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Plant Physiology 144:535-536 (2007) © 2007 American Society of Plant Biologists On the InsideMagnetic Resonance Imaging of Long-Distance TransportIt is difficult using conventional methods to measure transport within intact plants or to determine the conducting area of sap flow. Magnetic resonance imaging (MRI) is a promising noninvasive method for providing spatial and temporal quantitative information on water transport at different scales of resolution in intact plants. Scheenen et al. (pp. 1157–1165) have used MRI to quantify the long-distance xylem flow and hydraulics in intact cucumber (Cucumis sativus) plants. They demonstrated the accuracy of the MRI method to quantify sap flow and the conducting area of flow by measuring the flow characteristics of the water in a virtual slice through the stem and comparing the results with water uptake data and microscopy. The image resolution provided by MRI was high enough to distinguish large individual xylem vessels. Using MRI, the authors confirmed that cooling the roots of intact cucumber plants severely inhibited water uptake by the roots and increased the hydraulic resistance of the plant stem. This increase was (at least partially) due to the formation of embolisms in the xylem vessels. Refilling of the larger vessels was observed to be a lengthy process that began in the night after root cooling. Surprisingly, refilling continued while neighboring vessels at a distance of not more than 0.4 mm transported an equal amount of water as before root cooling. Relative differences in volume flow in different vascular bundles suggest differences in xylem tension in different vascular bundles. Toward a Hypoallergenic Peanut Cultivated peanut (Arachis hypogaea) is a valuable oilseed and food crop containing 45% to 53% oil and 24% to 29% protein in its seed. However, ingestion of peanut seeds is one of the most serious causes of fatal food-induced anaphylaxis (Fig. 1 ). The three major peanut allergens, Ara h 1, Ara h 2, and Ara h 3, are seed storage proteins and are generally recognized by more than 50% of peanut-allergic patients. Kang et al. (pp. 836–845) have characterized and compared the expression patterns of these three major peanut allergens during seed maturation and germination. They have also examined the spatial relationships between peanut allergen mRNAs and their corresponding proteins in seeds. Expression patterns were heterogeneous depending on the specific peanut allergen gene and the cultivars tested. However, ara h 3 expression patterns among the cultivars were more variable than ara h 1 and ara h 2. Transcripts were observed in seeds, but not in leaves, flowers, or roots, and were undetectable during seed germination. More ara h 1 and ara h 3 mRNA was detected in cotyledons relative to embryonic axes. Allergen polypeptide degradation patterns were different in embryonic axes compared with cotyledons during germination and seedling growth, with levels of Ara h 1 and Ara h 2 being dramatically reduced compared to the Ara h 3 polypeptides in embryonic axes. These characterization studies of major peanut allergen genes and their corresponding seed storage proteins provide basic information that is necessary for understanding possible mechanisms to control the synthesis and degradation of peanut allergens for future production of a hypoallergenic peanut.
Ca2+ Signaling in Arbuscular Mycorrhizal Symbiosis The generation of a transient Ca2+ elevation after perception of the rhizobial signaling molecule Nod factor is documented as one of the earliest plant responses in legume-rhizobia association. Oscillations in cytosolic free Ca2+ concentration ([Ca2+]cyt) have been observed in legume root hairs following the initial rapid [Ca2+]cyt change. Navazio et al. (pp. 673–681) now present evident that Ca2+ oscillations may also be involved in the formation of symbiotic interactions between roots and arbuscular mycorrhizae (AM). The authors have used soybean (Glycine max) cell cultures stably expressing the bioluminescent Ca2+ indicator aequorin to detect intracellular Ca2+ changes in response to the culture medium of spores of Gigaspora margarita germinating in the absence of the plant partner. Rapid and transient elevations in cytosolic free Ca2+ were recorded, indicating that diffusible molecules are released by the mycorrhizal fungus and perceived by host plant cells through Ca2+-mediated signaling pathways. Similar responses were also triggered by two Glomus isolates. The fungal molecules were found to be heat stable, lipophilic, and of low molecular mass (<3 kD). Evidence for the specificity of such an early fungal signal to the AM symbiosis is suggested by the lack of a Ca2+ response in cultured cells of the nonhost plant Arabidopsis (Arabidopsis thaliana). The up-regulation in soybean cells of genes encoding Ca2+/calmodulin-dependent protein kinases is also indicative of the involvement of a Ca2+ signaling transduction pathway associated with AM symbiosis. Apyrases Regulate Plant Growth Plant cells release significant quantities of ATP into their cell wall when