The continuing rise in atmospheric [CO2] is predicted to have diverse and dramatic effects on the productivity of agriculture, plant ecosystems and gas exchange. Stomatal pores in the epidermis provide gates for the exchange of CO2 and water between plants and the atmosphere, processes vital to plant life. Increased [CO2] has been shown to enhance anion channel activity proposed to mediate efflux of osmoregulatory anions (Cl- and malate(2-)) from guard cells during stomatal closure. However, the genes encoding anion efflux channels in plant plasma membranes remain unknown. Here we report the isolation of an Arabidopsis gene, SLAC1 (SLOW ANION CHANNEL-ASSOCIATED 1, At1g12480), which mediates CO2 sensitivity in regulation of plant gas exchange. The SLAC1 protein is a distant homologue of bacterial and fungal C4-dicarboxylate transporters, and is localized specifically to the plasma membrane of guard cells. It belongs to a protein family that in Arabidopsis consists of four structurally related members that are common in their plasma membrane localization, but show distinct tissue-specific expression patterns. The loss-of-function mutation in SLAC1 was accompanied by an over-accumulation of the osmoregulatory anions in guard cell protoplasts. Guard-cell-specific expression of SLAC1 or its family members resulted in restoration of the wild-type stomatal responses, including CO2 sensitivity, and also in the dissipation of the over-accumulated anions. These results suggest that SLAC1-family proteins have an evolutionarily conserved function that is required for the maintenance of organic/inorganic anion homeostasis on the cellular level.

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. Research on plant cell death has grown considerably in the past few years, owing to the importance of cell death for plant development and defense. Just as animal cells engage several mechanisms leading to death, the road to cell demise in plants can also vary. The long evolutionary distance and distinct cellular architecture between the two kingdoms may account for the differences between the mechanisms of plant and animal cell death. It is therefore appropriate to assess the relevance of animal cell death nomenclature 1 to plants. At present, there is confusion in cell death terminology in plant biology, which drives our attempt to formulate a more logical classification. Although our molecular understanding of plant cell death regulation and execution is insufficient to create definitive classifications based on precise biochemical pathways, it is possible to begin classifying plant cell death scenarios based on morphological criteria, as was initially the case in animal cell death research 2,3 and is still used for the classification of cell death in animal science. 1 This document attempts to provide a classification of plant cell death. We urge authors, reviewers and editors to follow this classification to facilitate communication between scientists and accelerate research in this field.

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.A lthough relatively little is known about the mechanistic details of cell death in plants, some aspects of the molecular machinery are conserved between plants and animals (1). It has been demonstrated that overexpression of Bax, which encodes a mammalian proapoptotic protein, is lethal in the budding yeast Saccharomyces cerevisiae (2-5), even though yeasts have neither Bcl-2-related proteins nor caspases. Lacomme and Santa Cruz (6) demonstrated that expression of Bax by using a tobacco mosaic virus (TMV) vector triggered cell death in tobacco leaf cells, which closely resembled the hypersensitive response (HR) induced by TMV in tobacco plants carrying the N gene. Conversely, overexpression of human Bcl-X L in transgenic tobacco suppressed HR and conferred stress tolerance (7). It was also reported that caspase-specific peptide inhibitors could abolish bacteria-induced plant programmed cell death (8). These observations clearly suggest some common features of animal and plant cell death processes.Xu and Reed (9) transformed yeast cells containing a galactose-inducible Bax plasmid by using a human cDNA library (in which cDNAs were fused to a constitutively active yeast promoter) and isolated cDNAs that prevented Bax-induced lethality in response to galactose. This resulted in the identification of a gene, termed BI-1 (Bax Inhibitor-1), which is identical to a previously isolated human gene of unknown function called TEGT (testis enhanced gene transcript; refs. 10 and 11). We have previously cloned plant BI-1 cDNAs (Arabidopsis AtBI-1, and rice OsBI-1) (12). Interestingly, Sanchez et al. (13) reported that AtBI-1, the same gene isolated by our group, was obtained by differential screening of genes from plants challenged with the phytopathogen Pseudomonas syringae. Expression of AtBI-1 was rapidly up-regulated in plants during wounding or pathogen challenge. Furthermore, accumulation of the AtBI-1 transcript is significantly delayed in coi1 plants, indicating that reduced AtBI-1 mRNA levels may contribute to the enhanced susceptibility exhibited by coi1 plants to infection by v...

Overexpression of plant Bax Inhibitor-1 (BI-1) was able to suppress Bax-mediated cell death in yeast and Arabidopsis. Here, we demonstrate that reactive oxygen species production induced by the ectopic expression of Bax was insensitive to the coexpression of AtBI-1. Similarly, H 2 O 2 -or salicylic acid-mediated cell death also was suppressed in tobacco BY-2 cells overexpressing AtBI-1. To define the functional domain of AtBI-1 as a cell death suppressor, a truncated series of the AtBI-1 protein was analyzed in yeast possessing a galactose-inducible mammalian Bax. The results showed that ⌬ C-AtBI-1 (with the C-terminal 14 amino acids deleted) lost the ability to sustain cell growth. Furthermore, a mutant protein in which the C-terminal seven amino acid residues of AtBI-1 were replaced with others lacking a coiled-coil structure failed to inhibit cell death, suggesting that the C-terminal region is essential for the inhibition of cell death. We also noted that the C-terminal hydrophilic region was interchangeable between animal and plant Bax inhibitors.

When ammonium is the sole nitrogen (N) source, plant growth is suppressed compared with the situation where nitrate is the N source. This is commonly referred to as ammonium toxicity. It is widely known that a combination of nitrate and ammonium as N source alleviates this ammonium toxicity (nitrate-dependent alleviation of ammonium toxicity), but the underlying mechanisms are still not completely understood. In plants, ammonium toxicity is often accompanied by a depletion of organic acids and inorganic cations, and by an accumulation of ammonium. All these factors have been considered as possible causes for ammonium toxicity. Thus, we hypothesized that nitrate could alleviate ammonium toxicity by lessening these symptoms. We analyzed growth, inorganic N and cation content and various primary metabolites in shoots of Arabidopsis thaliana seedlings grown on media containing various concentrations of nitrate and/or ammonium. Nitrate-dependent alleviation of ammonium toxicity was not accompanied by less depletion of organic acids and inorganic cations, and showed no reduction in ammonium accumulation. On the other hand, shoot growth was significantly correlated with the nitrate concentration in the shoots. This suggests that nitrate-dependent alleviation of ammonium toxicity is related to physiological processes that are closely linked to nitrate signaling, uptake and reduction. Based on transcript analyses of various genes related to nitrate signaling, uptake and reduction, possible underlying mechanisms for the nitrate-dependent alleviation are discussed.