Effects of water turbulence on variations in cell ultrastructure and metabolism of amino acids in the submersed macrophyte, Elodea nuttallii (Planch.) H. St. John.

The interactions between macrophytes and water movement are not yet fully understood, and the causes responsible for the metabolic and ultrastructural variations in plant cells as a consequence of turbulence are largely unknown. In the present study, growth, metabolism and ultrastructural changes were evaluated in the aquatic macrophyte Elodea nuttallii, after exposure to turbulence for 30 days. The turbulence was generated with a vertically oscillating horizontal grid. The turbulence reduced plant growth, plasmolysed leaf cells and strengthened cell walls, and plants exposed to turbulence accumulated starch granules in stem chloroplasts. The size of the starch granules increased with the magnitude of the turbulence. Using capillary electrophoresis-mass spectrometry (CE-MS), analysis of the metabolome found metabolite accumulation in response to the turbulence. Asparagine was the dominant amino acid that was concentrated in stressed plants, and organic acids such as citrate, ascorbate, oxalate and γ-amino butyric acid (GABA) also accumulated in response to turbulence. These results indicate that turbulence caused severe stress that affected plant growth, cell ultrastructure and some metabolic functions of E. nuttallii. Our findings offer insights to explain the effects of water movement on the functions of aquatic plants.

Effects of NAD kinase 2 overexpression on primary metabolite profiles in rice leaves under elevated carbon dioxide.

The concentration of carbon dioxide (CO2) in the atmosphere is projected to double by the end of the 21st century. In C3 plants, elevated CO2 concentrations promote photosynthesis but inhibit the assimilation of nitrate into organic nitrogen compounds. Several steps of nitrate assimilation depend on the availability of ATP and sources of reducing power, such as nicotinamide adenine dinucleotide phosphate (NADPH). Plastid-localised NAD kinase 2 (NADK2) plays key roles in increasing the ATP/ADP and NADP(H)/NAD(H) ratios. Here we examined the effects of NADK2 overexpression on primary metabolism in rice (Oryza sativa) leaves in response to elevated CO2. By using capillary electrophoresis mass spectrometry, we showed that the primary metabolite profile of NADK2-overexpressing plants clearly differed from that of wild-type plants under ambient and elevated CO2. In NADK2-overexpressing leaves, expression of the genes encoding glutamine synthetase and glutamate synthase was up-regulated, and the levels of Asn, Gln, Arg, and Lys increased in response to elevated CO2. The present study suggests that overexpression of NADK2 promotes the biosynthesis of nitrogen-rich amino acids under elevated CO2.

Bax inhibitor-1: a highly conserved endoplasmic reticulum-resident cell death suppressor.

In spite of fundamental differences between plant and animal cells, it is remarkable that some cell death regulators that were identified to control cell death in metazoans can also function in plants. The fact that most of these proteins do not have structural homologs in plant genomes suggests that they may be targeting a highly conserved 'core' mechanism with conserved functions that is present in all eukaryotes. The ubiquitous Bax inhibitor-1 (BI-1) is a common cell death suppressor in eukaryotes that has provided a potential portal to this cell death core. In this review, we will update the current status of our understanding on the function and activities of this intriguing protein. Genetic, molecular and biochemical studies have so far suggested a consistent view that BI-1 is an endoplasmic reticulum (ER)-resident transmembrane protein that can interact with multiple partners to alter intracellular Ca(2+) flux control and lipid dynamics. Functionally, the level of BI-1 protein has been hypothesized to have the role of a rheostat to regulate the threshold of ER-stress inducible cell death. Further, delineation of the cell death suppression mechanism by BI-1 should shed light on an ancient cell death core-control pathway in eukaryotes, as well as novel ways to improve stress tolerance.

Morphological classification of plant cell deaths.

Programmed cell death (PCD) is an integral part of plant development and of responses to abiotic stress or pathogens. Although the morphology of plant PCD is, in some cases, well characterised and molecular mechanisms controlling plant PCD are beginning to emerge, there is still confusion about the classification of PCD in plants. Here we suggest a classification based on morphological criteria. According to this classification, the use of the term 'apoptosis' is not justified in plants, but at least two classes of PCD can be distinguished: vacuolar cell death and necrosis. During vacuolar cell death, the cell contents are removed by a combination of autophagy-like process and release of hydrolases from collapsed lytic vacuoles. Necrosis is characterised by early rupture of the plasma membrane, shrinkage of the protoplast and absence of vacuolar cell death features. Vacuolar cell death is common during tissue and organ formation and elimination, whereas necrosis is typically found under abiotic stress. Some examples of plant PCD cannot be ascribed to either major class and are therefore classified as separate modalities. These are PCD associated with the hypersensitive response to biotrophic pathogens, which can express features of both necrosis and vacuolar cell death, PCD in starchy cereal endosperm and during self-incompatibility. The present classification is not static, but will be subject to further revision, especially when specific biochemical pathways are better defined.

Overexpression of Bax inhibitor suppresses the fungal elicitor-induced cell death in rice (Oryza sativa L) cells.

Treatment of suspension-cultured cells of rice (Oryza sativa L.) with cell wall extract of rice blast fungus (Magnaporthe grisea) elicits a rapid generation of H2O2, alkalinization of culture medium, and eventual cell death. To elucidate genes involved in these processes, we exploited SAGE (Serial Analysis of Gene Expression) technique for the molecular analysis of cell death in suspension-cultured cells treated with the elicitor. Among the downregulated genes in the elicitor-treated cells, a BI-1 gene coding for Bax inhibitor was identified. Transgenic rice cells overexpressing Arabidopsis BI-1 gene showed sustainable cell survival when challenged with M. grisea elicitor. Thus, the plant Bax inhibitor plays a functional role in regulating cell death in the rice cell culture system.

Mammalian Bax-induced plant cell death can be down-regulated by overexpression of Arabidopsis Bax Inhibitor-1 (AtBI-1).

We recently isolated the AtBI-1 (Arabidopsis Bax Inhibitor-1) gene, the expression of which suppressed Bax-induced cell death in yeast. To determine whether the same is true in the plant system, transgenic Arabidopsis plants overexpressing Bax protein under a dexamethasone (DEX)-inducible promoter were generated. On DEX treatment, such transgenic plants exhibited marked cell death at the whole-plant level, cell shrinkage, membranous destruction, and other apoptotic phenotypes. Transgenic Bax plants were retransformed with a vector containing the AtBI-1 gene (tagged with green fluorescent protein) under the control of the cauliflower mosaic virus 35S promoter. Plants expressing both Bax and AtBI-1 were able to maintain growth on DEX-treatment by sustaining intracellular integrity. Thus, we present here direct genetic evidence that the plant antiapoptotic protein AtBI-1 is biologically active in suppressing the mammalian Bax action in planta.