Differential expression of alternative oxidase genes in soybean cotyledons during postgerminative development

High Expression of ALTERNATIVE OXIDASE 2 in Latent Axillary Buds Suggests Its Key Role in Quiescence Maintenance in Rosebush

Most vegetative axes remain quiescent as dormant axillary buds until metabolic and hormonal signals, driven by environmental changes, trigger bud outgrowth. While the resumption of growth activity is well documented, the establishment and maintenance of quiescence is comparatively poorly understood, despite its major importance in the adaptation of plants to the seasonal cycle or in the establishment of their shape. Here, using the rosebush Rosa hybrida 'Radrazz' as a plant model, we highlighted that the quiescent state was the consequence of an internal and active energy control of buds, under the influence of hormonal factors previously identified in the bud outgrowth process. We found that the quiescent state in the non-growing vegetative axis of dormant axillary buds displayed a low energy state along with a high expression of the ALTERNATIVE OXIDASE 2 (AOX2) and the accumulation of the corresponding protein. Conversely, AOX2 expression and protein amount strongly decreased during bud burst as energy status shifted to a high state, allowing growth. Since AOX2 can deviate electrons from the cytochrome pathway in the mitochondrial respiratory chain, it could drastically reduce the formation of ATP, which would result in a low energy status unfavorable for growth activities. We provide evidence that the presence/absence of AOX2 in quiescent/growing vegetative axes of buds was under hormonal control and thus may constitute the mechanistic basis of both quiescence and sink strength manifestation, two important aspects of budbreak.

Modified Northern blot protocol for easy detection of mRNAs in total RNA using radiolabeled probes

BACKGROUND

Northern blotting is still used as a gold standard for validation of the data obtained from high-throughput whole transcriptome-based methods. However, its disadvantages of lower sensitivity, labor-intensive operation, and higher quality of RNA required limit its utilization in a routine molecular biology laboratory to monitor gene expression at RNA level. Therefore, it is necessary to optimize the traditional Northern protocol to make the technique more applicable for standard use.

RESULTS

In this paper, we report modifications and tips used to improve the traditional Northern protocol for the detection of mRNAs in total RNA. To maximize the retention of specifically bound radiolabeled probes on the blot, posthybridization washes were performed under only with moderate-stringency until the level of radioactivity retained on the filter decreased to 20~50 counts per second, rather than normally under high and low stringency sequentially for scheduled time or under only high stringent condition. Successful detection of the low-expression gene using heterologous DNA probes in 20 µg of total RNA after a two-day exposure suggested an improvement in detection sensitivity. Quantitatively controlled posthybridization washes combined with an ethidium bromide-prestaining RNA procedure to directly visualize prestained RNA bands at any time during electrophoresis or immediately after electrophoresis, which made the progress of the Northern procedure to be monitored and evaluated step by step, thereby making the experiment reliable and controllable. We also report tips used in the modified Northern protocol, including the moderate concentration of formaldehyde in the gel, the accessory capillary setup, and the staining jar placed into an enamel square tray with a lid used for hybridization. Using our modified Northern protocol, eight rounds of rehybridization could be performed on a single blot. The modification made and tips used ensured the efficient proceeding of the experiment and the resulting good performance, but without using special reagents or equipment.

CONCLUSIONS

The modified Northern protocol improved detection sensitivity and made the experiment easy, less expensive, reliable, and controllable, and can be employed in a routine molecular biology laboratory to detect low-expressed mRNAs with heterologous DNA probes in total RNA.

Legume Alternative Oxidase Isoforms Show Differential Sensitivity to Pyruvate Activation

Alternative oxidase (AOX) is an important component of the plant respiratory pathway, enabling a route for electrons that bypasses the energy-conserving, ROS-producing complexes of the mitochondrial electron transport chain. Plants contain numerous isoforms of AOX, classified as either AOX1 or AOX2. AOX1 isoforms have received the most attention due to their importance in stress responses across a wide range of species. However, the propensity for at least one isoform of AOX2 to accumulate to very high levels in photosynthetic tissues of all legumes studied to date, suggests that this isoform has specialized roles, but we know little of its properties. Previous studies with sub-mitochondrial particles of soybean cotyledons and roots indicated that differential expression of GmAOX1, GmAOX2A, and GmAOX2D across tissues might confer different activation kinetics with pyruvate. We have investigated this using recombinantly expressed isoforms of soybean AOX in a previously described bacterial system (Selinski et al., 2016, Physiologia Plantarum 157, 264-279). Pyruvate activation kinetics were similar between the two GmAOX2 isoforms but differed substantially from those of GmAOX1, suggesting that selective expression of AOX1 and 2 could determine the level of AOX activity. However, this alone cannot completely explain the differences seen in sub-mitochondrial particles isolated from different legume tissues and possible reasons for this are discussed.

Carrot AOX2a Transcript Profile Responds to Growth and Chilling Exposure

Alternative oxidase (AOX) is a key enzyme of the alternative respiration, known to be involved in plant development and in response to various stresses. To verify the role of DcAOX1 and DcAOX2a genes in carrot tap root growth and in response to cold stress, their expression was analyzed in two experiments: during root growth for 13 weeks and in response to a cold challenge trial of 7 days, in both cases using different carrot cultivars. Carrot root growth is initially characterized by an increase in length, followed by a strong increase in weight. DcAOX2a presented the highest expression levels during the initial stages of root growth for all cultivars, but DcAOX1 showed no particular trend in expression. Cold stress had a negative impact on root growth, and generally up-regulated DcAOX2a with no consistent effect on DcAOX1. The identification of cis-acting regulatory elements (CAREs) located at the promoters of both genes showed putative sequences involved in cold stress responsiveness, as well as growth. However, DcAOX2a promoter presented more CAREs related to hormonal pathways, including abscisic acid and gibberellins synthesis, than DcAOX1. These results point to a dual role of DcAOX2a on carrot tap root secondary growth and cold stress response.

Genomic structure and expression of alternative oxidase genes in legumes

Mitochondria isolated from chickpea (Cicer arietinum) possess substantial alternative oxidase (AOX) activity, even in non-stressed plants, and one or two AOX protein bands were detected immunologically, depending on the organ. Four different AOX isoforms were identified in the chickpea genome: CaAOX1 and CaAOX2A, B and D. CaAOX2A was the most highly expressed form and was strongly expressed in photosynthetic tissues, whereas CaAOX2D was found in all organs examined. These results are very similar to those of previous studies with soybean and siratro. Searches of available databases showed that this pattern of AOX genes and their expression was common to at least 16 different legume species. The evolution of the legume AOX gene family is discussed, as is the in vivo impact of an inherently high AOX capacity in legumes on growth and responses to environmental stresses.

