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Plant Physiology 139:1571-1572 (2005) © 2005 American Society of Plant Biologists On the InsideTranscriptome of the Female GametophyteBecause the female gametophytes of flowering plants are small and embedded within the maternal tissues of the ovule it is difficult to isolate them free of contaminating maternal tissue. Thus, relatively little is known about the genes that are expressed during female gametophyte development. It is not even known, for example, whether the female gametophyte transcriptome contains a major set of genes that are not expressed in the sporophyte or whether it is primarily a subset of the sporophytic transcriptome. Yu et al. (pp. 1853–1869) have sidestepped the problem of the female gametophyte being embedded within the maternal ovule tissue by utilizing the Arabidopsis (Arabidopsis thaliana) mutant sporocyteless that produces ovules lacking female gametophytes. Using an Arabidopsis whole-genome oligonucleotide array, they were able to identify a minimum of 225 genes as female gametophyte genes. Nearly 45% of the identified genes have not previously been detected by sporophytic expression profiling, suggesting that the female gametophyte transcriptome may contain a significant fraction of transcripts restricted to the gametophyte. Plant Defense against Aphids The green peach aphid (Myzus persicae; Fig. 1) has a wide host range covering greater than 50 families of plants and is the vector for more than 100 plant viruses. Unlike the better studied chewing insects, aphids do not cause extensive wounding to the plant host, suggesting that plant responses to phloem-feeding insects may differ significantly from those elicited by chewing insects. Aphid feeding causes changes in resource allocation in the host, resulting in increased nutrient flow to the insect-infested tissue. Pegadaraju et al. (pp. 1927–1934) have hypothesized that premature leaf senescence would be useful in limiting aphid growth. The authors report that green peach aphid feeding upon Arabidopsis leaves induced premature chlorosis and cell death and increased the expression of SENESCENCE ASSOCIATED GENES, all hallmarks of leaf senescence. Premature senescence was accompanied by enhanced resistance against green peach aphid in two mutants of Arabidopsis that have elevated expression of SENESCENCE ASSOCIATED GENES. In contrast, resistance against green peach aphid was compromised in the pad4 mutant plant. PAD4 is associated with the synthesis of camalexin, an antimicrobial phytoalexin, and with salicylic acid (SA) signaling. Studies with other mutants that are defective in camalexin synthesis or SA signaling, however, indicated that camalexin and SA have no bearing on aphid resistance. Hence, the involvement of PAD4 in Arabidopsis defense against green peach aphid is most likely independent of its role in SA signaling and camalexin biosynthesis.
Metabolite Profiling of Chlamydomonas Metabolic profiles reflect the dynamic response of biochemical pathway networks to environmental, genetic, or developmental signals. As such, they provide valuable information concerning how a living system adjusts to its changing environment. Bölling and Fiehn (pp. 1995–2005) have developed a metabolite-profiling technique for Chlamydomonas reinhardtii cells. The experimental protocol was optimized to quickly inactivate enzymatic activity, achieve maximum extraction capacity, and process large sample quantities. As a result of the rapid sampling, extraction, and analysis by gas chromatography coupled to time-of-flight mass spectrometry, more than 800 analytes from a single sample can be measured. To demonstrate the power of their technique, the authors analyzed how metabolite profiles change under conditions of nitrogen, sulfur, phosphorus, and iron depletion, respectively. Sulfur-depleted cells exhibited the largest increase of any single compound: 4-Hyp accumulated more than 50-fold compared to control conditions. Hyp is a prominent constituent in the Hyp-rich glycoprotein framework forming the Chlamydomonas cell wall. Cell wall proteins are extensively sulfated and rearranged during sulfur starvation while several prolyl 4-hydroxylases are down-regulated. Thus, the rise in 4-Hyp could be the result of enhanced degradation of cell wall proteins. Two other findings of note were that phosphate removal results in a 25-fold rise in intracellular Cys, whereas sulfate removal causes 2-ketovaline levels to plummet to 2% of control levels. Alternative Oxidase and Oxidative Stress The cyanide-resistant alternative oxidase (AOX) of plant mitochondria accepts electrons from the ubiquinone pool and uses them to reduce oxygen to water, with no conservation of energy by the formation of proton gradients across the inner mitochondrial membrane. This raises the question of why a seemingly