they are mechanically stimulated or wounded, or when they are engaged in activities that involve active secretion, such as growth. Moreover, plant cells respond to submicromolar concentrations of extracellular ATP (eATP), and the extensive depletion of eATP can lead to cell death. Conceivably, apyrases, which are far more efficient in removing phosphates from NTP/NDP than other phosphatases, might play an important role in regulating eATP levels in planta. Most apyrases are ectoapyrases, i.e. enzymes that are anchored in the plasma membrane with their active site facing the extracellular matrix of cells. In animal cells, ectoapyrases play a crucial role in terminating signal transduction initiated by extracellular nucleotides. Arabidopsis has seven apyrases, two of which, APY1 and APY2, are similar to the known pea ectoapyrase NTP9. Wu et al. (pp. 961–975) report that the highest expression of both apyrases in Arabidopsis was in rapidly growing tissues and/or tissues that accumulate high auxin levels. Red-light treatment of etiolated seedlings suppressed the protein and message level of both apyrases and inhibited hypocotyl growth. Adult apy1 and apy2 single mutants exhibited almost normal growth, but apy1apy2 double-knockout plants were dwarf. Pollen tubes and etiolated hypocotyls overexpressing an apyrase had enhanced growth rates. Elongating pollen tubes released ATP into the growth medium. The suppression of apyrase activity by antiapyrase antibodies or by inhibitors simultaneously increased medium ATP levels and inhibited pollen tube growth. These results imply that APY1 and APY2, like their homologs in animals, act to reduce the concentration of extracellular nucleotides, and that this function is important for the regulation of growth in Arabidopsis. Transcriptomics of Nematode Resistance Root-knot nematodes (Meloidogyne spp.) are obligate parasites of essentially all vascular plants and lower production of most crops. Central to the parasitic interaction is the ability of the nematode to reprogram root parenchyma cells to differentiate into highly specialized feeding cells called giant cells. Many plant processes and physiological parameters are affected by the induction of giant cells. Effective resistance genes do exist for a few plant species, including the Mi gene of tomato (Solanum lycopersicum). Schaff et al. (pp. 1079–1092) have compared the root transcriptomes of tomato cultivars resistant (‘Motelle’) and susceptible (‘Moneymaker’) before and after the addition of the root-knot nematode Meloidogyne incognita. In the absence of root-knot nematode infection, only one gene was found to be differentially regulated between the susceptible ‘Moneymaker’ and the resistant ‘Motelle’ transcriptomes—a gene encoding a glycosyltransferase. Experimental down-regulation of this gene via virus-induced gene silencing restored susceptibility to M. incognita in ‘Motelle,’ indicating that its function is necessary for Mi-mediated resistance. Glycosyltransferases have been implicated in carbohydrate biosynthesis and associated with plant stress and defense responses and with cell wall synthesis. This is the first report (to the authors' knowledge) of a role for a glycosyltransferase in nematode resistance. Root nematode infection also influenced the expression of broad suites of genes; more than half of the probes on the array identified differential gene regulation between infected and uninfected root tissue at some stage of root-knot nematode infection. Regulation of Leaf Size in Arabidopsis Recent work has provided evidence for the organ-wide coordination of cell proliferation and expansion. When cell proliferation in a leaf primordium is reduced because of certain mutations, the reduction in the final leaf area is compensated for by an increase in the size of individual leaf cells. This "compensation" phenomenon could aid in the understanding of the regulation of cell proliferation and expansion at the organ level. To elucidate the mechanisms of compensation, Ferjani et al. (pp. 988–999) have isolated five new Arabidopsis mutants (fugu1–fugu5) that exhibit compensation. These mutants were characterized together with angustifolia3, erecta, and a KIP-RELATED PROTEIN2 overexpressor, previously reported to exhibit compensation. The authors report that significant cell enlargement in the various mutants was caused by enhanced cell expansion either during cell proliferation or after mitosis. Furthermore, the increase in postmitotic cell expansion occurred in two ways: through either an increased expansion rate or an increased expansion period. Flow cytometric analyses revealed that increases in ploidy level are not always required to trigger compensation, suggesting that compensation is only partially mediated by ploidy-dependent processes. These results suggest that compensation reflects an organ-wide coordination of cell proliferation and expansion in determinate organs, and involves at least three different expansion pathways.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900227 Related articles in Plant Physiol.:
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