Alternative oxidase (AOX) over-expression improves cell expansion and elongation in cotton seedling exposed to cool temperatures

KEY MESSAGE

Evidence that supports a relation between AOX expression and improvement in plant height, internode length, and total leaf area under cool temperature is shown. Cell expansion and elongation appear to be enhanced when AOX expression was increased. Cotton growth is sensitive to cool temperature during germination and early seedling development. Delayed emergence, seedling damage, and increased risk to disease are common. Late seasonal cool weather is a major factor limiting the consistent production of high-quality cotton lint in West Texas. Alternative oxidase functions in the inner membrane of the mitochondria via an alternative respiration pathway and serves as a multifunctional system for amelioration of abiotic and biotic stresses. Cotton seedling emergence and growth exposed to cool temperatures was examined in plants with enhanced AOX expression. Thirteen T1 seed lines showed 3 to 1 segregation for the T-DNA containing the tobacco AOX1 gene. Two over-expressing, single-copy, homozygous AOX lines (94-20T and 66-6T) and Null line (94-3N) were selected for examination. The transcript levels were ≈ 2 to 6 fold higher in the AOX lines compared to those of the Null line and wild-type in stem, leaf, root and boll tissues. The research examined the hypothesis that transgenic cotton with enhanced AOX expression will have enhanced growth traits under suboptimal cool temperatures. Improved plant height, internode length, plant height and internode length from second node, and total leaf area under cool temperatures were observed in AOX over-expression lines. This may be attributed to improved cell expansion and elongation characteristics in the AOX line.

Alternative Oxidase Gene Family in Hypericum perforatum L.: Characterization and Expression at the Post-germinative Phase

Alternative oxidase (AOX) protein is located in the inner mitochondrial membrane and is encoded in the nuclear genome being involved in plant response upon a diversity of environmental stresses and also in normal plant growth and development. Here we report the characterization of the AOX gene family of Hypericum perforatum L. Two AOX genes were identified, both with a structure of four exons (HpAOX1, acc. KU674355 and HpAOX2, acc. KU674356). High variability was found at the N-terminal region of the protein coincident with the high variability identified at the mitochondrial transit peptide. In silico analysis of regulatory elements located at intronic regions identified putative sequences coding for miRNA precursors and trace elements of a transposon. Simple sequence repeats were also identified. Additionally, the mRNA levels for the HpAOX1 and HpAOX2, along with the ones for the HpGAPA (glyceraldehyde-3-phosphate dehydrogenase A subunit) and the HpCAT1 (catalase 1), were evaluated during the post-germinative development. Gene expression analysis was performed by RT-qPCR with accurate data normalization, pointing out HpHYP1 (chamba phenolic oxidative coupling protein 1) and HpH2A (histone 2A) as the most suitable reference genes (RGs) according to GeNorm algorithm. The HpAOX2 transcript demonstrated larger stability during the process with a slight down-regulation in its expression. Contrarily, HpAOX1 and HpGAPA (the corresponding protein is homolog to the chloroplast isoform involved in the photosynthetic carbon assimilation in other plant species) transcripts showed a marked increase, with a similar expression pattern between them, during the post-germinative development. On the other hand, the HpCAT1 (the corresponding protein is homolog to the major H2O2-scavenging enzyme in other plant species) transcripts showed an opposite behavior with a down-regulation during the process. In summary, our findings, although preliminary, highlight the importance to investigate in more detail the participation of AOX genes during the post-germinative development in H. perforatum, in order to explore their functional role in optimizing photosynthesis and in the control of reactive oxygen species (ROS) levels during the process.

Overexpression of alternative oxidase gene confers aluminum tolerance by altering the respiratory capacity and the response to oxidative stress in tobacco cells

Aluminum (Al) stress represses mitochondrial respiration and produces reactive oxygen species (ROS) in plants. Mitochondrial alternative oxidase (AOX) uncouples respiration from mitochondrial ATP production and may improve plant performance under Al stress by preventing excess accumulation of ROS. We tested respiratory changes and ROS production in isolated mitochondria and whole cell of tobacco (SL, ALT 301) under Al stress. Higher capacities of AOX pathways relative to cytochrome pathways were observed in both isolated mitochondria and whole cells of ALT301 under Al stress. AOX1 when studied showed higher AOX1 expression in ALT 301 than SL cells under stress. In order to study the function of tobacco AOX gene under Al stress, we produced transformed tobacco cell lines by introducing NtAOX1 expressed under the control of the cauliflower mosaic virus (CaMV) 35 S promoter in sensitive (SL) Nicotiana tabacum L. cell lines. The enhancement of endogenous AOX1 expression and AOX protein with or without Al stress was in the order of transformed tobacco cell lines > ALT301 > wild type (SL). A decreased respiratory inhibition and reduced ROS production with a better growth capability were the significant features that characterized AOX1 transformed cell lines under Al stress. These results demonstrated that AOX plays a critical role in Al stress tolerance with an enhanced respiratory capacity, reducing mitochondrial oxidative stress burden and improving the growth capability in tobacco cells.

A GmAOX2b antisense gene compromises vegetative growth and seed production in soybean

The alternative oxidase mediates the cyanide-resistant respiratory pathway in plant mitochondria. In non-thermogenic plants, the role of alternative oxidase in plant growth and development is not well understood. Soybean (Glycine max) lines carrying a GmAOX2b antisense gene had compromised vegetative growth and reproductive performance under typical glasshouse growth conditions. The reduction in vegetative growth was demonstrated by reduction in shoot height, the number of leaves per plant and the green leaf area. Antisense plants also had decreased pod formation and seed to pod ratios, which together led to a reduction in the number and total mass of seed produced. The negative effects of the antisense gene on pod set, seed set, ovule availability and total seed mass were primarily confined to the branches, rather than the main stem. The preferential effect of alternative oxidase suppression in the branches is discussed in relation to the reproductive potential of soybean under stress. Taken together, these results demonstrate that alternative oxidase provides the benefit of sustaining plant vegetative growth and reproductive capacity in soybean.

