During the course of their ontogenesis plants are continuously exposed to a large variety of abiotic stress factors which can damage tissues and jeopardize the survival of the organism unless properly countered. While animals can simply escape and thus evade stressors, plants as sessile organisms have developed complex strategies to withstand them. When the intensity of a detrimental factor is high, one of the defense programs employed by plants is the induction of programmed cell death (PCD). This is an active, genetically controlled process which is initiated to isolate and remove damaged tissues thereby ensuring the survival of the organism. The mechanism of PCD induction usually includes an increase in the levels of reactive oxygen species (ROS) which are utilized as mediators of the stress signal. Abiotic stress-induced PCD is not only a process of fundamental biological importance, but also of considerable interest to agricultural practice as it has the potential to significantly influence crop yield. Therefore, numerous scientific enterprises have focused on elucidating the mechanisms leading to and controlling PCD in response to adverse conditions in plants. This knowledge may help develop novel strategies to obtain more resilient crop varieties with improved tolerance and enhanced productivity. The aim of the present review is to summarize the recent advances in research on ROS-induced PCD related to abiotic stress and the role of the organelles in the process.

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Hydrogen peroxide (H2O2) has established itself as a key player in stress and programmed cell death responses, but little is known about the signaling pathways leading from H2O2 to programmed cell death in plants. Recently, identification of key regulatory mutants and near-full genome coverage microarray analysis of H2O2-induced cell death have begun to unravel the complexity of the H2O2 network. This review also describes a novel link between H2O2 and sphingolipids, two signals that can interplay and regulate plant cell death.

The sesquiterpenoid artemisinin, isolated these from the plant Artemisia annua L., and its semi-synthetic derivatives are a new and very effective group of antimalarial drugs. A branch point in the biosynthesis of this compound is the cyclisation of the ubiquitous precursor farnesyl diphosphate into the first specific precursor of artemisinin, namely amorpha-4,11-diene. Here we describe the isolation of a cDNA clone encoding amorpha-4,11-diene synthase. The deduced amino acid sequence exhibits the highest identity (50%) with a putative sesquiterpene cyclase of A. annua. When expressed in Escherichia coli, the recombinant enzyme catalyses the formation of amorpha-4,11-diene from farnesyl diphosphate. Introduction of the gene into tobacco (Nicotiana tabacum L.) resulted in the expression of an active enzyme and the accumulation of amorpha-4,11-diene ranging from 0.2 to 1.7 ng per g fresh weight.

These authors contributed equally to this work. SummaryThe nectrotrophic fungus Alternaria alternata f.sp. lycopersici infects tomato plants of the genotype asc/asc by utilizing a host-selective toxin, AAL-toxin, that kills the host cells by inducing programmed cell death. Asc-1 is homologous to genes found in most eukaryotes from yeast to humans, suggesting a conserved function. A yeast strain with deletions in the homologous genes LAG1 and LAC1 was functionally complemented by Asc-1, indicating that Asc-1 functions in an analogous manner to the yeast homologues. Examination of the yeast sphingolipids, which are almost absent in the lag1Dlac1D mutant, showed that Asc-1 was able to restore the synthesis of sphingolipids. We therefore examined the biosynthesis of sphingolipids in tomato by labeling leaf discs with L-[3-3 H]serine. In the absence of AAL-toxin, there was no detectable difference in sphingolipid labeling between leaf discs from Asc/Asc or asc/asc leaves. In the presence of pathologically significant concentrations of AAL-toxin however, asc/asc leaf discs showed severely reduced labeling of sphingolipids and increased label in dihydrosphingosine (DHS) and 3-ketodihydrosphingosine (3-KDHS). Leaf discs from Asc/Asc leaves responded to AAL-toxin treatment by incorporating label into different sphingolipid species. The effects of AAL-toxin on asc/asc leaflets could be partially blocked by the simultaneous application of AAL-toxin and myriocin. Leaf discs simultaneously treated with AAL-toxin and myriocin showed no incorporation of label into sphingolipids or long-chain bases as expected. These results indicate that the presence of Asc-1 is able to relieve an AAL-toxin-induced block on sphingolipid synthesis that would otherwise lead to programmed cell death.

