest that complex regulatory mechanisms function at both the gene and protein level to coordinate antioxidant responses and that a critical role is played by organellar localization and inter‐compartment coordination.