Stress-induced co-expression of two alternative oxidase (VuAox1 and 2b) genes in Vigna unguiculata

Cowpea (Vigna unguiculata) alternative oxidase is encoded by a small multigene family (Aox1, 2a and 2b) that is orthologous to the soybean Aox family. Like most of the identified Aox genes in plants, VuAox1 and VuAox2 consist of 4 exons interrupted by 3 introns. Alignment of the orthologous Aox genes revealed high identity of exons and intron variability, which is more prevalent in Aox1. In order to determine Aox gene expression in V. unguiculata, a steady-state analysis of transcripts involved in seed development (flowers, pods and dry seeds) and germination (soaked seeds) was performed and systemic co-expression of VuAox1 and VuAox2b was observed during germination. The analysis of Aox transcripts in leaves from seedlings under different stress conditions (cold, PEG, salicylate and H2O2 revealed stress-induced co-expression of both VuAox genes. Transcripts of VuAox2a and 2b were detected in all control seedlings, which was not the case for VuAox1 mRNA. Estimation of the primary transcript lengths of V. unguiculata and soybean Aox genes showed an intron length reduction for VuAox1 and 2b, suggesting that the two genes have converged in transcribed sequence length. Indeed, a bioinformatics analysis of VuAox1 and 2b promoters revealed a conserved region related to a cis-element that is responsive to oxidative stress. Taken together, the data provide evidence for co-expression of Aox1 and Aox2b in response to stress and also during the early phase of seed germination. The dual nature of VuAox2b expression (constitutive and induced) suggests that the constitutive Aox2b gene of V. unguiculata has acquired inducible regulatory elements.

Photosynthetic performance and fertility are repressed in GmAOX2b antisense soybean

The alternative oxidase (AOX) is a cyanide-resistant oxidase that provides an alternative outlet for electrons from the respiratory electron transport chain embedded in the inner membrane of plant mitochondria. Examination of soybean (Glycine max) plants carrying a GmAOX2b antisense gene showed AOX to have a central role in reproductive development and fecundity. In three independently transformed antisense lines, seed set was reduced by 16% to 43%, whereas ovule abortion increased by 1.2- to 1.7-fold when compared with nontransgenic transformation control plants. Reduced fecundity was associated with reductions in whole leaf cyanide-resistant, salicylhydroxamic acid-sensitive respiration and net photosynthesis, but there was no change in total respiration in the dark. The frequency of potential fertilization events was reduced by at least one-third in the antisense plants as a likely consequence of prefertilization defects. Pistils of the antisense plants contained a higher proportion of immature-sized, nonfertile embryo sacs compared with nontransgenic control plants. Increased rates of pollen abortion in vivo and reduced rates of pollen germination in vitro suggested that the antisense gene compromised pollen development and function. Reciprocal crosses between antisense and nontransgenic plants revealed that pollen produced by antisense plants was less active in fertilization. Taken together, the results presented here indicate that AOX expression has an important role in determining normal gametophyte development and function.

Aox gene structure, transcript variation and expression in plants

Alternative oxidase (Aox) has been proposed as a functional marker for breeding stress tolerant plant varieties. This requires presence of polymorphic Aox allele sequences in plants that affect plant phenotype in a recognizable way. In this review, we examine the hypothesis that organization of genomic Aox sequences and gene expression patterns are highly variable in relation to the possibility that such a variation may allow development of Aox functional markers in plants. Aox is encoded by a small multigene family, typically with four to five members in higher plants. The predominant structure of genomic Aox sequences is that of four exons interrupted by three introns at well conserved positions. Evolutionary intron loss and gain has resulted in the variation of intron numbers in some Aox members that may harbor two to four introns and three to five exons in their sequence. Accumulating evidence suggests that Aox gene structure is polymorphic enough to allow development of Aox markers in many plant species. However, the functional significance of Aox structural variation has not been examined exhaustively. Aox expression patterns display variability and typically Aox genes fall into two discrete subfamilies, Aox1 and Aox2, the former being present in all plants and the latter restricted in eudicot species. Typically, although not exclusively, the Aox1-type genes are induced by many different kinds of stress, whereas Aox2-type genes are expressed in a constitutive or developmentally regulated way. Specific Aox alleles are among the first and most intensively stress-induced genes in several experimental systems involving oxidative stress. Differential response of Aox genes to stress may provide a flexible plan of plant defense where an energy-dissipating system in mitochondria is involved. Evidence to link structural variation and differential allele expression patterns is scarce. Much research is still required to understand the significance of polymorphisms within AOX gene sequences for gene regulation and its potential for breeding on important agronomic traits. Association studies and mapping approaches will be helpful to advance future perspectives for application more efficiently.

Towards a structural elucidation of the alternative oxidase in plants

In addition to the conventional cytochrome c oxidase, mitochondria of all plants studied to date contain a second cyanide-resistant terminal oxidase or alternative oxidase (AOX). The AOX is located in the inner mitochondrial membrane and branches from the cytochrome pathway at the level of the quinone pool. It is non-protonmotive and couples the oxidation of ubiquinone to the reduction of oxygen to water. For many years, the AOX was considered to be confined to plants, fungi and a small number of protists. Recently, it has become apparent that the AOX occurs in wide range of organisms including prokaryotes and a moderate number of animal species. In this paper, we provide an overview of general features and current knowledge available about the AOX with emphasis on structure, the active site and quinone-binding site. Characterisation of the AOX has advanced considerably over recent years with information emerging about the role of the protein, regulatory regions and functional sites. The large number of sequences available is now enabling us to obtain a clearer picture of evolutionary origins and diversity.

Study of the effects of salicylic acid on soybean mitochondrial lipids and respiratory properties using the alternative oxidase as a stress-reporter protein

Biotic and abiotic stresses can lead to modifications in the lipid composition of cell membranes. Although mitochondria appear to be implicated in stress responses, little is known about the membrane lipid changes that occur in these organelles in plants. Besides cytochrome c oxidase, plant mitochondria have an alternative oxidase (AOX) that accepts electrons directly from ubiquinol, dissipating energy as heat. AOX upregulation occurs under a variety of stresses and its induction by salicylic acid (SA) has been observed in different plant species. AOX was also suggested to be used as a functional marker for cell reprogramming under stress. In the present study, we have used etiolated soybean (Glycine max (L.) Merr. cv Cresir) seedlings to study the effects of SA treatment on the lipid composition and the respiratory properties of hypocotyl mitochondria. AOX expression was studied in detail, as a reporter protein, to evaluate whether modifications in mitochondrial energy metabolism were occurring. In mitochondria extracted from SA-treated seedlings, AOX capacity and protein contents increased. Both AOX1 and AOX2b transcripts accumulated in response to SA, but with different kinetics. A reduction in external NADH oxidation capacity was observed, whereas succinate respiration remained unchanged. The phospholipid composition of mitochondria remained similar in control and SA-treated plants, but a reduction in the relative amount of linolenic acid was observed in phosphatidylcholine, phosphatidylethanolamine and cardiolipin. The possible causes of the fatty acid modifications observed, and the implications for mitochondrial metabolism are discussed.

