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Plant Physiology 133:425-426 (2003) © 2003 American Society of Plant Biologists Modeling Photosynthesis in an Ancient Polar ForestToday's permanent polar ice-sheets are a recent phenomenon that appeared some 34 million years ago (Ma) in Antarctica and 3 Ma in the Arctic. Earlier periods of global warmth extending back to 280 Ma enabled forests to cover the polar regions and reach latitudes as high as 85° in both hemispheres. These ancient high latitude forests grew in an environment unlike any on Earth today, with mean winter temperatures above freezing, and an atmospheric pCO2 enriched over current ambient levels. But, in common with modern polar vegetation, they would have experienced strong seasonality in daylength. Fossil evidence suggests that these polar forests were largely deciduous, but what advantage did these ancient forests gain by their deciduous habit? Many plant biologists have interpreted the deciduous habit in general as an adaptation for minimizing carbon losses during winter. Researchers, however, have recently simulated a warm climate of 100-40 Ma, and found that the quantity of carbon lost through complete abscission of deciduous leaf canopies greatly exceeds that lost through wintertime respiration and abscission from evergreen canopies. Despite the higher carbon loss, deciduous trees achieve similar rates of net primary production to their evergreen counterparts. In this issue, Osborne and Beerling (pp. 803-812) investigate mechanisms underlying this apparent paradox by measuring the seasonal patterns of leaf photosynthesis under pCO2 enrichment in three closely related 'living fossil' species of conifer: the evergreen Coastal Redwood (Sequoia sempervirens); the deciduous Dawn Redwood (Metasequoia glyptostroboides); and the deciduous Swamp Cypress (Taxodium distichum). Their calculations show that the carbon required for growth of the deciduous leaf canopies could be fixed in around 10 to 25 d, a period representing only 5% to 15% of the six-month growing season. These results suggest that the deciduous leaf habit may not have been as energetically costly to the polar forests as previously postulated. Aquaporins: A Role in Winter Embolism Recovery? Winter embolism, the generation of air bubbles in xylem vessels attributable to freezing and thawing, often leads to a loss of hydraulic conductivity in xylem vessels. Vulnerability to winter embolism affects the ability of plants to survive cold climates and the geographic distribution of species. Plants minimize the impact of winter embolism by replacing embolized vessels by new functional vessels every year and/or by refilling embolized vessels by generating positive xylem pressures. Although making new vessels is common to all the plants that exhibit secondary growth, the generation of positive xylem pressures has only been reported in a few species including walnut (Juglans regia). The refilling of embolized xylem vessels in walnut requires an osmotic force that is created by the breakdown of starch and the accumulation of soluble sugars in the vessels. The subsequent lowering of the vessels' water potentials leads to water movement from the parenchyma cells into the xylem vessels. In this issue, Sakr et al. (pp. 630-641) examine the possible role of plasma membrane aquaporins in winter embolism recovery in walnut They report their successful isolation of two PIP2 aquaporin genes (JrPIP2,1 and JrPIP2,2) that encode nearly identical proteins. The water channel activity of JrPIP2,1 was demonstrated by its expression in Xenopus laevis oocytes. Immunolocalization studies show that PIP2 aquaporins were mainly localized in vessel-associated cells in the winter. During winter, high levels of PIP2 mRNA and corresponding protein increased simultaneously with the rise in Suc. These results suggest that PIP2 aquaporins may play a role in water transport between xylem parenchyma cells and embolized vessels. ATP Signaling in Plants? In addition to being a ubiquitous energy source, ATP also acts extracellularly as a neurotransmitter. Although eATP has been reported to affect plant function, the mechanisms underlying these effects have largely been interpreted as involving supplementation of cellular energy or chelation of divalent cations rather than signaling activity. In this issue, Demidchik et al. (pp. 456-461) review the plant literature concerning the effects of eATP on plant function and conclude that much of this previous data is consistent with the existence of eATP signaling in plants They also report that eATP causes a dose-dependent increase in cytoplasmic calcium ([Ca2+]cyt) in aequorin-expressing Arabidopsis roots. Purines were found to be more effective than pyrimidine in eliciting these [Ca2+]cyt transients. Extracellular Ca2+ depletion abolished most of the [Ca2+]cyt increase induced by eATP, demonstrating eATP-activated Ca2+ entry from the extracellular space. Gd3+ as well as two purinoceptor inhibitors almost eliminated the eATP-induced [Ca2+]cyt transients. In animals, neurotransmitters establish and maintain a communication network between neurons. eATP could play a similar role in plants, perhaps providing intercellular communication to coordinate developmental programs or responses to environmental stimuli. Release of ATP could be from damaged cells or as a result of cell death. In this respect, a role in defense signaling could be envisaged. In contrast to the animal paradigm, certain plant cells could also be exposed to eATP released by other organisms. Since microbes can release ATP and other purines, plant-microbe interaction at the epidermis may perhaps involve eATP as a signaling agent. Regulation of DNA Synthesis by a G-Protein-Coupled Receptor and Phospholipase C