Ethylene can only induce senescence in leaves that have reached a defined age. Thus, ethylene-induced senescence depends on age-related changes (ARCs) of individual leaves. The relationship between ethylene and age in the induction of leaf senescence was tested in Arabidopsis Ler-0, Col-0, and Ws-0 accessions as well as in eight old (onset of leaf death) mutants, isolated from the Ler-0 background. Plants with a constant final age of 24 d were exposed to ethylene for 3-16 d. The wild-type accessions showed a common response to the ethylene treatment. Increasing ethylene treatments of 3-12 d caused an increase in the number of yellow leaves. However, an ethylene exposure time of 16 d resulted in a decrease in the amount of yellowing. Thus, ethylene can both positively and negatively influence ARCs and the subsequent induction of leaf senescence, depending on the length of the treatment. The old mutants showed altered responses to the ethylene treatments. old1 and old11 were hypersensitive to ethylene in the triple response assay and a 12-d ethylene exposure resulted in a decrease in the amount of yellow leaves. The other six mutants did not show a decrease in yellow leaves with an ethylene treatment of 16 d. The results revealed that the effect of ethylene on the induction of senescence can be modified by at least eight genes.

The phytopathogenic fungus Alternaria alternata f. sp. lycopersici (AAL) produces toxins that are essential for pathogenicity of the fungus on tomato (Lycopersicon esculentum). AAL toxins and fumonisins of the unrelated fungus Fusarium moniliforme are sphinganine-analog mycotoxins (SAMs), which cause inhibition of sphingolipid biosynthesis in vitro and are toxic for some plant species and mammalian cell lines. Sphingolipids can be determinants in the proliferation or death of cells. We investigated the tomato Alternaria stem canker (Asc) locus, which mediates resistance to SAM-induced apoptosis. Until now, mycotoxin resistance of plants has been associated with detoxification and altered affinity or absence of the toxin targets. Here we show that SAM resistance of tomato is determined by Asc-1, a gene homologous to the yeast longevity assurance gene LAG1 and that susceptibility is associated with a mutant Asc-1. Because both sphingolipid synthesis and LAG1 facilitate endocytosis of glycosylphosphatidylinositol-anchored proteins in yeast, we propose a role for Asc-1 in a salvage mechanism of sphingolipid-depleted plant cells. Mycotoxins are a continuous risk in the production and postharvest storage of crops, threatening animal and human health. They can be divided into the nonselective mycotoxins, phytotoxic to a broader range of organisms than the producing fungus infests and in the host-selective mycotoxins, primary determinants of phytopathogenicity (1). One class of host-selective mycotoxins is structurally related to sphinganine (2), a precursor in plant sphingolipid biosynthesis ( Fig. 1) (3). Two nonrelated fungi are well known for the production of sphinganine-analog mycotoxins (SAMs): Fusarium moniliforme, a fungal pathogen of maize that produces fumonisin B 1 (FB 1 ) (4) and Alternaria alternata f. sp. lycopersici (AAL), a tomato pathogen that produces AAL toxins (5). SAMs competitively inhibit de novo sphingolipid (ceramide) biosynthesis in vitro (see Fig. 1), which leads to a variety of cellular responses, including the accumulation of sphingoid bases in animal cells (6) and plants (7). Besides their role as structural components of biomembranes, sphingolipids can be determinants in the proliferation or death of cells, depending on the type of cells studied (6). SAMs are toxic to all tissues of sensitive tomato cultivars at low concentrations (5, 8) and induce apoptosis in sensitive tomato lines (9). SAMs also induce apoptosis in some mammalian cell lines (2) and are toxic to animals and suspected to be conditionally carcinogenic in humans (4). These data indicate the necessity to understand the mechanism of action of SAMs, which has been performed by studying SAM resistance of tomato.Until now, mycotoxin resistance of plants has been associated with detoxification and altered affinity or the absence of the toxin targets (1). Resistance or susceptibility to both AAL and SAMs in tomato is determined by the Alternaria stem canker (Asc) locus (5). It inherits in a single codominant fashion. Plants heter...

SummaryThe onset of leaf senescence is controlled by leaf age and ethylene can promote leaf senescence within a specific age window. We exploited the interaction between leaf age and ethylene and isolated mutants with altered leaf senescence that are named as onset of leaf death (old) mutants. Early leaf senescence mutants representing three genetic loci were selected and their senescence syndromes were characterised using phenotypical, physiological and molecular markers. old1 is represented by three recessive alleles and displayed earlier senescence both in air and upon ethylene exposure. The etiolated old1 seedlings exhibited a hypersensitive triple response. old2 is a dominant trait and the mutant plants were indistinguishable from the wild-type when grown in air but showed an earlier senescence syndrome upon ethylene treatment. old3 is a semi-dominant trait and its earlier onset of senescence is independent of ethylene treatment. Analyses of the chlorophyll degradation, ion leakage and SAG expression showed that leaf senescence was advanced in ethylene-treated old2 plants and in both air-grown and ethylene-treated old1 and old3 plants. Epistatic analysis indicated that OLD1 might act downstream of OLD2 and upstream of OLD3 and mediate the interaction between leaf age and ethylene. A genetic model was proposed that links the three OLD genes and ethylene into a regulatory pathway controlling the onset of leaf senescence.