The absence of ALTERNATIVE OXIDASE1a in Arabidopsis results in acute sensitivity to combined light and drought stress

Treatment of Arabidopsis (Arabidopsis thaliana) alternative oxidase1a (aox1a) mutant plants with moderate light under drought conditions resulted in a phenotypic difference compared with ecotype Columbia (Col-0), as evidenced by a 10-fold increase in the accumulation of anthocyanins in leaves, alterations in photosynthetic efficiency, and increased superoxide radical and reduced root growth at the early stages of seedling growth. Analysis of metabolite profiles revealed significant changes upon treatment in aox1a plants typical of combined stress treatments, and these were less pronounced or absent in Col-0 plants. These changes were accompanied by alteration in the abundance of a variety of transcripts during the stress treatment, providing a molecular fingerprint for the stress-induced phenotype of aox1a plants. Transcripts encoding proteins involved in the synthesis of anthocyanins, transcription factors, chloroplastic and mitochondrial components, cell wall synthesis, and sucrose and starch metabolism changed, indicating that effects were not confined to mitochondria, where the AOX1a protein is located. Microarray and quantitative reverse transcription-polymerase chain reaction analysis revealed that transcripts typically induced upon stress treatment or involved in antioxidant defense systems, especially chloroplast-located antioxidant defense components, had altered basal levels in untreated aox1a plants, suggesting a significant change in the basal equilibrium of signaling pathways that regulate these components. Taken together, these results indicate that aox1a plants have a greatly altered stress response even when mitochondria or the mitochondrial electron transport chain are not the primary target of the stress and that AOX1a plays a broad role in determining the normal redox balance in the cell.

Dinitro-o-cresol induces apoptosis-like cell death but not alternative oxidase expression in soybean cells

In plants, programmed cell death is thought to be activated during differentiation and in response to biotic and abiotic stresses. Although its mechanisms are far less clear, several morphological and biochemical features have been described in different experimental systems, including DNA laddering and cytosolic protease activation. Moreover, plant mitochondria have an alternative terminal oxidase (AOX), which is thought to be involved in protection against increased reactive oxygen species production, perhaps representing a mechanism to prevent programmed cell death. In this study, we analysed cell death induced by the herbicide dinitro-o-cresol (DNOC) in soybean (Glycine max) suspension cell cultures and evaluated biochemical and molecular events associated with programmed cell death. AOX capacity and expression were also determined. DNOC-treated cells showed fragmented nuclear DNA as assessed by an in situ assay that detects 3'-OH ends. In addition, specific colorimetric assays and immunoblot analysis revealed activation of caspase-3-like proteins and release of cytochrome c from mitochondria, respectively, confirming the apoptotic-like phenotype. Surprisingly, AOX capacity and protein levels decreased in DNOC-treated cells, suggesting no association between cell death and AOX under these experimental conditions. In conclusion, the results show that DNOC induces programmed cell death in soybean cells, suggesting that plants and animals might share similar pathways. Further, the role of AOX in cell death has not been confirmed, and may depend on the nature and intensity of stress conditions.

Alternative oxidase regulation in roots of Vigna unguiculata cultivars differing in drought/salt tolerance

The alternative oxidase (Aox) was studied at different levels (transcript, protein and capacity) in response to an osmotic shock applied to roots of cowpea (Vigna unguiculata). Two cultivars of V. unguiculata were used, Vita 3 and Vita 5, tolerant and sensitive to drought/saline stress respectively. The seedlings (17-day-old) were grown in hydroponic conditions and submitted to NaCl (100 and 200 mM) or 200.67 g L(-1) PEG 6000 (iso-osmotic condition to 100 mM NaCl). The VuAox1 and VuAox2a mRNA were not detected in either cultivar under all tested conditions while the VuAox2b gene was differently expressed. In the tolerant cultivar (Vita 3), the expression of VuAox2b gene was stimulated by an osmotic stress induced by PEG which was associated with a higher amount and capacity of the Aox protein. In the same cultivar, this gene was under-expressed in salt stress conditions with poor effect on the protein level. In the sensitive cultivar (Vita 5), the transcript level of the VuAox2b was unchanged in response to PEG treatment, even though the protein and the capacity tended to increase. Upon salt stress, the VuAox2b gene was over-expressed. At 100mM NaCl, this VuAox2b gene over-expression led to a higher amount and capacity of Aox. This effect was reduced at 200 mM NaCl. Overall, these results suggest complex mechanisms (transcriptional, translational and post-translational) for Aox regulation in response to osmotic stress.

Characterization of the regulatory and expression context of an alternative oxidase gene provides insights into cyanide-insensitive respiration during growth and development

Alternative oxidase (AOX) is encoded in small multigene families in plants. Functional analysis of the Arabidopsis (Arabidopsis thaliana) alternative oxidase 1c (AtAOX1c) promoter, an AOX gene not induced by oxidative stress, indicated that regulation of expression was complex, with the upstream promoter region containing positive and negative response regions. Comparison to the promoter region of soybean (Glycine max) alternative oxidase 2b (GmAOX2b), another AOX gene not induced by oxidative stress, revealed that they contained seven sequence elements in common. All elements were active in the promoter region of AtAOX1c in suspension cells and in leaf tissue from Columbia and mutant plants, where a mitochondrial protein import receptor was inactivated. Analysis of coexpressed and putatively coregulated genes, the latter defined as containing five or more sequence elements functional in AtAOX1c, indicated that AtAOX1c was coregulated with components involved with cell division and growth. Consistent with this analysis, we demonstrated that site II elements, previously shown to regulate the proliferating cell nuclear antigen, are present in the upstream promoter region of AtAOX1c and were strong negative regulators of AtAOX1c expression. It was demonstrated that NDB4, a gene encoding an external NAD(P)H dehydrogenase, displayed strong coexpression with AtAOX1c. Overall, these results indicate that AtAOX1c is regulated by growth and developmental signals.

Alternative oxidases in Arabidopsis: A comparative analysis of differential expression in the gene family provides new insights into function of non-phosphorylating bypasses

The emergence of Arabidopsis as a model plant provides an opportunity to gain insights into the role of the alternative oxidase that cannot be as readily achieved in other plant species. The analysis of extensive mRNA expression data indicates that all five Aox genes (Aox1a, 1b, 1c, 1d and 2) are expressed, but organ and developmental regulation are evident, suggesting regulatory specialisation of Aox gene members. The stress-induced nature of the alternative pathway in a variety of plants is further supported in Arabidopsis as Aox1a and Aox1d are amongst the most stress responsive genes amongst the hundreds of known genes encoding mitochondrial proteins. Analysis of genes co-expressed with Aoxs from studies of responses to various treatments altering mitochondrial functions and/or from plants with altered Aox levels reveals that: (i) this gene set encodes more functions outside the mitochondrion than functions in mitochondria, (ii) several pathways for induction exist and there is a difference in the magnitude of the induction in each pathway, (iii) the magnitude of induction may depend on the endogenous levels of Aox, and (iv) induction of Aox can be oxidative stress-dependent or -independent depending on the gene member and the tissue analysed. An overall role for Aox in re-programming cellular metabolism in response to the ever changing environment encountered by plants is proposed.