Many lines of evidence suggest that specific events during the cell cycle are mediated by a heterotrimeric G-protein activated by a cognate G-protein coupled receptor. However, coupling between the only known G Maize (Zea mays) Nitrilase and IAA Biosynthesis The details of IAA biosynthesis are not fully understood, probably because there are parallel pathways that may work together or could be differentially regulated dependent on organs, developmental stages or environmental conditions. Originally, the amino acid Trp was identified as the precursor of IAA, and IAA synthesis was suggested to occur by deamination and decarboxylation of Trp. There is growing evidence, however, that both Trp-dependent and Trp-independent pathways of auxin biosynthesis can operate in a single plant. The alternative pathway of forming IAA may involve the conversion of indole-3-acetaldoxime to indole-3-acetonitrile (IAN). Nitrilases could then potentially hydrolyze IAN to its corresponding carboxylic acid (IAA). In this issue, Park et al. (pp. 794-802) report upon their successful cloning of two nitrilase genes ZmNIT1 and ZmNIT2 from maize. ZmNIT2 converts IAN to IAA 7 to 20 times more efficiently than its counterparts in Arabidopsis. Quantitative real time PCR revealed the gene expression of both nitrilase types in maize kernels. Nitrilase protein and endogenous nitrilase activity are also present in maize kernels together with the substrate IAN. These results suggest a role for ZmNIT2 in auxin biosynthesis. Transgenic Plants Reduced in Heavy Metal Uptake Large parts of agricultural soil are contaminated with lead (Pb) and cadmium (Cd). To reduce heavy metal in the food chain, plants that transfer less heavy metals to the shoot are required. In this issue, Lee et al. (pp. 589-596) report that an Escherichia coli gene ZntA, which encodes a Pb(II)/Cd(II)/Zn(II) pump, is useful for developing plants with reduced heavy metal content. In Arabidopsis plants transformed with ZntA, ZntA was localized at the plasma membrane and improved the resistance of the plants to Pb(II) and Cd(II). The shoots of the transgenic plants had decreased Pb and Cd content. Moreover, the transgenic protoplasts showed lower accumulation of Cd and faster release of preloaded Cd than wild-type protoplasts. These results show that a bacterial transporter gene, ZntA, can be functionally expressed in plant cells, and that that it may be useful for the development of crop plants that are safe from heavy metal contamination. Crystal Matrix Protein Associated with Calcium Oxalate Precipitation Many plants produce calcium (Ca) oxalate as crystalline deposits. The formation of calcium oxalate crystals is regarded as a high capacity mechanism for regulating Ca. More than 90% of tissue Ca can be stored as this compound in some species. Ca oxalate crystals usually occur within the vacuole of specialized cells called idioblasts. Dissolution of crystals from Pistia stratiotes leaves behind a crystal shaped matrix "ghost" which is capable of precipitation of calcium oxalate in the original crystal morphology. To assess whether this matrix has a protein component, Li et al. (pp. 736-747) isolated purified crystals and analyzed them for internal protein. PAGE revealed the presence of one major polypeptide of about 55 kD and two minor species of 60 kD and 63 kD. Amino acid analyses indicated that the matrix protein is relatively high in acidic amino acids, and 45Ca binding assays demonstrated that the matrix protein has a strong affinity for Ca. Immunocytochemical localization using antibody raised to the isolated protein showed that the matrix protein is specific to idioblasts. The anti-matrix protein serum labeled both the surface and the internal structures of two Ca oxalate crystal forms within the vacuole. These results demonstrate that a specific Ca binding protein exists as an integral component of calcium oxalate crystals.
Department of Natural Sciences Mercy College Dobbs Ferry, NY 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.900092. Related articles in Plant Physiol.:
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-subunit of the heterotrimeric G-protein (GPA1) and the only putative G-protein coupled receptor (GCR1) of plants has never been shown. In this issue, Apone et al. (