energy-wasteful pathway operates in plant mitochondria. One hypothesis is that AOX may be involved in acclimation to oxidative stress: AOX may function to prevent the formation of reactive oxygen species (ROS) by diverting reductants in excess of cytochrome pathway capacity down the AOX pathway. To date, the AOX connection with mitochondrial ROS has been investigated only in isolated mitochondria and suspension culture cells. To study ROS and AOX in whole plants, Umbach et al. (pp. 1806–1820) generated three classes of transformed lines of Arabidopsis: AtAOX1a overexpressors, AtAOX1a anti-sense plants, and overexpressors of a mutated, constitutively active AtAOX1a. In the presence of KCN, leaf tissue of AOX overexpressors showed no increase in oxidative damage, whereas anti-sense lines had levels of damage greater than those observed for untransformed leaves. Similarly, ROS production increased markedly in response to KCN treatment in anti-sense and untransformed, but not overexpressor, roots. Thus, AOX functions in leaves and roots, as in suspension cells, to ameliorate ROS production when the cytochrome pathway is chemically inhibited. In contrast to previous reports using suspension culture cells, no changes in leaf transcript levels of selected electron transport components or oxidative-stress-related enzymes were detected. Furthermore, a microarray study using an anti-sense line showed AOX influences processes outside mitochondria, particularly in chloroplasts and several carbon metabolism pathways. These results illustrate the value of expanding AOX transformant studies to whole tissues. The different transcriptional responses of leaves and cultured cells indicate that the effects of altered AOX levels need to be examined in intact tissues to achieve a better understanding of AOX function and its interaction with other metabolic systems in whole plants. In a companion paper, Fiorani et al. (pp. 1795–1805) tested the hypothesis that AOX plays a role in ameliorating plant stress under cold conditions by preventing excess accumulation of ROS. In plants grown at 12°C, AOX anti-sense plants showed 27% reduced leaf area and 25% smaller rosettes. In AOX overexpressors, the leaf areas and rosette size were 30% and 33% larger, respectively. These phenotypic differences were not the result of major alterations in tissue redox state because the changes in levels of lipid peroxidation products, reflecting oxidative damage, and the expression of genes encoding antioxidant and electron transfer chain redox enzymes did not correspond with the shoot phenotypes. These results demonstrate that: (1) AOX activity plays a role in shoot acclimation to low temperature in Arabidopsis and (2) AOX not only functions to prevent excess ROS formation in whole tissues under stressful environmental conditions but also affects metabolism through more pervasive effects, including some that are extramitochondrial. Rapamycin Sensitivity in C. reinhardtii The antibiotic rapamycin is a potent antifungal agent that also exhibits immunosuppressive activity due to its capacity to block the growth and proliferation of T cells. Although the vegetative growth of higher plants is unaffected by rapamycin, many players in the rapamycin signal transduction pathway are present in higher plants. Rapamycin's mechanism of action, based on research with Saccharomyces cerevisiae, involves the binding of rapamycin to the FK506-binding protein (FKBP12). This complex then inhibits a Ser/Thr kinase called TOR (target of rapamycin). Studies have identified FKBP12 in bacteria, fungi, animals, plants, and more recently in the green alga C. reinhardtii. However, the physiological function of this protein is still poorly understood in plants. In Arabidopsis, FKBP12 interacts with AtFIP37, a phosphatidylinositol kinase essential for development, whereas TOR kinase plays an important role in controlling plant cell growth and disruption of its gene leads to the premature arrest of endosperm and embryo development. Whereas Arabidopsis is insensitive to rapamycin, this drug does inhibit the growth of Chlamydomonas. Crespo et al. (pp. 1736–1749) report that Chlamydomonas lacking FKBP12 are fully resistant to the drug, indicating that this protein mediates rapamycin's inhibitory effect on cell growth. The authors also demonstrate that Chlamydomonas FKBP12, unlike its higher plant homolog, exhibits high affinity to rapamycin in vivo, and that TOR binds FKBP12 in the presence of rapamycin. It is also reported that rapamycin treatment results in a pronounced increase of vacuole size that resembles autophagy. These findings suggest that Chlamydomonas cell growth is positively controlled by a conserved TOR kinase.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900181. Related articles in Plant Physiol.:
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