Characterization of Mitochondrial Alternative NAD(P)H Dehydrogenases in Arabidopsis: Intraorganelle Location and Expression

The intramitochondrial location of putative type II NAD(P)H dehydrogenases (NDs) in Arabidopsis was investigated by measuring the ability of isolated mitochondria to take up precursor proteins generated from cDNAs using an in vitro translation system. The mature proteins of NDA1, NDA2 and NDC1 were judged to be located on the inside of the inner membrane because they were protected from protease added after the mitochondrial outer membrane had been ruptured. In contrast, NDB1, NDB2 and NDB4 were not protected from protease digestion in mitochondria with ruptured outer membranes and were deemed to be located on the outside of the inner membrane. Expression of all ND genes was measured using quantitative reverse transcription-PCR (RT-PCR) to determine transcript abundance, and compared with expression of alternative oxidase, uncoupler proteins and selected components of the oxidative phosphorylation complexes. NDA1 and NDB2 were the most prominently expressed members in a variety of tissues, and were up-regulated in the early daytime in a diurnal manner. Analysis of array data suggested that NDA1 clustered closest to the gene encoding the P-subunit of glycine decarboxylase. Taken together with the diurnal regulation of NDA1 observed here and in other studies, this suggests that NDA1 plays a role in integrating metabolic activities of chloroplasts and mitochondria. NDA2, NDB2 and Aox1a were up-regulated in a coordinated manner under various treatments, potentially forming a complete respiratory chain capable of oxidizing matrix and cytosolic NAD(P)H. NDB1 and NDC1 were down-regulated under the same conditions and may be regarded as housekeeping genes.

Ordered assembly of mitochondria during rice germination begins with pro-mitochondrial structures rich in components of the protein import apparatus

Mitochondrial maturation during imbibition of rice embryos follows the transition of unstructured double membrane bound pro-mitochondria to the typical cristae-rich mitochondrial structures observed in mature plant cells. During the first 48 h following imbibition, an ordered increase in the abundance of transcripts encoding mitochondrial proteins was observed. Co-incident with these changes in transcript levels was dynamic and rapid changes in mitochondrial protein content and mitochondrial function. Proteins representing components of the mitochondrial protein import apparatus are strikingly abundant in dry seeds, and a functional import apparatus was shown to operate 2 h after imbibition. Interestingly, this import process was best driven by the oxidation of NADH from outside the mitochondrial inner membrane. In later developmental stages the capacity for matrix organic acid metabolism was evident, accompanied by the appearance of proteins for TCA cycle components, and coordination of electron transport chain assembly through components encoded in both mitochondrial and nuclear genomes. Together these events provide new insights into the understanding of mitochondrial maturation and the nature of pro-mitochondrial structures in plant cells.

Stress-induced co-expression of alternative respiratory chain components in Arabidopsis thaliana

Plant mitochondria contain non-phosphorylating bypasses of the respiratory chain, catalysed by the alternative oxidase (AOX) and alternative NADH dehydrogenases (NDH), as well as uncoupling (UCP) protein. Each of these components either circumvents or short-circuits proton translocation pathways, and each is encoded by a small gene family in Arabidopsis. Whole genome microarray experiments were performed with suspension cell cultures to examine the effects of various 3 h treatments designed to induce abiotic stress. The expression of over 60 genes encoding components of the classical, phosphorylating respiratory chain and tricarboxylic acid cycle remained largely constant when cells were subjected to a broad range of abiotic stresses, but expression of the alternative components responded differentially to the various treatments. In detailed time-course quantitative PCR analysis, specific members of both AOX and NDH gene families displayed coordinated responses to treatments. In particular, the co-expression of AOX1a and NDB2 observed under a number of treatments suggested co-regulation that may be directed by common sequence elements arranged hierarchically in the upstream promoter regions of these genes. A series of treatment sets were identified, representing the response of specific AOX and NDH genes to mitochondrial inhibition, plastid inhibition and abiotic stresses. These treatment sets emphasise the multiplicity of pathways affecting alternative electron transport components in plants.

Stress-induced co-expression of alternative respiratory chain components in Arabidopsis thaliana

Plant mitochondria contain non-phosphorylating bypasses of the respiratory chain, catalysed by the alternative oxidase (AOX) and alternative NADH dehydrogenases (NDH), as well as uncoupling (UCP) protein. Each of these components either circumvents or short-circuits proton translocation pathways, and each is encoded by a small gene family in Arabidopsis. Whole genome microarray experiments were performed with suspension cell cultures to examine the effects of various 3 h treatments designed to induce abiotic stress. The expression of over 60 genes encoding components of the classical, phosphorylating respiratory chain and tricarboxylic acid cycle remained largely constant when cells were subjected to a broad range of abiotic stresses, but expression of the alternative components responded differentially to the various treatments. In detailed time-course quantitative PCR analysis, specific members of both AOX and NDH gene families displayed coordinated responses to treatments. In particular, the co-expression of AOX1a and NDB2 observed under a number of treatments suggested co-regulation that may be directed by common sequence elements arranged hierarchically in the upstream promoter regions of these genes. A series of treatment sets were identified, representing the response of specific AOX and NDH genes to mitochondrial inhibition, plastid inhibition and abiotic stresses. These treatment sets emphasise the multiplicity of pathways affecting alternative electron transport components in plants.

A reporter gene system used to study developmental expression of alternative oxidase and isolate mitochondrial retrograde regulation mutants in Arabidopsis

Perturbation of mitochondrial function causes altered nuclear gene expression in plants. To study this response, called mitochondrial retrograde regulation, and developmental gene expression, a transgenic Arabidopsis thaliana (Col-0) line containing a firefly luciferase gene controlled by a promoter region of the Arabidopsis alternative oxidase 1a gene (AtAOX1a) was created. The transgene and the endogenous gene were developmentally induced in young cotyledons to a level higher than in older cotyledons and leaves. Analysis of transgene expression suggests that this is true for emerging leaves as well. Antimycin A (AA), a mitochondrial electron transport chain inhibitor, and monofluroacetate (MFA), a TCA cycle inhibitor, induced expression of the transgene and the endogenous gene in parallel. The following comparative responses of the transgene to inhibitors were observed: (a) the response in cotyledons to AA treatment differed greatly in magnitude from the response in leaves; (b) the induction kinetics in cotyledons following MFA treatment differed greatly from the kinetics in leaves; and (c) the induction kinetics following MFA treatment differed from the kinetics of AA in both leaves and cotyledons. The transgenic line was used in a genetic screen to isolate mutants with greatly decreased transgene and AtAOX1a induction in response to AA. Some of these mutant lines showed greatly decreased induction by MFA, but one did not. Taken altogether, the data provide genetic evidence that suggests that induction of the AtAOX1a gene by distinct mitochondrial perturbations are via distinct, but overlapping signaling pathways that are tissue specific.

A reporter gene system used to study developmental expression of alternative oxidase and isolate mitochondrial retrograde regulation mutants in Arabidopsis

Perturbation of mitochondrial function causes altered nuclear gene expression in plants. To study this response, called mitochondrial retrograde regulation, and developmental gene expression, a transgenic Arabidopsis thaliana (Col-0) line containing a firefly luciferase gene controlled by a promoter region of the Arabidopsis alternative oxidase 1a gene (AtAOX1a) was created. The transgene and the endogenous gene were developmentally induced in young cotyledons to a level higher than in older cotyledons and leaves. Analysis of transgene expression suggests that this is true for emerging leaves as well. Antimycin A (AA), a mitochondrial electron transport chain inhibitor, and monofluroacetate (MFA), a TCA cycle inhibitor, induced expression of the transgene and the endogenous gene in parallel. The following comparative responses of the transgene to inhibitors were observed: (a) the response in cotyledons to AA treatment differed greatly in magnitude from the response in leaves; (b) the induction kinetics in cotyledons following MFA treatment differed greatly from the kinetics in leaves; and (c) the induction kinetics following MFA treatment differed from the kinetics of AA in both leaves and cotyledons. The transgenic line was used in a genetic screen to isolate mutants with greatly decreased transgene and AtAOX1a induction in response to AA. Some of these mutant lines showed greatly decreased induction by MFA, but one did not. Taken altogether, the data provide genetic evidence that suggests that induction of the AtAOX1a gene by distinct mitochondrial perturbations are via distinct, but overlapping signaling pathways that are tissue specific.

Analysis of the alternative oxidase promoters from soybean

Alternative oxidase (Aox) is a nuclear-encoded mitochondrial protein. In soybean (Glycine max), the three members of the gene family have been shown to be differentially expressed during normal plant development and in response to stresses. To examine the function of the Aox promoters, genomic fragments were obtained for all three soybean genes: Aox1, Aox2a, and Aox2b. The regions of these fragments immediately upstream of the coding regions were used to drive beta-glucuronidase (GUS) expression during transient transformation of soybean suspension culture cells and stable transformation of Arabidopsis. The expression patterns of the GUS reporter genes in soybean cells were in agreement with the presence or absence of the various endogenous Aox proteins, determined by immunoblotting. Deletion of different portions of the upstream regions identified sequences responsible for both positive and negative regulation of Aox gene expression in soybean cells. Reporter gene analysis in Arabidopsis plants showed differential tissue expression patterns driven by the three upstream regions, similar to those reported for the endogenous proteins in soybean. The expression profiles of all five members of the Arabidopsis Aox gene family were examined also, to compare with GUS expression driven by the soybean upstream fragments. Even though the promoter activity of the upstream fragments from soybean Aox2a and Aox2b displayed the same tissue specificity in Arabidopsis as they do in soybean, the most prominently expressed endogenous genes in all tissues of Arabidopsis were of the Aox1 type. Thus although regulation of Aox expression generally appears to involve the same signals in different species, different orthologs of Aox may respond variously to these signals. A comparison of upstream sequences between soybean Aox genes and similarly expressed Arabidopsis Aox genes identified common motifs.

A tomato alternative oxidase protein with altered regulatory properties

We have investigated the expression and regulatory properties of the two alternative oxidase (Aox) proteins that are expressed in tomato (Lycopersicon esculentum L. Mill cv. Sweetie) after storage of green fruit at 4 degrees C. Four Aox genes were identified in the tomato genome, of which two (LeAox1a and LeAox1b) were demonstrated to be expressed in cold-treated fruit. The activity and regulatory properties of LeAox1a and LeAox1b were assayed after expression of each protein in yeast cells (Saccharomyces cerevisiae), proving that each is an active Aox protein. The LeAox1b protein was shown to have altered regulatory properties due to the substitution of a Ser for the highly conserved Cys(I) residue. LeAox1b could not form inactive disulfide-linked dimers and was activated by succinate instead of pyruvate. This is the first example of a dicot species expressing a natural Cys(I)/Ser isoform. The implications of the existence and expression of such Aox isoforms is discussed in the light of the hypothesised role for Aox in plant metabolism.

Mosaic (MSC) cucumbers regenerated from independent cell cultures possess different mitochondrial rearrangements

Passage of the highly inbred cucumber ( Cucumis sativus L.) line B through cell culture produces progenies with paternally transmitted, mosaic (MSC) phenotypes. Because the mitochondrial genome of cucumber shows paternal transmission, we evaluated for structural polymorphisms by hybridizing cosmids spanning the entire mitochondrial genome of Arabidopsis thaliana L. to DNA-gel blots of four independently generated MSC and four wild-type cucumbers. Polymorphisms were identified by cosmids carrying rrn18, nad5-exon2, rpl5, and the previously described JLV5 deletion. Polymorphisms revealed by rrn18 and nad5-exon2 were due to one rearrangement bringing together these two coding regions. The polymorphism revealed by rpl5 was unique to MSC16 and was due to rearrangement(s) placing the rpl5 region next to the forward junction of the JLV5 deletion. The rearrangement near rpl5 existed as a sublimon in wild-type inbred B, but was not detected in the cultivar Calypso. Although RNA-gel blots revealed reduced transcription of rpl5 in MSC16 relative to wild-type cucumber, Western analyses revealed no differences for the RPL5 protein and the genetic basis of the MSC16 phenotype remains enigmatic. We evaluated 17 MSC and wild-type lines regenerated from independent cell-culture experiments for these structural polymorphisms and identified eight different patterns, indicating that the passage of cucumber through cell culture may be a unique mechanism to induce or select for novel rearrangements affecting mitochondrial gene expression.

Respiratory gene expression in soybean cotyledons during post-germinative development

Gene expression for nuclear- and mitochondrial-encoded subunits of respiratory chain components was measured in developing soybean cotyledons and compared to the abundance of the relevant proteins. Overall respiratory gene expression peaked at day 16, close to the peak in cytochrome chain and TCA cycle activities from day 10 to 15. Protein abundance followed transcript abundance for all components examined with the exception of the F1beta subunit of ATP synthase. A dramatic peak in F1beta transcript levels early in development (day 5 to 7) was not mirrored by an increase in protein suggesting translational or post-translational control. Mitochondrial-encoded transcripts were at least 10-fold more abundant than nuclear-encoded transcripts. The pattern of transcript and protein abundance for uncoupling proteins displayed a trend similar to other respiratory proteins examined, implicating similar control mechanisms. The expression of alternative oxidase differed, increasing throughout development with protein peaking at day 20, perhaps suggesting a role in senescence. Overall, this study indicated that respiratory gene expression and protein abundance is co-ordinated with respiratory activity for most components but that some components, such as the F1beta subunit may be under discrete forms of regulation.

Differential expression of alternative oxidase genes in maize mitochondrial mutants

We have examined the expression of three alternative oxidase (aox) genes in two types of maize mitochondrial mutants. Nonchromosomal stripe (NCS) mutants carry mitochondrial DNA deletions that affect subunits of respiratory complexes and show constitutively defective growth. Cytoplasmic male-sterile (CMS) mutants have mitochondrial DNA rearrangements, but they are impaired for mitochondrial function only during anther development. In contrast to normal plants, which have very low levels of AOX, NCS mutants exhibit high expression of aox genes in all nonphotosynthetic tissues tested. The expression pattern is specific for each type of mitochondrial lesion: the NADH dehydrogenase-defective NCS2 mutant has high expression of aox2, whereas the cytochrome oxidase-defective NCS6 mutant predominantly expresses aox3. Similarly, aox2 and aox3 can be induced differentially in normal maize seedlings by specific inhibitors of these two respiratory complexes. Translation-defective NCS4 plants show induction of both aox2 and aox3. AOX2 and AOX3 proteins differ in their ability to be regulated by reversible dimerization. CMS mutants show relatively high levels of aox2 mRNAs in young tassels but none in ear shoots. Significant expression of aox1 is detected only in NCS and CMS tassels. The induction pattern of maize aox genes could serve as a selective marker for diverse mitochondrial defects.

Regulation of alternative oxidase gene expression in soybean

Soybean (Glycine max cv. Stevens) suspension cells were used to investigate the expression of the alternative oxidase (Aox) multigene family. Suspension cells displayed very high rates of cyanide-insensitive respiration, but Aox3 was the only isoform detected in untreated cells. Incubation with antimycin A, citrate, salicylic acid or at low temperature (10 degrees C) specifically induced the accumulation of the Aox1 isoform. Aox2 was not observed under any conditions in the cells. Increases in Aox1 protein correlated with increases in Aox1 mRNA. Treatment of soybean cotyledons with norflurazon also induced expression of Aox1. Reactive oxygen species (ROS) were detected upon incubation of cells with antimycin, salicylic acid or at low temperature, but not during incubation with citrate. Aox1 induction by citrate, but not by antimycin, was prevented by including the protein kinase inhibitor staurosporine in the medium. The results suggest that multiple pathways exist in soybean to regulate expression of Aox genes and that Aox1 specifically is induced by a variety of stress and metabolic conditions via at least two independent signal transduction pathways.

AOX1c, a novel rice gene for alternative oxidase; comparison with rice AOX1a and AOX1b

A novel gene for alternative oxidase (AOX) was isolated from rice (Oryza sativa L.) and characterized. The deduced amino acid sequence of the novel AOX gene contains features that are conserved among other AOXs. This AOX gene was designated AOX1c based on a phylogenetic analysis of the AOX genes. Northern hybridization analyses revealed that AOX1c and AOX1a/AOX1b transcripts accumulated differently in various rice organs and rice seedlings under low temperature conditions. AOX1c mRNA was mainly present in young leaves under constant light, mature leaves and panicles after heading, but it was not detected in young etiolated leaves and young roots of seedlings or young panicles. On the other hand, the mRNAs of the rice AOX1a and AOX1b genes were mainly present in young roots and mature leaves. Under low temperature conditions, the steady-state mRNA levels of the rice AOX1a and AOX1b genes clearly increased with time but the rice AOX1c gene was apparently not responsive to low temperature. The rice AOX gene family and differences in their regulation are discussed.

Regulation of alternative oxidase activity during phosphate deficiency in bean roots (Phaseolus vulgaris)

Cyanide-resistant respiration was studied in mitochondria isolated from the roots of bean plants (Phaseolus vulgaris L. cv. Złota Saxa) grown hydroponically up to 16 days on a phosphate-sufficient (+P, control) or phosphate-deficient (-P) medium. Western blotting indicated that the alternative oxidase (AOX) was present only in its reduced (active) form, both in phosphate-sufficient and phosphate-deficient roots, but in the latter, the amount of AOX protein was greater. Addition of pyruvate to the isolation, washing and reaction media made mitochondria from +P roots cyanide-insensitive, similar to mitochondria from -P roots. The doubled activity of NAD-malic enzyme (NAD-ME) in -P compared with +P root mitochondria may suggest increased pyruvate production in -P mitochondria. Lower cytochrome c oxidase (COX) activity and no uncoupler effect on respiration indicated limited cytochrome chain activity in -P mitochondria. In -P mitochondria, the oxygen uptake decreased and the level of Q reduction increased from 60 to 80%. With no pyruvate present (AOX not fully activated), inhibition of the cytochrome pathway resulted in an increased level of the ratio of reduced ubiquinone (Qr) to total ubiquinone (Qt) (Qr/Qt) in +P mitochondria, but did not change Qr/Qt in -P mitochondria. When pyruvate was present, the kinetics for AOX were similar in mitochondria from -P and +P roots. It is suggested that AOX participation in -P respiration may provide an acclimation to phosphate deficiency. Stabilization of the ubiquinone reduction level by AOX might prevent the harmful effect of an increased formation of reactive oxygen species.

The expression of alternative oxidase and uncoupling protein during fruit ripening in mango

The expression of alternative oxidase (Aox) and uncoupling proteins (Ucp) was investigated during ripening in mango (Mangifera indica) and compared with the expression of peroxisomal thiolase, a previously described ripening marker in mango. The multigene family for the Aox in mango was expressed differentially during ripening. Abundance of Aox message and protein both peaked at the ripe stage. Expression of the single gene for the Ucp peaked at the turning stage and the protein abundance peaked at the ripe stage. Proteins of the cytochrome chain peaked at the mature stage of ripening. The pattern of protein accumulation suggested that increases in cytochrome chain components played an important role in facilitating the climacteric burst of respiration and that the Aox and Ucp may play a role in post-climacteric senescent processes. Because both message and protein for the Aox and Ucp increased in a similar pattern, it suggests that their expression is not controlled in a reciprocal manner but may be active simultaneously.

Characterization of the gene family encoding alternative oxidase from Candida albicans

Candida albicans possesses a cyanide-resistant respiratory pathway mediated by alternative oxidase (AOX), which seems to be encoded by a gene family with two members. Cloning and expression of AOX1a, one of the genes encoding alternative oxidase from C. albicans, has previously been reported [Huh and Kang (1999) J. Bacteriol. 181, 4098-4102]. In the present study we report the isolation of another gene coding for alternative oxidase, designated AOX1b. AOX1b contains a continuous open reading frame that encodes a polypeptide consisting of 365 amino acids. Interestingly, AOX1a and AOX1b were found to be located in tandem on one of the chromosomes of C. albicans. The presence of cyanide in the culture medium remarkably retarded the growth of the aox1a/aox1a mutants. The growth of the aox1b/aox1b mutants and the aox1a/aox1a aox1b/aox1b double mutants was almost completely inhibited in the same medium. beta-Galactosidase reporter assays indicated that, whereas AOX1a was expressed constitutively, the expression of AOX1b was dependent on growth phase and was induced by treatment with cyanide, antimycin A, H(2)O(2), menadione and paraquat. Growth of the cells in media with non-fermentable carbon sources also enhanced the expression of AOX1b. CaSLN1, which encodes a histidine kinase, seems to be involved in the regulation of AOX expression in C. albicans on the basis of the observation that the activity of cyanide-resistant respiration and the expression level of AOX in the casln1/casln1 mutants were found to be significantly low under normal conditions and slightly increased in the presence of respiratory inhibitors compared with the wild-type strain. Like AOX1a, AOX1b could also be functionally expressed in AOX-deficient Saccharomyces cerevisiae and confer cyanide-resistant respiration on the organism.

The gene for alternative oxidase-2 (AOX2) from Arabidopsis thaliana consists of five exons unlike other AOX genes and is transcribed at an early stage during germination

We investigated the expressions of genes for alternative oxidase (AOX1a, AOX1b, AOX1c and AOX2) and genes for cytochrome c oxidase (COX5b and COX6b) during germination of Arabidopsis thaliana, and examined oxygen uptakes of the alternative respiration and the cytochrome respiration in imbibed Arabidopsis seeds. A Northern blot analysis showed that AOX2 mRNA has already accumulated in dry seeds and subsequently decreased, whereas accumulation ofAOX1a mRNA was less abundant from 0 hours to 48 hours after imbibition and then increased. The increase of the capacity of the alternative pathway appeared to be dependent on the expressions of both AOX2 and AOX1a. On the other hand, steady-state mRNA levels of COX5b and COX6b were gradually increased during germination, and the capacity of the cytochrome pathway was correlated with the increase of expressions of the COX genes. Antimycin A, the respiratory inhibitor, strongly increased the expression of AOX1a but had no effect on the expression of AOX2. A 5'RACE analysis showed that AOX2 consists of five exons, which is different from the case of most AOX genes identified so far. Analysis of subcellular localization of AOX2 using green fluorescent protein indicated that the AOX2 protein is imported into the mitochondria.

Control of plant mitochondrial respiration

Plant mitochondria are characterised by the presence of both phosphorylating (cytochrome) and non-phosphorylating (alternative) respiratory pathways, the relative activities of which directly affect the efficiency of mitochondrial energy conservation. Different approaches to study the regulation of the partitioning of reducing equivalents between these routes are critically reviewed. Furthermore, an updated view is provided regarding the understanding of plant mitochondrial respiration in terms of metabolic control. We emphasise the extent to which kinetic modelling and 'top-down' metabolic control analysis improve the insight in phenomena related to plant mitochondrial respiration. This is illustrated with an example regarding the affinity of the plant alternative oxidase for oxygen.

Arachidonic acid stimulates cell growth and forms a novel oxygenated metabolite in Candida albicans

Infection of human tissues by Candida albicans has been reported to cause the release of arachidonic acid (AA), eicosanoids and other proinflammatory mediators from host cells. Therefore, we investigated the interaction of this pathogen with AA. AA stimulated cell growth at micromolar concentrations when used as a sole carbon source. Moreover, it selectively inhibited the antimycin A-resistant alternative oxidase. [1-(14)C]AA was completely metabolised by C. albicans. Only one-seventh of the radioactivity metabolised was found in CO(2), whereas two-thirds occurred in carbohydrates suggesting a predominant role of the glyoxalate shunt of citrate cycle. About 1% of radioactivity was found in polar lipids including eicosanoids. A novel AA metabolite, which revealed immunoreactivity with an antibody against 3(R)-hydroxy-oxylipins, was identified as 3, 18-dihydroxy-5,8,11,14-eicosatetraenoic acid. Using immunofluorescence microscopy, endogenous 3(R)-hydroxy-oxylipins were found in hyphae but not in yeast cells. Such compounds have recently been shown to be connected with the sexual stage of the life cycle of Dipodascopsis uninucleata. Together, we propose that infection-mediated release of AA from host cells may modulate cell growth, morphogenesis and invasiveness of C. albicans by several modes. A better understanding of its role is thus promising for novel approaches towards the treatment of human mycoses.

Import of precursor proteins into mitochondria from soybean tissues during development

Characterisation of the amount of protein import of the alternative oxidase (AOX) and the F(A)d precursor proteins (previously shown to use different import pathways) into mitochondria from developing soybean tissues indicated that they displayed different patterns. Import of the AOX declined in both cotyledon and root mitochondria with increasing age, whereas the import of the F(A)d into cotyledon mitochondria remained high throughout the same period. Using primary leaf mitochondria, it was evident that import of AOX remained high while it declined in cotyledon and root mitochondria. The amount of import of the AOX into mitochondria from different tissues closely matched the amount of the Tom 20 receptor.

An alternative oxidase monoclonal antibody recognises a highly conserved sequence among alternative oxidase subunits

The alternative oxidase is found in the inner mitochondrial membranes of plants and some fungi and protists. A monoclonal antibody raised against the alternative oxidase from the aroid lily Sauromatum guttatum has been used extensively to detect the enzyme in these organisms. Using an immunoblotting strategy, the antibody binding site has been localised to the sequence RADEAHHRDVNH within the soybean alternative oxidase 2 protein. Examination of sequence variants showed that A2 and residues C-terminal to H7 are required for recognition by the monoclonal antibody raised against the alternative oxidase. The recognition sequence is highly conserved among all alternative oxidase proteins and is absolutely conserved in 12 of 14 higher plant sequences, suggesting that this antibody will continue to be extremely useful in studying the expression and synthesis of the alternative oxidase.